I'm 13 days late, but 🎉 ReC98 is now 10 years old! 🎉 On June 26, 2014, I first tried exporting IDA's disassembly of TH05's OP.EXE and reassembling and linking the resulting file back into a binary, and was amazed that it actually yielded an identical binary. Now, this doesn't actually mean that I've spent 10 years working on this project; priorities have been shifting and continue to shift, and time-consuming mistakes were certainly made. Still, it's a good occasion to finally fully realize the good future for ReC98 that GhostPhanom invested in with the very first financial contribution back in 2018, deliver the last three of the first four reserved pushes, cross another piece of time-consuming maintenance off the list, and prepare the build process for hopefully the next 10 years.
But why did it take 8 pushes and over two months to restore feature parity with the old system? 🥲
The original plan for ReC98's good future was quite different from what I ended up shipping here. Before I started writing the code for this website in August 2019, I focused on feature-completing the experimental 16-bit DOS build system for Borland compilers that I'd been developing since 2018, and which would form the foundation of my internal development work in the following years. Eventually, I wanted to polish and publicly release this system as soon as people stopped throwing money at me. But as of November 2019, just one month after launch, the store kept selling out with everyone investing into all the flashier goals, so that release never happened.
In theory, this build system remains the optimal way of developing with old Borland compilers on a real PC-98 (or any other 32-bit single-core system) and outside of Borland's IDE, even after the changes introduced by this delivery. In practice though, you're soon going to realize that there are lots of issues I'd have to revisit in case any PC-98 homebrew developers are interested in funding me to finish and release this tool…
The main idea behind the system still has its charm: Your build script is a regular C++ program that #includes the build system as a static library and passes fixed structures with names of source files and build flags. By employing static constructors, even a 1994 Turbo C++ would let you define the whole build at compile time, although this certainly requires some dank preprocessor magic to remain anywhere near readable at ReC98 scale. 🪄 While this system does require a bootstrapping process, the resulting binary can then use the same dependency-checking mechanisms to recompile and overwrite itself if you change the C++ build code later. Since DOS just simply loads an entire binary into RAM before executing it, there is no lock to worry about, and overwriting the originating binary is something you can just do.
Later on, the system also made use of batched compilation: By passing more than one source file to TCC.EXE, you get to avoid TCC's quite noticeable startup times, thus speeding up the build proportional to the number of translation units in each batch. Of course, this requires that every passed source file is supposed to be compiled with the same set of command-line flags, but that's a generally good complexity-reducing guideline to follow in a build script. I went even further and enforced this guideline in the system itself, thus truly making per-file compiler command line switches considered harmful. Thanks to Turbo C++'s #pragma option, changing the command line isn't even necessary for the few unfortunate cases where parts of ZUN's code were compiled with inconsistent flags.
I combined all these ideas with a general approach of "targeting DOSBox": By maximizing DOS syscalls and minimizing algorithms and data structures, we spend as much time as possible in DOSBox's native-code DOS implementation, which should give us a performance advantage over DOS-native implementations of MAKE that typically follow the opposite approach.
Of course, all this only matters if the system is correct and reliable at its core. Tup teaches us that it's fundamentally impossible to have a reliable generic build system without
augmenting the build graph with all actual files read and written by each invoked build tool, which involves tracing all file-related syscalls, and
persistently serializing the full build graph every time the system runs, allowing later runs to detect every possible kind of change in the build script and rebuild or clean up accordingly.
Unfortunately, the design limitations of my system only allowed half-baked attempts at solving both of these prerequisites:
If your build system is not supposed to be generic and only intended to work with specific tools that emit reliable dependency information, you can replace syscall tracing with a parser for those specific formats. This is what my build system was doing, reading dependency information out of each .OBJ file's OMF COMENT record.
Since DOS command lines are limited to 127 bytes, DOS compilers support reading additional arguments from response files, typically indicated with an @ next to their path on the command line. If we now put every parameter passed to TCC or TLINK into a response file and leave these files on disk afterward, we've effectively serialized all command-line arguments of the entire build into a makeshift database. In later builds, the system can then detect changed command-line arguments by comparing the existing response files from the previous run with the new contents it would write based on the current build structures. This way, we still only recompile the parts of the codebase that are affected by the changed arguments, which is fundamentally impossible with Makefiles.
But this strategy only covers changes within each binary's compile or link arguments, and ignores the required deletions in "the database" when removing binaries between build runs. This is a non-issue as long as we keep decompiling on master, but as soon as we switch between master and similarly old commits on the debloated/anniversary branches, we can get very confusing errors:
The symptom is a calling convention mismatch: The two vector functions use __cdecl on master and pascal on debloated/anniversary. We've switched from anniversary (which compiles to ANNIV.EXE) back to master (which compiles to REIIDEN.EXE) here, so the .obj file on disk still uses the pascal calling convention. The build system, however, only checks the response files associated with the current target binary (REIIDEN.EXE) and therefore assumes that the .obj files still reflect the (unchanged) command-line flags in the TCC response file associated with this binary. And if none of the inputs of these .obj files changed between the two branches, they aren't rebuilt after switching, even though they would need to be.
Apparently, there's also such a thing as "too much batching", because TCC would suddenly stop applying certain compiler optimizations at very specific places if too many files were compiled within a single process? At least you quickly remember which source files you then need to manually touch and recompile to make the binaries match ZUN's original ones again…
But the final nail in the coffin was something I'd notice on every single build: 5 years down the line, even the performance argument wasn't convincing anymore. The strategy of minimizing emulated code still left me with an 𝑂(𝑛) algorithm, and with this entire thing still being single-threaded, there was no force to counteract the dependency check times as they grew linearly with the number of source files.
At P0280, each build run would perform a total of 28,130 file-related DOS syscalls to figure out which source files have changed and need to be rebuilt. At some point, this was bound to become noticeable even despite these syscalls being native, not to mention that they're still surrounded by emulator code that must convert their parameters and results to and from the DOS ABI. And with the increasing delays before TCC would do its actual work, the entire thing started feeling increasingly jankier.
While this system was waiting to be eventually finished, the public master branch kept using the Makefile that dates back to early 2015. Back then, it didn't takelong for me to abandon raw dumb batch files because Make was simply the most straightforward way of ensuring that the build process would abort on the first compile error.
The following years also proved that Makefile syntax is quite well-suited for expressing the build rules of a codebase at this scale. The built-in support for automatically turning long commands into response files was especially helpful because of how naturally it works together with batched compilation. Both of these advantages culminate in this wonderfully arcane incantation of ASCII special characters and syntactically significant linebreaks:
tcc … @&&|
$**
|
Which translates to "take the filenames of all dependents of this explicit rule, write them into a temporary file with an autogenerated name, insert this filename into the tcc … @ command line, and delete the file after the command finished executing". The @ is part of TCC's command-line interface, the rest is all MAKE syntax.
But 📝 as we all know by now, these surface-level niceties change nothing about Makefiles inherently being unreliable trash due to implementing none of the aforementioned two essential properties of a generic build system. Borland got so close to a correct and reliable implementation of autodependencies, but that would have just covered one of the two properties. Due to this unreliability, the old build16b.bat called Borland's MAKER.EXE with the -B flag, recompiling everything all the time. Not only did this leave modders with a much worse build process than I was using internally, but it also eventually got old for me to merge my internal branch onto master before every delivery. Let's finally rectify that and work towards a single good build process for everyone.
As you would expect by now, I've once again migrated to Tup's Lua syntax. Rewriting it all makes you realize once again how complex the PC-98 Touhou build process is: It has to cover 2 programming languages, 2 pipeline steps, and 3 third-party libraries, and currently generates a total of 39 executables, including the small programs I wrote for research. The final Lua code comprises over 1,300 lines – but then again, if I had written it in 📝 Zig, it would certainly be as long or even longer due to manual memory management. The Tup building blocks I constructed for Shuusou Gyoku quickly turned out to be the wrong abstraction for a project that has no debug builds, but their 📝 basic idea of a branching tree of command-line options remained at the foundation of this script as well.
This rewrite also provided an excellent opportunity for finally dumping all the intermediate compilation outputs into a separate dedicated obj/ subdirectory, finally leaving bin/ nice and clean with only the final executables. I've also merged this new system into most of the public branches of the GitHub repo.
As soon as I first tried to build it all though, I was greeted with a particularly nasty Tup bug. Due to how DOS specified file metadata mutation, MS-DOS Player has to open every file in a way that current Tup treats as a write access… but since unannotated file writes introduce the risk of a malformed build graph if these files are read by another build command later on, Tup providently deletes these files after the command finished executing. And by these files, I mean TCC.EXE as well as every one of its C library header files opened during compilation.
Due to a minor unsolved question about a failing test case, my fix has not been merged yet. But even if it was, we're now faced with a problem: If you previously chose to set up Tup for ReC98 or 📝 Shuusou Gyoku and are maybe still running 📝 my 32-bit build from September 2020, running the new build.bat would in fact delete the most important files of your Turbo C++ 4.0J installation, forcing you to reinstall it or restore it from a backup. So what do we do?
Should my custom build get a special version number so that the surrounding batch file can fail if the version number of your installed Tup is lower?
Or do I just put a message somewhere, which some people invariably won't read?
The easiest solution, however, is to just put a fixed Tup binary directly into the ReC98 repo. This not only allows me to make Tup mandatory for 64-bit builds, but also cuts out one step in the build environment setup that at least one person previously complained about. *nix users might not like this idea all too much (or do they?), but then again, TASM32 and the Windows-exclusive MS-DOS Player require Wine anyway. Running Tup through Wine as well means that there's only one PATH to worry about, and you get to take advantage of the tool checks in the surrounding batch file.
If you're one of those people who doesn't trust binaries in Git repos, the repo also links to instructions for building this binary yourself. Replicating this specific optimized binary is slightly more involved than the classic ./configure && make && make install trinity, so having these instructions is a good idea regardless of the fact that Tup's GPL license requires it.
One particularly interesting aspect of the Lua code is the way it handles sprite dependencies:
If build commands read from files that were created by other build commands, Tup requires these input dependencies to be spelled out so that it can arrange the build graph and parallelize the build correctly. We could simply put every sprite into a single array and automatically pass that as an extra input to every source file, but that would effectively split the build into a "sprite convert" and "code compile" phase. Spelling out every individual dependency allows such source files to be compiled as soon as possible, before (and in parallel to) the rest of the sprites they don't depend on. Similarly, code files without sprite dependencies can compile before the first sprite got converted, or even before the sprite converter itself got compiled and linked, maximizing the throughput of the overall build process.
Running a 30-year-old DOS toolchain in a parallel build system also introduces new issues, though. The easiest and recommended way of compiling and linking a program in Turbo C++ is a single tcc invocation:
tcc … main.cpp utils.cpp master.lib
This performs a batched compilation of main.cpp and utils.cpp within a single TCC process, and then launches TLINK to link the resulting .obj files into main.exe, together with the C++ runtime library and any needed objects from master.lib. The linking step works by TCC generating a TLINK command line and writing it into a response file with the fixed name turboc.$ln… which obviously can't work in a parallel build where multiple TCC processes will want to link different executables via the same response file.
Therefore, we have to launch TLINK with a custom response file ourselves. This file is echo'd as a separate parallel build rule, and the Lua code that constructs its contents has to replicate TCC's logic for picking the correct C++ runtime .lib file for the selected memory model.
The response file for TH02's ZUN_RES.COM, consisting of the C++ standard library, two files of ZUN code, and master.lib.
While this does add more string formatting logic, not relying on TCC to launch TLINK actually removes the one possible PATH-related error case I previously documented in the README. Back in 2021 when I first stumbled over the issue, it took a few hours of RE to figure this out. I don't like these hours to go to waste, so here's a Gist, and here's the text replicated for SEO reasons:
Issue: TCC compiles, but fails to link, with Unable to execute command 'tlink.exe'
Cause: This happens when invoking TCC as a compiler+linker, without the -c flag. To locate TLINK, TCC needlessly copies the PATH environment variable into a statically allocated 128-byte buffer. It then constructs absolute tlink.exe filenames for each of the semicolon- or \0-terminated paths, writing these into a buffer that immediately follows the 128-byte PATH buffer in memory. The search is finished as soon as TCC finds an existing file, which gives precedence to earlier paths in the PATH. If the search didn't complete until a potential "final" path that runs past the 128 bytes, the final attempted filename will consist of the part that still managed to fit into the buffer, followed by the previously attempted path.
Workaround: Make sure that the BIN\ path to Turbo C++ is fully contained within the first 127 bytes of the PATH inside your DOS system. (The 128th byte must either be a separating ; or the terminating \0 of the PATH string.)
Now that DOS emulation is an integral component of the single-part build process, it even makes sense to compile our pipeline tools as 16-bit DOS executables and then emulate them as part of the build. Sure, it's technically slower, but realistically it doesn't matter: Our only current pipeline tools are 📝 the converter for hardcoded sprites and the 📝 ZUN.COM generators, both of which involve very little code and are rarely run during regular development after the initial full build. In return, we get to drop that awkward dependency on the separate Borland C++ 5.5 compiler for Windows and yet another additional manual setup step. 🗑️ Once PC-98 Touhou becomes portable, we're probably going to require a modern compiler anyway, so you can now delete that one as well.
That gives us perfect dependency tracking and minimal parallel rebuilds across the whole codebase! While MS-DOS Player is noticeably slower than DOSBox-X, it's not going to matter all too much; unless you change one of the more central header files, you're rarely if ever going to cause a full rebuild. Then again, given that I'm going to use this setup for at least a couple of years, it's worth taking a closer look at why exactly the compilation performance is so underwhelming …
On the surface, MS-DOS Player seems like the right tool for our job, with a lot of advantages over DOSBox:
It doesn't spawn a window that boots an entire emulated PC, but is instead
perfectly integrated into the Windows console. Using it in a modern developer console would allow you to click on a compile error and have your editor immediately open the relevant file and jump to that specific line! With DOSBox, this basic comfort feature was previously unthinkable.
Heck, Takeda Toshiya originally developed it to run the equally vintage LSI C-86 compiler on 64-bit Windows. Fixing any potential issues we'd run into would be well within the scope of the project.
It consists of just a single comparatively small binary that we could just drop into the ReC98 repo. No manual setup steps required.
But once I began integrating it, I quickly noticed two glaring flaws:
Back in 2009, Takeda Toshiya chose to start the project by writing a custom DOS implementation from scratch. He was aware of DOSBox, but only adapted small tricky parts of its source code rather than starting with the DOSBox codebase and ripping out everything he didn't need. This matches the more research-oriented nature that all of his projects appear to follow, where the primary goal of writing the code is a personal understanding of the problem domain rather than a widely usable piece of software. MS-DOS Player is even the outlier in this regard, with Takeda Toshiya describing it as 珍しく実用的かもしれません. I am definitely sympathetic to this mindset; heck, my old internal build system falls under this category too, being so specialized and narrow that it made little sense to use it outside of ReC98. But when you apply it to emulators for niche systems, you end up with exactly the current PC-98 emulation scene, where there's no single universally good emulator because all of them have some inaccuracy somewhere. This scene is too small for you not to eventually become part of someone else's supply chain… 🥲
Emulating DOS is a particularly poor fit for a research/NIH project because it's Hyrum's Law incarnate. With the lack of memory protection in Real Mode, programs could freely access internal DOS (and even BIOS) data structures if they only knew where to look, and frequently did. It might look as if "DOS command-line tools" just equals x86 plus INT 21h, but soon you'll also be emulating the BIOS, PIC, PIT, EMS, XMS, and probably a few more things, all with their individual quirks that some application out there relies on. DOSBox simply had much more time to grow and mature and figure out all of these details by trial and error. If you start a DOS emulator from scratch, you're bound to duplicate all this research as people want to use your emulator to run more and more programs, until you've ended up with what's effectively a clone of DOSBox's exact logic. Unless, of course, if you draw a line somewhere and limit the scope of the DOS and BIOS emulation. But given how many people have wanted to use MS-DOS Player for running DOS TUIs in arbitrarily sized terminal windows with arbitrary fonts, that's not what happened. I guess it made sense for this use case before DOSBox-X gained a TTF output mode in late 2020?
As usual, I wouldn't mention this if I didn't run into twobugs when combining MS-DOS Player with Turbo C++ and Tup. Both of these originated from workarounds for inaccuracies in the DOS emulation that date back to MS-DOS Player's initial release and were thankfully no longer necessary with the accuracy improvements implemented in the years since.
For CPU emulation, MS-DOS Player can use either MAME's or Neko Project 21/W's x86 core, both of which are interpreters and won't win any performance contests. The NP21/W core is significantly better optimized and runs ≈41% faster, but still pales in comparison to DOSBox-X's dynamic recompiler. Running the same sequential commands that the P0280 Makefile would execute, the upstream 2024-03-02 NP21/W core build of MS-DOS Player would take to compile the entire ReC98 codebase on my system, whereas DOSBox-X's dynamic core manages the same in , or 94% faster.
Granted, even the DOSBox-X performance is much slower than we would like it to be. Most of it can be blamed on the awkward time in the early-to-mid-90s when Turbo C++ 4.0J came out. This was the time when DOS applications had long grown past the limitations of the x86 Real Mode and required DOS extenders or even sillier hacks to actually use all the RAM in a typical system of that period, but Win32 didn't exist yet to put developers out of this misery. As such, this compiler not only requires at least a 386 CPU, but also brings its own DOS extender (DPMI16BI.OVL) plus a loader for said extender (RTM.EXE), both of which need to be emulated alongside the compiler, to the great annoyance of emulator maintainers 30 years later. Even MS-DOS Player's README file notes how Protected Mode adds a lot of complexity and slowdown:
8086 binaries are much faster than 80286/80386/80486/Pentium4/IA32 binaries.
If you don't need the protected mode or new mnemonics added after 80286,
I recommend i86_x86 or i86_x64 binary.
The immediate reaction to these performance numbers is obvious: Let's just put DOSBox-X's dynamic recompiler into MS-DOS Player, right?! 🙌 Except that once you look at DOSBox-X, you immediately get why Takeda Toshiya might have preferred to start from scratch. Its codebase is a historically grown tangled mess, requiring intimate familiarity and a significant engineering effort to isolate the dynamic core in the first place. I did spend a few days trying to untangle and copy it all over into MS-DOS Player… only to be greeted with an infinite loop as soon as everything compiled for the first time. 😶 Yeah, no, that's bound to turn into a budget-exceeding maintenance nightmare.
Instead, let's look at squeezing at least some additional performance out of what we already have. A generic emulator for the entire CISCy instruction set of the 80386, with complete support for Protected Mode, but it's only supposed to run the subset of instructions and features used by a specific compiler and linker as fast as possible… wait a moment, that sounds like a use case for profile-guided optimization! This is the first time I've encountered a situation that would justify the required 2-phase build process and lengthy profile collection – after all, writing into some sort of database for every function call does slow down MS-DOS Player by roughly 15×. However, profiling just the compilation of our most complex translation unit (📝 TH01 YuugenMagan) and the linking of our largest executable (TH01's REIIDEN.EXE) should be representative enough.
I'll get to the performance numbers later, but even the build output is quite intriguing. Based on this profile, Visual Studio chooses to optimize only 104 out of MS-DOS Player's 1976 functions for speed and the rest for size, shaving off a nice 109 KiB from the binary. Presumably, keeping rare code small is also considered kind of fast these days because it takes up less space in your CPU's instruction cache once it does get executed?
With PGO as our foundation, let's run a performance profile and see if there are any further code-level optimizations worth trying out:
Removing redundant memset() calls: MS-DOS Player is written in a very C-like style of C++, and initializes a bunch of its statically allocated data by memset()ing it with 00 bytes at startup. This is strictly redundant even in C; Section 6.7.9/10 of the C standard mandates that all static data is zero-initialized by default. In turn, the program loaders of modern operating systems employ all sorts of paging tricks to reduce the CPU cost (and actual RAM usage!) of this initialization as much as possible. If you manually memset() afterward, you throw all these advantages out of the window.
Of course, these calls would only ever show up among the top CPU consumers in a performance profile if a program uses a large amount of static data, but the hardcoded 32 MiB of emulated RAM in ≥i386-supporting builds definitely qualifies. Zeroing 32.8 MiB of memory makes up a significant chunk of the runtime of some of the shorter build steps and quickly adds up; a full rebuild of the ReC98 codebase currently spawns a total of 361 MS-DOS Player instances, totaling 11.5 GiB of needless memory writes.
Limiting the emulated instruction set: NP21/W's x86 core emulates everything up to the SSE3 extension from 2004, but Turbo C++ 4.0J's x86 instruction set usage doesn't stretch past the 386. It doesn't even need the x87 FPU for compiling code that involves floating-point constants. Disabling all these unneeded extensions speeds up x86's infamously annoying instruction decoding, and also reduces the size of the MS-DOS Player binary by another 149.5 KiB. The source code already had macros for this purpose, and only needed a slight fix for the code to compile with these macros disabled.
Removing x86 paging: Borland's DOS extender uses segmented memory addressing even in Protected Mode. This allows us to remove the MMU emulation and the corresponding "are we paging" check for every memory access.
Removing cycle counting: When emulating a whole system, counting the cycles of each instruction is important for accurately synchronizing the CPU with other pieces of hardware. As hinted above, MS-DOS Player does emulate and periodically update a few pieces of hardware outside the CPU, but we need none of them for a build tool.
Testing Takeda Toshiya's optimizations: In a nice turn of events, Takeda Toshiya merged every single one of my bugfixes and optimization flags into his upstream codebase. He even agreed with my memset() and cycle counting removal optimizations, which are now part of all upstream builds as of 2024-06-24. For the 2024-06-27 build, he claims to have gone even further than my more minimal optimization, so let's see how these additional changes affect our build process.
Further risky optimizations: A lot of the remaining slowness of x86 emulation comes from the segmentation and protection fault checks required for every memory access. If we assume that the emulator only ever executes correct code, we can remove these checks and implement further shortcuts based on their absence.
The L[DEFGS]S group of instructions that load a segment and offset register from a 32-bit far pointer, for example, are both frequently used in Turbo C++ 4.0J code and particularly expensive to emulate. Intel specified their Real Mode operation as loading the segment and offset part in two separate 16-bit reads. But if we assume that neither of those reads can fault, we can compress them into a single 32-bit read and thus only perform the costly address translation once rather than twice. Emulator authors are probably rolling their eyes at this gross violation of Intel documentation now, but it's at least worth a try to see just how much performance we could get out of it.
Measured on a 6-year-old 6-core Intel Core i5 8400T on Windows 11. The first number in each column represents the codebase before the #include cleanup explained below, and the second one corresponds to this commit. All builds are 64-bit, 32-bit builds were ≈5% slower across the board. I kept the fastest run within three attempts; as Tup parallelizes the build process across all CPU cores, it's common for the long-running full build to take up to a few seconds longer depending on what else is running on your system. Tup's standard output is also redirected to a file here; its regular terminal output and nice progress bar will add more slowdown on top.
The key takeaways:
By merely disabling certain x86 features from MS-DOS Player and retaining the accuracy of the remaining emulation, we get speedups of ≈60% (full build), ≈70% (median TU), and ≈80% (largest TU).
≈25% (full build), ≈29% (median TU), and ≈41% (largest TU) of this speedup came from Visual Studio's profile-guided optimization, with no changes to the MS-DOS Player codebase.
The effects of removing cycle counting are the biggest surprise. Between ≈17% and ≈23%, just for removing one subtraction per emulated instruction? Turns out that in the absence of a "target cycle amount" setting, the x86 emulation loop previously ran for only a single cycle. This caused the PIC check to run after every instruction, followed by PIT, serial I/O, keyboard, mouse, and CRTC update code every millisecond. Without cycle counting, the x86 loop actually keeps running until a CPU exception is raised or the emulated process terminates, skipping the hardware code during the vast majority of the program's execution time.
While Takeda Toshiya's changes in the 2024-06-27 build completely throw out the cycle counter and clean up process termination, they also reintroduce the hardware updates that made up the majority of the cycle removal speedup. This explains the results we're getting: The small speedup for full rebuilds is too insignificant to bother with and might even fall within a statistical margin of error, but the build slows down more and more the longer the emulated process runs. Compiling and linking YuugenMagan takes a whole 14% longer on generic builds, and ≈9-12% longer on PGO builds. I did another in-between test that just removed the x86 loop from the cycle removal version, and got exactly the same numbers. This just goes to show how much removing two writes to a fixed memory address per emulated instruction actually matters. Let's not merge back this one, and stay on top of 2024-06-24 for the time being.
The risky optimizations of ignoring segment limits and speeding up 32-bit segment+offset pointer load instructions could yield a further speedup. However, most of these changes boil down to removing branches that would never be taken when emulating correct x86 code. Consequently, these branches get recorded as unlikely during PGO training, which then causes the profile-guided rebuild to rearrange the instructions on these branches in a way that favors the common case, leaving the rest of their effective removal to your CPU's branch predictor. As such, the 10%-15% speedup we can observe in generic builds collapses down to 2%-6% in PGO builds. At this rate and with these absolute durations, it's not worth it to maintain what's strictly a more inaccurate fork of Neko Project 21/W's x86 core.
The redundant header inclusions afforded by #include guards do in fact have a measurable performance cost on Turbo C++ 4.0J, slowing down compile times by 5%.
But how does this compare to DOSBox-X's dynamic core? Dynamic recompilers need some kind of cache to ensure that every block of original ASM gets recompiled only once, which gives them an advantage in long-running processes after the initial warmup. As a result, DOSBox-X compiles and links YuugenMagan in , ≈92% faster than even our optimized MS-DOS Player build. That percentage resembles the slowdown we were initially getting when comparing full rebuilds between DOSBox-X and MS-DOS Player, as if we hadn't optimized anything.
On paper, this would mean that DOSBox-X barely lost any of its huge advantage when it comes to single-threaded compile+link performance. In practice, though, this metric is supposed to measure a typical decompilation or modding workflow that focuses on repeatedly editing a single file. Thus, a more appropriate comparison would also have to add the aforementioned constant 28,130 syscalls that my old build system required to detect that this is the one file/binary that needs to be recompiled/relinked. The video at the top of this blog post happens to capture the best time () I got for the detection process on DOSBox-X. This is almost as slow as the compilation and linking itself, and would have only gotten slower as we continue decompiling the rest of the games. Tup, on the other hand, performs its filesystem scan in a near-constant , matching the claim in Section 4.7 of its paper, and thus shrinking the performance difference to ≈14% after all. Sure, merging the dynamic core would have been even better (contribution-ideas, anyone?), but this is good enough for now.
Just like with Tup, I've also placed this optimized binary directly into the ReC98 repo and added the specific build instructions to the GitHub release page.
I do have more far-reaching ideas for further optimizing Neko Project 21/W's x86 core for this specific case of repeated switches between Real Mode and Protected Mode while still retaining the interpreted nature of this core, but these already strained the budget enough.
The perhaps more important remaining bottleneck, however, is hiding in the actual DOS emulation. Right now, a Tup-driven full rebuild spawns a total of 361 MS-DOS Player processes, which means that we're booting an emulated DOS 361 times. This isn't as bad as it sounds, as "booting DOS" basically just involves initializing a bunch of internal DOS structures in conventional memory to meaningful values. However, these structures also include a few environment variables like PATH, APPEND, or TEMP/TMP, which MS-DOS Player seamlessly integrates by translating them from their value on the Windows host system to the DOS 8.3 format. This could be one of the main reasons why MS-DOS Player is a native Windows program rather than being cross-platform:
On Windows, this path translation is as simple as calling GetShortPathNameA(), which returns a unique 8.3 name for every component along the path.
Also, drive letters are an integralpart of the DOS INT 21h API, and Windows still uses them as well.
However, the NT kernel doesn't actually use drive letters either, and views them as just a legacy abstraction over its reality of volume GUIDs. Converting paths back and forth between these two views therefore requires it to communicate with a
mount point manager service, which can coincidentally also be observed in debug builds of Tup.
As a result, calling any path-retrieving API is a surprisingly expensive operation on modern Windows. When running a small sprite through our 📝 sprite converter, MS-DOS Player's boot process makes up 56% of the runtime, with 64% of that boot time (or 36% of the entire runtime) being spent on path translation. The actual x86 emulation to run the program only takes up 6.5% of the runtime, with the remaining 37.5% spent on initializing the multithreaded C++ runtime.
But then again, the truly optimal solution would not involve MS-DOS Player at all. If you followed general video game hacking news in May, you'll probably remember the N64 community putting the concept of statically recompiled game ports on the map. In case you're wondering where this seemingly sudden innovation came from and whether a reverse-engineered decompilation project like ReC98 is obsolete now, I wrote a new FAQ entry about why this hype, although justified, is at least in part misguided. tl;dr: None of this can be meaningfully applied to PC-98 games at the moment.
On the other hand, recompiling our compiler would not only be a reasonable thing to attempt, but exactly the kind of problem that recompilation solves best. A 16-bit command-line tool has none of the pesky hardware factors that drag down the usefulness of recompilations when it comes to game ports, and a recompiled port could run even faster than it would on 32-bit Windows. Sure, it's not as flashy as a recompiled game, but if we got a few generous backers, it would still be a great investment into improving the state of static x86 recompilation by simply having another open-source project in that space. Not to mention that it would be a great foundation for improving Turbo C++ 4.0J's code generation and optimizations, which would allow us to simplify lots of awkward pieces of ZUN code… 🤩
That takes care of building ReC98 on 64-bit platforms, but what about the 32-bit ones we used to support? The previous split of the build process into a Tup-driven 32-bit part and a Makefile-driven 16-bit part sure was awkward and I'm glad it's gone, but it did give you the choice between 1) emulating the 16-bit part or 2) running both parts natively on 32-bit Windows. While Tup's upstream Windows builds are 64-bit-only, it made sense to 📝 compile a custom 32-bit version and thus turn any 32-bit Windows ≥Vista into the perfect build platform for ReC98. Older Windows versions that can't run Tup had to build the 32-bit part using a separately maintained dumb batch script created by tup generate, but again, due to Make being trash, they were fully rebuilding the entire codebase every time anyway.
Driving the entire build via Tup changes all of that. Now, it makes little sense to continue using 32-bit Tup:
We need to DLL-inject into a 64-bit MS-DOS Player. Sure, we could compile a 32-bit build of MS-DOS Player, but why would we? If we look at current marketshares, nobody runs 32-bit Windows anymore, not even by accident. If you run 32-bit Windows in 2024, it's because you know what you're doing and made a conscious choice for the niche use case of natively running DOS programs. Emulating them defeats the whole point of setting up this environment to begin with.
It would make sense if Tup could inject into DOS programs, but it can't.
Also, as we're going to see later, requiring Windows ≥Vista goes in the opposite direction of what we want for a 32-bit build. The earlier the Windows version, the better it is at running native DOS tools.
This means that we could now only support 32-bit Windows via an even larger tup generated batch file. We'd have to move the MS-DOS Player prefix of the respective command lines into an environment variable to make Tup use the same rules for both itself and the batch file, but the result seems to work…
…but it's really slow, especially on Windows 9x. 🐌 If we look back at the theory behind my previous custom build system, we can already tell why: Efficiently building ReC98 requires a completely different approach depending on whether you're running a typical modern multi-core 64-bit system or a vintage single-core 32-bit system. On the former, you'd want to parallelize the slow emulation as much as you can, so you maximize the amount of TCC processes to keep all CPU cores as busy as possible. But on the latter, you'd want the exact opposite – there, the biggest annoyance is the repeated startup and shutdown of the VDM, TCC, and its DOS extender, so you want to continue batching translation units into as few TCC processes as possible.
CMake fans will probably feel vindicated now, thinking "that sounds exactly like you need a meta build system 🤪". Leaving aside the fact that the output vomited by all of CMake's Makefile generators is a disgusting monstrosity that's far removed from addressing any performance concerns, we sure could solve this problem by adding another layer of abstraction. But then, I'd have to rewrite my working Lua script into either C++ or (heaven forbid) Batch, which are the only options we'd have for bootstrapping without adding any further dependencies, and I really wouldn't want to do that. Alternatively, we could fork Tup and modify tup generate to rewrite the low-level build rules that end up in Tup's database.
But why should we go for any of these if the Lua script already describes the build in a high-level declarative way? The most appropriate place for transforming the build rules is the Lua script itself…
… if there wasn't the slight problem of Tup forbidding file writes from Lua. 🥲 Presumably, this limitation exists because there is no way of replicating these writes in a tup generated dumb shell script, and it does make sense from that point of view.
But wait, printing to stdout or stderr works, and we always invoke Tup from a batch file anyway. You can now tell where this is going. Hey, exfiltrating commands from a build script to the build system via standard I/O streams works for Rust's Cargo too!
Just like Cargo, we want to add a sufficiently unique prefix to every line of the generated batch script to distinguish it from Tup's other output. Since Tup only reruns the Lua script – and would therefore print the batch file – if the script changed between the previous and current build run, we only want to overwrite the batch file if we got one or more lines. Getting all of this to work wasn't all too easy; we're once again entering the more awful parts of Batch syntax here, which apparently are so terrible that Wine doesn't even bother to correctly implement parts of it. 😩
Most importantly, we don't really want to redirect any of Tup's standard I/O streams. Redirecting stdout disables console output coloring and the pretty progress bar at the bottom, and looping over stderr instead of stdout in Batch is incredibly awkward. Ideally, we'd run a second Tup process with a sub-command that would just evaluate the Lua script if it changed - and fortunately, tup parse does exactly that. 😌
In the end, the optimally fast and ERRORLEVEL-preserving solution involves two temporary files. But since creating files between two Tup runs causes it to reparse the Lua code, which would print the batch file to the unfiltered stdout, we have to hide these temporary files from Tup by placing them into its .tup/ database directory. 🤪
On a more positive note, programmatically generating batches from single-file TCC rules turned out to be a great idea. Since the Lua code maps command-line flags to arrays of input files, it can also batch across binaries, surpassing my old system in this regard. This works especially well on the debloated and anniversary branches, which replace ZUN's little command-line flag inconsistencies with a single set of good optimization flags that every translation unit is compiled with.
Time to fire up some VMs then… only to see the build failing on Windows 9x with multiple unhelpful Bad command or file name errors. Clearly, the long echo lines that write our response files run up against some length limit in command.com and need to be split into multiple ones. Windows 9x's limit is larger than the 127 characters of DOS, that's for sure, and the exact number should just be one search away…
…except that it's not the 1024 characters recounted in a surviving newsgroup post. Sure, lines are truncated to 1023 bytes and that off-by-one error is no big deal in this context, but that's not the whole story:
: This not unrealistic command line is 137 bytes long and fails on Windows 9x?!
> echo -DA=1 2 3 a/b/c/d/1 a/b/c/d/2 a/b/c/d/3 a/b/c/d/4 a/b/c/d/5 a/b/c/d/6 a/b/c/d/7 a/b/c/d/8 a/b/c/d/9 a/b/c/d/10 a/b/c/d/11 a/b/c/d/12
Bad command or file name
Wait, what, something about / being the SWITCHAR? And not even just that…
: Down to 132 bytes… and 32 "assignments"?
> echo a=0 b=1 c=2 d=3 e=4 f=5 g=6 h=7 i=8 j=9 k=0 l=1 m=2 n=3 o=4 p=5 q=6 r=7 s=8 t=9 u=0 v=1 w=2 x=3 y=4 z=5 a=0 b=1 c=2 d=3 e=4 f=5
Bad command or file name
And what's perhaps the worst example:
: 64 slashes. Works on DOS, works on `cmd.exe`, fails on 9x.
> echo ////////////////////////////////////////////////////////////////
Bad command or file name
My complete set of test cases: 2024-07-09-Win9x-batch-tokenizer-tests.bat
So, time to load command.com into DOSBox-X's debugger and step through some code. 🤷 The earliest NT-based Windows versions were ported to a variety of CPUs and therefore received the then-all-new cmd.exe shell written in C, whereas Windows 9x's command.com was still built on top of the dense hand-written ASM code that originated in the very first DOS versions. Fortunately though, Microsoft open-sourced one of the later DOS versions in April. This made it somewhat easier to cross-reference the disassembly even though the Windows 9x version significantly diverged in the parts we're interested in.
And indeed: After truncating to 1023 bytes and parsing out any redirectors, each line is split into tokens around whitespace and = signs and before every occurrence of the SWITCHAR. These tokens are written into a statically allocated 64-element array, and once the code tries to write the 65th element, we get the Bad command or file name error instead.
#
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
String
echo
-DA
1
2
3
a
/B
/C
/D
/1
a
/B
/C
/D
/2
Switch flag
🚩
🚩
🚩
🚩
🚩
🚩
🚩
🚩
The first few elements of command.com's internal argument array after calling the Windows 9x equivalent of parseline with my initial example string. Note how all the "switches" got capitalized and annotated with a flag, whereas the = sign no longer appears in either string or flag form.
Needless to say, this makes no sense. Both DOS and Windows pass command lines as a single string to newly created processes, and since this tokenization is lossy, command.com will just have to pass the original string anyway. If your shell wants to handle tokenization at a central place, it should happen after it decided that the command matches a builtin that can actually make use of a pointer to the resulting token array – or better yet, as the first call of each builtin's code. Doing it before is patently ridiculous.
I don't know what's worse – the fact that Windows 9x blindly grinds each batch line through this tokenizer, or the fact that no documentation of this behavior has survived on today's Internet, if any even ever existed. The closest thing I found was this page that doesn't exist anymore, and it also just contains a mere hint rather than a clear description of the issue. Even the usual Batch experts who document everything else seem to have a blind spot when it comes to this specific issue. As do emulators: DOSBox and FreeDOS only reimplement the sane DOS versions of command.com, and Wine only reimplements cmd.exe.
Oh well. 71 lines of Lua later, the resulting batch file does in fact work everywhere:
The clear performance winner at 11.15 seconds after the initial tool check, though sadly bottlenecked by strangely long TASM32 startup times. As for TCC though, even this performance is the slowest a recompiled port would be. Modern compiler optimizations are probably going to shave off another second or two, and implementing support for #pragma once into the recompiled code will get us the aforementioned 5% on top.
If you run this on VirtualBox on modern Windows, make sure to disable Hyper-V to avoid the slower snail execution mode. 🐢
Building in Windows XP under Hyper-V exchanges Windows 98's slow TASM32 startup times for slightly slower DOS performance, resulting in a still decent 13.4 seconds.
29.5 seconds?! Surely something is getting emulated here. And this is the best time I randomly got; my initial preview recording took 55 seconds which is closer to DOSBox-X's dynamic core than it is to Windows 9x. Given how poorly 32-bit Windows 10 performs, Microsoft should have probably discontinued 32-bit Windows after 8 already. If any 16-bit program you could possibly want to run is either too slow or likely to exhibit other compatibility issues (📝 Shuusou Gyoku, anyone?), the existence of 32-bit Windows 10 is nothing but a maintenance burden. Especially because Windows 10 simultaneously overhauled the console subsystem, which is bound to cause compatibility issues anyway. It sure did for me back in 2019 when I tried to get my build system to work…
But wait, there's more! The codebase now compiles on all 32-bit Windows systems I've tested, and yields binaries that are equivalent to ZUN's… except on 32-bit Windows 10. 🙄 Suddenly, we're facing the exact same batched compilation bug from my custom build system again, with REIIDEN.EXE being 16 bytes larger than it's supposed to be.
Looks like I have to look into that issue after all, but figuring out the exact cause by debugging TCC would take ages again. Thankfully, trial and error quickly revealed a functioning workaround: Separating translation unit filenames in the response file with two spaces rather than one. Really, I couldn't make this up. This is the most ridiculous workaround for a bug I've encountered in a long time.
The TCC response file generation code for all current decompiled TH04 code, split into multiple echo calls based on the Windows 9x batch tokenizer rules and with double spaces between each parameter for added "safety". Would this also have been the solution for the batched compilation bugs I was experiencing with my old build system in DOSBox? I suddenly was unable to reproduce these bugs, so we won't know for the time being…
Hopefully, you've now got the impression that supporting any kind of 32-bit Windows build is way more of a liability than an asset these days, at least for this specific project. "Real hardware", "motivating a TCC recompilation", and "not dropping previous features" really were the only reasons for putting up with the sheer jank and testing effort I had to go through. And I wouldn't even be surprised if real-hardware developers told me that the first reason doesn't actually hold up because compiling ReC98 on actual PC-98 hardware is slow enough that they'd rather compile it on their main machine and then transfer the binaries over some kind of network connection.
I guess it also made for some mildly interesting blog content, but this was definitely the last time I bothered with such a wide variety of Windows versions without being explicitly funded to do so. If I ever get to recompile TCC, it will be 64-bit only by default as well.
Instead, let's have a tier list of supported build platforms that clearly defines what I am maintaining, with just the most convincing 32-bit Windows version in Tier 1. Initially, that was supposed to be Windows 98 SE due to its superior performance, but that's just unreasonable if key parts of the OS remain undocumented and make no sense. So, XP it is.
*nix fans will probably once again be disappointed to see their preferred OS in Tier 2. But at least, all we'd need for that to move up to Tier 1 is a CI configuration, contributed either via funding me or sending a PR. (Look, even more contribution-ideas!)
Getting rid of the Wine requirement for a fully cross-platform build process wouldn't be too unrealistic either, but would require us to make a few quality decisions, as usual:
Do we run the DOS tools by creating a cross-platform MS-DOS Player fork, or do we statically recompile them?
Do we replace 32-bit Windows TASM with the 16-bit DOS TASM.EXE or TASMX.EXE, which we then either run through our forked MS-DOS Player or recompile? This would further slow down the build and require us to get rid of these nice long non-8.3 filenames… 😕 I'd only recommend this after the looming librarization of ZUN's master.lib fork is completed.
Or do we try migrating to JWasm again? As an open-source assembler that aims for MASM compatibility, it's the closest we can get to TASM, but it's not a drop-in replacement by any means. I already tried in late 2014, but encountered too many issues and quickly abandoned the idea. Maybe it works better now that we have less ASM? In any case, this migration would only get easier the less ASM code we have remaining in the codebase as we get closer to the 100% finalization mark.
Y'know what I think would be the best idea for right now, though? Savoring this new build system and spending an extended amount of time doing actual decompilation or modding for a change.
Now that even full rebuilds are decently fast, let's make use of that productivity boost by doing some urgent and far-reaching code cleanup that touches almost every single C++ source file. The most immediately annoying quirk of this codebase was the silly way each translation unit #included the headers it needed. Many years ago, I measured that repeatedly including the same header did significantly impact Turbo C++ 4.0J's compilation times, regardless of any include guards inside. As a consequence of this discovery, I slightly overreacted and decided to just not use any include guards, ever. After all, this emulated build process is slow enough, and we don't want it to needlessly slow down even more! This way, redundantly including any file that adds more than just a few #define macros won't even compile, throwing lots of Multiple definition errors.
Consequently, the headers themselves #included almost nothing. Starting a new translation unit therefore always involved figuring and spelling out the transitive dependencies of the headers the new unit actually wants to use, in a short trial-and-error process. While not too bad by itself, this was bound to become quite counterproductive once we get closer to porting these games: If some inlined function in a header needed access to, let's say, PC-98-specific I/O ports as an implementation detail, the header would have externalized this dependency to the top-level translation unit, which in turn made that that unit appear to contain PC-98-native code even if the unit's code itself was perfectly portable.
But once we start making some of these implicit transitive dependencies optional, it all stops being justifiable. Sometimes, a.hpp declared things that required declarations from b.hpp but these things are used so rarely that it didn't justify adding #include "b.hpp" to all translation units that #include "a.hpp". So how about conditionally declaring these things based on previously #included headers?
#if (defined(SUBPIXEL_HPP) && defined(PLANAR_H))
// Sets the [tile_ring] tile at (x, y) to the given VRAM offset.
void tile_ring_set_vo(subpixel_t x, subpixel_t y, vram_offset_t image_vo);
#endif
You can maybe do this in a project that consistently sorts the #include lists in every translation unit… err, no, don't do this, ever, it's awful. Just separate that declaration out into another header.
Now that we've measured that the sane alternative of include guards comes with a performance cost of just 5% and we've further reduced its effective impact by parallelizing the build, it's worth it to take that cost in exchange for a tidy codebase without such surprises. From now on, every header file will #include its own dependencies and be a valid translation unit that must compile on its own without errors. In turn, this allows us to remove at least 1,000 #include of transitive dependencies from .cpp files. 🗑️
However, that 5% number was only measured after I reduced these redundant #includes to their absolute minimum. So it still makes sense to only add include guards where they are absolutely necessary – i.e., transitively dependent headers included from more than one other file – and continue to (ab)use the Multiple definition compiler errors as a way of communicating "you're probably #including too many headers, try removing a few". Certainly a less annoying error than Undefined symbol.
Since all of this went way over the 7-push mark, we've got some small bits of RE and PI work to round it all out. The .REC loader in TH04 and TH05 is completely unremarkable, but I've got at least a bit to say about TH02's High Score menu. I already decompiled MAINE.EXE's post-Staff Roll variant in 2015, so we were only missing the almost identical MAIN.EXE variant shown after a Game Over or when quitting out of the game. The two variants are similar enough that it mostly needed just a small bit of work to bring my old 2015 code up to current standards, and allowed me to quickly push TH02 over the 40% RE mark.
Functionally, the two variants only differ in two assignments, but ZUN once again chose to copy-paste the entire code to handle them. This was one of ZUN's better copy-pasting jobs though – and honestly, I can't even imagine how you would mess up a menu that's entirely rendered on the PC-98's text RAM. It almost makes you wonder whether ZUN actually used the same #if ENDING preprocessor branching that my decompilation uses… until the visual inconsistencies in the alignment of the place numbers and the and labels clearly give it away as copy-pasted:
Next up: Starting the big Seihou summer! Fortunately, waiting two more months was worth it: In mid-June, Microsoft released a preview version of Visual Studio that, in response to my bug report, finally, finally makes C++ standard library modules fully usable. Let's clean up that codebase for real, and put this game into a window.
Oh, it's 2024 already and I didn't even have a delivery for December or January? Yeah… I can only repeat what I said at the end of November, although the finish line is actually in sight now. With 10 pushes across 4 repositories and a blog post that has already reached a word count of 9,240, the Shuusou Gyoku SC-88Pro BGM release is going to break 📝 both the push record set by TH01 Sariel two years ago, and 📝 the blog post length record set by the last Shuusou Gyoku delivery. Until that's done though, let's clear some more PC-98 Touhou pushes out of the backlog, and continue the preparation work for the non-ASCII translation project starting later this year.
But first, we got another free bugfix according to my policy! 📝 Back in April 2022 when I researched the Divide Error crash that can occur in TH04's Stage 4 Marisa fight, I proposed and implemented four possible workarounds and let the community pick one of them for the generally recommended small bugfix mod. I still pushed the others onto individual branches in case the gameplay community ever wants to look more closely into them and maybe pick a different one… except that I accidentally pushed the wrong code for the warp workaround, probably because I got confused with the second warp variant I developed later on.
Fortunately, I still had the intended code for both variants lying around, and used the occasion to merge the current master branch into all of these mod branches. Thanks to wyatt8740 for spotting and reporting this oversight!
As the final piece of code shared in largely identical form between 4 of the 5 games, the Music Rooms were the biggest remaining piece of low-hanging fruit that guaranteed big finalization% gains for comparatively little effort. They seemed to be especially easy because I already decompiled TH02's Music Room together with the rest of that game's OP.EXE back in early 2015, when this project focused on just raw decompilation with little to no research. 9 years of increased standards later though, it turns out that I missed a lot of details, and ended up renaming most variables and functions. Combined with larger-than-expected changes in later games and the usual quality level of ZUN's menu code, this ended up taking noticeably longer than the single push I expected.
The undoubtedly most interesting part about this screen is the animation in the background, with the spinning and falling polygons cutting into a single-color background to reveal a spacey image below. However, the only background image loaded in the Music Room is OP3.PI (TH02/TH03) or MUSIC3.PI (TH04/TH05), which looks like this in a .PI viewer or when converted into another image format with the usual tools:
Let's call this "the blank image".
That is definitely the color that appears on top of the polygons, but where is the spacey background? If there is no other .PI file where it could come from, it has to be somewhere in that same file, right?
And indeed: This effect is another bitplane/color palette trick, exactly like the 📝 three falling stars in the background of TH04's Stage 5. If we set every bit on the first bitplane and thus change any of the resulting even hardware palette color indices to odd ones, we reveal a full second 8-color sub-image hiding in the same .PI file:
The spacey sub-image. Never before seen!1!! …OK, touhou-memories beat me by a month. Let's add each image's full 16-color palette to deliver some additional value.
On a high level, the first bitplane therefore acts as a stencil buffer that selects between the blank and spacey sub-image for every pixel. The important part here, however, is that the first bitplane of the blank sub-images does not consist entirely of 0 bits, but does have 1 bits at the pixels that represent the caption that's supposed to be overlaid on top of the animation. Since there now are some pixels that should always be taken from the spacey sub-image regardless of whether they're covered by a polygon, the game can no longer just clear the first bitplane at the start of every frame. Instead, it has to keep a separate copy of the first bitplane's original state (called nopoly_B in the code), captured right after it blitted the .PI image to VRAM. Turns out that this copy also comes in quite handy with the text, but more on that later.
Then, the game simply draws polygons onto only the reblitted first bitplane to conditionally set the respective bits. ZUN used master.lib's grcg_polygon_c() function for this, which means that we can entirely thank the uncredited master.lib developers for this iconic animation – if they hadn't included such a function, the Music Rooms would most certainly look completely different.
This is where we get to complete the series on the PC-98 GRCG chip with the last remaining four bits of its mode register. So far, we only needed the highest bit (0x80) to either activate or deactivate it, and the bit below (0x40) to choose between the 📝 RMW and 📝 TCR/📝 TDW modes. But you can also use the lowest four bits to restrict the GRCG's operations to any subset of the four bitplanes, leaving the other ones untouched:
// Enable the GRCG (0x80) in regular RMW mode (0x40). All bitplanes are
// enabled and written according to the contents of the tile register.
outportb(0x7C, 0xC0);
// The same, but limiting writes to the first bitplane by disabling the
// second (0x02), third (0x04), and fourth (0x08) one, as done in the
// PC-98 Touhou Music Rooms.
outportb(0x7C, 0xCE);
// Regular GRCG blitting code to any VRAM segment…
pokeb(0xA8000, offset, …);
// We're done, turn off the GRCG.
outportb(0x7C, 0x00);
This could be used for some unusual effects when writing to two or three of the four planes, but it seems rather pointless for this specific case at first. If we only want to write to a single plane, why not just do so directly, without the GRCG? Using that chip only involves more hardware and is therefore slower by definition, and the blitting code would be the same, right?
This is another one of these questions that would be interesting to benchmark one day, but in this case, the reason is purely practical: All of master.lib's polygon drawing functions expect the GRCG to be running in RMW mode. They write their pixels as bitmasks where 1 and 0 represent pixels that should or should not change, and leave it to the GRCG to combine these masks with its tile register and OR the result into the bitplanes instead of doing so themselves. Since GRCG writes are done via MOV instructions, not using the GRCG would turn these bitmasks into actual dot patterns, overwriting any previous contents of each VRAM byte that gets modified.
Technically, you'd only have to replace a few MOV instructions with OR to build a non-GRCG version of such a function, but why would you do that if you haven't measured polygon drawing to be an actual bottleneck.
An example with three polygons drawn from top to bottom. Without the GRCG, edges of later polygons overwrite any previously drawn pixels within the same VRAM byte. Note how treating bitmasks as dot patterns corrupts even those areas where the background image had nonzero bits in its first bitplane.
As far as complexity is concerned though, the worst part is the implicit logic that allows all this text to show up on top of the polygons in the first place. If every single piece of text is only rendered a single time, how can it appear on top of the polygons if those are drawn every frame?
Depending on the game (because of course it's game-specific), the answer involves either the individual bits of the text color index or the actual contents of the palette:
Colors 0 or 1 can't be used, because those don't include any of the bits that can stay constant between frames.
If the lowest bit of a palette color index has no effect on the displayed color, text drawn in either of the two colors won't be visually affected by the polygon animation and will always appear on top. TH04 and TH05 rely on this property with their colors 2/3, 4/5, and 6/7 being identical, but this would work in TH02 and TH03 as well.
But this doesn't apply to TH02 and TH03's palettes, so how do they do it? The secret: They simply include all text pixels in nopoly_B. This allows text to use any color with an odd palette index – the lowest bit then won't be affected by the polygons ORed into the first bitplane, and the other bitplanes remain unchanged.
TH04 is a curious case. Ostensibly, it seems to remove support for odd text colors, probably because the new 10-frame fade-in animation on the comment text would require at least the comment area in VRAM to be captured into nopoly_B on every one of the 10 frames. However, the initial pixels of the tracklist are still included in nopoly_B, which would allow those to still use any odd color in this game. ZUN only removed those from nopoly_B in TH05, where it had to be changed because that game lets you scroll and browse through multiple tracklists.
The contents of nopoly_B with each game's first track selected.
Finally, here's a list of all the smaller details that turn the Music Rooms into such a mess:
Due to the polygon animation, the Music Room is one of the few double-buffered menus in PC-98 Touhou, rendering to both VRAM pages on alternate frames instead of using the other page to store a background image. Unfortunately though, this doesn't actually translate to tearing-free rendering because ZUN's initial implementation for TH02 mixed up the order of the required operations. You're supposed to first wait for the GDC's VSync interrupt and then, within the display's vertical blanking interval, write to the relevant I/O ports to flip the accessed and shown pages. Doing it the other way around and flipping as soon as you're finished with the last draw call of a frame means that you'll very likely hit a point where the (real or emulated) electron beam is still traveling across the screen. This ensures that there will be a tearing line somewhere on the screen on all but the fastest PC-98 models that can render an entire frame of the Music Room completely within the vertical blanking interval, causing the very issue that double-buffering was supposed to prevent.
ZUN only fixed this landmine in TH05.
The polygons have a fixed vertex count and radius depending on their index, everything else is randomized. They are also never reinitialized while OP.EXE is running – if you leave the Music Room and reenter it, they will continue animating from the same position.
TH02 and TH04 don't handle it at all, causing held keys to be processed again after about a second.
TH03 and TH05 correctly work around the quirk, at the usual cost of a 614.4 µs delay per frame. Except that the delay is actually twice as long in frames in which a previously held key is released, because this code is a mess.
But even in 2024, DOSBox-X is the only emulator that actually replicates this detail of real hardware. On anything else, keyboard input will behave as ZUN intended it to. At least I've now mentioned this once for every game, and can just link back to this blog post for the other menus we still have to go through, in case their game-specific behavior matches this one.
TH02 is the only game that
separately lists the stage and boss themes of the main game, rather than following the in-game order of appearance,
continues playing the selected track when leaving the Music Room,
always loads both MIDI and PMD versions, regardless of the currently selected mode, and
does not stop the currently playing track before loading the new one into the PMD and MMD drivers.
The combination of 2) and 3) allows you to leave the Music Room and change the music mode in the Option menu to listen to the same track in the other version, without the game changing back to the title screen theme. 4), however, might cause the PMD and MMD drivers to play garbage for a short while if the music data is loaded from a slow storage device that takes longer than a single period of the OPN timer to fill the driver's song buffer. Probably not worth mentioning anymore though, now that people no longer try fitting PC-98 Touhou games on floppy disks.
Exactly 40 (TH02/TH03) / 38 (TH04/TH05) visible bytes per line,
padded with 2 bytes that can hold a CR/LF newline sequence for easier editing.
Every track starts with a title line that mostly just duplicates the names from the hardcoded tracklist,
followed by a fixed 19 (TH02/TH03/TH04) / 9 (TH05) comment lines.
In TH04 and TH05, lines can start with a semicolon (;) to prevent them from being rendered. This is purely a performance hint, and is visually equivalent to filling the line with spaces.
All in all, the quality of the code is even slightly below the already poor standard for PC-98 Touhou: More VRAM page copies than necessary, conditional logic that is nested way too deeply, a distinct avoidance of state in favor of loops within loops, and – of course – a couple of gotos to jump around as needed.
In TH05, this gets so bad with the scrolling and game-changing tracklist that it all gives birth to a wonderfully obscure inconsistency: When pressing both ⬆️/⬇️ and ⬅️/➡️ at the same time, the game first processes the vertical input and then the horizontal one in the next frame, making it appear as if the latter takes precedence. Except when the cursor is highlighting the first (⬆️ ) or 12th (⬇️ ) element of the list, and said list element is not the first track (⬆️ ) or the quit option (⬇️ ), in which case the horizontal input is ignored.
And that's all the Music Rooms! The OP.EXE binaries of TH04 and especially TH05 are now very close to being 100% RE'd, with only the respective High Score menus and TH04's title animation still missing. As for actual completion though, the finalization% metric is more relevant as it also includes the ZUN Soft logo, which I RE'd on paper but haven't decompiled. I'm 📝 still hoping that this will be the final piece of code I decompile for these two games, and that no one pays to get it done earlier…
For the rest of the second push, there was a specific goal I wanted to reach for the remaining anything budget, which was blocked by a few functions at the beginning of TH04's and TH05's MAINE.EXE. In another anticlimactic development, this involved yet another way too early decompilation of a main() function…
Generally, this main() function just calls the top-level functions of all other ending-related screens in sequence, but it also handles the TH04-exclusive congratulating All Clear images within itself. After a 1CC, these are an additional reward on top of the Good Ending, showing the player character wearing a different outfit depending on the selected difficulty. On Easy Mode, however, the Good Ending is unattainable because the game always ends after Stage 5 with a Bad Ending, but ZUN still chose to show the EASY ALL CLEAR!! image in this case, regardless of how many continues you used.
While this might seem inconsistent with the other difficulties, it is consistent within Easy Mode itself, as the enforced Bad Ending after Stage 5 also doesn't distinguish between the number of continues. Also, Try to Normal Rank!! could very well be ZUN's roundabout way of implying "because this is how you avoid the Bad Ending".
With that out of the way, I was finally able to separate the VRAM text renderer of TH04 and TH05 into its own assembly unit, 📝 finishing the technical debt repayment project that I couldn't complete in 2021 due to assembly-time code segment label arithmetic in the data segment. This now allows me to translate this undecompilable self-modifying mess of ASM into C++ for the non-ASCII translation project, and thus unify the text renderers of all games and enhance them with support for Unicode characters loaded from a bitmap font. As the final finalized function in the SHARED segment, it also allowed me to remove 143 lines of particularly ugly segmentation workarounds 🙌
The remaining 1/6th of the second push provided the perfect occasion for some light TH02 PI work. The global boss position and damage variables represented some equally low-hanging fruit, being easily identified global variables that aren't part of a larger structure in this game. In an interesting twist, TH02 is the only game that uses an increasing damage value to track boss health rather than decreasing HP, and also doesn't internally distinguish between bosses and midbosses as far as these variables are concerned. Obviously, there's quite a bit of state left to be RE'd, not least because Marisa is doing her own thing with a bunch of redundant copies of her position, but that was too complex to figure out right now.
Also doing their own thing are the Five Magic Stones, which need five positions rather than a single one. Since they don't move, the game doesn't have to keep 📝 separate position variables for both VRAM pages, and can handle their positions in a much simpler way that made for a nice final commit.
And for the first time in a long while, I quite like what ZUN did there!
Not only are their positions stored in an array that is indexed with a consistent ID for every stone, but these IDs also follow the order you fight the stones in: The two inner ones use 0 and 1, the two outer ones use 2 and 3, and the one in the center uses 4. This might look like an odd choice at first because it doesn't match their horizontal order on the playfield. But then you notice that ZUN uses this property in the respective phase control functions to iterate over only the subrange of active stones, and you realize how brilliant it actually is.
This seems like a really basic thing to get excited about, especially since the rest of their data layout sure isn't perfect. Splitting each piece of state and even the individual X and Y coordinates into separate 5-element arrays is still counter-productive because the game ends up paying more memory and CPU cycles to recalculate the element offsets over and over again than this would have ever saved in cache misses on a 486. But that's a minor issue that could be fixed with a few regex replacements, not a misdesigned architecture that would require a full rewrite to clean it up. Compared to the hardcoded and bloated mess that was 📝 YuugenMagan's five eyes, this is definitely an improvement worthy of the good-code tag. The first actual one in two years, and a welcome change after the Music Room!
These three pieces of data alone yielded a whopping 5% of overall TH02 PI in just 1/6th of a push, bringing that game comfortably over the 60% PI mark. MAINE.EXE is guaranteed to reach 100% PI before I start working on the non-ASCII translations, but at this rate, it might even be realistic to go for 100% PI on MAIN.EXE as well? Or at least technical position independence, without the false positives.
Next up: Shuusou Gyoku SC-88Pro BGM. It's going to be wild.
And once again, the Shuusou Gyoku task was too complex to be satisfyingly solved within a single month. Even just finding provably correct loop sections in both the original and arranged MIDI files required some rather involved detection algorithms. I could have just defined what sounded like correct loops, but the results of these algorithms were quite surprising indeed. Turns out that not even Seihou is safe from ZUN quirks, and some tracks technically loop much later than you'd think they do, or don't loop at all. And since I then wanted to put these MIDI loops back into the game to ensure perfect synchronization between the recordings and MIDI versions, I ended up rewriting basically all the MIDI code in a cross-platform way. This rewrite also uncovered a pbg bug that has traveled from Shuusou Gyoku into Windows Touhou, where it survived until ZUN ultimately removed all MIDI code in TH11 (!)…
Fortunately, the backlog still had enough general PC-98 Touhou funds that I could spend on picking some soon-important low-hanging fruit, giving me something to deliver for the end of the month after all. TH04 and TH05 use almost identical code for their main/option menus, so decompiling it would make number go up quite significantly and the associated blog post won't be that long…
Wait, what's this, a bug report from touhou-memories concerning the website?
Tab switchers tended to break on certain Firefox versions, and
video playback didn't work on Microsoft Edge at all?
Those are definitely some high-priority bugs that demand immediate attention.
The tab switcher issue was easily fixed by replacing the previous z-index trickery with a more robust solution involving the hidden attribute. The second one, however, is much more aggravating, because video playback on Edge has been broken ever since I 📝 switched the preferred video codec to AV1.
This goes so far beyond not supporting a specific codec. Usually, unsupported codecs aren't supposed to be an issue: As soon as you start using the HTML <video> tag, you'll learn that not every browser supports all codecs. And so you set up an encoding pipeline to serve each video in a mix of new and ancient formats, put the <source> tag of the most preferred codec first, and rest assured that browsers will fall back on the best-supported option as necessary. Except that Edge doesn't even try, and insists on staying on a non-playing AV1 video. 🙄
The codecs parameter for the <source> type attribute was the first potential solution I came across. Specifying the video codec down to the finest encoding details right in the HTML markup sounds like a good idea, similar to specifying sizes of images and videos to prevent layout reflows on long pages during the initial page load. So why was this the first time I heard of this feature? The fact that there isn't a simple ffprobe -show_html_codecs_string command to retrieve this string might already give a clue about how useful it is in practice. Instead, you have to manually piece the string together by grepping your way through all of a video's metadata…
…and then it still doesn't change anything about Edge's behavior, even when also specifying the string for the VP9 and VP8 sources. Calling the infamously ridiculous HTMLMediaElement.canPlayType() method with a representative parameter of "video/webm; codecs=av01.1.04M.08.0.000.01.13.00.0" explains why: Both the AV1-supporting Chrome and Edge return "probably", but only the former can actually play this format. 🤦
But wait, there is an AV1 video extension in the Microsoft Store that would add support to any unspecified favorite video app. Except that it stopped working inside Edge as of version 116. And even if it did: If you can't query the presence of this extension via JavaScript, it might as well not exist at all.
Not to mention that the favorite video app part is obviously a lie as a lot of widely preferred Windows video apps are bundled with their own codecs, and have probably long supported AV1.
In the end, there's no way around the utter desperation move of removing the AV1 <source> for Edge users. Serving each video in two other formats means that we can at least do something here – try visiting the GitHub release page of the P0234-1 TH01 Anniversary Edition build in Edge and you also don't get to see anything, because that video uses AV1 and GitHub understandably doesn't re-encode every uploaded video into a variety of old formats.
Just for comparison, I tried both that page and the ReC98 blog on an old Android 6 phone from 2014, and even that phone picked and played the AV1 videos with the latest available Chrome and Firefox versions. This was the phone whose available Firefox version didn't support VP9 in 2019, which was my initial reason for adding the VP8 versions. Looks like it's finally time to drop those… 🤔 Maybe in the far future once I start running out of space on this server.
Removing the <source> tags can be done in one of two places:
server-side, detecting Edge via the User-Agent header, or
I went with 2) because more dynamic server-side code would only move us further away from static site generation, which would make a lot of sense as the next evolutionary step in the architecture of this website. The client-side solution is much simpler too, and we can defer the deletion until a user actually hovers over a specific video.
And while we're at it, let's also add a popup complaining about this whole state of affairs. Edge is heavily marketed inside Windows as "the modern browser recommended by Microsoft", and you sure wouldn't expect low-quality chroma-subsampled VP9 from such a tagline. With such a level of anti-support for AV1, Edge users deserve to know exactly what's going on, especially since this post also explains what they will encounter on other websites.
That's the polite way of putting it.
Alright, where was I? For TH01, the main menu was the last thing I decompiled before the 100% finalization mark, so it's rather anticlimactic to already cover the TH04/TH05 one now, with both of the games still being very far away from 100%, just because people will soon want to translate the description text in the bottom-right corner of the screen. But then again, the ZUN Soft logo animation would make for an even nicer final piece of decompiled code, especially since the bouncing-ball logo from TH01, TH02, and TH03 was the very first decompilation I did, all the way back in 2015.
The code quality of ZUN's VRAM-based menus has barely increased between TH01 and TH05. Both the top-level and option menu still need to know the bounding rectangle of the other one to unblit the right pixels when switching between the two. And since ZUN sure loved hardcoded and copy-pasted numbers in the PC-98 days, the coordinates both tend to be excessively large, and excessively wrong. Luckily, each menu item comes with its own correct unblitting rectangle, which avoids any graphical glitches that would otherwise occur.
As for actual observable quirks and bugs, these menus only contain one of each, and both are exclusive to TH04:
Quitting out of the Music Room moves the cursor to the Start option. In TH05, it stays on Music Room.
Changing the S.E. mode seems to do nothing within TH04's menus, and would only take effect if you also change the Music mode afterward, or launch into the game.
And yes, these videos do have a frame rate of 2 FPS.
Now that 100% finalization of their OP.EXE binaries is within reach, all this bloat made me think about the viability of a 📝 single-executable build for TH04's and TH05's debloated and anniversary versions. It would be really nice to have such a build ready before I start working on the non-ASCII translations – not just because they will be based on the anniversary branch by default, but also because it would significantly help their development if there are 4 fewer executables to worry about.
However, it's not as simple for these games as it was for TH01. The unique code in their OP.EXE and MAINE.EXE binaries is much larger than Borland's easily removed C++ exception handler, so I'd have to remove a lot more bloat to keep the resulting single binary at or below the size of the original MAIN.EXE. But I'm sure going to try.
Speaking of code that can be debloated for great effect: The second push of this delivery focused on the first-launch sound setup menu, whose BGM and sound effect submenus are almost complete code duplicates of each other. The debloated branch could easily remove more than half of the code in there, yielding another ≈800 bytes in case we need them.
If hex-editing MIKO.CFG is more convenient for you than deleting that file, you can set its first byte to FF to re-trigger this menu. Decompiling this screen was not only relevant now because it contains text rendered with font ROM glyphs and it would help dig our way towards more important strings in the data segment, but also because of its visual style. I can imagine many potential mods that might want to use the same backgrounds and box graphics for their menus.
How about an initial language selection menu in the same style?
With the two submenus being shown in a fixed sequence, there's not a lot of room for the code to do anything wrong, and it's even more identical between the two games than the main menu already was. Thankfully, ZUN just reblits the respective options in the new color when moving the cursor, with no 📝 palette tricks. TH04's background image only uses 7 colors, so he could have easily reserved 3 colors for that. In exchange, the TH05 image gets to use the full 16 colors with no change to the code.
Rounding out this delivery, we also got TH05's rolling Yin-Yang Orb animation before the title screen… and it's just more bloat and landmines on a smaller scale that might be noticeable on slower PC-98 models. In total, there are three unnecessary inter-page copies of the entire VRAM that can easily insert lag frames, and two minor page-switching landmines that can potentially lead to tearing on the first frame of the roll or fade animation. Clearly, ZUN did not have smoothness or code quality in mind there, as evidenced by the fact that this animation simply displays 8 .PI files in sequence. But hey, a short animation like this is 📝 another perfectly appropriate place for a quick-and-dirty solution if you develop with a deadline.
And that's 1.30% of all PC-98 Touhou code finalized in two pushes! We're slowly running out of these big shared pieces of ASM code…
I've been neglecting TH03's OP.EXE quite a bit since it simply doesn't contain any translatable plaintext outside the Music Room. All menu labels are gaiji, and even the character selection menu displays its monochrome character names using the 4-plane sprites from CHNAME.BFT. Splitting off half of its data into a separate .ASM file was more akin to getting out a jackhammer to free up the room in front of the third remaining Music Room, but now we're there, and I can decompile all three of them in a natural way, with all referenced data.
Next up, therefore: Doing just that, securing another important piece of text for the upcoming non-ASCII translations and delivering another big piece of easily finalized code. I'm going to work full-time on ReC98 for almost all of December, and delivering that and the Shuusou Gyoku SC-88Pro recording BGM back-to-back should free up about half of the slightly higher cap for this month.
And we're back to PC-98 Touhou for a brief interruption of the ongoing Shuusou Gyoku Linux port.
Let's clear some of the Touhou-related progress from the backlog, and use
the unconstrained nature of these contributions to prepare the
📝 upcoming non-ASCII translations commissioned by Touhou Patch Center.
The current budget won't cover all of my ambitions, but it would at least be
nice if all text in these games was feasibly translatable by the time I
officially start working on that project.
At a little over 3 pushes, it might be surprising to see that this took
longer than the
📝 TH03/TH04/TH05 cutscene system. It's
obvious that TH02 started out with a different system for in-game dialog,
but while TH04 and TH05 look identical on the surface, they only
actually share 30% of their dialog code. So this felt more like decompiling
2.4 distinct systems, as opposed to one identical base with tons of
game-specific differences on top.
The table of contents was pretty popular last time around, so let's have
another one:
Let's start with the ones from TH04 and TH05, since they are not that
broken. For TH04, ZUN started out by copy-pasting the cutscene system,
causing the result to inherit many of the caveats I already described in the
cutscene blog post:
It's still a plaintext format geared exclusively toward full-width
Japanese text.
The parser still ignores all whitespace, forcing ASCII text into hacks
with unassigned Shift-JIS lead bytes outside the second byte of a 2-byte
chunk.
Commands are still preceded by a 0x5C byte, which renders
as either a \ or a ¥ depending on your font and
interpretation of Shift-JIS.
Command parameters are parsed in exactly the same way, with all the same
limits.
A lot of the same script commands are identical, including 7 of them
that were not used in TH04's original dialog scripts.
Then, however, he greatly simplified the system. Mainly, this was done by
moving text rendering from the PC-98 graphics chip to the text chip, which
avoids the need for any text-related unblitting code, but ZUN also added a
bunch of smaller changes:
The player must advance through every dialog box by releasing any held
keys and then pressing any key mapped to a game action. There are no
timeouts.
The delay for every 2 bytes of text was doubled to 2 frames, and can't
be overridden.
Instead of holding ESC to fast-forward, pressing any key
will immediately print the entire rest of a text box.
Dialogs run in their own single-buffered frame loop, interrupting the
rest of the game. The other VRAM page keeps the background pixels required
for unblitting the face images.
All script commands that affect the graphics layer are preceded by a
1-frame delay. ZUN most likely did this because of the single-buffered
nature, as it prevents tearing on the first frame by waiting for the CRT
beam to return to the top-left corner before changing any pixels.
Both boxes are intended to contain up to 30 half-width characters on
each of their up to 3 lines, but nothing in the code enforces these limits.
There is no support for automatic line breaks or starting new boxes.
While it would seem that TH05 has no issues with ASCII 0x20
spaces, the text as a whole is still blindly processed two bytes at a
time, and any commands can only appear at even byte positions within a
line. I dimmed the VRAM pixels to 25% of their original brightness to make the
text easier to read.
The same text backported to TH04, additionally demonstrating how that
game's dialog system inherited the whitespace skipping behavior of
TH03's cutscene system. Just like there, ASCII 0x20 spaces
only work at odd byte positions because the game treats them as the
trailing byte of a full-width Shift-JIS codepoint. I don't know how
large the budget for the upcoming non-ASCII translations will be, but
I'm going to fix this even in the very basic fully static variant.
I dimmed the VRAM pixels to 25% of their original brightness to make the
text easier to read.
TH05 then moved from TH04's plaintext scripts to the binary
.TX2 format while removing all the unused commands copy-pasted
from the cutscene system. Except for a
single additional command intended to clear a text box, TH05's dialog
system only supports a strict subset of the features of TH04's system.
This change also introduced the following differences compared to TH04:
The game now stores the dialog of all 4 playable characters in the same
file, with a (4 + 1)-word header that indicates the byte offset
and length of each character's script. This way, it can load only the one
script for the currently played character.
Since there is no need for whitespace in a binary format, you can now
use ASCII 0x20 spaces even as the first byte of a 2-byte text
chunk! 🥳
All command parameters are now mandatory.
Filenames are now passed directly by pointer to the respective game
function. Therefore, they now need to be null-terminated, but can in turn be
as long as
📝 the number of remaining bytes in the allocated dialog segment.
In practice though, the game still runs on DOS and shares its restriction of
8.3 filenames…
When starting a new dialog box, any existing text in the other box is
now colored blue.
Thanks to ZUN messing up the return values of the command-interpreting
switch function, you can effectively use only line break and gaiji commands in the middle of text. All other
commands do execute, but the interpreter then also treats their command byte
as a Shift-JIS lead byte and places it in text RAM together with whatever
other byte follows in the script.
This is why TH04 can and does put its \= commandsinto the boxes
started with the 0 or 1 commands, but TH05 has to
put its 0x02 commands before the equivalent 0x0D.
Writing the 0x02 byte to text RAM results in an character, which is simply the PC-98 font ROM's glyph for that
Shift-JIS codepoint. Also note how each face change is now
preceded by two frames of delay.
No problem in TH04. Note how the dialog also runs a bit faster – TH04
only adds the aforementioned one frame of delay to each face change, and
has fewer two-byte chunks of text to display overall.
For modding these files, you probably want to use TXDEF from
-Tom-'s MysticTK. It decodes these
files into a text representation, and its encoder then takes care of the
character-specific byte offsets in the 10-byte header. This text
representation simplifies the format a lot by avoiding all corner cases and
landmines you'd experience during hex-editing – most notably by interpreting
the box-starting 0x0D as a
command to show text that takes a string parameter, avoiding the broken
calls to script commands in the middle of text. However, you'd still have to
manually ensure an even number of bytes on every line of text.
In the entry function of TH05's dialog loop, we also encounter the hack that
is responsible for properly handling
📝 ZUN's hidden Extra Stage replay. Since the
dialog loop doesn't access the replay inputs but still requires key presses
to advance through the boxes, ZUN chose to just skip the dialog altogether in the
specific case of the Extra Stage replay being active, and replicated all
sprite management commands from the dialog script by just hardcoding
them.
And you know what? Not only do I not mind this hack, but I would have
preferred it over the actual dialog system! The aforementioned sprite
management commands effectively boil down to manual memory management,
deallocating all stage enemy and midboss sprites and thus ensuring that the
boss sprites end up at specific master.lib sprite IDs (patnums). The
hardcoded boss rendering function then expects these sprites to be available
at these exact IDs… which means that the otherwise hardcoded bosses can't
render properly without the dialog script running before them.
There is absolutely no excuse for the game to burden dialog scripts with
this functionality. Sure, delayed deallocation would allow them to blit
stage-specific sprites, but the original games don't do that; probably
because none of the two games feature an unblitting command. And even if
they did, it would have still been cleaner to expose the boss-specific
sprite setup as a single script command that can then also be called from
game code if the script didn't do so. Commands like these just are a recipe
for crashes, especially with parsers that expect fullwidth Shift-JIS
text and where misaligned ASCII text can easily cause these commands to be
skipped.
But then again, it does make for funny screenshot material if you
accidentally the deallocation and then see bosses being turned into stage
enemies:
Some of the more amusing consequences of not calling the
sprite-deallocating
\c /
0x04 command inside a dialog
script.
In the case of 4️⃣, the game then even crashes on this frame at the end
of the dialog, in a way that resembles the infamous
📝 TH04 crash before Stage 5 Yuuka if no EMS driver is loaded.
Both the stage- and boss-specific BFNT sprites are loaded into memory at
this point, leaving no room for the 256×256-pixel background image on
the size-limited master.lib heap.
With all the general details out of the way, here's the command reference:
0 1
0x00 0x01
Selects either the player character (0) or the boss (1) as the
currently speaking character, and moves the cursor to the beginning of
the text box. In TH04, this command also directly starts the new dialog
box, which is probably why it's not prefixed with a \ as it
only makes sense outside of text. TH05 requires a separate 0x0D command to do the
same.
\=1
0x02 0x!!
Replaces the face portrait of the currently active speaking
character with image #1 within her .CD2
file.
\=255
0x02 0xFF
Removes the face portrait from the currently active text box.
\l,filename
0x03 filename 0x00
Calls master.lib's super_entry_bfnt() function, which
loads sprites from a BFNT file to consecutive IDs starting at the
current patnum write cursor.
\c
0x04
Deallocates all stage-specific BFNT sprites (i.e., stage enemies and
midbosses), freeing up conventional RAM for the boss sprites and
ensuring that master.lib's patnum write cursor ends up at
128 /
180.
In TH05's Extra Stage, this command also replaces
📝 the sprites loaded from MIKO16.BFT with the ones from ST06_16.BFT.
\d
Deallocates all face portrait images.
The game automatically does this at the end of each dialog sequence.
However, ZUN wanted to load Stage 6 Yuuka's 76 KiB of additional
animations inside the script via \l, and would have once again
run up against the master.lib heap size limit without that extra free
memory.
\m,filename
0x05 filename 0x00
Stops the currently playing BGM, loads a new one from the given
file, and starts playback.
\m$
0x05 $ 0x00
Stops the currently playing BGM.
Note that TH05 interprets $ as a null-terminated filename as
well.
\m*
Restarts playback of the currently loaded BGM from the
beginning.
\b0,0,0
0x06 0x!!!!0x!!!!0x!!
Blits the master.lib patnum with the ID indicated by the third
parameter to the current VRAM page at the top-left screen position
indicated by the first two parameters.
\e0
Plays the sound effect with the given ID.
\t100
Sets palette brightness via master.lib's
palette_settone() to any value from 0 (fully black) to 200
(fully white). 100 corresponds to the palette's original colors.
\fo1
\fi1
Calls master.lib's palette_black_out() or
palette_black_in() to play a hardware palette fade
animation from or to black, spending roughly 1 frame on each of the 16 fade steps.
\wo1
\wi1
0x09 0x!!
0x0A 0x!!
Calls master.lib's palette_white_out() or
palette_white_in() to play a hardware palette fade
animation from or to white, spending roughly 1 frame on each of the 16 fade steps. The
TH05 version of 0x09 also clears the text in both boxes
before the animation.
\n
0x0B
Starts a new line by resetting the X coordinate of the TRAM cursor
to the left edge of the text area and incrementing the Y coordinate.
The new line will always be the next one below the last one that was
properly started, regardless of whether the text previously wrapped to
the next TRAM row at the edge of the screen.
\g8
Plays a blocking 8-frame screen shake
animation. Copy-pasted from the cutscene parser, but actually used right
at the end of the dialog shown before TH04's Bad Ending.
\ga0
0x0C 0x!!
Shows the gaiji with the given ID from 0 to 255
at the current cursor position, ignoring the per-glyph delay.
\k0
Waits 0 frames (0 = forever) for any key
to be pressed before continuing script execution.
Takes the current dialog cursor as the top-left corner of a
240×48-pixel rectangle, and replaces all text RAM characters within that
rectangle with whitespace.
This is only used to clear the player character's text box before
Shinki's final いくよ‼ box. Shinki has two
consecutive text boxes in all 4 scripts here, and ZUN probably wanted to
clear the otherwise blue text to imply a dramatic pause before Shinki's
final sentence. Nice touch.
(You could, however, also use it after a
box-ending 0xFF command to mess with text RAM in
general.)
\#
Quits the currently running loop. This returns from either the text
loop to the command loop, or it ends the dialog sequence by returning
from the command loop back to gameplay. If this stage of the game later
starts another dialog sequence, it will start at the next script
byte.
\$
Like \#, but first waits for any key to be
pressed.
0xFF
Behaves like TH04's \$ in the text loop, and like
\# in the command loop. Hence, it's not possible in TH05 to
automatically end a text box and advance to the next one without waiting
for a key press.
Unused commands are in gray.
At the end of the day, you might criticize the system for how its landmines
make it annoying to mod in ASCII text, but it all works and does what it's
supposed to. ZUN could have written the cleanest single and central
Shift-JIS iterator that properly chunks a byte buffer into halfwidth and
fullwidth codepoints, and I'd still be throwing it out for the upcoming
non-ASCII translations in favor of something that either also supports UTF-8
or performs dictionary lookups with a full box of text.
The only actual bug can be found in the input detection, which once
again doesn't correctly handle the infamous key
up/key down scancode quirk of PC-98 keyboards. All it takes
is one wrongly placed input polling call, and suddenly you have to think
about how the update cycle behind the PC-98 keyboard state bytes
might cause the game to run the regular 2-frame delay for a single
2-byte chunk of text before it shows the full text of a box after
all… But even this bug is highly theoretical and could probably only be
observed very, very rarely, and exclusively on real hardware.
The same can't be said about TH02 though, but more on that later. Let's
first take a look at its data, which started out much simpler in that game.
The STAGE?.TXT files contain just raw Shift-JIS text with no
trace of commands or structure. Turning on the whitespace display feature in
your editor reveals how the dialog system even assumes a fixed byte
length for each box: 36 bytes per line which will appear on screen, followed
by 4 bytes of padding, which the original files conveniently use to visually
split the lines via a CR/LF newline sequence. Make sure to disable trimming
of trailing whitespace in your editor to not ruin the file when modding the
text…
Two boxes from TH02's STAGE5.TXT with visualized whitespace.
These also demonstrate how the CR/LF newlines only make up 2 of the 4
padding bytes, and require each line to be padded with two more bytes; you
could not use these trailing spaces for actual text. Also note how
the exquisite mixture of fullwidth and halfwidth spaces demands the text to
be viewed with only the most metrically consistent monospace fonts to
preserve the intended alignment. 🍷 It appears quite misaligned on my phone.
Consequently, everything else is hardcoded – every effect shown between text
boxes, the face portrait shown for each box, and even how many boxes are
part of each dialog sequence. Which means that the source code now contains
a
long hardcoded list of face IDs for most of the text boxes in the game,
with the rest being part of the
dedicated hardcoded dialog scripts for 2/3 of the
game's stages.
Without the restriction to a fixed set of scripting commands, TH02 naturally
gravitated to having the most varied dialog sequences of all PC-98 Touhou
games. This flexibility certainly facilitated Mima's grand entrance
animation in Stage 4, or the different lines in Stage 4 and 5 depending on
whether you already used a continue or not. Marisa's post-boss dialog even
inserts the number of continues into the text itself – by, you guessed it,
writing to hardcoded byte offsets inside the dialog text before printing it
to the screen. But once again, I have nothing to
criticize here – not even the fact that the alternate dialog scripts have to
mutate the "box cursor" to jump to the intended boxes within the file. I
know that some people in my audience like VMs, but I would have considered
it more bloated if ZUN had implemented a full-blown scripting
language just to handle all these special cases.
Another unique aspect of TH02 is the way it stores its face portraits, which
are infamous for how hard they are to find in the original data files. These
sprites are actually map tiles, stored in MIKO_K.MPN,
and drawn using the same functions used to blit the regular map tiles to the
📝 tile source area in VRAM. We can only guess
why ZUN chose this one out of the three graphics formats he used in TH02:
BFNT supports transparency, but sacrifices one of the 16 colors to do
so. ZUN only used 15 colors for the face portraits, but might have wanted to
keep open the option to use that 16th color. The detailed
backgrounds also suggest that these images were never supposed to be
transparent to begin with.
PI is used for all bigger and non-transparent images, but ZUN would have
had to write a separate small function to blit a 48×48 subsection of such an
image. That certainly wouldn't have stopped him in the TH01 days, but he
probably was already past that point by this game.
That only leaves .MPN. Sure, he did have to slice each face into 9
separate 16×16 "map" tiles to use this format, but that's a small price to
pay in exchange for not having to write any new low-level blitting code,
especially since he must have already had an asset pipeline to generate
these files.
TH02's MIKO_K.PTN, arranged into a 16×16-tile layout that
reveals how these tiles are combined into face portraits. MPNDEF from -Tom-'s MysticTK conveniently uses
this exact layout in its .BMP output. Earlier MPNDEF
versions crashed when converting this file as its 256 tiles led to an
8-bit overflow bug, so make sure you've updated to the current version
from the end of October 2023 if you want to convert this file yourself.
The format stores the 4 bitplanes of each 16×16 tile in order, so good
luck finding a different planar image viewer that would support both
such a tiled layout and a custom palette. Sometimes, a weird
internal format is the best type of obfuscation.
And since you're certainly wondering about all these black tiles at the
edges: Yes, these are not only part of the file and pad it from the required
240×192 pixels to 256×256, but also kept in memory during a stage, wasting
9.5 KiB of conventional RAM. That's 172 seconds of potential input
replay data, just for those people who might still think that we need EMS
for replays.
Alright, we've got the text, we've got the faces, let's slide in the box and
display it all on screen. Apparently though, we also have to blit the player
and option sprites using raw, low-level master.lib function calls in the
process? This can't be right, especially because ZUN
always blits the option sprite associated with the Reimu-A shot type,
regardless of which one the player actually selected. And if you keep moving
above the box area before the dialog starts, you get to see exactly how
wrong this is:
Let's look closer at Reimu's sprite during the slide-in animation, and in
the two frames before:
This one image shows off no less than 4 bugs:
ZUN blits the stationary player sprite here, regardless of whether the
player was previously moving left or right. This is a nice way of indicating
that Reimu stops moving once the dialog starts, but maybe ZUN should
have unblitted the old sprite so that the new one wouldn't have appeared on
top. The game only unblits the 384×64 pixels covered by the dialog box on
every frame of the slide-in animation, so Reimu would only appear correctly
if her sprite happened to be entirely located within that area.
All sprites are shifted up by 1 pixel in frame 2️⃣. This one is not a
bug in the dialog system, but in the main game loop. The game runs the
relevant actions in the following order:
Invalidate any map tiles covered by entities
Redraw invalidated tiles
Decrement the Y coordinate at the top of VRAM according to the
scroll speed
Update and render all game entities
Scroll in new tiles as necessary according to the scroll speed, and
report whether the game has scrolled one pixel past the end of the
map
If that happened, pretend it didn't by incrementing the value
calculated in #3 for all further frames and skipping to
#8.
Issue a GDC SCROLL command to reflect the line
calculated in #3 on the display
Wait for VSync
Flip VRAM pages
Start boss if we're past the end of the map
The problem here: Once the dialog starts, the game has already rendered
an entire new frame, with all sprites being offset by a new Y scroll
offset, without adjusting the graphics GDC's scroll registers to
compensate. Hence, the Y position in 3️⃣ is the correct one, and the
whole existence of frame 2️⃣ is a bug in itself. (Well… OK, probably a
quirk because speedrunning exists, and it would be pretty annoying to
synchronize any video regression tests of the future TH02 Anniversary
Edition if it renders one fewer frame in the middle of a stage.)
ZUN blits the option sprites to their position from frame 1️⃣. This
brings us back to
📝 TH02's special way of retaining the previous and current position in a two-element array, indexed with a VRAM page ID.
Normally, this would be equivalent to using dedicated prev and
cur structure fields and you'd just index it with the back page
for every rendering call. But if you then decide to go single-buffered for
dialogs and render them onto the front page instead…
Note that fixing bug #2 would not cancel out this one – the sprites would
then simply be rendered to their position in the frame before 1️⃣.
And of course, the fixed option sprite ID also counts as a bug.
As for the boxes themselves, it's yet another loop that prints 2-byte chunks
of Shift-JIS text at an even slower fixed interval of 3 frames. In an
interesting quirk though, ZUN assumes that every box starts with the name of
the speaking character in its first two fullwidth Shift-JIS characters,
followed by a fullwidth colon. These 6 bytes are displayed immediately at
the start of every box, without the usual delay. The resulting alignment
looks rather janky with Genjii, whose single right-padded 亀
kanji looks quite awkward with the fullwidth space between the name
and the colon. Kind of makes you wonder why ZUN just didn't spell out his
proper name, 玄爺, instead, but I get the stylistic
difference.
In Stage 4, the two-kanji assumption then breaks with Marisa's three-kanji
name, which causes the full-width colon to be printed as the first delayed
character in each of her boxes:
That's all the issues and quirks in the system itself. The scripts
themselves don't leave much room for bugs as they basically just loop over
the hardcoded face ID array at this level… until we reach the end of the
game. Previously, the slide-in animation could simply use the tile
invalidation and re-rendering system to unblit the box on each frame, which
also explained why Reimu had to be separately rendered on top. But this no
longer works with a custom-rendered boss background, and so the game just
chooses to flood-fill the area with graphics chip color #0:
Then again, transferring pixels from the back page would be just
as wrong as they lag one frame behind. No way around capturing these 384×64
pixels to main memory here… Oh well, this flood-fill at least adds even more
legibility on top of the already half-transparent text box. A property that
the following dialog sequence unfortunately lacks…
For Mima's final defeat dialog though, ZUN chose to not even show the box.
He might have realized the issue by that point, or simply preferred the more
dramatic effect this had on the lines. The resulting issues, however, might
even have ramifications for such un-technical things as lore and
character dynamics. As it turns out, the code
for this dialog sequence does in fact render Mima's smiling face for all
boxes?! You only don't see it in the original game because it's rendered to
the other VRAM page that remains invisible during the dialog sequence:
Caution, flashing lights.
Here's how I interpret the situation:
The function that launches into the final part of the dialog script
starts with dedicated
code to re-render Mima to the back page, on top of the previously
rendered planet background. Since the entire script runs on the front
page (and thus, on top of the previous frame) and the game launches into
the ending immediately after, you don't ever get to see this new partial
frame in the original game.
Showing this partial frame would also ensure that you can actually
read the dialog text without a surrounding box. Then, the white
letters won't ever be put on top of any white bullets – or, worse, be completely invisible if the
dialog is triggered in the middle of Reimu-B's bomb animation, which
fills VRAM with lots of white pixels.
Hence, we've got enough evidence to classify not showing the back page
as a ZUN
bug. 🐞
However, Mima's smiling face jars with the words she says here. Adding
the face would deviate more significantly from the original game than
removing the player shot, item, bullet, or spark sprites would. It's
imaginable that ZUN just forgot about the dedicated code that
re-rendered just Mima to the back page, but the faces add
something to the dialog, and ZUN would have clearly noticed and
fixed it if their absence wasn't intended. Heck, ZUN might have just put
something related to Mima into the code because TH02's dialog system has
no way of not drawing a face for a dialog box. Filling the face
area with graphics chip color #0, as seen in the first and third boxes
of the Extra Stage pre-boss dialog, would have been an alternative, but
that would have been equally wrong with regard to the background.
Hence, the invisible face portrait from the original game is a ZUN
quirk. 🎺
So, the future TH02 Anniversary Edition will fix the bug by showing
the back page, but retain the quirk by rewriting the dialog code to
not blit the face.
And with that, we've secured all in-game dialog for the upcoming non-ASCII
translations! The remaining 2/3 of the last push made
for a good occasion to also decompile the small amount of code related to
TH03's win messages, stored in the @0?TX.TXT files. Similar to
TH02's dialog format, these files are also split into fixed-size blocks of
3×60 bytes. But this time, TH03 loads all 60 bytes of a line, including the
CR/LF line breaking codepoints in the original files, into the statically
allocated buffer that it renders from. These control characters are then
only filtered to whitespace by ZUN's graph_putsa_fx() function.
If you remove the line breaks, you get to use the full 60 bytes on every
line.
The final commits went to the MIKO.CFG loading and saving
functions used in TH04's and TH05's OP.EXE, as well as TH04's
game startup code to finally catch up with
📝 TH05's counterpart from over 3 years ago.
This brought us right in front of the main menu rendering code in both TH04
and TH05, which is identical in both games and will be tackled in the next
PC-98 Touhou delivery.
Next up, though: Returning to Shuusou Gyoku, and adding support for SC-88Pro
recordings as BGM. Which may or may not come with a slight controversy…
And then, the supposed boilerplate code revealed yet another confusing issue
that quickly forced me back to serial work, leading to no parallel progress
made with Shuusou Gyoku after all. 🥲 The list of functions I put together
for the first ½ of this push seemed so boring at first, and I was so sure
that there was almost nothing I could possibly talk about:
TH02's gaiji animations at the start and end of each stage, resembling
opening and closing window blind slats. ZUN should have maybe not defined
the regular whitespace gaiji as what's technically the last frame of the
closing animation, but that's a minor nitpick. Nothing special there
otherwise.
The remaining spawn functions for TH04's and TH05's gather circles. The
only dumb antic there is the way ZUN initializes the template for bullets
fired at the end of the animation, featuring ASM instructions that are
equivalent to what Turbo C++ 4.0J generates for the __memcpy__
intrinsic, but show up in a different order. Which means that they must have
been handwritten. I already figured that out in 2022
though, so this was just more of the same.
EX-Alice's override for the game's main 16×16 sprite sheet, loaded
during her dialog script. More of a naming and consistency challenge, if
anything.
The regular version of TH05's big 16×16 sprite sheet.
EX-Alice's variant of TH05's big 16×16 sprite sheet.
The rendering function for TH04's Stage 4 midboss, which seems to
feature the same premature clipping quirk we've seen for
📝 TH05's Stage 5 midboss, 7 months ago?
The rendering function for the big 48×48 explosion sprite, which also
features the same clipping quirk?
That's three instances of ZUN removing sprites way earlier than you'd want
to, intentionally deciding against those sprites flying smoothly in and out
of the playfield. Clearly, there has to be a system and a reason behind it.
Turns out that it can be almost completely blamed on master.lib. None of the
super_*() sprite blitting functions can clip the rendered
sprite to the edges of VRAM, and much less to the custom playfield rectangle
we would actually want here. This is exactly the wrong choice to make for a
game engine: Not only is the game developer now stuck with either rendering
the sprite in full or not at all, but they're also left with the burden of
manually calculating when not to display a sprite.
However, strictly limiting the top-left screen-space coordinate to
(0, 0) and the bottom-right one to (640, 400) would actually
stop rendering some of the sprites much earlier than the clipping conditions
we encounter in these games. So what's going on there?
The answer is a combination of playfield borders, hardware scrolling, and
master.lib needing to provide at least some help to support the
latter. Hardware scrolling on PC-98 works by dividing VRAM into two vertical
partitions along the Y-axis and telling the GDC to display one of them at
the top of the screen and the other one below. The contents of VRAM remain
unmodified throughout, which raises the interesting question of how to deal
with sprites that reach the vertical edges of VRAM. If the top VRAM row that
starts at offset 0x0000 ends up being displayed below
the bottom row of VRAM that starts at offset 0x7CB0 for 399 of
the 400 possible scrolling positions, wouldn't we then need to vertically
wrap most of the rendered sprites?
For this reason, master.lib provides the super_roll_*()
functions, which unconditionally perform exactly this vertical wrapping. But
this creates a new problem: If these functions still can't clip, and don't
even know which VRAM rows currently correspond to the top and bottom row of
the screen (since master.lib's graph_scrollup() function
doesn't retain this information), won't we also see sprites wrapping around
the actual edges of the screen? That's something we certainly
wouldn't want in a vertically scrolling game…
The answer is yes, and master.lib offers no solution for this issue. But
this is where the playfield borders come in, and helpfully cover 16 pixels
at the top and 16 pixels at the bottom of the screen. As a result, they can
hide up to 32 rows of potentially wrapped sprite pixels below them:
•
The earliest possible frame that TH05 can start rendering the Stage 5
midboss on. Hiding the text layer reveals how master.lib did in fact
"blindly" render the top part of her sprite to the bottom of the
playfield. That's where her sprite starts before it is correctly
wrapped around to the top of VRAM.
If we scrolled VRAM by another 200 pixels (and faked an equally shifted
TRAM for demonstration purposes), we get an equally valid game scene
that points out why a vertically scrolling PC-98 game must wrap all sprites
at the vertical edges of VRAM to begin with.
Also, note how the HP bar has filled up quite a bit before the midboss can
actually appear on screen.
And that's how the lowest possible top Y coordinate for sprites blitted
using the master.lib super_roll_*() functions during the
scrolling portions of TH02, TH04, and TH05 is not 0, but -16. Any lower, and
you would actually see some of the sprite's upper pixels at the
bottom of the playfield, as there are no more opaque black text cells to
cover them. Theoretically, you could lower this number for
some animation frames that start with multiple rows of transparent
pixels, but I thankfully haven't found any instance of ZUN using such a
hack. So far, at least…
Visualized like that, it all looks quite simple and logical, but for days, I
did not realize that these sprites were rendered to a scrolling VRAM.
This led to a much more complicated initial explanation involving the
invisible extra space of VRAM between offsets 0x7D00 and
0x7FFF that effectively grant a hidden additional 9.6 lines
below the playfield. Or even above, since PC-98 hardware ignores the highest
bit of any offset into a VRAM bitplane segment
(& 0x7FFF), which prevents blitting operations from
accidentally reaching into a different bitplane. Together with the
aforementioned rows of transparent pixels at the top of these midboss
sprites, the math would have almost worked out exactly.
The need for manual clipping also applies to the X-axis. Due to the lack of
scrolling in this dimension, the boundaries there are much more
straightforward though. The minimum left coordinate of a sprite can't fall
below 0 because any smaller coordinate would wrap around into the
📝 tile source area and overwrite some of the
pixels there, which we obviously don't want to re-blit every frame.
Similarly, the right coordinate must not extend into the HUD, which starts
at 448 pixels.
The last part might be surprising if you aren't familiar with the PC-98 text
chip. Contrary to the CGA and VGA text modes of IBM-compatibles, PC-98 text
cells can only use a single color for either their foreground or
background, with the other pixels being transparent and always revealing the
pixels in VRAM below. If you look closely at the HUD in the images above,
you can see how the background of cells with gaiji glyphs is slightly
brighter (◼ #100) than the opaque black
cells (◼ #000) surrounding them. This
rather custom color clearly implies that those pixels must have been
rendered by the graphics GDC. If any other sprite was rendered below the
HUD, you would equally see it below the glyphs.
So in the end, I did find the clear and logical system I was looking for,
and managed to reduce the new clipping conditions down to a
set of basic rules for each edge. Unfortunately, we also need a second
macro for each edge to differentiate between sprites that are smaller or
larger than the playfield border, which is treated as either 32×32 (for
super_roll_*()) or 32×16 (for non-"rolling"
super_*() functions). Since smaller sprites can be fully
contained within this border, the games can stop rendering them as soon as
their bottom-right coordinate is no longer seen within the playfield, by
comparing against the clipping boundaries with <= and
>=. For example, a 16×16 sprite would be completely
invisible once it reaches (16, 0), so it would still be rendered at
(17, 1). A larger sprite during the scrolling part of a stage, like,
say, the 64×64 midbosses, would still be rendered if their top-left
coordinate was (0, -16), so ZUN used < and
> comparisons to at least get an additional pixel before
having to stop rendering such a sprite. Turbo C++ 4.0J sadly can't
constant-fold away such a difference in comparison operators.
And for the most part, ZUN did follow this system consistently. Except for,
of course, the typical mistakes you make when faced with such manual
decisions, like how he treated TH04's Stage 4 midboss as a "small" sprite
below 32×32 pixels (it's 64×64), losing that precious one extra pixel. Or
how the entire rendering code for the 48×48 boss explosion sprite pretends
that it's actually 64×64 pixels large, which causes even the initial
transformation into screen space to be misaligned from the get-go.
But these are additional bugs on top of the single
one that led to all this research.
Because that's what this is, a bug. 🐞 Every resulting pixel boundary is a
systematic result of master.lib's unfortunate lack of clipping. It's as much
of a bug as TH01's byte-aligned rendering of entities whose internal
position is not byte-aligned. In both cases, the entities are alive,
simulated, and partake in collision detection, but their rendered appearance
doesn't accurately reflect their internal position.
Initially, I classified
📝 the sudden pop-in of TH05's Stage 5 midboss
as a quirk because we had no conclusive evidence that this wasn't
intentional, but now we do. There have been multiple explanations for why
ZUN put borders around the playfield, but master.lib's lack of sprite
clipping might be the biggest reason.
And just like byte-aligned rendering, the clipping conditions can easily be
removed when porting the game away from PC-98 hardware. That's also what
uth05win chose to do: By using OpenGL and not having to rely on hardware
scrolling, it can simply place every sprite as a textured quad at its exact
position in screen space, and then draw the black playfield borders on top
in the end to clip everything in a single draw call. This way, the Stage 5
midboss can smoothly fly into the playfield, just as defined by its movement
code:
The entire smooth Stage 5 midboss entrance animation as shown in
uth05win. If the simultaneous appearance of the Enemy!! label
doesn't lend further proof to this having been ZUN's actual intention, I
don't know what will.
Meanwhile, I designed the interface of the 📝 generic blitter used in the TH01 Anniversary Edition entirely around
clipping the blitted sprite at any explicit combination of VRAM edges. This
was nothing I tacked on in the end, but a core aspect that informed the
architecture of the code from the very beginning. You really want to
have one and only one place where sprite clipping is done right – and
only once per sprite, regardless of how many bitplanes you want to write to.
Which brings us to the goal that the final ¼ of this push went toward. I
thought I was going to start cleaning up the
📝 player movement and rendering code, but
that turned out too complicated for that amount of time – especially if you
want to start with just cleanup, preserving all original bugs for the
time being.
Fixing and smoothening player and Orb movement would be the next big task in
Anniversary Edition development, needing about 3 pushes. It would start with
more performance research into runtime-shifting of larger sprites, followed
by extending my generic blitter according to the results, writing new
optimized loaders for the original image formats, and finally rewriting all
rendering code accordingly. With that code in place, we can then start
cleaning up and fixing the unique code for each boss, one by one.
Until that's funded, the code still contains a few smaller and easier pieces
of code that are equally related to rendering bugs, but could be dealt with
in a more incremental way. Line rendering is one of those, and first needs
some refactoring of every call site, including
📝 the rotating squares around Mima and
📝 YuugenMagan's pentagram. So far, I managed
to remove another 1,360 bytes from the binary within this final ¼ of a push,
but there's still quite a bit to do in that regard.
This is the perfect kind of feature for smaller (micro-)transactions. Which
means that we've now got meaningful TH01 code cleanup and Anniversary
Edition subtasks at every price range, no matter whether you want to invest
a lot or just a little into this goal.
If you can, because Ember2528 revealed the plan behind
his Shuusou Gyoku contributions: A full-on Linux port of the game, which
will be receiving all the funding it needs to happen. 🐧 Next up, therefore:
Turning this into my main project within ReC98 for the next couple of
months, and getting started by shipping the long-awaited first step towards
that goal.
I've raised the cap to avoid the potential of rounding errors, which might
prevent the last needed Shuusou Gyoku push from being correctly funded. I
already had to pick the larger one of the two pending TH02 transactions for
this push, because we would have mathematically ended up
1/25500 short of a full push with the smaller
transaction. And if I'm already at it, I might
as well free up enough capacity to potentially ship the complete OpenGL
backend in a single delivery, which is currently estimated to cost 7 pushes
in total.
🎉 After almost 3 years, TH04 finally caught up to TH05 and is now 100%
position-independent as well! 🎉
For a refresher on what this means and does not mean, check the
announcements from back in 2019 and 2020 when we chased the goal for TH05's
📝 OP.EXE and
📝 the rest of the game. These also feature
some demo videos that show off the kind of mods you were able to efficiently
code back then. With the occasional reverse-engineering attention it
received over the years, TH04's code should now be slightly easier to work
with than TH05's was back in the day. Although not by much – TH04 has
remained relatively unpopular among backers, and only received more than the
funded attention because it shares most of its core code with the more
popular TH05. Which, coincidentally, ended up becoming
📝 the reason for getting this done now.
Not that it matters a lot. Ever since we reached 100% PI for TH05, community
and backer interest in position independence has dropped to near zero. We
just didn't end up seeing the expected large amount of community-made mods
that PI was meant to facilitate, and even the
📝 100% decompilation of TH01 changed nothing
about that. But that's OK; after all, I do appreciate the business of
continually getting commissioned for all the
📝 large-scale mods. Not focusing on PI is
also the correct choice for everyone who likes reading these blog posts, as
it often means that I can't go that much into detail due to cutting corners
and piling up technical debt left and right.
Surprisingly, this only took 1.25 pushes, almost twice as fast as expected.
As that's closer to 1 push than it is to 2, I'm OK with releasing it like
this – especially since it was originally meant to come out three days ago.
🍋 Unfortunately, it was delayed thanks to surprising
website bugs and a certain piece of code that was way more difficult to
document than it was to decompile… The next push will have slightly less
content in exchange, though.
📝 P0240 and P0241 already covered the final
remaining structures, so I only needed to do some superficial RE to prove
the remaining numeric literals as either constants or memory addresses. For
example, I initially thought I'd have to decompile the dissolve animations
in the staff roll, but I only needed to identify a single function pointer
type to prove all false positives as screen coordinates there. Now, the TH04
staff roll would be another fast and cheap decompilation, similar to the
custom entity types of TH04. (And TH05 as well!)
The one piece of code I did have to decompile was Stage 4's carpet
lighting animation, thanks to hex literals that were way too complicated to
leave in ASM. And this one probably takes the crown for TH04's worst set of
landmines and bloat that still somehow results in no observable bugs or
quirks.
This animation starts at frame 1664, roughly 29.5 seconds into the stage,
and quickly turns the stage background into a repeated row of dark-red plaid
carpet tiles by moving out from the center of the playfield towards the
edges. Afterward, the animation repeats with a brighter set of tiles that is
then used for the rest of the stage. As I explained
📝 a while ago in the context of TH02, the
stage tile and map formats in PC-98 Touhou can't express animations, so all
of this needed to be hardcoded in the binary.
The repeating 384×16 row of carpet tiles at the beginning of TH04's
Stage 4 in all three light levels, shown twice for better visibility.
And ZUN did start out making the right decision by only using fully-lit
carpet tiles for all tile sections defined in ST03.MAP. This
way, the animation can simply disable itself after it completed, letting the
rest of the stage render normally and use new tile sections that are only
defined for the final light level. This means that the "initial" dark
version of the carpet is as much a result of hardcoded tile manipulation as
the animation itself.
But then, ZUN proceeded to implement it all by directly manipulating the
ring buffer of on-screen tiles. This is the lowest level before the tiles
are rendered, and rather detached from the defined content of the
📝 .MAP tile sections. Which leads to a whole
lot of problems:
If you decide to do this kind of tile ring modification, it should ideally
happen at a very specific point: after scrolling in new tiles into
the ring buffer, but before blitting any scrolled or invalidated
tiles to VRAM based on the ring buffer. Which is not where ZUN chose to put
it, as he placed the call to the stage-specific render function after both
of those operations. By the time the function is
called, the tile renderer has already blitted a few lines of the fully-lit
carpet tiles from the defined .MAP tile section, matching the scroll speed.
Fortunately, these are hidden behind the black TRAM cells above and below
the playfield…
Still, the code needs to get rid of them before they would become visible.
ZUN uses the regular tile invalidation function for this, which will only
cause actual redraws on the next frame. Again, the tile rendering call has
already happened by the time the Stage 4-specific rendering function gets
called.
But wait, this game also flips VRAM pages between frames to provide a
tear-free gameplay experience. This means that the intended redraw of the
new tiles actually hits the wrong VRAM page.
And sure, the code does attempt to invalidate these newly blitted lines
every frame – but only relative to the current VRAM Y coordinate that
represents the top of the hardware-scrolled screen. Once we're back on the
original VRAM page on the next frame, the lines we initially set out to
remove could have already scrolled past that point, making it impossible to
ever catch up with them in this way.
The only real "solution": Defining the height of the tile invalidation
rectangle at 3× the scroll speed, which ensures that each invalidation call
covers 3 frames worth of newly scrolled-in lines. This is not intuitive at
all, and requires an understanding of everything I have just written to even
arrive at this conclusion. Needless to say that ZUN didn't comprehend it
either, and just hardcoded an invalidation height that happened to be enough
for the small scroll speeds defined in ST03.STD for the first
30 seconds of the stage.
The effect must consistently modify the tile ring buffer to "fix" any new
tiles, overriding them with the intended light level. During the animation,
the code not only needs to set the old light level for any tiles that are
still waiting to be replaced, but also the new light level for any tiles
that were replaced – and ZUN forgot the second part. As a result, newly scrolled-in tiles within the already animated
area will "remain" untouched at light level 2 if the scroll speed is fast
enough during the transition from light level 0 to 1.
All that means that we only have to raise the scroll speed for the effect to
fall apart. Let's try, say, 4 pixels per frame rather than the original
0.25:
By hiding the text RAM layer and revealing what's below the usually
opaque black cells above and below the playfield, we can observe all
three landmines – 1) and 2) throughout light level 0, and 3) during the
transition from level 0 to 1.
All of this could have been so much simpler and actually stable if ZUN
applied the tile changes directly onto the .MAP. This is a much more
intuitive way of expressing what is supposed to happen to the map, and would
have reduced the code to the actually necessary tile changes for the first
frame and each individual frame of the animation. It would have still
required a way to force these changes into the tile ring buffer, but ZUN
could have just used his existing full-playfield redraw functions for that.
In any case, there would have been no need for any per-frame tile
fixing and redrawing. The CPU cycles saved this way could have then maybe
been put towards writing the tile-replacing part of the animation in C++
rather than ASM…
Wow, that was an unreasonable amount of research into a feature that
superficially works fine, just because its decompiled code didn't make
sense. To end on a more positive note, here are
some minor new discoveries that might actually matter to someone:
The laser part of Marisa's Illusion Laser shot type always does 3
points of damage per frame, regardless of the player's power level. Its
hitbox also remains identical on all power levels, no matter how wide the
laser appears on screen. The strength difference between the levels purely
comes from the number of frames the laser stays active before a fixed
non-damaging 32-frame cooldown time:
Power level
Frames per cycle (including 32-frame cooldown)
2
64
3
72
4
88
5
104
6
128
7
144
8
168
9
192
The decay animation for player shots is faster in TH05 (12 frames) than in
TH04 (16 frames).
In the first phase of her Stage 6 fight, Yuuka moves along one of two
randomly chosen hardcoded paths, defined as a set of 5 movement angles.
After reaching the final point and firing a danmaku pattern, she teleports
back to her initial position to repeat the path one more time before the
phase times out.
Similarly, TH04's Stage 3 midboss also goes through 12 fixed movement angles
before flying off the playfield.
The formulas for calculating the skill rating on both TH04's and TH05's
final verdict screen are going to be very long and complicated.
Next up: ¾ of a push filled with random boilerplate, finalization, and TH01
code cleanup work, while I finish the preparations for Shuusou Gyoku's
OpenGL backend. This month, everything should finally work out as intended:
I'll complete both tasks in parallel, ship the former to free up the cap,
and then ship the latter once its 5th push is fully funded.
Well, well. My original plan was to ship the first step of Shuusou Gyoku
OpenGL support on the next day after this delivery. But unfortunately, the
complications just kept piling up, to a point where the required solutions
definitely blow the current budget for that goal. I'm currently sitting on
over 70 commits that would take at least 5 pushes to deliver as a meaningful
release, and all of that is just rearchitecting work, preparing the
game for a not too Windows-specific OpenGL backend in the first place. I
haven't even written a single line of OpenGL yet… 🥲
This shifts the intended Big Release Month™ to June after all. Now I know
that the next round of Shuusou Gyoku features should better start with the
SC-88Pro recordings, which are much more likely to get done within their
current budget. At least I've already completed the configuration versioning
system required for that goal, which leaves only the actual audio part.
So, TH04 position independence. Thanks to a bit of funding for stage
dialogue RE, non-ASCII translations will soon become viable, which finally
presents a reason to push TH04 to 100% position independence after
📝 TH05 had been there for almost 3 years. I
haven't heard back from Touhou Patch Center about how much they want to be
involved in funding this goal, if at all, but maybe other backers are
interested as well.
And sure, it would be entirely possible to implement non-ASCII translations
in a way that retains the layout of the original binaries and can be easily
compared at a binary level, in case we consider translations to be a
critical piece of infrastructure. This wouldn't even just be an exercise in
needless perfectionism, and we only have to look to Shuusou Gyoku to realize
why: Players expected
that my builds were compatible with existing SpoilerAL SSG files, which
was something I hadn't even considered the need for. I mean, the game is
open-source 📝 and I made it easy to build.
You can just fork the code, implement all the practice features you want in
a much more efficient way, and I'd probably even merge your code into my
builds then?
But I get it – recompiling the game yields just yet another build that can't
be easily compared to the original release. A cheat table is much more
trustworthy in giving players the confidence that they're still practicing
the same original game. And given the current priorities of my backers,
it'll still take a while for me to implement proof by replay validation,
which will ultimately free every part of the community from depending on the
original builds of both Seihou and PC-98 Touhou.
However, such an implementation within the original binary layout would
significantly drive up the budget of non-ASCII translations, and I sure
don't want to constantly maintain this layout during development. So, let's
chase TH04 position independence like it's 2020, and quickly cover a larger
amount of PI-relevant structures and functions at a shallow level. The only
parts I decompiled for now contain calculations whose intent can't be
clearly communicated in ASM. Hitbox visualizations or other more in-depth
research would have to wait until I get to the proper decompilation of these
features.
But even this shallow work left us with a large amount of TH04-exclusive
code that had its worst parts RE'd and could be decompiled fairly quickly.
If you want to see big TH04 finalization% gains, general TH04 progress would
be a very good investment.
The first push went to the often-mentioned stage-specific custom entities
that share a single statically allocated buffer. Back in 2020, I
📝 wrongly claimed that these were a TH05 innovation,
but the system actually originated in TH04. Both games use a 26-byte
structure, but TH04 only allocates a 32-element array rather than TH05's
64-element one. The conclusions from back then still apply, but I also kept
wondering why these games used a static array for these entities to begin
with. You know what they call an area of memory that you can cleanly
repurpose for things? That's right, a heap!
And absolutely no one would mind one additional heap allocation at the start
of a stage, next to the ones for all the sprites and portraits.
However, we are still running in Real Mode with segmented memory. Accessing
anything outside a common data segment involves modifying segment registers,
which has a nonzero CPU cycle cost, and Turbo C++ 4.0J is terrible at
optimizing away the respective instructions. Does this matter? Probably not,
but you don't take "risks" like these if you're in a permanent
micro-optimization mindset…
In TH04, this system is used for:
Kurumi's symmetric bullet spawn rays, fired from her hands towards the left
and right edges of the playfield. These are rather infamous for being the
last thing you see before
📝 the Divide Error crash that can happen in ZUN's original build.
Capped to 6 entities.
The 4 📝 bits used in Marisa's Stage 4 boss
fight. Coincidentally also related to the rare Divide Error
crash in that fight.
Stage 4 Reimu's spinning orbs. Note how the game uses two different sets
of sprites just to have two different outline colors. This was probably
better than messing with the palette, which can easily cause unintended
effects if you only have 16 colors to work with. Heck, I have an entire blog post tag just to highlight
these cases. Capped to the full 32 entities.
The chasing cross bullets, seen in Phase 14 of the same Stage 6 Yuuka
fight. Featuring some smart sprite work, making use of point symmetry to
achieve a fluid animation in just 4 frames. This is
good-code in sprite form. Capped to 31 entities, because the 32nd custom entity during this fight is defined to be…
The single purple pulsating and shrinking safety circle, seen in Phase 4 of
the same fight. The most interesting aspect here is actually still related
to the cross bullets, whose spawn function is wrongly limited to 32 entities
and could theoretically overwrite this circle. This
is strictly landmine territory though:
Yuuka never uses these bullets and the safety circle
simultaneously
She never spawns more than 24 cross bullets
All cross bullets are fast enough to have left the screen by the
time Yuuka restarts the corresponding subpattern
The cross bullets spawn at Yuuka's center position, and assign its
Q12.4 coordinates to structure fields that the safety circle interprets
as raw pixels. The game does try to render the circle afterward, but
since Yuuka's static position during this phase is nowhere near a valid
pixel coordinate, it is immediately clipped.
The flashing lines seen in Phase 5 of the Gengetsu fight,
telegraphing the slightly random bullet columns.
These structures only took 1 push to reverse-engineer rather than the 2 I
needed for their TH05 counterparts because they are much simpler in this
game. The "structure" for Gengetsu's lines literally uses just a single X
position, with the remaining 24 bytes being basically padding. The only
minor bug I found on this shallow level concerns Marisa's bits, which are
clipped at the right and bottom edges of the playfield 16 pixels earlier
than you would expect:
The remaining push went to a bunch of smaller structures and functions:
The structure for the up to 2 "thick" (a.k.a. "Master Spark") lasers. Much
saner than the
📝 madness of TH05's laser system while being
equally customizable in width and duration.
The structure for the various monochrome 16×16 shapes in the background of
the Stage 6 Yuuka fight, drawn on top of the checkerboard.
The rendering code for the three falling stars in the background of Stage 5.
The effect here is entirely palette-related: After blitting the stage tiles,
the 📝 1bpp star image is ORed
into only the 4th VRAM plane, which is equivalent to setting the
highest bit in the palette color index of every pixel within the star-shaped
region. This of course raises the question of how the stage would look like
if it was fully illuminated:
The full tile map of TH04's Stage 5, in both dark and fully
illuminated views. Since the illumination effect depends on two
matching sets of palette colors that are distinguished by a single
bit, the illuminated view is limited to only 8 of the 16 colors. The
dark view, on the other hand, can freely use colors from the
illuminated set, since those are unaffected by the OR
operation.
Most code that modifies a stage's tile map, and directly specifies tiles via
their top-left offset in VRAM.
Thanks to code alignment reasons, this forced a much longer detour into the
.STD format loader. Nothing all too noteworthy there since we're still
missing the enemy script and spawn structures before we can call .STD
"reverse-engineered", but maybe still helpful if you're looking for an
overview of the format. Also features a buffer overflow landmine if a .STD
file happens to contain more than 32 enemy scripts… you know, the usual
stuff.
To top off the second push, we've got the vertically scrolling checkerboard
background during the Stage 6 Yuuka fight, made up of 32×32 squares. This
one deserves a special highlight just because of its needless complexity.
You'd think that even a performant implementation would be pretty simple:
Set the GRCG to TDW mode
Set the GRCG tile to one of the two square colors
Start with Y as the current scroll offset, and X
as some indicator of which color is currently shown at the start of each row
of squares
Iterate over all lines of the playfield, filling in all pixels that
should be displayed in the current color, skipping over the other ones
Count down Y for each line drawn
If Y reaches 0, reset it to 32 and flip X
At the bottom of the playfield, change the GRCG tile to the other color,
and repeat with the initial value of X flipped
The most important aspect of this algorithm is how it reduces GRCG state
changes to a minimum, avoiding the costly port I/O that we've identified
time and time again as one of the main bottlenecks in TH01. With just 2
state variables and 3 loops, the resulting code isn't that complex either. A
naive implementation that just drew the squares from top to bottom in a
single pass would barely be simpler, but much slower: By changing the GRCG
tile on every color, such an implementation would burn a low 5-digit number
of CPU cycles per frame for the 12×11.5-square checkerboard used in the
game.
And indeed, ZUN retained all important aspects of this algorithm… but still
implemented it all in ASM, with a ridiculous layer of x86 segment arithmetic
on top? Which blows up the complexity to 4 state
variables, 5 nested loops, and a bunch of constants in unusual units. I'm
not sure what this code is supposed to optimize for, especially with that
rather questionable register allocation that nevertheless leaves one of the
general-purpose registers unused. Fortunately,
the function was still decompilable without too many code generation hacks,
and retains the 5 nested loops in all their goto-connected
glory. If you want to add a checkerboard to your next PC-98
demo, just stick to the algorithm I gave above.
(Using a single XOR for flipping the starting X offset between 32 and 64
pixels is pretty nice though, I have to give him that.)
This makes for a good occasion to talk about the third and final GRCG mode,
completing the series I started with my previous coverage of the
📝 RMW and
📝 TCR modes. The TDW (Tile Data Write) mode
is the simplest of the three and just writes the 8×1 GRCG tile into VRAM
as-is, without applying any alpha bitmask. This makes it perfect for
clearing rectangular areas of pixels – or even all of VRAM by doing a single
memset():
// Set up the GRCG in TDW mode.
outportb(0x7C, 0x80);
// Fill the tile register with color #7 (0111 in binary).
outportb(0x7E, 0xFF); // Plane 0: (B): (********)
outportb(0x7E, 0xFF); // Plane 1: (R): (********)
outportb(0x7E, 0xFF); // Plane 2: (G): (********)
outportb(0x7E, 0x00); // Plane 3: (E): ( )
// Set the 32 pixels at the top-left corner of VRAM to the exact contents of
// the tile register, effectively repeating the tile 4 times. In TDW mode, the
// GRCG ignores the CPU-supplied operand, so we might as well just pass the
// contents of a register with the intended width. This eliminates useless load
// instructions in the compiled assembly, and even sort of signals to readers
// of this code that we do not care about the source value.
*reinterpret_cast<uint32_t far *>(MK_FP(0xA800, 0)) = _EAX;
// Fill the entirety of VRAM with the GRCG tile. A simple C one-liner that will
// probably compile into a single `REP STOS` instruction. Unfortunately, Turbo
// C++ 4.0J only ever generates the 16-bit `REP STOSW` here, even when using
// the `__memset__` intrinsic and when compiling in 386 mode. When targeting
// that CPU and above, you'd ideally want `REP STOSD` for twice the speed.
memset(MK_FP(0xA800, 0), _AL, ((640 / 8) * 400));
However, this might make you wonder why TDW mode is even necessary. If it's
functionally equivalent to RMW mode with a CPU-supplied bitmask made up
entirely of 1 bits (i.e., 0xFF, 0xFFFF, or
0xFFFFFFFF), what's the point? The difference lies in the
hardware implementation: If all you need to do is write tile data to
VRAM, you don't need the read and modify parts of RMW mode
which require additional processing time. The PC-9801 Programmers'
Bible claims a speedup of almost 2× when using TDW mode over equivalent
operations in RMW mode.
And that's the only performance claim I found, because none of these old
PC-98 hardware and programming books did any benchmarks. Then again, it's
not too interesting of a question to benchmark either, as the byte-aligned
nature of TDW blitting severely limits its use in a game engine anyway.
Sure, maybe it makes sense to temporarily switch from RMW to TDW mode
if you've identified a large rectangular and byte-aligned section within a
sprite that could be blitted without a bitmask? But the necessary
identification work likely nullifies the performance gained from TDW mode,
I'd say. In any case, that's pretty deep
micro-optimization territory. Just use TDW mode for the
few cases it's good at, and stick to RMW mode for the rest.
So is this all that can be said about the GRCG? Not quite, because there are
4 bits I haven't talked about yet…
And now we're just 5.37% away from 100% position independence for TH04! From
this point, another 2 pushes should be enough to reach this goal. It might
not look like we're that close based on the current estimate, but a
big chunk of the remaining numbers are false positives from the player shot
control functions. Since we've got a very special deadline to hit, I'm going
to cobble these two pushes together from the two current general
subscriptions and the rest of the backlog. But you can, of course, still
invest in this goal to allow the existing contributions to go to something
else.
… Well, if the store was actually open. So I'd better
continue with a quick task to free up some capacity sooner rather than
later. Next up, therefore: Back to TH02, and its item and player systems.
Shouldn't take that long, I'm not expecting any surprises there. (Yeah, I
know, famous last words…)
So, TH02! Being the only game whose main binary hadn't seen any dedicated
attention ever, we get to start the TH02-related blog posts at the very
beginning with the most foundational pieces of code. The stage tile system
is the best place to start here: It not only blocks every entity that is
rendered on top of these tiles, but is curiously placed right next to
master.lib code in TH02, and would need to be separated out into its own
translation unit before we can do the same with all the master.lib
functions.
In late 2018, I already RE'd
📝 TH04's and TH05's stage tile implementation, but haven't properly documented it on this
blog yet, so this post is also going to include the details that are unique
to those games. On a high level, the stage tile system works identically in
all three games:
The tiles themselves are 16×16 pixels large, and a stage can use 100 of
them at the same time.
The optimal way of blitting tiles would involve VRAM-to-VRAM copies
within the same page using the EGC, and that's exactly what the games do.
All tiles are stored on both VRAM pages within the rightmost 64×400 pixels
of the screen just right next to the HUD, and you only don't see them
because the games cover the same area in text RAM with black cells:
The initial screen of TH02's Stage 1, with the tile source
area uncovered by filling the same area in text RAM with transparent
cells instead of black ones. In TH02, this also reveals how the tile
area ends with a bunch of glitch tiles, tinted blue in the image. These
are the result of ZUN unconditionally blitting 100 tile images every
time, regardless of how many are actually contained in an
.MPN file.
These glitch tiles are another good example of a ZUN
landmine. Their appearance is the result of reading heap memory
outside allocated boundaries, which can easily cause segmentation faults
when porting the game to a system with virtual memory. Therefore, these
would not just be removed in this game's Anniversary Edition, but on the
more conservative debloated branch as well. Since the game
never uses these tiles and you can't observe them unless you manipulate
text RAM from outside the confines of the game, it's not a bug
according to our definition.
To reduce the memory required for a map, tiles are arranged into fixed
vertical sections of a game-specific constant size.
The 6 24×8-tile sections defined in TH02's STAGE0.MAP, in
reverse order compared to how they're defined in the file. Note the
duplicated row at the top of the final section: The boss fight starts
once the game scrolled the last full row of tiles onto the top of the
screen, not the playfield. But since the PC-98 text chip
covers the top tile row of the screen with black cells, this final row
is never visible, which effectively reduces a map's final tile section
to 7 rows rather than 8.
The actual stage map then is simply a list of these tile sections,
ordered from the start/bottom to the top/end.
Any manipulation of specific tiles within the fixed tile sections has to
be hardcoded. An example can be found right in Stage 1, where the Shrine
Tank leaves track marks on the tiles it appears to drive over:
This video also shows off the two issues with Touhou's first-ever
midboss: The replaced tiles are rendered below the midboss
during their first 4 frames, and maybe ZUN should have stopped the
tile replacements one row before the timeout. The first one is
clearly a bug, but it's not so clear-cut with the second one. I'd
need to look at the code to tell for sure whether it's a quirk or a
bug.
The differences between the three games can best be summarized in a table:
TH02
TH04
TH05
Tile image file extension
.MPN
Tile section format
.MAP
Tile section order defined as part of
.DT1
.STD
Tile section index format
0-based ID
0-based ID × 2
Tile image index format
Index between 0 and 100, 1 byte
VRAM offset in tile source area, 2 bytes
Scroll speed control
Hardcoded
Part of the .STD format, defined per referenced tile
section
Redraw granularity
Full tiles (16×16)
Half tiles (16×8)
Rows per tile section
8
5
Maximum number of tile sections
16
32
Lowest number of tile sections used
5 (Stage 3 / Extra)
8 (Stage 6)
11 (Stage 2 / 4)
Highest number of tile sections used
13 (Stage 4)
19 (Extra)
24 (Stage 3)
Maximum length of a map
320 sections (static buffer)
256 sections (format limitation)
Shortest map
14 sections (Stage 5)
20 sections (Stage 5)
15 sections (Stage 2)
Longest map
143 sections (Stage 4)
95 sections (Stage 4)
40 sections (Stage 1 / 4 / Extra)
The most interesting part about stage tiles is probably the fact that some
of the .MAP files contain unused tile sections. 👀 Many
of these are empty, duplicates, or don't really make sense, but a few
are unique, fit naturally into their respective stage, and might have
been part of the map during development. In TH02, we can find three unused
sections in Stage 5:
The non-empty tile sections defined in TH02's STAGE4.MAP,
showing off three unused ones.
These unused tile sections are much more common in the later games though,
where we can find them in TH04's Stage 3, 4, and 5, and TH05's Stage 1, 2,
and 4. I'll document those once I get to finalize the tile rendering code of
these games, to leave some more content for that blog post. TH04/TH05 tile
code would be quite an effective investment of your money in general, as
most of it is identical across both games. Or how about going for a full-on
PC-98 Touhou map viewer and editor GUI?
Compared to TH04 and TH05, TH02's stage tile code definitely feels like ZUN
was just starting to understand how to pull off smooth vertical scrolling on
a PC-98. As such, it comes with a few inefficiencies and suboptimal
implementation choices:
The redraw flag for each tile is stored in a 24×25 bool
array that does nothing with 7 of the 8 bits.
During bombs and the Stage 4, 5, and Extra bosses, the game disables the
tile system to render more elaborate backgrounds, which require the
playfield to be flood-filled with a single color on every frame. ZUN uses
the GRCG's RMW mode rather than TDW mode for this, leaving almost half of
the potential performance on the table for no reason. Literally,
changing modes only involves changing a single constant.
The scroll speed could theoretically be changed at any time. However,
the function that scrolls in new stage tiles can only ever blit part of a
single tile row during every call, so it's up to the caller to ensure
that scrolling always ends up on an exact 16-pixel boundary. TH02 avoids
this problem by keeping the scroll speed constant across a stage, using 2
pixels for Stage 4 and 1 pixel everywhere else.
Since the scroll speed is given in pixels, the slowest speed would be 1
pixel per frame. To allow the even slower speeds seen in the final game,
TH02 adds a separate scroll interval variable that only runs the
scroll function every 𝑛th frame, effectively adding a prescaler to the
scroll speed. In TH04 and TH05, the speed is specified as a Q12.4 value
instead, allowing true fractional speeds at any multiple of
1/16 pixels. This also necessitated a fixed algorithm
that correctly blits tile lines from two rows.
Finally, we've got a few inconsistencies in the way the code handles the
two VRAM pages, which cause a few unnecessary tiles to be rendered to just
one of the two pages. Mentioning that just in case someone tries to play
this game with a fully cleared text RAM and wonders where the flickering
tiles come from.
Even though this was ZUN's first attempt at scrolling tiles, he already saw
it fit to write most of the code in assembly. This was probably a reaction
to all of TH01's performance issues, and the frame rate reduction
workarounds he implemented to keep the game from slowing down too much in
busy places. "If TH01 was all C++ and slow, TH02 better contain more ASM
code, and then it will be fast, right?"
Another reason for going with ASM might be found in the kind of
documentation that may have been available to ZUN. Last year, the PC-98
community discovered and scanned two new game programming tutorial books
from 1991 (1, 2).
Their example code is not only entirely written in assembly, but restricts
itself to the bare minimum of x86 instructions that were available on the
8086 CPU used by the original PC-9801 model 9 years earlier. Such code is
not only suboptimal
on the 486, but can often be actually worse than what your C++
compiler would generate. TH02 is where the trend of bad hand-written ASM
code started, and it
📝 only intensified in ZUN's later games. So,
don't copy code from these books unless you absolutely want to target the
earlier 8086 and 286 models. Which,
📝 as we've gathered from the recent blitting benchmark results,
are not all too common among current real-hardware owners.
That said, all that ASM code really only impacts readability and
maintainability. Apart from the aforementioned issues, the algorithms
themselves are mostly fine – especially since most EGC and GRCG operations
are decently batched this time around, in contrast to TH01.
Luckily, the tile functions merely use inline assembly within a
typical C function and can therefore be at least part of a C++ source file,
even if the result is pretty ugly. This time, we can actually be sure that
they weren't written directly in a .ASM file, because they feature x86
instruction encodings that can only be generated with Turbo C++ 4.0J's
inline assembler, not with TASM. The same can't unfortunately be said about
the following function in the same segment, which marks the tiles covered by
the spark sprites for redrawing. In this one, it took just one dumb hand-written ASM
inconsistency in the function's epilog to make the entire function
undecompilable.
The standard x86 instruction sequence to set up a stack frame in a function prolog looks like this:
PUSH BP
MOV BP, SP
SUB SP, ?? ; if the function needs the stack for local variables
When compiling without optimizations, Turbo C++ 4.0J will
replace this sequence with a single ENTER instruction. That one
is two bytes smaller, but much slower on every x86 CPU except for the 80186
where it was introduced.
In functions without local variables, BP and SP
remain identical, and a single POP BP is all that's needed in
the epilog to tear down such a stack frame before returning from the
function. Otherwise, the function needs an additional MOV SP,
BP instruction to pop all local variables. With x86 being the helpful
CISC architecture that it is, the 80186 also introduced the
LEAVE instruction to perform both tasks. Unlike
ENTER, this single instruction
is faster than the raw two instructions on a lot of x86 CPUs (and
even current ones!), and it's always smaller, taking up just 1 byte instead
of 3. So what if you use LEAVE even if your function
doesn't use local variables? The fact that the
instruction first does the equivalent of MOV SP, BP doesn't
matter if these registers are identical, and who cares about the additional
CPU cycles of LEAVE compared to just POP BP,
right? So that's definitely something you could theoretically do, but
not something that any compiler would ever generate.
And so, TH02 MAIN.EXE decompilation already hits the first
brick wall after two pushes. Awesome! Theoretically,
we could slowly mash through this wall using the 📝 code generator. But having such an inconsistency in the
function epilog would mean that we'd have to keep Turbo C++ 4.0J from
emitting any epilog or prolog code so that we can write our
own. This means that we'd once again have to hide any use of the
SI and DI registers from the compiler… and doing
that requires code generation macros for 22 of the 49 instructions of
the function in question, almost none of which we currently have. So, this
gets quite silly quite fast, especially if we only need to do it
for one single byte.
Instead, wouldn't it be much better if we had a separate build step between
compile and link time that allowed us to replicate mistakes like these by
just patching the compiled .OBJ files? These files still contain the names
of exported functions for linking, which would allow us to look up the code
of a function in a robust manner, navigate to specific instructions using a
disassembler, replace them, and write the modified .OBJ back to disk before
linking. Such a system could then naturally expand to cover all other
decompilation issues, culminating in a full-on optimizer that could even
recreate ZUN's self-modifying code. At that point, we would have sealed away
all of ZUN's ugly ASM code within a separate build step, and could finally
decompile everything into readable C++.
Pulling that off would require a significant tooling investment though.
Patching that one byte in TH02's spark invalidation function could be done
within 1 or 2 pushes, but that's just one issue, and we currently have 32
other .ASM files with undecompilable code. Also, note that this is
fundamentally different from what we're doing with the
debloated branch and the Anniversary Editions. Mistake patching
would purely be about having readable code on master that
compiles into ZUN's exact binaries, without fixing weird
code. The Anniversary Editions go much further and rewrite such code in
a much more fundamental way, improving it further than mistake patching ever
could.
Right now, the Anniversary Editions seem much more
popular, which suggests that people just want 100% RE as fast as
possible so that I can start working on them. In that case, why bother with
such undecompilable functions, and not just leave them in raw and unreadable
x86 opcode form if necessary… But let's first
see how much backer support there actually is for mistake patching before
falling back on that.
The best part though: Once we've made a decision and then covered TH02's
spark and particle systems, that was it, and we will have already RE'd
all ZUN-written PC-98-specific blitting code in this game. Every further
sprite or shape is rendered via master.lib, and is thus decently abstracted.
Guess I'll need to update
📝 the assessment of which PC-98 Touhou game is the easiest to port,
because it sure isn't TH01, as we've seen with all the work required for the first Anniversary Edition build.
Until then, there are still enough parts of the game that don't use any of
the remaining few functions in the _TEXT segment. Previously, I
mentioned in the 📝 status overview blog post
that TH02 had a seemingly weird sprite system, but the spark and point popup
() structures showed that the game just
stores the current and previous position of its entities in a slightly
different way compared to the rest of PC-98 Touhou. Instead of having
dedicated structure fields, TH02 uses two-element arrays indexed with the
active VRAM page. Same thing, and such a pattern even helps during RE since
it's easy to spot once you know what to look for.
There's not much to criticize about the point popup system, except for maybe
a landmine that causes sprite glitches when trying to display more than
99,990 points. Sadly, the final push in this delivery was rounded out by yet
another piece of code at the opposite end of the quality spectrum. The
particle and smear effects for Reimu's bomb animations consist almost
entirely of assembly bloat, which would just be replaced with generic calls
to the generic blitter in this game's future Anniversary Edition.
If I continue to decompile TH02 while avoiding the brick wall, items would
be next, but they probably require two pushes. Next up, therefore:
Integrating Stripe as an alternative payment provider into the order form.
There have been at least three people who reported issues with PayPal, and
Stripe has been working much better in tests. In the meantime, here's a temporary Stripe
order link for everyone. This one is not connected to the cap yet, so
please make sure to stay within whatever value is currently shown on the
front page – I will treat any excess money as donations.
If there's some time left afterward, I might
also add some small improvements to the TH01 Anniversary Edition.
Starting the year with a delivery that wasn't delayed until the last
day of the month for once, nice! Still, very soon and
high-maintenance did not go well together…
It definitely wasn't Sara's fault though. As you would expect from a Stage 1
Boss, her code was no challenge at all. Most of the TH02, TH04, and TH05
bosses follow the same overall structure, so let's introduce a new table to
replace most of the boilerplate overview text:
Phase #
Patterns
HP boundary
Timeout condition
(Entrance)
4,650
288 frames
2
4
2,550
2,568 frames
(= 32 patterns)
3
4
450
5,296 frames
(= 24 patterns)
4
1
0
1,300 frames
Total
9
9,452 frames
In Phases 2 and 3, Sara cycles between waiting, moving randomly for a
fixed 28 frames, and firing a random pattern among the 4 phase-specific
ones. The pattern selection makes sure to never
pick any pattern twice in a row. Both phases contain spiral patterns that
only differ in the clockwise or counterclockwise turning direction of the
spawner; these directions are treated as individual unrelated patterns, so
it's possible for the "same" pattern to be fired multiple times in a row
with a flipped direction.
The two phases also differ in the wait and pattern durations:
In Phase 2, the wait time starts at 64 frames and decreases by 12
frames after the first 5 patterns each, ending on a minimum of 4 frames.
In Phase 3, it's a constant 16 frames instead.
All Phase 2 patterns are fired for 28 frames, after a 16-frame
gather animation. The Phase 3 pattern time starts at 80 frames and
increases by 24 frames for the first 6 patterns, ending at 200 frames
for all later ones.
Phase 4 consists of the single laser corridor pattern with additional
random bullets every 16 frames.
And that's all the gameplay-relevant detail that ZUN put into Sara's code. It doesn't even make sense to describe the remaining
patterns in depth, as their groups can significantly change between
difficulties and rank values. The
📝 general code structure of TH05 bosses
won't ever make for good-code, but Sara's code is just a
lesser example of what I already documented for Shinki.
So, no bugs, no unused content, only inconsequential bloat to be found here,
and less than 1 push to get it done… That makes 9 PC-98 Touhou bosses
decompiled, with 22 to go, and gets us over the sweet 50% overall
finalization mark! 🎉 And sure, it might be possible to pass through the
lasers in Sara's final pattern, but the boss script just controls the
origin, angle, and activity of lasers, so any quirk there would be part of
the laser code… wait, you can do what?!?
TH05 expands TH04's one-off code for Yuuka's Master and Double Sparks into a
more featureful laser system, and Sara is the first boss to show it off.
Thus, it made sense to look at it again in more detail and finalize the code
I had purportedly
📝 reverse-engineered over 4 years ago.
That very short delivery notice already hinted at a very time-consuming
future finalization of this code, and that prediction certainly came true.
On the surface, all of the low-level laser ray rendering and
collision detection code is undecompilable: It uses the SI and
DI registers without Turbo C++'s safety backups on the stack,
and its helper functions take their input and output parameters from
convenient registers, completely ignoring common calling conventions. And
just to raise the confusion even further, the code doesn't just set
these registers for the helper function calls and then restores their
original values, but permanently shifts them via additions and
subtractions. Unfortunately, these convenient registers also include the
BP base pointer to the stack frame of a function… and shifting
that register throws any intuition behind accessed local variables right out
of the window for a good part of the function, requiring a correctly shifted
view of the stack frame just to make sense of it again.
How could such code even have been written?! This
goes well beyond the already wrong assumption that using more stack space is
somehow bad, and straight into the territory of self-inflicted pain.
So while it's not a lot of instructions, it's quite dense and really hard to
follow. This code would really benefit from a decompilation that
anchors all this madness as much as possible in existing C++ structures… so
let's decompile it anyway?
Doing so would involve emitting lots of raw machine code bytes to hide the
SI and DI registers from the compiler, but I
already had a certain
📝 batshit insane compiler bug workaround abstraction
lying around that could make such code more readable. Hilariously, it only
took this one additional use case for that abstraction to reveal itself as
premature and way too complicated. Expanding
the core idea into a full-on x86 instruction generator ended up simplifying
the code structure a lot. All we really want there is a way to set all
potential parameters to e.g. a specific form of the MOV
instruction, which can all be expressed as the parameters to a force-inlined
__emit__() function. Type safety can help by providing
overloads for different operand widths here, but there really is no need for
classes, templates, or explicit specialization of templates based on
classes. We only need a couple of enums with opcode, register,
and prefix constants from the x86 reference documentation, and a set of
associated macros that token-paste pseudoregisters onto the prefixes of
these enum constants.
And that's how you get a custom compile-time assembler in a 1994 C++
compiler and expand the limits of decompilability even further. What's even
truly left now? Self-modifying code, layout tricks that can't be replicated
with regularly structured control flow… and that's it. That leaves quite a
few functions I previously considered undecompilable to be revisited once I
get to work on making this game more portable.
With that, we've turned the low-level laser code into the expected horrible
monstrosity that exposes all the hidden complexity in those few ASM
instructions. The high-level part should be no big deal now… except that
we're immediately bombarded with Fixup overflow errors at link
time? Oh well, time to finally learn the true way of fixing this highly
annoying issue in a second new piece of decompilation tech – and one
that might actually be useful for other x86 Real Mode retro developers at
that.
Earlier in the RE history of TH04 and TH05, I often wrote about the need to
split the two original code segments into multiple segments within two
groups, which makes it possible to slot in code from different
translation units at arbitrary places within the original segment. If we
don't want to define a unique segment name for each of these slotted-in
translation units, we need a way to set custom segment and group names in C
land. Turbo C++ offers two #pragmas for that:
#pragma option -zCsegment -zPgroup – preferred in most
cases as it's equivalent to setting the default segment and group via the
command line, but can only be used at the beginning of a translation unit,
before the first non-preprocessor and non-comment C language token
#pragma codeseg segment <group> – necessary if a
translation unit needs to emit code into two or more segments
For the most part, these #pragmas work well, but they seemed to
not help much when it came to calling near functions declared
in different segments within the same group. It took a bit of trial and
error to figure out what was actually going on in that case, but there
is a clear logic to it:
Symbols are allocated to the segment and group that's active during
their first appearance, no matter whether that appearance is a declaration
or definition. Any later appearance of the function in a different segment
is ignored.
The linker calculates the 16-bit offsets of such references relative to
the symbol's declared segment, not its actual one. Turbo C++ does
not show an error or warning if the declared and actual segments are
different, as referencing the same symbol from multiple segments is a valid
use case. The linker merely throws the Fixup overflow error if
the calculated distance exceeds 64 KiB and thus couldn't possibly fit
within a near reference. With a wrong segment declaration
though, your code can be incorrect long before a fixup hits that limit.
Summarized in code:
#pragma option -zCfoo_TEXT -zPfoo
void bar(void);
void near qux(void); // defined somewhere else, maybe in a different segment
#pragma codeseg baz_TEXT baz
// Despite the segment change in the line above, this function will still be
// put into `foo_TEXT`, the active segment during the first appearance of the
// function name.
void bar(void) {
}
// This function hasn't been declared yet, so it will go into `baz_TEXT` as
// expected.
void baz(void) {
// This `near` function pointer will be calculated by subtracting the
// flat/linear address of qux() inside the binary from the base address
// of qux()'s declared segment, i.e., `foo_TEXT`.
void (near *ptr_to_qux)(void) = qux;
}
So yeah, you might have to put #pragma codeseg into your
headers to tell the linker about the correct segment of a
near function in advance. 🤯 This is an important insight for
everyone using this compiler, and I'm shocked that none of the Borland C++
books documented the interaction of code segment definitions and
near references at least at this level of clarity. The TASM
manuals did have a few pages on the topic of groups, but that syntax
obviously doesn't apply to a C compiler. Fixup overflows in particular are
such a common error and really deserved better than the unhelpful 🤷
of an explanation that ended up in the User's Guide. Maybe this whole
technique of custom code segment names was considered arcane even by 1993,
judging from the mere three sentences that #pragma codeseg was
documented with? Still, it must have been common knowledge among Amusement
Makers, because they couldn't have built these exact binaries without
knowing about these details. This is the true solution to
📝 any issues involving references to near functions,
and I'm glad to see that ZUN did not in fact lie to the compiler. 👍
OK, but now the remaining laser code compiles, and we get to write
C++ code to draw some hitboxes during the two collision-detected states of
each laser. These confirm what the low-level code from earlier already
uncovered: Collision detection against lasers is done by testing a
12×12-pixel box at every 16 pixels along the length of a laser, which leaves
obvious 4-pixel gaps at regular intervals that the player can just pass
through. This adds
📝 yet📝 another📝 quirk to the growing list of quirks that
were either intentional or must have been deliberately left in the game
after their initial discovery. This is what constants were invented for, and
there really is no excuse for not using them – especially during
intoxicated coding, and/or if you don't have a compile-time abstraction for
Q12.4 literals.
When detecting laser collisions, the game checks the player's single
center coordinate against any of the aforementioned 12×12-pixel boxes.
Therefore, it's correct to split these 12×12 pixels into two 6×6-pixel
boxes and assign the other half to the player for a more natural
visualization. Always remember that hitbox visualizations need to keep
all colliding entities in mind –
📝 assigning a constant-sized hitbox to "the player" and "the bullets" will be wrong in most other cases.
Using subpixel coordinates in collision detection also introduces a slight
inaccuracy into any hitbox visualization recorded in-engine on a 16-color
PC-98. Since we have to render discrete pixels, we cannot exactly place a
Q12.4 coordinate in the 93.75% of cases where the fractional part is
non-zero. This is why pretty much every laser segment hitbox in the video
above shows up as 7×7 rather than 6×6: The actual W×H area of each box is 13
pixels smaller, but since the hitbox lies between these pixels, we
cannot indicate where it lies exactly, and have to err on the
side of caution. It's also why Reimu's box slightly changes size as she
moves: Her non-diagonal movement speed is 3.5 pixels per frame, and the
constant focused movement in the video above halves that to 1.75 pixels,
making her end up on an exact pixel every 4 frames. Looking forward to the
glorious future of displays that will allow us to scale up the playfield to
16× its original pixel size, thus rendering the game at its exact internal
resolution of 6144×5888 pixels. Such a port would definitely add a lot of
value to the game…
The remaining high-level laser code is rather unremarkable for the most
part, but raises one final interesting question: With no explicitly defined
limit, how wide can a laser be? Looking at the laser structure's 1-byte
width field and the unsigned comparisons all throughout the update and
rendering code, the answer seems to be an obvious 255 pixels. However, the
laser system also contains an automated shrinking state, which can be most
notably seen in Mai's wheel pattern. This state shrinks a laser by 2 pixels
every 2 frames until it reached a width of 0. This presents a problem with
odd widths, which would fall below 0 and overflow back to 255 due to the
unsigned nature of this variable. So rather than, I don't know, treating
width values of 0 as invalid and stopping at a width of 1, or even adding a
condition for that specific case, the code just performs a signed
comparison, effectively limiting the width of a shrinkable laser to a
maximum of 127 pixels. This small signedness
inconsistency now forces the distinction between shrinkable and
non-shrinkable lasers onto every single piece of code that uses lasers. Yet
another instance where
📝 aiming for a cinematic 30 FPS look
made the resulting code much more complicated than if ZUN had just evenly
spread out the subtraction across 2 frames. 🤷
Oh well, it's not as if any of the fixed lasers in the original scripts came
close to any of these limits. Moving lasers are much more streamlined and
limited to begin with: Since they're hardcoded to 6 pixels, the game can
safely assume that they're always thinner than the 28 pixels they get
gradually widened to during their decay animation.
Finally, in case you were missing a mention of hitboxes in the previous
paragraph: Yes, the game always uses the aforementioned 12×12 boxes,
regardless of a laser's width.
This video also showcases the 127-pixel limit because I wanted
to include the shrink animation for a seamless loop.
That was what, 50% of this blog post just being about complications that
made laser difficult for no reason? Next up: The first TH01 Anniversary
Edition build, where I finally get to reap the rewards of having a 100%
decompiled game and write some good code for once.
More than three months without any reverse-engineering progress! It's been
way too long. Coincidentally, we're at least back with a surprising 1.25% of
overall RE, achieved within just 3 pushes. The ending script system is not
only more or less the same in TH04 and TH05, but actually originated in
TH03, where it's also used for the cutscenes before stages 8 and 9. This
means that it was one of the final pieces of code shared between three of
the four remaining games, which I got to decompile at roughly 3× the usual
speed, or ⅓ of the price.
The only other bargains of this nature remain in OP.EXE. The
Music Room is largely equivalent in all three remaining games as well, and
the sound device selection, ZUN Soft logo screens, and main/option menus are
the same in TH04 and TH05. A lot of that code is in the "technically RE'd
but not yet decompiled" ASM form though, so it would shift Finalized% more
significantly than RE%. Therefore, make sure to order the new
Finalization option rather than Reverse-engineering if you
want to make number go up.
So, cutscenes. On the surface, the .TXT files look simple enough: You
directly write the text that should appear on the screen into the file
without any special markup, and add commands to define visuals, music, and
other effects at any place within the script. Let's start with the basics of
how text is rendered, which are the same in all three games:
First off, the text area has a size of 480×64 pixels. This means that it
does not correspond to the tiled area painted into TH05's
EDBK?.PI images:
The yellow area is designated for character names.
Since the font weight can be customized, all text is rendered to VRAM.
This also includes gaiji, despite them ignoring the font weight
setting.
The system supports automatic line breaks on a per-glyph basis, which
move the text cursor to the beginning of the red text area. This might seem like a piece of long-forgotten
ancient wisdom at first, considering the absence of automatic line breaks in
Windows Touhou. However, ZUN probably implemented it more out of pure
necessity: Text in VRAM needs to be unblitted when starting a new box, which
is way more straightforward and performant if you only need to worry
about a fixed area.
The system also automatically starts a new (key press-separated) text
box after the end of the 4th line. However, the text cursor is
also unconditionally moved to the top-left corner of the yellow name
area when this happens, which is almost certainly not what you expect, given
that automatic line breaks stay within the red area. A script author might
as well add the necessary text box change commands manually, if you're
forced to anticipate the automatic ones anyway…
Due to ZUN forgetting an unblitting call during the TH05 refactoring of the
box background buffer, this feature is even completely broken in that game,
as any new text will simply be blitted on top of the old one:
Wait, why are we already talking about game-specific differences after
all? Also, note how the ⏎ animation appears one line below where you'd
expect it.
Overall, the system is geared toward exclusively full-width text. As
exemplified by the 2014 static English patches and the screenshots in this
blog post, half-width text is possible, but comes with a lot of
asterisks attached:
Each loop of the script interpreter starts by looking at the next
byte to distinguish commands from text. However, this step also skips
over every ASCII space and control character, i.e., every byte
≤ 32. If you only intend to display full-width glyphs anyway, this
sort of makes sense: You gain complete freedom when it comes to the
physical layout of these script files, and it especially allows commands
to be freely separated with spaces and line breaks for improved
readability. Still, enforcing commands to be separated exclusively by
line breaks might have been even better for readability, and would have
freed up ASCII spaces for regular text…
Non-command text is blindly processed and rendered two bytes at a
time. The rendering function interprets these bytes as a Shift-JIS
string, so you can use half-width characters here. While the
second byte can even be an ASCII 0x20 space due to the
parser's blindness, all half-width characters must still occur in pairs
that can't be interrupted by commands:
As a workaround for at least the ASCII space issue, you can replace
them with any of the unassigned
Shift-JIS lead bytes – 0x80, 0xA0, or
anything between 0xF0 and 0xFF inclusive.
That's what you see in all screenshots of this post that display
half-width spaces.
Finally, did you know that you can hold ESC to fast-forward
through these cutscenes, which skips most frame delays and reduces the rest?
Due to the blocking nature of all commands, the ESC key state is
only updated between commands or 2-byte text groups though, so it can't
interrupt an ongoing delay.
Superficially, the list of game-specific differences doesn't look too long,
and can be summarized in a rather short table:
It's when you get into the implementation that the combined three systems
reveal themselves as a giant mess, with more like 56 differences between the
games. Every single new weird line of code opened up
another can of worms, which ultimately made all of this end up with 24
pieces of bloat and 14 bugs. The worst of these should be quite interesting
for the general PC-98 homebrew developers among my audience:
The final official 0.23 release of master.lib has a bug in
graph_gaiji_put*(). To calculate the JIS X 0208 code point for
a gaiji, it is enough to ADD 5680h onto the gaiji ID. However,
these functions accidentally use ADC instead, which incorrectly
adds the x86 carry flag on top, causing weird off-by-one errors based on the
previous program state. ZUN did fix this bug directly inside master.lib for
TH04 and TH05, but still needed to work around it in TH03 by subtracting 1
from the intended gaiji ID. Anyone up for maintaining a bug-fixed master.lib
repository?
The worst piece of bloat comes from TH03 and TH04 needlessly
switching the visibility of VRAM pages while blitting a new 320×200 picture.
This makes it much harder to understand the code, as the mere existence of
these page switches is enough to suggest a more complex interplay between
the two VRAM pages which doesn't actually exist. Outside this visibility
switch, page 0 is always supposed to be shown, and page 1 is always used
for temporarily storing pixels that are later crossfaded onto page 0. This
is also the only reason why TH03 has to render text and gaiji onto both VRAM
pages to begin with… and because TH04 doesn't, changing the picture in the
middle of a string of text is technically bugged in that game, even though
you only get to temporarily see the new text on very underclocked PC-98
systems.
These performance implications made me wonder why cutscenes even bother with
writing to the second VRAM page anyway, before copying each crossfade step
to the visible one.
📝 We learned in June how costly EGC-"accelerated" inter-page copies are;
shouldn't it be faster to just blit the image once rather than twice?
Well, master.lib decodes .PI images into a packed-pixel format, and
unpacking such a representation into bitplanes on the fly is just about the
worst way of blitting you could possibly imagine on a PC-98. EGC inter-page
copies are already fairly disappointing at 42 cycles for every 16 pixels, if
we look at the i486 and ignore VRAM latencies. But under the same
conditions, packed-pixel unpacking comes in at 81 cycles for every 8
pixels, or almost 4× slower. On lower-end systems, that can easily sum up to
more than one frame for a 320×200 image. While I'd argue that the resulting
tearing could have been an acceptable part of the transition between two
images, it's understandable why you'd want to avoid it in favor of the
pure effect on a slower framerate.
Really makes me wonder why master.lib didn't just directly decode .PI images
into bitplanes. The performance impact on load times should have been
negligible? It's such a good format for
the often dithered 16-color artwork you typically see on PC-98, and
deserves better than master.lib's implementation which is both slow to
decode and slow to blit.
That brings us to the individual script commands… and yes, I'm going to
document every single one of them. Some of their interactions and edge cases
are not clear at all from just looking at the code.
Almost all commands are preceded by… well, a 0x5C lead byte.
Which raises the question of whether we should
document it as an ASCII-encoded \ backslash, or a Shift-JIS-encoded
¥ yen sign. From a gaijin perspective, it seems obvious that it's a
backslash, as it's consistently displayed as one in most of the editors you
would actually use nowadays. But interestingly, iconv
-f shift-jis -t utf-8 does convert any 0x5C
lead bytes to actual ¥ U+00A5 YEN SIGN code points
.
Ultimately, the distinction comes down to the font. There are fonts
that still render 0x5C as ¥, but mainly do so out
of an obvious concern about backward compatibility to JIS X 0201, where this
mapping originated. Unsurprisingly, this group includes MS Gothic/Mincho,
the old Japanese fonts from Windows 3.1, but even Meiryo and Yu
Gothic/Mincho, Microsoft's modern Japanese fonts. Meanwhile, pretty much
every other modern font, and freely licensed ones in particular, render this
code point as \, even if you set your editor to Shift-JIS. And
while ZUN most definitely saw it as a ¥, documenting this code
point as \ is less ambiguous in the long run. It can only
possibly correspond to one specific code point in either Shift-JIS or UTF-8,
and will remain correct even if we later mod the cutscene system to support
full-blown Unicode.
Now we've only got to clarify the parameter syntax, and then we can look at
the big table of commands:
Numeric parameters are read as sequences of up to 3 ASCII digits. This
limits them to a range from 0 to 999 inclusive, with 000 and
0 being equivalent. Because there's no further sentinel
character, any further digit from the 4th one onwards is
interpreted as regular text.
Filename parameters must be terminated with a space or newline and are
limited to 12 characters, which translates to 8.3 basenames without any
directory component. Any further characters are ignored and displayed as
text as well.
Each .PI image can contain up to four 320×200 pictures ("quarters") for
the cutscene picture area. In the script commands, they are numbered like
this:
0
1
2
3
\@
Clears both VRAM pages by filling them with VRAM color 0. 🐞
In TH03 and TH04, this command does not update the internal text area
background used for unblitting. This bug effectively restricts usage of
this command to either the beginning of a script (before the first
background image is shown) or its end (after no more new text boxes are
started). See the image below for an
example of using it anywhere else.
\b2
Sets the font weight to a value between 0 (raw font ROM glyphs) to 3
(very thicc). Specifying any other value has no effect.
🐞 In TH04 and TH05, \b3 leads to glitched pixels when
rendering half-width glyphs due to a bug in the newly micro-optimized
ASM version of
📝 graph_putsa_fx(); see the image below for an example.
In these games, the parameter also directly corresponds to the
graph_putsa_fx() effect function, removing the sanity check
that was present in TH03. In exchange, you can also access the four
dissolve masks for the bold font (\b2) by specifying a
parameter between 4 (fewest pixels) to 7 (most
pixels). Demo video below.
\c15
Changes the text color to VRAM color 15.
\c=字,15
Adds a color map entry: If 字 is the first code point
inside the name area on a new line, the text color is automatically set
to 15. Up to 8 such entries can be registered
before overflowing the statically allocated buffer.
🐞 The comma is assumed to be present even if the color parameter is omitted.
\e0
Plays the sound effect with the given ID.
\f
(no-op)
\fi1
\fo1
Calls master.lib's palette_black_in() or
palette_black_out() to play a hardware palette fade
animation from or to black, spending roughly 1 frame on each of the 16 fade steps.
\fm1
Fades out BGM volume via PMD's AH=02h interrupt call,
in a non-blocking way. The fade speed can range from 1 (slowest) to 127 (fastest).
Values from 128 to 255 technically correspond to
AH=02h's fade-in feature, which can't be used from cutscene
scripts because it requires BGM volume to first be lowered via
AH=19h, and there is no command to do that.
\g8
Plays a blocking 8-frame screen shake
animation.
\ga0
Shows the gaiji with the given ID from 0 to 255
at the current cursor position. Even in TH03, gaiji always ignore the
text delay interval configured with \v.
@3
TH05's replacement for the \ga command from TH03 and
TH04. The default ID of 3 corresponds to the
gaiji. Not to be confused with \@, which starts with a backslash,
unlike this command.
@h
Shows the gaiji.
@t
Shows the gaiji.
@!
Shows the gaiji.
@?
Shows the gaiji.
@!!
Shows the gaiji.
@!?
Shows the gaiji.
\k0
Waits 0 frames (0 = forever) for an advance key to be pressed before
continuing script execution. Before waiting, TH05 crossfades in any new
text that was previously rendered to the invisible VRAM page…
🐞 …but TH04 doesn't, leaving the text invisible during the wait time.
As a workaround, \vp1 can be
used before \k to immediately display that text without a
fade-in animation.
\m$
Stops the currently playing BGM.
\m*
Restarts playback of the currently loaded BGM from the
beginning.
\m,filename
Stops the currently playing BGM, loads a new one from the given
file, and starts playback.
\n
Starts a new line at the leftmost X coordinate of the box, i.e., the
start of the name area. This is how scripts can "change" the name of the
currently speaking character, or use the entire 480×64 pixels without
being restricted to the non-name area.
Note that automatic line breaks already move the cursor into a new line.
Using this command at the "end" of a line with the maximum number of 30
full-width glyphs would therefore start a second new line and leave the
previously started line empty.
If this command moved the cursor into the 5th line of a box,
\s is executed afterward, with
any of \n's parameters passed to \s.
\p
(no-op)
\p-
Deallocates the loaded .PI image.
\p,filename
Loads the .PI image with the given file into the single .PI slot
available to cutscenes. TH04 and TH05 automatically deallocate any
previous image, 🐞 TH03 would leak memory without a manual prior call to
\p-.
\pp
Sets the hardware palette to the one of the loaded .PI image.
\p@
Sets the loaded .PI image as the full-screen 640×400 background
image and overwrites both VRAM pages with its pixels, retaining the
current hardware palette.
\p=
Runs \pp followed by \p@.
\s0
\s-
Ends a text box and starts a new one. Fades in any text rendered to
the invisible VRAM page, then waits 0 frames
(0 = forever) for an advance key to be
pressed. Afterward, the new text box is started with the cursor moved to
the top-left corner of the name area. \s- skips the wait time and starts the new box
immediately.
\t100
Sets palette brightness via master.lib's
palette_settone() to any value from 0 (fully black) to 200
(fully white). 100 corresponds to the palette's original colors.
Preceded by a 1-frame delay unless ESC is held.
\v1
Sets the number of frames to wait between every 2 bytes of rendered
text.
Sets the number of frames to spend on each of the 4 fade
steps when crossfading between old and new text. The game-specific
default value is also used before the first use of this command.
\v2
\vp0
Shows VRAM page 0. Completely useless in
TH03 (this game always synchronizes both VRAM pages at a command
boundary), only of dubious use in TH04 (for working around a bug in \k), and the games always return to
their intended shown page before every blitting operation anyway. A
debloated mod of this game would just remove this command, as it exposes
an implementation detail that script authors should not need to worry
about. None of the original scripts use it anyway.
\w64
\w and \wk wait for the given number
of frames
\wm and \wmk wait until PMD has played
back the current BGM for the total number of measures, including
loops, given in the first parameter, and fall back on calling
\w and \wk with the second parameter as
the frame number if BGM is disabled.
🐞 Neither PMD nor MMD reset the internal measure when stopping
playback. If no BGM is playing and the previous BGM hasn't been
played back for at least the given number of measures, this command
will deadlock.
Since both TH04 and TH05 fade in any new text from the invisible VRAM
page, these commands can be used to simulate TH03's typing effect in
those games. Demo video below.
Contrary to \k and \s, specifying 0 frames would
simply remove any frame delay instead of waiting forever.
The TH03-exclusive k variants allow the delay to be
interrupted if ⏎ Return or Shot are held down.
TH04 and TH05 recognize the k as well, but removed its
functionality.
All of these commands have no effect if ESC is held.
\wm64,64
\wk64
\wmk64,64
\wi1
\wo1
Calls master.lib's palette_white_in() or
palette_white_out() to play a hardware palette fade
animation from or to white, spending roughly 1 frame on each of the 16 fade steps.
\=4
Immediately displays the given quarter of the loaded .PI image in
the picture area, with no fade effect. Any value ≥ 4 resets the picture area to black.
\==4,1
Crossfades the picture area between its current content and quarter
#4 of the loaded .PI image, spending 1 frame on each of the 4 fade steps unless
ESC is held. Any value ≥ 4 is
replaced with quarter #0.
\$
Stops script execution. Must be called at the end of each file;
otherwise, execution continues into whatever lies after the script
buffer in memory.
TH05 automatically deallocates the loaded .PI image, TH03 and TH04
require a separate manual call to \p- to not leak its memory.
Bold values signify the default if the parameter
is omitted; \c is therefore
equivalent to \c15.
The \@ bug. Yes, the ¥ is fake. It
was easier to GIMP it than to reword the sentences so that the backslashes
landed on the second byte of a 2-byte half-width character pair.
The font weights and effects available through \b, including the glitch with
\b3 in TH04 and TH05.
Font weight 3 is technically not rendered correctly in TH03 either; if
you compare 1️⃣ with 4️⃣, you notice a single missing column of pixels
at the left side of each glyph, which would extend into the previous
VRAM byte. Ironically, the TH04/TH05 version is more correct in
this regard: For half-width glyphs, it preserves any further pixel
columns generated by the weight functions in the high byte of the 16-dot
glyph variable. Unlike TH03, which still cuts them off when rendering
text to unaligned X positions (3️⃣), TH04 and TH05 do bit-rotate them
towards their correct place (4️⃣). It's only at byte-aligned X positions
(2️⃣) where they remain at their internally calculated place, and appear
on screen as these glitched pixel columns, 15 pixels away from the glyph
they belong to. It's easy to blame bugs like these on micro-optimized
ASM code, but in this instance, you really can't argue against it if the
original C++ version was equally incorrect.
Combining \b and s- into a partial dissolve
animation. The speed can be controlled with \v.
Simulating TH03's typing effect in TH04 and TH05 via \w. Even prettier in TH05 where we
also get an additional fade animation
after the box ends.
So yeah, that's the cutscene system. I'm dreading the moment I will have to
deal with the other command interpreter in these games, i.e., the
stage enemy system. Luckily, that one is completely disconnected from any
other system, so I won't have to deal with it until we're close to finishing
MAIN.EXE… that is, unless someone requests it before. And it
won't involve text encodings or unblitting…
The cutscene system got me thinking in greater detail about how I would
implement translations, being one of the main dependencies behind them. This
goal has been on the order form for a while and could soon be implemented
for these cutscenes, with 100% PI being right around the corner for the TH03
and TH04 cutscene executables.
Once we're there, the "Virgin" old-school way of static translation patching
for Latin-script languages could be implemented fairly quickly:
Establish basic UTF-8 parsing for less painful manual editing of the
source files
Procedurally generate glyphs for the few required additional letters
based on existing font ROM glyphs. For example, we'd generate ä
by painting two short lines on top of the font ROM's a glyph,
or generate ¿ by vertically flipping the question mark. This
way, the text retains a consistent look regardless of whether the translated
game is run with an NEC or EPSON font ROM, or the that Neko Project II auto-generates if you
don't provide either.
(Optional) Change automatic line breaks to work on a per-word
basis, rather than per-glyph
That's it – script editing and distribution would be handled by your local
translation group. It might seem as if this would also work for Greek and
Cyrillic scripts due to their presence in the PC-98 font ROM, but I'm not
sure if I want to attempt procedurally shrinking these glyphs from 16×16 to
8×16… For any more thorough solution, we'd need to go for a more "Chad" kind
of full-blown translation support:
Implement text subdivisions at a sensible granularity while retaining
automatic line and box breaks
Compile translatable text into a Japanese→target language dictionary
(I'm too old to develop any further translation systems that would overwrite
modded source text with translations of the original text)
Implement a custom Unicode font system (glyphs would be taken from GNU
Unifont unless translators provide a different 8×16 font for their
language)
Combine the text compiler with the font compiler to only store needed
glyphs as part of the translation's font file (dealing with a multi-MB font
file would be rather ugly in a Real Mode game)
Write a simple install/update/patch stacking tool that supports both
.HDI and raw-file DOSBox-X scenarios (it's different enough from thcrap to
warrant a separate tool – each patch stack would be statically compiled into
a single package file in the game's directory)
Add a nice language selection option to the main menu
(Optional) Support proportional fonts
Which sounds more like a separate project to be commissioned from
Touhou Patch Center's Open Collective funds, separate from the ReC98 cap.
This way, we can make sure that the feature is completely implemented, and I
can talk with every interested translator to make sure that their language
works.
It's still cheaper overall to do this on PC-98 than to first port the games
to a modern system and then translate them. On the other hand, most
of the tasks in the Chad variant (3, 4, 5, and half of 2) purely deal with
the difficulty of getting arbitrary Unicode characters to work natively in a
PC-98 DOS game at all, and would be either unnecessary or trivial if we had
already ported the game. Depending on where the patrons' interests lie, it
may not be worth it. So let's see what all of you think about which
way we should go, or whether it's worth doing at all. (Edit
(2022-12-01): With Splashman's
order towards the stage dialogue system, we've pretty much confirmed that it
is.) Maybe we want to meet in the middle – using e.g. procedural glyph
generation for dynamic translations to keep text rendering consistent with
the rest of the PC-98 system, and just not support non-Latin-script
languages in the beginning? In any case, I've added both options to the
order form. Edit (2023-07-28):Touhou Patch Center has agreed to fund
a basic feature set somewhere between the Virgin and Chad level. Check the
📝 dedicated announcement blog post for more
details and ideas, and to find out how you can support this goal!
Surprisingly, there was still a bit of RE work left in the third push after
all of this, which I filled with some small rendering boilerplate. Since I
also wanted to include TH02's playfield overlay functions,
1/15 of that last push went towards getting a
TH02-exclusive function out of the way, which also ended up including that
game in this delivery.
The other small function pointed out how TH05's Stage 5 midboss pops into
the playfield quite suddenly, since its clipping test thinks it's only 32
pixels tall rather than 64:
Good chance that the pop-in might have been intended. Edit (2023-06-30): Actually, it's a
📝 systematic consequence of ZUN having to work around the lack of clipping in master.lib's sprite functions.
There's even another quirk here: The white flash during its first frame
is actually carried over from the previous midboss, which the
game still considers as actively getting hit by the player shot that
defeated it. It's the regular boilerplate code for rendering a
midboss that resets the responsible damage variable, and that code
doesn't run during the defeat explosion animation.
Next up: Staying with TH05 and looking at more of the pattern code of its
boss fights. Given the remaining TH05 budget, it makes the most sense to
continue in in-game order, with Sara and the Stage 2 midboss. If more money
comes in towards this goal, I could alternatively go for the Mai & Yuki
fight and immediately develop a pretty fix for the cheeto storage
glitch. Also, there's a rather intricate
pull request for direct ZMBV decoding on the website that I've still got
to review…
TH05 has passed the 50% RE mark, with both MAIN.EXE and the
game as a whole! With that, we've also reached what -Tom-
wanted out of the project, so he's suspending his discount offer for a
bit.
Curve bullets are now officially called cheetos! 76.7% of
fans prefer this term, and it fits into the 8.3 DOS filename scheme much
better than homing lasers (as they're called in
OMAKE.TXT) or Taito
lasers (which would indeed have made sense as well).
…oh, and I managed to decompile Shinki within 2 pushes after all. That
left enough budget to also add the Stage 1 midboss on top.
So, Shinki! As far as final boss code is concerned, she's surprisingly
economical, with 📝 her background animations
making up more than ⅓ of her entire code. Going straight from TH01's
📝 final📝 bosses
to TH05's final boss definitely showed how much ZUN had streamlined
danmaku pattern code by the end of PC-98 Touhou. Don't get me wrong, there
is still room for improvement: TH05 not only
📝 reuses the same 16 bytes of generic boss state we saw in TH04 last month,
but also uses them 4× as often, and even for midbosses. Most importantly
though, defining danmaku patterns using a single global instance of the
group template structure is just bad no matter how you look at it:
The script code ends up rather bloated, with a single MOV
instruction for setting one of the fields taking up 5 bytes. By comparison,
the entire structure for regular bullets is 14 bytes large, while the
template structure for Shinki's 32×32 ball bullets could have easily been
reduced to 8 bytes.
Since it's also one piece of global state, you can easily forget to set
one of the required fields for a group type. The resulting danmaku group
then reuses these values from the last time they were set… which might have
been as far back as another boss fight from a previous stage.
And of course, I wouldn't point this out if it
didn't actually happen in Shinki's pattern code. Twice.
Declaring a separate structure instance with the static data for every
pattern would be both safer and more space-efficient, and there's
more than enough space left for that in the game's data segment.
But all in all, the pattern functions are short, sweet, and easy to follow.
The "devil"
patternis significantly more complex than the others, but still
far from TH01's final bosses at their worst. I especially like the clear
architectural separation between "one-shot pattern" functions that return
true once they're done, and "looping pattern" functions that
run as long as they're being called from a boss's main function. Not many
all too interesting things in these pattern functions for the most part,
except for two pieces of evidence that Shinki was coded after Yumeko:
The gather animation function in the first two phases contains a bullet
group configuration that looks like it's part of an unused danmaku
pattern. It quickly turns out to just be copy-pasted from a similar function
in Yumeko's fight though, where it is turned into actual
bullets.
As one of the two places where ZUN forgot to set a template field, the
lasers at the end of the white wing preparation pattern reuse the 6-pixel
width of Yumeko's final laser pattern. This actually has an effect on
gameplay: Since these lasers are active for the first 8 frames after
Shinki's wings appear on screen, the player can get hit by them in the last
2 frames after they grew to their final width.
Of course, there are more than enough safespots between the lasers.
Speaking about that wing sprite: If you look at ST05.BB2 (or
any other file with a large sprite, for that matter), you notice a rather
weird file layout:
A large sprite split into multiple smaller ones with a width of
64 pixels each? What's this, hardware sprite limitations? On my
PC-98?!
And it's not a limitation of the sprite width field in the BFNT+ header
either. Instead, it's master.lib's BFNT functions which are limited to
sprite widths up to 64 pixels… or at least that's what
MASTER.MAN claims. Whatever the restriction was, it seems to be
completely nonexistent as of master.lib version 0.23, and none of the
master.lib functions used by the games have any issues with larger
sprites.
Since ZUN stuck to the supposed 64-pixel width limit though, it's now the
game that expects Shinki's winged form to consist of 4 physical
sprites, not just 1. Any conversion from another, more logical sprite sheet
layout back into BFNT+ must therefore replicate the original number of
sprites. Otherwise, the sequential IDs ("patnums") assigned to every newly
loaded sprite no longer match ZUN's hardcoded IDs, causing the game to
crash. This is exactly what used to happen with -Tom-'s
MysticTK automation scripts,
which combined these exact sprites into a single large one. This issue has
now been fixed – just in case there are some underground modders out there
who used these scripts and wonder why their game crashed as soon as the
Shinki fight started.
And then the code quality takes a nosedive with Shinki's main function.
Even in TH05, these boss and midboss update
functions are still very imperative:
The origin point of all bullet types used by a boss must be manually set
to the current boss/midboss position; there is no concept of a bullet type
tracking a certain entity.
The same is true for the target point of a player's homing shots…
… and updating the HP bar. At least the initial fill animation is
abstracted away rather decently.
Incrementing the phase frame variable also must be done manually. TH05
even "innovates" here by giving the boss update function exclusive ownership
of that variable, in contrast to TH04 where that ownership is given out to
the player shot collision detection (?!) and boss defeat helper
functions.
Speaking about collision detection: That is done by calling different
functions depending on whether the boss is supposed to be invincible or
not.
Timeout conditions? No standard way either, and all done with manual
if statements. In combination with the regular phase end
condition of lowering (mid)boss HP to a certain value, this leads to quite a
convoluted control flow.
The manual calls to the score bonus functions for cleared phases at least provide some sense of orientation.
One potentially nice aspect of all this imperative freedom is that
phases can end outside of HP boundaries… by manually incrementing the
phase variable and resetting the phase frame variable to 0.
The biggest WTF in there, however, goes to using one of the 16 state bytes
as a "relative phase" variable for differentiating between boss phases that
share the same branch within the switch(boss.phase)
statement. While it's commendable that ZUN tried to reduce code duplication
for once, he could have just branched depending on the actual
boss.phase variable? The same state byte is then reused in the
"devil" pattern to track the activity state of the big jerky lasers in the
second half of the pattern. If you somehow managed to end the phase after
the first few bullets of the pattern, but before these lasers are up,
Shinki's update function would think that you're still in the phase
before the "devil" pattern. The main function then sequence-breaks
right to the defeat phase, skipping the final pattern with the burning Makai
background. Luckily, the HP boundaries are far away enough to make this
impossible in practice.
The takeaway here: If you want to use the state bytes for your custom
boss script mods, alias them to your own 16-byte structure, and limit each
of the bytes to a clearly defined meaning across your entire boss script.
One final discovery that doesn't seem to be documented anywhere yet: Shinki
actually has a hidden bomb shield during her two purple-wing phases.
uth05win got this part slightly wrong though: It's not a complete
shield, and hitting Shinki will still deal 1 point of chip damage per
frame. For comparison, the first phase lasts for 3,000 HP, and the "devil"
pattern phase lasts for 5,800 HP.
And there we go, 3rd PC-98 Touhou boss
script* decompiled, 28 to go! 🎉 In case you were expecting a fix for
the Shinki death glitch: That one
is more appropriately fixed as part of the Mai & Yuki script. It also
requires new code, should ideally look a bit prettier than just removing
cheetos between one frame and the next, and I'd still like it to fit within
the original position-dependent code layout… Let's do that some other
time.
Not much to say about the Stage 1 midboss, or midbosses in general even,
except that their update functions have to imperatively handle even more
subsystems, due to the relative lack of helper functions.
The remaining ¾ of the third push went to a bunch of smaller RE and
finalization work that would have hardly got any attention otherwise, to
help secure that 50% RE mark. The nicest piece of code in there shows off
what looks like the optimal way of setting up the
📝 GRCG tile register for monochrome blitting
in a variable color:
mov ah, palette_index ; Any other non-AL 8-bit register works too.
; (x86 only supports AL as the source operand for OUTs.)
rept 4 ; For all 4 bitplanes…
shr ah, 1 ; Shift the next color bit into the x86 carry flag
sbb al, al ; Extend the carry flag to a full byte
; (CF=0 → 0x00, CF=1 → 0xFF)
out 7Eh, al ; Write AL to the GRCG tile register
endm
Thanks to Turbo C++'s inlining capabilities, the loop body even decompiles
into a surprisingly nice one-liner. What a beautiful micro-optimization, at
a place where micro-optimization doesn't hurt and is almost expected.
Unfortunately, the micro-optimizations went all downhill from there,
becoming increasingly dumb and undecompilable. Was it really necessary to
save 4 x86 instructions in the highly unlikely case of a new spark sprite
being spawned outside the playfield? That one 2D polar→Cartesian
conversion function then pointed out Turbo C++ 4.0J's woefully limited
support for 32-bit micro-optimizations. The code generation for 32-bit
📝 pseudo-registers is so bad that they almost
aren't worth using for arithmetic operations, and the inline assembler just
flat out doesn't support anything 32-bit. No use in decompiling a function
that you'd have to entirely spell out in machine code, especially if the
same function already exists in multiple other, more idiomatic C++
variations.
Rounding out the third push, we got the TH04/TH05 DEMO?.REC
replay file reading code, which should finally prove that nothing about the
game's original replay system could serve as even just the foundation for
community-usable replays. Just in case anyone was still thinking that.
Next up: Back to TH01, with the Elis fight! Got a bit of room left in the
cap again, and there are a lot of things that would make a lot of
sense now:
TH04 would really enjoy a large number of dedicated pushes to catch up
with TH05. This would greatly support the finalization of both games.
Continuing with TH05's bosses and midbosses has shown to be good value
for your money. Shinki would have taken even less than 2 pushes if she
hadn't been the first boss I looked at.
Oh, and I also added Seihou as a selectable goal, for the two people out
there who genuinely like it. If I ever want to quit my day job, I need to
branch out into safer territory that isn't threatened by takedowns, after
all.
Slight change of plans, because we got instructions for
reliably reproducing the TH04 Kurumi Divide Error crash! Major thanks to
Colin Douglas Howell. With those, it also made sense to immediately look at
the crash in the Stage 4 Marisa fight as well. This way, I could release
both of the obligatory bugfix mods at the same time.
Especially since it turned out that I was wrong: Both crashes are entirely
unrelated to the custom entity structure that would have required PI-centric
progress. They are completely specific to Kurumi's and Marisa's
danmaku-pattern code, and really are two separate bugs
with no connection to each other. All of the necessary research nicely fit
into Arandui's 0.5 pushes, with no further deep understanding
required here.
But why were there still three weeks between Colin's message and this blog
post? DMCA distractions aside: There are no easy fixes this time, unlike
📝 back when I looked at the Stage 5 Yuuka crash.
Just like how division by zero is undefined in mathematics, it's also,
literally, undefined what should happen instead of these two
Divide error crashes. This means that any possible "fix" can
only ever be a fanfiction interpretation of the intentions behind ZUN's
code. The gameplay community should be aware of this, and
might decide to handle these cases differently. And if we
have to go into fanfiction territory to work around crashes in the
canon games, we'd better document what exactly we're fixing here and how, as
comprehensible as possible.
With that out of the way, let's look at Kurumi's crash first, since it's way
easier to grasp. This one is known to primarily happen to new players, and
it's easy to see why:
In one of the patterns in her third phase, Kurumi fires a series of 3
aimed rings from both edges of the playfield. By default (that is, on Normal
and with regular rank), these are 6-way rings.
6 happens to be quite a peculiar number here, due to how rings are
(manually) tuned based on the current "rank" value (playperf)
before being fired. The code, abbreviated for clarity:
Let's look at the range of possible playperf values per
difficulty level:
Easy
Normal
Hard
Lunatic
Extra
playperf_min
4
11
20
22
16
playperf_max
16
24
32
34
20
Edit (2022-05-24): This blog post initially had
26 instead of 16 for playperf_min for the Extra Stage. Thanks
to Popfan for pointing out that typo!
Reducing rank to its minimum on Easy mode will therefore result in a
0-ring after tuning.
To calculate the individual angles of each bullet in a ring, ZUN divides
360° (or, more correctly,
📝 0x100) by the total number of
bullets…
Boom, division by zero.
The pattern that causes the crash in Kurumi's fight. Also
demonstrates how the number of bullets in a ring is always halved on
Easy Mode after the rank-based tuning, leading to just a 3-ring on
playperf = 16.
So, what should the workaround look like? Obviously, we want to modify
neither the default number of ring bullets nor the tuning algorithm – that
would change all other non-crashing variations of this pattern on other
difficulties and ranks, creating a fork of the original gameplay. Instead, I
came up with four possible workarounds that all seemed somewhat logical to
me:
Firing no bullet, i.e., interpreting 0-ring literally. This would
create the only constellation in which a call to the bullet group spawn
functions would not spawn at least one new bullet.
Firing a "1-ring", i.e., a single bullet. This would be consistent with
how the bullet spawn functions behave for "0-way" stack and spread
groups.
Firing a "∞-ring", i.e., 200 bullets, which is as much as the game's cap
on 16×16 bullets would allow. This would poke fun at the whole "division by
zero" idea… but given that we're still talking about Easy Mode (and
especially new players) here, it might be a tad too cruel. Certainly the
most trollish interpretation.
Triggering an immediate Game Over, exchanging the hard crash for a
softer and more controlled shutdown. Certainly the option that would be
closest to the behavior of the original games, and perhaps the only one to
be accepted in Serious, High-Level Play™.
As I was writing this post, it felt increasingly wrong for me to make this
decision. So I once again went to Twitter, where 56.3%
voted in favor of the 1-bullet option. Good that I asked! I myself was
more leaning towards the 0-bullet interpretation, which only got 28.7% of
the vote. Also interesting are the 2.3% in favor of the Game Over option but
I get it, low-rank Easy Mode isn't exactly the most competitive mode of
playing TH04.
There are reports of Kurumi crashing on higher difficulties as well, but I
could verify none of them. If they aren't fixed by this workaround, they're
caused by an entirely different bug that we have yet to discover.
Onto the Stage 4 Marisa crash then, which does in fact apply to all
difficulty levels. I was also wrong on this one – it's a hell of a lot more
intricate than being just a division by the number of on-screen bits.
Without having decompiled the entire fight, I can't give a completely
accurate picture of what happens there yet, but here's the rough idea:
Marisa uses different patterns, depending on whether at least one of her
bits is still alive, or all of them have been destroyed.
Destroying the last bit will immediately switch to the bit-less
counterpart of the current pattern.
The bits won't respawn before the pattern ended, which ensures that the
bit-less version is always shown in its entirety after being started or
switched into.
In two of the bit-less patterns, Marisa gradually moves to the point
reflection of her position at the start of the pattern across the playfield
coordinate of (192, 112), or (224, 128) on screen.
Reference points for Marisa's point-reflected movement. Cyan:
Marisa's position, green: (192, 112), yellow: the intended end
point.
The velocity of this movement is determined by both her distance to that
point and the total amount of frames that this instance of the bit-less
pattern will last.
Since this frame amount is directly tied to the frame the player
destroyed the last bit on, it becomes a user-controlled variable. I think
you can see where this is going…
The last 12 frames of this duration, however, are always reserved for a
"braking phase", where Marisa's velocity is halved on each frame.
This part of the code only runs every 4 frames though. This expands the
time window for this crash to 4 frames, rather than just the two frames you
would expect from looking at the division itself.
Both of the broken patterns run for a maximum of 160 frames. Therefore,
the crash will occur when Marisa's last bit is destroyed between frame 152
and 155 inclusive. On these frames, the
last_frame_with_bits_alive variable is set to 148, which is the
crucial 12 duration frames away from the maximum of 160.
Interestingly enough, the calculated velocity is also only
applied every 4 frames, with Marisa actually staying still for the 3 frames
inbetween. As a result, she either moves
too slowly to ever actually reach the yellow point if the last bit
was destroyed early in the pattern (see destruction frames 68 or
112),
or way too quickly, and almost in a jerky, teleporting way (see
destruction frames 144 or 148).
Finally, as you may have already gathered from the formula: Destroying
the last bit between frame 156 and 160 inclusive results in
duration values of 8 or 4. These actually push Marisa
away from the intended point, as the divisor becomes negative.
One of the two patterns in TH04's Stage 4 Marisa boss fight that feature
frame number-dependent point-reflected movement. The bits were hacked to
self-destruct on the respective frame.
tl;dr: "Game crashes if last bit destroyed within 4-frame window near end of
two patterns". For an informed decision on a new movement behavior for these
last 8 frames, we definitely need to know all the details behind the crash
though. Here's what I would interpret into the code:
Not moving at all, i.e., interpreting 0 as the middle ground between
positive and negative movement. This would also make sense because a
12-frame duration implies 100% of the movement to consist of
the braking phase – and Marisa wasn't moving before, after all.
Move at maximum speed, i.e., dividing by 1 rather than 0. Since the
movement duration is still 12 in this case, Marisa will immediately start
braking. In total, she will move exactly ¾ of the way from her initial
position to (192, 112) within the 8 frames before the pattern
ends.
Directly warping to (192, 112) on frame 0, and to the
point-reflected target on 4, respectively. This "emulates" the division by
zero by moving Marisa at infinite speed to the exact two points indicated by
the velocity formula. It also fits nicely into the 8 frames we have to fill
here. Sure, Marisa can't reach these points at any other duration, but why
shouldn't she be able to, with infinite speed? Then again, if Marisa
is far away enough from (192, 112), this workaround would warp her
across the entire playfield. Can Marisa teleport according to lore? I
have no idea…
Triggering an immediate Game O– hell no, this is the Stage 4 boss,
people already hate losing runs to this bug!
Asking Twitter worked great for the Kurumi workaround, so let's do it again!
Gotta attach a screenshot of an earlier draft of this blog post though,
since this stuff is impossible to explain in tweets…
…and it went
through the roof, becoming the most successful ReC98 tweet so far?!
Apparently, y'all really like to just look at descriptions of overly complex
bugs that I'd consider way beyond the typical attention span that can be
expected from Twitter. Unfortunately, all those tweet impressions didn't
quite translate into poll turnout. The results
were pretty evenly split between 1) and 2), with option 1) just coming out
slightly ahead at 49.1%, compared to 41.5% of option 2).
(And yes, I only noticed after creating the poll that warping to both the
green and yellow points made more sense than warping to just one of the two.
Let's hope that this additional variant wouldn't have shifted the results
too much. Both warp options only got 9.4% of the vote after all, and no one
else came up with the idea either. In the end,
you can always merge together your preferred combination of workarounds from
the Git branches linked below.)
So here you go: The new definitive version of TH04, containing not only the
community-chosen Kurumi and Stage 4 Marisa workaround variant, but also the
📝 No-EMS bugfix from last year.
Edit (2022-05-31): This package is outdated, 📝 the current version is here!2022-04-18-community-choice-fixes.zip
Oh, and let's also add spaztron64's TH03 GDC clock fix
from 2019 because why not. This binary was built from the community_choice_fixes
branch, and you can find the code for all the individual workarounds on
these branches:
Again, because it can't be stated often enough: These fixes are
fanfiction. The gameplay community should be aware of
this, and might decide to handle these cases differently.
With all of that taking way more time to evaluate and document, this
research really had to become part of a proper push, instead of just being
covered in the quick non-push blog post I initially intended. With ½ of a
push left at the end, TH05's Stage 1-5 boss background rendering functions
fit in perfectly there. If you wonder how these static backdrop images even
need any boss-specific code to begin with, you're right – it's basically the
same function copy-pasted 4 times, differing only in the backdrop image
coordinates and some other inconsequential details.
Only Sara receives a nice variation of the typical
📝 blocky entrance animation: The usually
opaque bitmap data from ST00.BB is instead used as a transition
mask from stage tiles to the backdrop image, by making clever use of the
tile invalidation system:
TH04 uses the same effect a bit more frequently, for its first three bosses.
Next up: Shinki, for real this time! I've already managed to decompile 10 of
her 11 danmaku patterns within a little more than one push – and yes,
that one is included in there. Looks like I've slightly
overestimated the amount of work required for TH04's and TH05's bosses…
Did you know that moving on top of a boss sprite doesn't kill the player in
TH04, only in TH05?
Yup, Reimu is not getting hit… yet.
That's the first of only three interesting discoveries in these 3 pushes,
all of which concern TH04. But yeah, 3 for something as seemingly simple as
these shared boss functions… that's still not quite the speed-up I had hoped
for. While most of this can be blamed, again, on TH04 and all of its
hardcoded complexities, there still was a lot of work to be done on the
maintenance front as well. These functions reference a bunch of code I RE'd
years ago and that still had to be brought up to current standards, with the
dependencies reaching from 📝 boss explosions
over 📝 text RAM overlay functionality up to
in-game dialog loading.
The latter provides a good opportunity to talk a bit about x86 memory
segmentation. Many aspiring PC-98 developers these days are very scared
of it, with some even going as far as to rather mess with Protected Mode and
DOS extenders just so that they don't have to deal with it. I wonder where
that fear comes from… Could it be because every modern programming language
I know of assumes memory to be flat, and lacks any standard language-level
features to even express something like segments and offsets? That's why
compilers have a hard time targeting 16-bit x86 these days: Doing anything
interesting on the architecture requires giving the programmer full
control over segmentation, which always comes down to adding the
typical non-standard language extensions of compilers from back in the day.
And as soon as DOS stopped being used, these extensions no longer made sense
and were subsequently removed from newer tools. A good example for this can
be found in an old version of the
NASM manual: The project started as an attempt to make x86 assemblers
simple again by throwing out most of the segmentation features from
MASM-style assemblers, which made complete sense in 1996 when 16-bit DOS and
Windows were already on their way out. But there was a point to all
those features, and that's why ReC98 still has to use the supposedly
inferior TASM.
Not that this fear of segmentation is completely unfounded: All the
segmentation-related keywords, directives, and #pragmas
provided by Borland C++ and TASM absolutely can be the cause of many
weird runtime bugs. Even if the compiler or linker catches them, you are
often left with confusing error messages that aged just as poorly as memory
segmentation itself.
However, embracing the concept does provide quite the opportunity for
optimizations. While it definitely was a very crazy idea, there is a small
bit of brilliance to be gained from making proper use of all these
segmentation features. Case in point: The buffer for the in-game dialog
scripts in TH04 and TH05.
// Thanks to the semantics of `far` pointers, we only need a single 32-bit
// pointer variable for the following code.
extern unsigned char far *dialog_p;
// This master.lib function returns a `void __seg *`, which is a 16-bit
// segment-only pointer. Converting to a `far *` yields a full segment:offset
// pointer to offset 0000h of that segment.
dialog_p = (unsigned char far *)hmem_allocbyte(/* … */);
// Running the dialog script involves pointer arithmetic. On a far pointer,
// this only affects the 16-bit offset part, complete with overflow at 64 KiB,
// from FFFFh back to 0000h.
dialog_p += /* … */;
dialog_p += /* … */;
dialog_p += /* … */;
// Since the segment part of the pointer is still identical to the one we
// allocated above, we can later correctly free the buffer by pulling the
// segment back out of the pointer.
hmem_free((void __seg *)dialog_p);
If dialog_p was a huge pointer, any pointer
arithmetic would have also adjusted the segment part, requiring a second
pointer to store the base address for the hmem_free call. Doing
that will also be necessary for any port to a flat memory model. Depending
on how you look at it, this compression of two logical pointers into a
single variable is either quite nice, or really, really dumb in its
reliance on the precise memory model of one single architecture.
Why look at dialog loading though, wasn't this supposed to be all about
shared boss functions? Well, TH04 unnecessarily puts certain stage-specific
code into the boss defeat function, such as loading the alternate Stage 5
Yuuka defeat dialog before a Bad Ending, or initializing Gengetsu after
Mugetsu's defeat in the Extra Stage.
That's TH04's second core function with an explicit conditional branch for
Gengetsu, after the
📝 dialog exit code we found last year during EMS research.
And I've heard people say that Shinki was the most hardcoded fight in PC-98
Touhou… Really, Shinki is a perfectly regular boss, who makes proper use of
all internal mechanics in the way they were intended, and doesn't blast
holes into the architecture of the game. Even within TH05, it's Mai and Yuki
who rely on hacks and duplicated code, not Shinki.
The worst part about this though? How the function distinguishes Mugetsu
from Gengetsu. Once again, it uses its own global variable to track whether
it is called the first or the second time within TH04's Extra Stage,
unrelated to the same variable used in the dialog exit function. But this
time, it's not just any newly created, single-use variable, oh no. In a
misguided attempt to micro-optimize away a few bytes of conventional memory,
TH04 reserves 16 bytes of "generic boss state", which can (and are) freely
used for anything a boss doesn't want to store in a more dedicated
variable.
It might have been worth it if the bosses actually used most of these
16 bytes, but the majority just use (the same) two, with only Stage 4 Reimu
using a whopping seven different ones. To reverse-engineer the various uses
of these variables, I pretty much had to map out which of the undecompiled
danmaku-pattern functions corresponds to which boss
fight. In the end, I assigned 29 different variable names for each of the
semantically different use cases, which made up another full push on its
own.
Now, 16 bytes of wildly shared state, isn't that the perfect recipe for
bugs? At least during this cursory look, I haven't found any obvious ones
yet. If they do exist, it's more likely that they involve reused state from
earlier bosses – just how the Shinki death glitch in
TH05 is caused by reusing cheeto data from way back in Stage 4 – and
hence require much more boss-specific progress.
And yes, it might have been way too early to look into all these tiny
details of specific boss scripts… but then, this happened:
Looks similar to another
screenshot of a crash in the same fight that was reported in December,
doesn't it? I was too much in a hurry to figure it out exactly, but notice
how both crashes happen right as the last of Marisa's four bits is destroyed.
KirbyComment has suspected
this to be the cause for a while, and now I can pretty much confirm it
to be an unguarded division by the number of on-screen bits in
Marisa-specific pattern code. But what's the cause for Kurumi then?
As for fixing it, I can go for either a fast or a slow option:
Superficially fixing only this crash will probably just take a fraction
of a push.
But I could also go for a deeper understanding by looking at TH04's
version of the 📝 custom entity structure. It
not only stores the data of Marisa's bits, but is also very likely to be
involved in Kurumi's crash, and would get TH04 a lot closer to 100%
PI. Taking that look will probably need at least 2 pushes, and might require
another 3-4 to completely decompile Marisa's fight, and 2-3 to decompile
Kurumi's.
OK, now that that's out of the way, time to finish the boss defeat function…
but not without stumbling over the third of TH04's quirks, relating to the
Clear Bonus for the main game or the Extra Stage:
To achieve the incremental addition effect for the in-game score display
in the HUD, all new points are first added to a score_delta
variable, which is then added to the actual score at a maximum rate of
61,110 points per frame.
There are a fixed 416 frames between showing the score tally and
launching into MAINE.EXE.
As a result, TH04's Clear Bonus is effectively limited to
(416 × 61,110) = 25,421,760 points.
Only TH05 makes sure to commit the entirety of the
score_delta to the actual score before switching binaries,
which fixes this issue.
And after another few collision-related functions, we're now truly,
finally ready to decompile bosses in both TH04 and TH05! Just as the
anything funds were running out… The
remaining ¼ of the third push then went to Shinki's 32×32 ball bullets,
rounding out this delivery with a small self-contained piece of the first
TH05 boss we're probably going to look at.
Next up, though: I'm not sure, actually. Both Shinki and Elis seem just a
little bit larger than the 2¼ or 4 pushes purchased so far, respectively.
Now that there's a bunch of room left in the cap again, I'll just let the
next contribution decide – with a preference for Shinki in case of a tie.
And if it will take longer than usual for the store to sell out again this
time (heh), there's still the
📝 PC-98 text RAM JIS trail word rendering research
waiting to be documented.
Two years after
📝 the first look at TH04's and TH05's bullets,
we finally get to finish their logic code by looking at the special motion
types. Bullets as a whole still aren't completely finished as the
rendering code is still waiting to be RE'd, but now we've got everything
about them that's required for decompiling the midboss and boss fights of
these games.
Just like the motion types of TH01's pellets, the ones we've got here really
are special enough to warrant an enum, despite all the
overlap in the "slow down and turn" and "bounce at certain edges of the
playfield" types. Sure, including them in the bitfield I proposed two years
ago would have allowed greater variety, but it wouldn't have saved any
memory. On the contrary: These types use a single global state variable for
the maximum turn count and delta speed, which a proper customizable
architecture would have to integrate into the bullet structure. Maybe it is
possible to stuff everything into the same amount of bytes, but not without
first completely rearchitecting the bullet structure and removing every
single piece of redundancy in there. Simply extending the system by adding a
new enum value for a new motion type would be way more
straightforward for modders.
Speaking about memory, TH05 already extends the bullet structure by 6 bytes
for the "exact linear movement" type exclusive to that game. This type is
particularly interesting for all the prospective PC-98 game developers out
there, as it nicely points out the precision limits of Q12.4 subpixels.
Regular bullet movement works by adding a Q12.4 velocity to a Q12.4 position
every frame, with the velocity typically being calculated only once on spawn
time from an 8-bit angle and a Q12.4 speed. Quantization errors from this
initial calculation can quickly compound over all the frames a bullet spends
moving across the playfield. If a bullet is only supposed to move on a
straight line though, there is a more precise way of calculating its
position: By storing the origin point, movement angle, and total distance
traveled, you can perform a full polar→Cartesian transformation every frame.
Out of the 10 danmaku patterns in TH05 that use this motion type, the
difference to regular bullet movement can be best seen in Louise's final
pattern:
Louise's final pattern in its original form, demonstrating
exact linear bullet movement. Note how each bullet spawns slightly
behind the delay cloud: ZUN simply forgot to shift the fixed origin
point along with it.The same pattern with standard bullet movement, corrupting
its intended appearance. No delay cloud-related oversights here though,
at least.
Not far away from the regular bullet code, we've also got the movement
function for the infamous curve / "cheeto" bullets. I would have almost
called them "cheetos" in the code as well, which surely fits more nicely
into 8.3 filenames than "curve bullets" does, but eh, trademarks…
As for hitboxes, we got a 16×16 one on the head node, and a 12×12 one on the
16 trail nodes. The latter simply store the position of the head node during
the last 16 frames, Snake style. But what you're all here for is probably
the turning and homing algorithm, right? Boiled down to its essence, it
works like this:
// [head] points to the controlled "head" part of a curve bullet entity.
// Angles are stored with 8 bits representing a full circle, providing free
// normalization on arithmetic overflow.
// The directions are ordered as you would expect:
// • 0x00: right (sin(0x00) = 0, cos(0x00) = +1)
// • 0x40: down (sin(0x40) = +1, cos(0x40) = 0)
// • 0x80: left (sin(0x80) = 0, cos(0x80) = -1)
// • 0xC0: up (sin(0xC0) = -1, cos(0xC0) = 0)
uint8_t angle_delta = (head->angle - player_angle_from(
head->pos.cur.x, head->pos.cur.y
));
// Stop turning if the player is 1/128ths of a circle away from this bullet
const uint8_t SNAP = 0x02;
// Else, turn either clockwise or counterclockwise by 1/256th of a circle,
// depending on what would reach the player the fastest.
if((angle_delta > SNAP) && (angle_delta < static_cast<uint8_t>(-SNAP))) {
angle_delta = (angle_delta >= 0x80) ? -0x01 : +0x01;
}
head_p->angle -= angle_delta;
5 lines of code, and not all too difficult to follow once you are familiar
with 8-bit angles… unlike what ZUN actually wrote. Which is 26 lines,
and includes an unused "friction" variable that is never set to any value
that makes a difference in the formula. uth05win
correctly saw through that all and simplified this code to something
equivalent to my explanation. Redoing that work certainly wasted a bit of my
time, and means that I now definitely need to spend another push on RE'ing
all the shared boss functions before I can start with Shinki.
So while a curve bullet's speed does get faster over time, its
angular velocity is always limited to 1/256th of a
circle per frame. This reveals the optimal strategy for dodging them:
Maximize this delta angle by staying as close to 180° away from their
current direction as possible, and let their acceleration do the rest.
At least that's the theory for dodging a single one. As a danmaku
designer, you can now of course place other bullets at these technically
optimal places to prevent a curve bullet pattern from being cheesed like
that. I certainly didn't record the video above in a single take either…
After another bunch of boring entity spawn and update functions, the
playfield shaking feature turned out as the most notable (and tricky) one to
round out these two pushes. It's actually implemented quite well in how it
simply "un-shakes" the screen by just marking every stage tile to be
redrawn. In the context of all the other tile invalidation that can take
place during a frame, that's definitely more performant than
📝 doing another EGC-accelerated memmove().
Due to these two games being double-buffered via page flipping, this
invalidation only really needs to happen for the frame after the next
one though. The immediately next frame will show the regular, un-shaken
playfield on the other VRAM page first, except during the multi-frame
shake animation when defeating a midboss, where it will also appear shifted
in a different direction… 😵 Yeah, no wonder why ZUN just always invalidates
all stage tiles for the next two frames after every shaking animation, which
is guaranteed to handle both sporadic single-frame shakes and continuous
ones. So close to good-code here.
Finally, this delivery was delayed a bit because -Tom-
requested his round-up amount to be limited to the cap in the future. Since
that makes it kind of hard to explain on a static page how much money he
will exactly provide, I now properly modeled these discounts in the website
code. The exact round-up amount is now included in both the pre-purchase
breakdown, as well as the cap bar on the main page.
With that in place, the system is now also set up for round-up offers from
other patrons. If you'd also like to support certain goals in this way, with
any amount of money, now's the time for getting in touch with me about that.
Known contributors only, though! 😛
Next up: The final bunch of shared boring boss functions. Which certainly
will give me a break from all the maintenance and research work, and speed
up delivery progress again… right?
EMS memory! The
infamous stopgap measure between the 640 KiB ("ought to be enough for
everyone") of conventional
memory offered by DOS from the very beginning, and the later XMS standard for
accessing all the rest of memory up to 4 GiB in the x86 Protected Mode. With
an optionally active EMS driver, TH04 and TH05 will make use of EMS memory
to preload a bunch of situational .CDG images at the beginning of
MAIN.EXE:
The "eye catch" game title image, shown while stages are loaded
The character-specific background image, shown while bombing
The player character dialog portraits
TH05 additionally stores the boss portraits there, preloading them
at the beginning of each stage. (TH04 instead keeps them in conventional
memory during the entire stage.)
Once these images are needed, they can then be copied into conventional
memory and accessed as usual.
Uh… wait, copied? It certainly would have been possible to map EMS
memory to a regular 16-bit Real Mode segment for direct access,
bank-switching out rarely used system or peripheral memory in exchange for
the EMS data. However, master.lib doesn't expose this functionality, and
only provides functions for copying data from EMS to regular memory and vice
versa.
But even that still makes EMS an excellent fit for the large image files
it's used for, as it's possible to directly copy their pixel data from EMS
to VRAM. (Yes, I tried!) Well… would, because ZUN doesn't do
that either, and always naively copies the images to newly allocated
conventional memory first. In essence, this dumbs down EMS into just another
layer of the memory hierarchy, inserted between conventional memory and
disk: Not quite as slow as disk, but still requiring that
memcpy() to retrieve the data. Most importantly though: Using
EMS in this way does not increase the total amount of memory
simultaneously accessible to the game. After all, some other data will have
to be freed from conventional memory to make room for the newly loaded data.
The most idiomatic way to define the game-specific layout of the EMS area
would be either a struct or an enum.
Unfortunately, the total size of all these images exceeds the range of a
16-bit value, and Turbo C++ 4.0J supports neither 32-bit enums
(which are silently degraded to 16-bit) nor 32-bit structs
(which simply don't compile). That still leaves raw compile-time constants
though, you only have to manually define the offset to each image in terms
of the size of its predecessor. But instead of doing that, ZUN just placed
each image at a nice round decimal offset, each slightly larger than the
actual memory required by the previous image, just to make sure that
everything fits. This results not only in quite
a bit of unnecessary padding, but also in technically the single
biggest amount of "wasted" memory in PC-98 Touhou: Out of the 180,000 (TH04)
and 320,000 (TH05) EMS bytes requested, the game only uses 135,552 (TH04)
and 175,904 (TH05) bytes. But hey, it's EMS, so who cares, right? Out of all
the opportunities to take shortcuts during development, this is among the
most acceptable ones. Any actual PC-98 model that could run these two games
comes with plenty of memory for this to not turn into an actual issue.
On to the EMS-using functions themselves, which are the definition of
"cross-cutting concerns". Most of these have a fallback path for the non-EMS
case, and keep the loaded .CDG images in memory if they are immediately
needed. Which totally makes sense, but also makes it difficult to find names
that reflect all the global state changed by these functions. Every one of
these is also just called from a single place, so inlining
them would have saved me a lot of naming and documentation trouble
there.
The TH04 version of the EMS allocation code was actually displayed on ZUN's monitor in the
2010 MAG・ネット documentary; WindowsTiger already transcribed the low-quality video image
in 2019. By 2015 ReC98 standards, I would have just run with that, but
the current project goal is to write better code than ZUN, so I didn't. 😛
We sure ain't going to use magic numbers for EMS offsets.
The dialog init and exit code then is completely different in both games,
yet equally cross-cutting. TH05 goes even further in saving conventional
memory, loading each individual player or boss portrait into a single .CDG
slot immediately before blitting it to VRAM and freeing the pixel data
again. People who play TH05 without an active EMS driver are surely going to
enjoy the hard drive access lag between each portrait change…
TH04, on the other hand, also abuses the dialog
exit function to preload the Mugetsu defeat / Gengetsu entrance and
Gengetsu defeat portraits, using a static variable to track how often the
function has been called during the Extra Stage… who needs function
parameters anyway, right?
This is also the function in which TH04 infamously crashes after the Stage 5
pre-boss dialog when playing with Reimu and without any active EMS driver.
That crash is what motivated this look into the games' EMS usage… but the
code looks perfectly fine? Oh well, guess the crash is not related to EMS
then. Next u–
OK, of course I can't leave it like that. Everyone is expecting a fix now,
and I still got half of a push left over after decompiling the regular EMS
code. Also, I've now RE'd every function that could possibly be involved in
the crash, and this is very likely to be the last time I'll be looking at
them.
Turns out that the bug has little to do with EMS, and everything to do with
ZUN limiting the amount of conventional RAM that TH04's
MAIN.EXE is allowed to use, and then slightly miscalculating
this upper limit. Playing Stage 5 with Reimu is the most asset-intensive
configuration in this game, due to the combination of
6 player portraits (Marisa has only 5), at 128×128 pixels each
a 288×256 background for the boss fight, tied in size only with the
ones in the Extra Stage
the additional 96×80 image for the vertically scrolling stars during
the stage, wastefully stored as 4 bitplanes rather than a single one.
This image is never freed, not even at the end of the stage.
The star image used in TH04's Stage 5.
Remove any single one of the above points, and this crash would have never
occurred. But with all of them combined, the total amount of memory consumed
by TH04's MAIN.EXE just barely exceeds ZUN's limit of 320,000
bytes, by no more than 3,840 bytes, the size of the star image.
But wait: As we established earlier, EMS does nothing to reduce the amount
of conventional memory used by the game. In fact, if you disabled TH04's EMS
handling, you'd still get this crash even if you are running an EMS
driver and loaded DOS into the High Memory Area to free up as much
conventional RAM as possible. How can EMS then prevent this crash in the
first place?
The answer: It's only because ZUN's usage of EMS bypasses the need to load
the cached images back out of the XOR-encrypted 東方幻想.郷
packfile. Leaving aside the general
stupidity of any game data file encryption*, master.lib's decryption
implementation is also quite wasteful: It uses a separate buffer that
receives fixed-size chunks of the file, before decrypting every individual
byte and copying it to its intended destination buffer. That really
resembles the typical slowness of a C fread() implementation
more than it does the highly optimized ASM code that master.lib purports to
be… And how large is this well-hidden decryption buffer? 4 KiB.
So, looking back at the game, here is what happens once the Stage 5
pre-battle dialog ends:
Reimu's bomb background image, which was previously freed to make space
for her dialog portraits, has to be loaded back into conventional memory
from disk
BB0.CDG is found inside the 東方幻想.郷
packfile
file_ropen() ends up allocating a 4 KiB buffer for the
encrypted packfile data, getting us the decisive ~4 KiB closer to the memory
limit
The .CDG loader tries to allocate 52 608 contiguous bytes for the
pixel data of Reimu's bomb image
This would exceed the memory limit, so hmem_allocbyte()
fails and returns a nullptr
ZUN doesn't check for this case (as usual)
The pixel data is loaded to address 0000:0000,
overwriting the Interrupt Vector Table and whatever comes after
The game crashes
The final frame rendered by a crashing TH04.
The 4 KiB encryption buffer would only be freed by the corresponding
file_close() call, which of course never happens because the
game crashes before it gets there. At one point, I really did suspect the
cause to be some kind of memory leak or fragmentation inside master.lib,
which would have been quite delightful to fix.
Instead, the most straightforward fix here is to bump up that memory limit
by at least 4 KiB. Certainly easier than squeezing in a
cdg_free() call for the star image before the pre-boss dialog
without breaking position dependence.
Or, even better, let's nuke all these memory limits from orbit
because they make little sense to begin with, and fix every other potential
out-of-memory crash that modders would encounter when adding enough data to
any of the 4 games that impose such limits on themselves. Unless you want to
launch other binaries (which need to do their own memory allocations) after
launching the game, there's really no reason to restrict the amount of
memory available to a DOS process. Heck, whenever DOS creates a new one, it
assigns all remaining free memory by default anyway.
Removing the memory limits also removes one of ZUN's few error checks, which
end up quitting the game if there isn't at least a given maximum amount of
conventional RAM available. While it might be tempting to reserve enough
memory at the beginning of execution and then never check any allocation for
a potential failure, that's exactly where something like TH04's crash
comes from.
This game is also still running on DOS, where such an initial allocation
failure is very unlikely to happen – no one fills close to half of
conventional RAM with TSRs and then tries running one of these games. It
might have been useful to detect systems with less than 640 KiB of
actual, physical RAM, but none of the PC-98 models with that little amount
of memory are fast enough to run these games to begin with. How ironic… a
place where ZUN actually added an error check, and then it's mostly
pointless.
Here's an archive that contains both fix variants, just in case. These were
compiled from the th04_noems_crash_fix
and mem_assign_all
branches, and contain as little code changes as possible. Edit (2022-04-18): For TH04, you probably want to download
the 📝 community choice fix package instead,
which contains this fix along with other workarounds for the Divide
error crashes.
2021-11-29-Memory-limit-fixes.zip
So yeah, quite a complex bug, leaving no time for the TH03 scorefile format
research after all. Next up: Raising prices.
…or maybe not that soon, as it would have only wasted time to
untangle the bullet update commits from the rest of the progress. So,
here's all the bullet spawning code in TH04 and TH05 instead. I hope
you're ready for this, there's a lot to talk about!
(For the sake of readability, "bullets" in this blog post refers to the
white 8×8 pellets
and all 16×16 bullets loaded from MIKO16.BFT, nothing else.)
But first, what was going on📝 in 2020? Spent 4 pushes on the basic types
and constants back then, still ended up confusing a couple of things, and
even getting some wrong. Like how TH05's "bullet slowdown" flag actually
always prevents slowdown and fires bullets at a constant speed
instead. Or how "random spread" is not the
best term to describe that unused bullet group type in TH04.
Or that there are two distinct ways of clearing all bullets on screen,
which deserve different names:
Mechanic #1: Clearing bullets for a custom amount of
time, awarding 1000 points for all bullets alive on the first frame,
and 100 points for all bullets spawned during the clear time.
Mechanic #2: Zapping bullets for a fixed 16 frames,
awarding a semi-exponential and loudly announced Bonus!! for all
bullets alive on the first frame, and preventing new bullets from being
spawned during those 16 frames. In TH04 at least; thanks to a ZUN bug,
zapping got reduced to 1 frame and no animation in TH05…
Bullets are zapped at the end of most midboss and boss phases, and
cleared everywhere else – most notably, during bombs, when losing a
life, or as rewards for extends or a maximized Dream bonus. The
Bonus!! points awarded for zapping bullets are calculated iteratively,
so it's not trivial to give an exact formula for these. For a small number
𝑛 of bullets, it would exactly be 5𝑛³ - 10𝑛² + 15𝑛
points – or, using uth05win's (correct) recursive definition,
Bonus(𝑛) = Bonus(𝑛-1) + 15𝑛² - 5𝑛 + 10.
However, one of the internal step variables is capped at a different number
of points for each difficulty (and game), after which the points only
increase linearly. Hence, "semi-exponential".
On to TH04's bullet spawn code then, because that one can at least be
decompiled. And immediately, we have to deal with a pointless distinction
between regular bullets, with either a decelerating or constant
velocity, and special bullets, with preset velocity changes during
their lifetime. That preset has to be set somewhere, so why have
separate functions? In TH04, this separation continues even down to the
lowest level of functions, where values are written into the global bullet
array. TH05 merges those two functions into one, but then goes too far and
uses self-modifying code to save a grand total of two local variables…
Luckily, the rest of its actual code is identical to TH04.
Most of the complexity in bullet spawning comes from the (thankfully
shared) helper function that calculates the velocities of the individual
bullets within a group. Both games handle each group type via a large
switch statement, which is where TH04 shows off another Turbo
C++ 4.0 optimization: If the range of case values is too
sparse to be meaningfully expressed in a jump table, it usually generates a
linear search through a second value table. But with the -G
command-line option, it instead generates branching code for a binary
search through the set of cases. 𝑂(log 𝑛) as the worst case for a
switch statement in a C++ compiler from 1994… that's so cool.
But still, why are the values in TH04's group type enum all
over the place to begin with?
Unfortunately, this optimization is pretty rare in PC-98 Touhou. It only
shows up here and in a few places in TH02, compared to at least 50
switch value tables.
In all of its micro-optimized pointlessness, TH05's undecompilable version
at least fixes some of TH04's redundancy. While it's still not even
optimal, it's at least a decently written piece of ASM…
if you take the time to understand what's going on there, because it
certainly took quite a bit of that to verify that all of the things which
looked like bugs or quirks were in fact correct. And that's how the code
for this function ended up with 35% comments and blank lines before I could
confidently call it "reverse-engineered"…
Oh well, at least it finally fixes a correctness issue from TH01 and TH04,
where an invalid bullet group type would fill all remaining slots in the
bullet array with identical versions of the first bullet.
Something that both games also share in these functions is an over-reliance
on globals for return values or other local state. The most ridiculous
example here: Tuning the speed of a bullet based on rank actually mutates
the global bullet template… which ZUN then works around by adding a wrapper
function around both regular and special bullet spawning, which saves the
base speed before executing that function, and restores it afterward.
Add another set of wrappers to bypass that exact
tuning, and you've expanded your nice 1-function interface to 4 functions.
Oh, and did I mention that TH04 pointlessly duplicates the first set of
wrapper functions for 3 of the 4 difficulties, which can't even be
explained with "debugging reasons"? That's 10 functions then… and probably
explains why I've procrastinated this feature for so long.
At this point, I also finally stopped decompiling ZUN's original ASM just
for the sake of it. All these small TH05 functions would look horribly
unidiomatic, are identical to their decompiled TH04 counterparts anyway,
except for some unique constant… and, in the case of TH05's rank-based
speed tuning function, actually become undecompilable as soon as we
want to return a C++ class to preserve the semantic meaning of the return
value. Mainly, this is because Turbo C++ does not allow register
pseudo-variables like _AX or _AL to be cast into
class types, even if their size matches. Decompiling that function would
have therefore lowered the quality of the rest of the decompiled code, in
exchange for the additional maintenance and compile-time cost of another
translation unit. Not worth it – and for a TH05 port, you'd already have to
decompile all the rest of the bullet spawning code anyway!
The only thing in there that was still somewhat worth being
decompiled was the pre-spawn clipping and collision detection function. Due
to what's probably a micro-optimization mistake, the TH05 version continues
to spawn a bullet even if it was spawned on top of the player. This might
sound like it has a different effect on gameplay… until you realize that
the player got hit in this case and will either lose a life or deathbomb,
both of which will cause all on-screen bullets to be cleared anyway.
So it's at most a visual glitch.
But while we're at it, can we please stop talking about hitboxes? At least
in the context of TH04 and TH05 bullets. The actual collision detection is
described way better as a kill delta of 8×8 pixels between the
center points of the player and a bullet. You can distribute these pixels
to any combination of bullet and player "hitboxes" that make up 8×8. 4×4
around both the player and bullets? 1×1 for bullets, and 8×8 for the
player? All equally valid… or perhaps none of them, once you keep in mind
that other entity types might have different kill deltas. With that in
mind, the concept of a "hitbox" turns into just a confusing abstraction.
The same is true for the 36×44 graze box delta. For some reason,
this one is not exactly around the center of a bullet, but shifted to the
right by 2 pixels. So, a bullet can be grazed up to 20 pixels right of the
player, but only up to 16 pixels left of the player. uth05win also spotted
this… and rotated the deltas clockwise by 90°?!
Which brings us to the bullet updates… for which I still had to
research a decompilation workaround, because
📝 P0148 turned out to not help at all?
Instead, the solution was to lie to the compiler about the true segment
distance of the popup function and declare its signature far
rather than near. This allowed ZUN to save that ridiculous overhead of 1 additional far function
call/return per frame, and those precious 2 bytes in the BSS segment
that he didn't have to spend on a segment value.
📝 Another function that didn't have just a
single declaration in a common header file… really,
📝 how were these games even built???
The function itself is among the longer ones in both games. It especially
stands out in the indentation department, with 7 levels at its most
indented point – and that's the minimum of what's possible without
goto. Only two more notable discoveries there:
Bullets are the only entity affected by Slow Mode. If the number of
bullets on screen is ≥ (24 + (difficulty * 8) + rank) in TH04,
or (42 + (difficulty * 8)) in TH05, Slow Mode reduces the frame
rate by 33%, by waiting for one additional VSync event every two frames.
The code also reveals a second tier, with 50% slowdown for a slightly
higher number of bullets, but that conditional branch can never be executed
Bullets must have been grazed in a previous frame before they can
be collided with. (Note how this does not apply to bullets that spawned
on top of the player, as explained earlier!)
Whew… When did ReC98 turn into a full-on code review?! 😅 And after all
this, we're still not done with TH04 and TH05 bullets, with all the
special movement types still missing. That should be less than one push
though, once we get to it. Next up: Back to TH01 and Konngara! Now have fun
rewriting the Touhou Wiki Gameplay pages 😛
Back after taking way too long to get Touhou Patch Center's MediaWiki
update feature complete… I'm still waiting for more translators to test and
review the new translation interface before delivering and deploying it
all, which will most likely lead to another break from ReC98 within the
next few months. For now though, I'm happy to have mostly addressed the
nagging responsibility I still had after willing that site into existence,
and to be back working on ReC98. 🙂
As announced, the next few pushes will focus on TH04's and TH05's bullet
spawning code, before I get to put all that accumulated TH01 money towards
finishing all of konngara's code in TH01. For a full
picture of what's happening with bullets, we'd really also like to
have the bullet update function as readable C code though.
Clearing all bullets on the playfield will trigger a Bonus!! popup,
displayed as 📝 gaiji in that proportional
font. Unfortunately, TLINK refused to link the code as soon as I referenced
the function for animating the popups at the top of the playfield? Which
can only mean that we have to decompile that function first…
So, let's turn that piece of technical debt into a full push, and first
decompile another random set of previously reverse-engineered TH04 and TH05
functions. Most of these are stored in a different place within the two
MAIN.EXE binaries, and the tried-and-true method of matching
segment names would therefore have introduced several unnecessary
translation units. So I resorted to a segment splitting technique I should
have started using way earlier: Simply creating new segments with names
derived from their functions, at the exact positions they're needed. All
the new segment start and end directives do bloat the ASM code somewhat,
and certainly contributed to this push barely removing any actual lines of
code. However, what we get in return is total freedom as far as
decompilation order is concerned,
📝 which should be the case for any ReC project, really.
And in the end, all these tiny code segments will cancel out anyway.
If only we could do the same with the data segment…
The popup function happened to be the final one I RE'd before my long break
in the spring of 2019. Back then, I didn't even bother looking into that
64-frame delay between changing popups, and what that meant for the game.
Each of these popups stays on screen for 128 frames, during which, of
course, another popup-worthy event might happen. Handling this cleanly
without removing previous popups too early would involve some sort of event
queue, whose size might even be meaningfully limited to the number of
distinct events that can happen. But still, that'd be a data structure, and
we're not gonna have that! Instead, ZUN
simply keeps two variables for the new and current popup ID. During an
active popup, any change to that ID will only be committed once the current
popup has been shown for at least 64 frames. And during that time,
that new ID can be freely overwritten with a different one, which drops any
previous, undisplayed event. But surely, there won't be more than two
events happening within 63 frames, right?
The rest was fairly uneventful – no newly RE'd functions in this push,
after all – until I reached the widely used helper function for applying
the current vertical scrolling offset to a Y coordinate. Its combination of
a function parameter, the pascal calling convention, and no
stack frame was previously thought to be undecompilable… except that it
isn't, and the decompilation didn't even require any new workarounds to be
developed? Good thing that I already forgot how impossible it was to
decompile the first function I looked at that fell into this category!
Oh well, this discovery wasn't too groundbreaking. Looking back at
all the other functions with that combination only revealed a grand total
of 1 additional one where a decompilation made sense: TH05's version of
snd_kaja_interrupt(), which is now compiled from the same C++
file for all 4 games that use it. And well, looks like some quirks really
remain unnoticed and undocumented until you look at a function for the 11th
time: Its return value is undefined if BGM is inactive – that is, if the
user disabled it, or if no FM board is installed. Not that it matters for
the original code, which never uses this function to retrieve anything from
KAJA's drivers. But people apparently do copy ReC98 code into their own
projects, so it is something to keep in mind.
All in all, nothing quite at jank level in this one, but we were surely grazing that tag. Next up, with that out of the way: The bullet update/step function! Very soon in fact, since I've mostly got it done already.
Didn't quite get to cover background rendering for TH05's Stage 1-5
bosses in this one, as I had to reverse-engineer two more fundamental parts
involved in boss background rendering before.
First, we got the those blocky transitions from stage tiles to bomb and
boss backgrounds, loaded from BB*.BB and ST*.BB,
respectively. These files store 16 frames of animation, with every bit
corresponding to a 16×16 tile on the playfield. With 384×368 pixels to be
covered, that would require 69 bytes per frame. But since that's a very odd
number to work with in micro-optimized ASM, ZUN instead stores 512×512
pixels worth of bits, ending up with a frame size of 128 bytes, and a
per-frame waste of 59 bytes. At least it was
possible to decompile the core blitting function as __fastcall
for once.
But wait, TH05 comes with, and loads, a bomb .BB file for every character,
not just for the Reimu and Yuuka bomb transitions you see in-game… 🤔
Restoring those unused stage tile → bomb image transition
animations for Mima and Marisa isn't that trivial without having decompiled
their actual bomb animation functions before, so stay tuned!
Interestingly though, the code leaves out what would look like the most
obvious optimization: All stage tiles are unconditionally redrawn
each frame before they're erased again with the 16×16 blocks, no matter if
they weren't covered by such a block in the previous frame, or are
going to be covered by such a block in this frame. The same is true
for the static bomb and boss background images, where ZUN simply didn't
write a .CDG blitting function that takes the dirty tile array into
account. If VRAM writes on PC-98 really were as slow as the games'
README.TXT files claim them to be, shouldn't all the
optimization work have gone towards minimizing them?
Oh well, it's not like I have any idea what I'm talking about here. I'd
better stop talking about anything relating to VRAM performance on PC-98…
Second, it finally was time to solve the long-standing confusion about all
those callbacks that are supposed to render the playfield background. Given
the aforementioned static bomb background images, ZUN chose to make this
needlessly complicated. And so, we have two callback function
pointers: One during bomb animations, one outside of bomb
animations, and each boss update function is responsible for keeping the
former in sync with the latter.
Other than that, this was one of the smoothest pushes we've had in a while;
the hardest parts of boss background rendering all were part of
📝 the last push. Once you figured out that
ZUN does indeed dynamically change hardware color #0 based on the current
boss phase, the remaining one function for Shinki, and all of EX-Alice's
background rendering becomes very straightforward and understandable.
Meanwhile, -Tom- told me about his plans to publicly
release 📝 his TH05 scripting toolkit once
TH05's MAIN.EXE would hit around 50% RE! That pretty much
defines what the next bunch of generic TH05 pushes will go towards:
bullets, shared boss code, and one
full, concrete boss script to demonstrate how it's all combined. Next up,
therefore: TH04's bullet firing code…? Yes, TH04's. I want to see what I'm
doing before I tackle the undecompilable mess that is TH05's bullet firing
code, and you all probably want readable code for that feature as
well. Turns out it's also the perfect place for Blue Bolt's
pending contributions.
Y'know, I kinda prefer the pending crowdfunded workload to stay more near
the middle of the cap, rather than being sold out all the time. So to reach
this point more quickly, let's do the most relaxing thing that can be
easily done in TH05 right now: The boss backgrounds, starting with Shinki's,
📝 now that we've got the time to look at it in detail.
… Oh come on, more things that are borderline undecompilable, and
require new workarounds to be developed? Yup, Borland C++ always optimizes
any comparison of a register with a literal 0 to OR reg, reg,
no matter how many calculations and inlined function calls you replace the
0 with. Shinki's background particle rendering function contains a
CMP AX, 0 instruction though… so yeah,
📝 yet another piece of custom ASM that's worse
than what Turbo C++ 4.0J would have generated if ZUN had just written
readable C. This was probably motivated by ZUN insisting that his modified
master.lib function for blitting particles takes its X and Y parameters as
registers. If he had just used the __fastcall convention, he
also would have got the sprite ID passed as a register. 🤷
So, we really don't want to be forced into inline assembly just
because of the third comparison in the otherwise perfectly decompilable
four-comparison if() expression that prevents invisible
particles from being drawn. The workaround: Comparing to a pointer
instead, which only the linker gets to resolve to the actual value of 0.
This way, the compiler has to make room for
any 16-bit literal, and can't optimize anything.
And then we go straight from micro-optimization to
waste, with all the duplication in the code that
animates all those particles together with the zooming and spinning lines.
This push decompiled 1.31% of all code in TH05, and thanks to alignment,
we're still missing Shinki's high-level background rendering function that
calls all the subfunctions I decompiled here.
With all the manipulated state involved here, it's not at all trivial to
see how this code produces what you see in-game. Like:
If all lines have the same Y velocity, how do the other three lines in
background type B get pushed down into this vertical formation while the
top one stays still? (Answer: This velocity is only applied to the top
line, the other lines are only pushed based on some delta.)
How can this delta be calculated based on the distance of the top line
with its supposed target point around Shinki's wings? (Answer: The velocity
is never set to 0, so the top line overshoots this target point in every
frame. After calculating the delta, the top line itself is pushed down as
well, canceling out the movement. )
Why don't they get pushed down infinitely, but stop eventually?
(Answer: We only see four lines out of 20, at indices #0, #6, #12, and
#18. In each frame, lines [0..17] are copied to lines [1..18], before
anything gets moved. The invisible lines are pushed down based on the delta
as well, which defines a distance between the visible lines of (velocity *
array gap). And since the velocity is capped at -14 pixels per frame, this
also means a maximum distance of 84 pixels between the midpoints of each
line.)
And why are the lines moving back up when switching to background type
C, before moving down? (Answer: Because type C increases the
velocity rather than decreasing it. Therefore, it relies on the previous
velocity state from type B to show a gapless animation.)
So yeah, it's a nice-looking effect, just very hard to understand. 😵
With the amount of effort I'm putting into this project, I typically
gravitate towards more descriptive function names. Here, however,
uth05win's simple and seemingly tiny-brained "background type A/B/C/D" was
quite a smart choice. It clearly defines the sequence in which these
animations are intended to be shown, and as we've seen with point 4
from the list above, that does indeed matter.
Next up: At least EX-Alice's background animations, and probably also the
high-level parts of the background rendering for all the other TH05 bosses.
Technical debt, part 10… in which two of the PMD-related functions came
with such complex ramifications that they required one full push after
all, leaving no room for the additional decompilations I wanted to do. At
least, this did end up being the final one, completing all
SHARED segments for the time being.
The first one of these functions determines the BGM and sound effect
modes, combining the resident type of the PMD driver with the Option menu
setting. The TH04 and TH05 version is apparently coded quite smartly, as
PC-98 Touhou only needs to distinguish "OPN- /
PC-9801-26K-compatible sound sources handled by PMD.COM"
from "everything else", since all other PMD varieties are
OPNA- / PC-9801-86-compatible.
Therefore, I only documented those two results returned from PMD's
AH=09h function. I'll leave a comprehensive, fully documented
enum to interested contributors, since that would involve research into
basically the entire history of the PC-9800 series, and even the clearly
out-of-scope PC-88VA. After all, distinguishing between more versions of
the PMD driver in the Option menu (and adding new sprites for them!) is
strictly mod territory.
The honor of being the final decompiled function in any SHARED
segment went to TH04's snd_load(). TH04 contains by far the
sanest version of this function: Readable C code, no new ZUN bugs (and
still missing file I/O error handling, of course)… but wait, what about
that actual file read syscall, using the INT 21h, AH=3Fh DOS
file read API? Reading up to a hardcoded number of bytes into PMD's or
MMD's song or sound effect buffer, 20 KiB in TH02-TH04, 64 KiB in
TH05… that's kind of weird. About time we looked closer into this.
Turns out that no, KAJA's driver doesn't give you the full 64 KiB of one
memory segment for these, as especially TH05's code might suggest to
anyone unfamiliar with these drivers. Instead,
you can customize the size of these buffers on its command line. In
GAME.BAT, ZUN allocates 8 KiB for FM songs, 2 KiB for sound
effects, and 12 KiB for MMD files in TH02… which means that the hardcoded
sizes in snd_load() are completely wrong, no matter how you
look at them. Consequently, this read syscall
will overflow PMD's or MMD's song or sound effect buffer if the
given file is larger than the respective buffer size.
Now, ZUN could have simply hardcoded the sizes from GAME.BAT
instead, and it would have been fine. As it also turns out though,
PMD has an API function (AH=22h) to retrieve the actual
buffer sizes, provided for exactly that purpose. There is little excuse
not to use it, as it also gives you PMD's default sizes if you don't
specify any yourself.
(Unless your build process enumerates all PMD files that are part of the
game, and bakes the largest size into both snd_load() and
GAME.BAT. That would even work with MMD, which doesn't have
an equivalent for AH=22h.)
What'd be the consequence of loading a larger file then? Well, since we
don't get a full segment, let's look at the theoretical limit first.
PMD prefers to keep both its driver code and the data buffers in a single
memory segment. As a result, the limit for the combined size of the song,
instrument, and sound effect buffer is determined by the amount of
code in the driver itself. In PMD86 version 4.8o (bundled with TH04
and TH05) for example, the remaining size for these buffers is exactly
45,555 bytes. Being an actually good programmer who doesn't blindly trust
user input, KAJA thankfully validates the sizes given via the
/M, /V, and /E command-line options
before letting the driver reside in memory, and shuts down with an error
message if they exceed 40 KiB. Would have been even better if he calculated
the exact size – even in the current
PMD version 4.8s from
January 2020, it's still a hardcoded value (see line 8581).
Either way: If the file is larger than this maximum, the concrete effect
is down to the INT 21h, AH=3Fh implementation in the
underlying DOS version. DOS 3.3 treats the destination address as linear
and reads past the end of the segment,
DOS
5.0 and DOSBox-X truncate the number of bytes to not exceed the remaining
space in the segment, and maybe there's even a DOS that wraps around
and ends up overwriting the PMD driver code. In any case: You will
overwrite what's after the driver in memory – typically, the game .EXE and
its master.lib functions.
It almost feels like a happy accident that this doesn't cause issues in
the original games. The largest PMD file in any of the 4 games, the -86
version of 幽夢 ～ Inanimate Dream, takes up 8,099 bytes,
just under the 8,192 byte limit for BGM. For modders, I'd really recommend
implementing this properly, with PMD's AH=22h function and
error handling, once position independence has been reached.
Whew, didn't think I'd be doing more research into KAJA's drivers during
regular ReC98 development! That's probably been the final time though, as
all involved functions are now decompiled, and I'm unlikely to iterate
over them again.
And that's it! Repaid the biggest chunk of technical debt, time for some
actual progress again. Next up: Reopening the store tomorrow, and waiting
for new priorities. If we got nothing by Sunday, I'm going to put the
pending [Anonymous] pushes towards some work on the website.
Whoops, the build was broken again? Since
P0127 from
mid-November 2020, on TASM32 version 5.3, which also happens to be the
one in the DevKit… That version changed the alignment for the default
segments of certain memory models when requesting .386
support. And since redefining segment alignment apparently is highly
illegal and absolutely has to be a build error, some of the stand-alone
.ASM translation units didn't assemble anymore on this version. I've only
spotted this on my own because I casually compiled ReC98 somewhere else –
on my development system, I happened to have TASM32 version 5.0 in the
PATH during all this time.
At least this was a good occasion to
get rid of some
weird segment alignment workarounds from 2015, and replace them with the
superior convention of using the USE16 modifier for the
.MODEL directive.
ReC98 would highly benefit from a build server – both in order to
immediately spot issues like this one, and as a service for modders.
Even more so than the usual open-source project of its size, I would say.
But that might be exactly
because it doesn't seem like something you can trivially outsource
to one of the big CI providers for open-source projects, and quickly set
it up with a few lines of YAML.
That might still work in the beginning, and we might get by with a regular
64-bit Windows 10 and DOSBox running the exact build tools from the DevKit.
Ideally, though, such a server should really run the optimal configuration
of a 32-bit Windows 10, allowing both the 32-bit and the 16-bit build step
to run natively, which already is something that no popular CI service out
there offers. Then, we'd optimally expand to Linux, every other Windows
version down to 95, emulated PC-98 systems, other TASM versions… yeah, it'd
be a lot. An experimental project all on its own, with additional hosting
costs and probably diminishing returns, the more it expands…
I've added it as a category to the order form, let's see how much interest
there is once the store reopens (which will be at the beginning of May, at
the latest). That aside, it would 📝 also be
a great project for outside contributors!
So, technical debt, part 8… and right away, we're faced with TH03's
low-level input function, which
📝 once📝 again📝 insists on being word-aligned in a way we
can't fake without duplicating translation units.
Being undecompilable isn't exactly the best property for a function that
has been interesting to modders in the past: In 2018,
spaztron64 created an
ASM-level mod that hardcoded more ergonomic key bindings for human-vs-human
multiplayer mode: 2021-04-04-TH03-WASD-2player.zip
However, this remapping attempt remained quite limited, since we hadn't
(and still haven't) reached full position independence for TH03 yet.
There's quite some potential for size optimizations in this function, which
would allow more BIOS key groups to already be used right now, but it's not
all that obvious to modders who aren't intimately familiar with x86 ASM.
Therefore, I really wouldn't want to keep such a long and important
function in ASM if we don't absolutely have to…
… and apparently, that's all the motivation I needed? So I took the risk,
and spent the first half of this push on reverse-engineering
TCC.EXE, to hopefully find a way to get word-aligned code
segments out of Turbo C++ after all.
And there is! The -WX option, used for creating
DPMI
applications, messes up all sorts of code generation aspects in weird
ways, but does in fact mark the code segment as word-aligned. We can
consider ourselves quite lucky that we get to use Turbo C++ 4.0, because
this feature isn't available in any previous version of Borland's C++
compilers.
That allowed us to restore all the decompilations I previously threw away…
well, two of the three, that lookup table generator was too much of a mess
in C. But what an abuse this is. The
subtly different code generation has basically required one creative
workaround per usage of -WX. For example, enabling that option
causes the regular PUSH BP and POP BP prolog and
epilog instructions to be wrapped with INC BP and
DEC BP, for some reason:
a_function_compiled_with_wx proc
inc bp ; ???
push bp
mov bp, sp
; [… function code …]
pop bp
dec bp ; ???
ret
a_function_compiled_with_wx endp
Luckily again, all the functions that currently require -WX
don't set up a stack frame and don't take any parameters.
While this hasn't directly been an issue so far, it's been pretty
close: snd_se_reset(void) is one of the functions that require
word alignment. Previously, it shared a translation unit with the
immediately following snd_se_play(int new_se), which does take
a parameter, and therefore would have had its prolog and epilog code messed
up by -WX.
Since the latter function has a consistent (and thus, fakeable) alignment,
I simply split that code segment into two, with a new -WX
translation unit for just snd_se_reset(void). Problem solved –
after all, two C++ translation units are still better than one ASM
translation unit. Especially with all the
previous #include improvements.
The rest was more of the usual, getting us 74% done with repaying the
technical debt in the SHARED segment. A lot of the remaining
26% is TH04 needing to catch up with TH03 and TH05, which takes
comparatively little time. With some good luck, we might get this
done within the next push… that is, if we aren't confronted with all too
many more disgusting decompilations, like the two functions that ended this
push.
If we are, we might be needing 10 pushes to complete this after all, but
that piece of research was definitely worth the delay. Next up: One more of
these.
Alright, no more big code maintenance tasks that absolutely need to be
done right now. Time to really focus on parts 6 and 7 of repaying
technical debt, right? Except that we don't get to speed up just yet, as
TH05's barely decompilable PMD file loading function is rather…
complicated.
Fun fact: Whenever I see an unusual sequence of x86 instructions in PC-98
Touhou, I first consult the disassembly of Wolfenstein 3D. That game was
originally compiled with the quite similar Borland C++ 3.0, so it's quite
helpful to compare its ASM to the
officially released source
code. If I find the instructions in question, they mostly come from
that game's ASM code, leading to the amusing realization that "even John
Carmack was unable to get these instructions out of this compiler"
This time though, Wolfenstein 3D did point me
to Borland's intrinsics for common C functions like memcpy()
and strchr(), available via #pragma intrinsic.
Bu~t those unfortunately still generate worse code than what ZUN
micro-optimized here. Commenting how these sequences of instructions
should look in C is unfortunately all I could do here.
The conditional branches in this function did compile quite nicely
though, clarifying the control flow, and clearly exposing a ZUN
bug: TH05's snd_load() will hang in an infinite loop when
trying to load a non-existing -86 BGM file (with a .M2
extension) if the corresponding -26 BGM file (with a .M
extension) doesn't exist either.
Unsurprisingly, the PMD channel monitoring code in TH05's Music Room
remains undecompilable outside the two most "high-level" initialization
and rendering functions. And it's not because there's data in the
middle of the code segment – that would have actually been possible with
some #pragmas to ensure that the data and code segments have
the same name. As soon as the SI and DI registers are referenced
anywhere, Turbo C++ insists on emitting prolog code to save these
on the stack at the beginning of the function, and epilog code to restore
them from there before returning.
Found that out in
September 2019, and confirmed that there's no way around it. All the
small helper functions here are quite simply too optimized, throwing away
any concern for such safety measures. 🤷
Oh well, the two functions that were decompilable at least indicate
that I do try.
Within that same 6th push though, we've finally reached the one function
in TH05 that was blocking further progress in TH04, allowing that game
to finally catch up with the others in terms of separated translation
units. Feels good to finally delete more of those .ASM files we've
decompiled a while ago… finally!
But since that was just getting started, the most satisfying development
in both of these pushes actually came from some more experiments with
macros and inline functions for near-ASM code. By adding
"unused" dummy parameters for all relevant registers, the exact input
registers are made more explicit, which might help future port authors who
then maybe wouldn't have to look them up in an x86 instruction
reference quite as often. At its best, this even allows us to
declare certain functions with the __fastcall convention and
express their parameter lists as regular C, with no additional
pseudo-registers or macros required.
As for output registers, Turbo C++'s code generation turns out to be even
more amazing than previously thought when it comes to returning
pseudo-registers from inline functions. A nice example for
how this can improve readability can be found in this piece of TH02 code
for polling the PC-98 keyboard state using a BIOS interrupt:
inline uint8_t keygroup_sense(uint8_t group) {
_AL = group;
_AH = 0x04;
geninterrupt(0x18);
// This turns the output register of this BIOS call into the return value
// of this function. Surprisingly enough, this does *not* naively generate
// the `MOV AL, AH` instruction you might expect here!
return _AH;
}
void input_sense(void)
{
// As a result, this assignment becomes `_AH = _AH`, which Turbo C++
// never emits as such, giving us only the three instructions we need.
_AH = keygroup_sense(8);
// Whereas this one gives us the one additional `MOV BH, AH` instruction
// we'd expect, and nothing more.
_BH = keygroup_sense(7);
// And now it's obvious what both of these registers contain, from just
// the assignments above.
if(_BH & K7_ARROW_UP || _AH & K8_NUM_8) {
key_det |= INPUT_UP;
}
// […]
}
I love it. No inline assembly, as close to idiomatic C code as something
like this is going to get, yet still compiling into the minimum possible
number of x86 instructions on even a 1994 compiler. This is how I keep
this project interesting for myself during chores like these.
We might have even reached peak
inline already?
And that's 65% of technical debt in the SHARED segment repaid
so far. Next up: Two more of these, which might already complete that
segment? Finally!
Technical debt, part 5… and we only got TH05's stupidly optimized
.PI functions this time?
As far as actual progress is concerned, that is. In maintenance news
though, I was really hyped for the #include improvements I've
mentioned in 📝 the last post. The result: A
new x86real.h file, bundling all the declarations specific to
the 16-bit x86 Real Mode in a smaller file than Turbo C++'s own
DOS.H. After all, DOS is something else than the underlying
CPU. And while it didn't speed up build times quite as much as I had hoped,
it now clearly indicates the x86-specific parts of PC-98 Touhou code to
future port authors.
After another couple of improvements to parameter declaration in ASM land,
we get to TH05's .PI functions… and really, why did ZUN write all of
them in ASM? Why (re)declare all the necessary structures and data in
ASM land, when all these functions are merely one layer of abstraction
above master.lib, which does all the actual work?
I get that ZUN might have wanted masked blitting to be faster, which is
used for the fade-in effect seen during TH05's main menu animation and the
ending artwork. But, uh… he knew how to modify master.lib. In fact, he
did already modify the graph_pack_put_8() function
used for rendering a single .PI image row, to ignore master.lib's VRAM
clipping region. For this effect though, he first blits each row regularly
to the invisible 400th row of VRAM, and then does an EGC-accelerated
VRAM-to-VRAM blit of that row to its actual target position with the mask
enabled. It would have been way more efficient to add another version of
this function that takes a mask pattern. No amount of REP
MOVSW is going to change the fact that two VRAM writes per line are
slower than a single one. Not to mention that it doesn't justify writing
every other .PI function in ASM to go along with it…
This is where we also find the most hilarious aspect about this: For most
of ZUN's pointless micro-optimizations, you could have maybe made the
argument that they do save some CPU cycles here and there, and
therefore did something positive to the final, PC-98-exclusive result. But
some of the hand-written ASM here doesn't even constitute a
micro-optimization, because it's worse than what you would have got
out of even Turbo C++ 4.0J with its 80386 optimization flags!
At least it was possible to "decompile" 6 out of the 10 functions
here, making them easy to clean up for future modders and port authors.
Could have been 7 functions if I also decided to "decompile"
pi_free(), but all the C++ code is already surrounded by ASM,
resulting in 2 ASM translation units and 2 C++ translation units.
pi_free() would have needed a single translation unit by
itself, which wasn't worth it, given that I would have had to spell out
every single ASM instruction anyway.
There you go. What about this needed to be written in ASM?!?
The function calls between these small translation units even seemed to
glitch out TASM and the linker in the end, leading to one CALL
offset being weirdly shifted by 32 bytes. Usually, TLINK reports a fixup
overflow error when this happens, but this time it didn't, for some reason?
Mirroring the segment grouping in the affected translation unit did solve
the problem, and I already knew this, but only thought of it after spending
quite some RTFM time… during which I discovered the -lE
switch, which enables TLINK to use the expanded dictionaries in
Borland's .OBJ and .LIB files to speed up linking. That shaved off roughly
another second from the build time of the complete ReC98 repository. The
more you know… Binary blobs compiled with non-Borland tools would be the
only reason not to use this flag.
So, even more slowdown with this 5th dedicated push, since we've still only
repaid 41% of the technical debt in the SHARED segment so far.
Next up: Part 6, which hopefully manages to decompile the FM and SSG
channel animations in TH05's Music Room, and hopefully ends up being the
final one of the slow ones.
Wow, 31 commits in a single push? Well, what the last push had in
progress, this one had in maintenance. The
📝 master.lib header transition absolutely
had to be completed in this one, for my own sanity. And indeed,
it reduced the build time for the entirety of ReC98 to about 27 seconds on
my system, just as expected in the original announcement. Looking forward
to even faster build times with the upcoming #include
improvements I've got up my sleeve! The port authors of the future are
going to appreciate those quite a bit.
As for the new translation units, the funniest one is probably TH05's
function for blitting the 1-color .CDG images used for the main menu
options. Which is so optimized that it becomes decompilable again,
by ditching the self-modifying code of its TH04 counterpart in favor of
simply making better use of CPU registers. The resulting C code is still a
mess, but what can you do.
This was followed by even more TH05 functions that clearly weren't
compiled from C, as evidenced by their padding
bytes. It's about time I've documented my lack of ideas of how to get
those out of Turbo C++.
And just like in the previous push, I also had to 📝 throw away a decompiled TH02 function purely due to alignment issues. Couldn't have been a better one though, no one's going to miss a residency check for the MMD driver that is largely identical to the corresponding (and indeed decompilable) function for the PMD driver. Both of those should have been merged into a single function anyway, given how they also mutate the game's sound configuration flags…
In the end, I've slightly slowed down with this one, with only 37% of technical debt done after this 4th dedicated push. Next up: One more of these, centered around TH05's stupidly optimized .PI functions. Maybe also with some more reverse-engineering, after not having done any for 1½ months?
Alright, back to continuing the master.hpp transition started
in P0124, and repaying technical debt. The last blog post already
announced some ridiculous decompilations… and in fact, not a single
one of the functions in these two pushes was decompilable into
idiomatic C/C++ code.
As usual, that didn't keep me from trying though. The TH04 and TH05
version of the infamous 16-pixel-aligned, EGC-accelerated rectangle
blitting function from page 1 to page 0 was fairly average as far as
unreasonable decompilations are concerned.
The big blocker in TH03's MAIN.EXE, however, turned out to be
the .MRS functions, used to render the gauge attack portraits and bomb
backgrounds. The blitting code there uses the additional FS and GS segment
registers provided by the Intel 386… which
are not supported by Turbo C++'s inline assembler, and
can't be turned into pointers, due to a compiler bug in Turbo C++ that
generates wrong segment prefix opcodes for the _FS and
_GS pseudo-registers.
Apparently I'm the first one to even try doing that with this compiler? I
haven't found any other mention of this bug…
Compiling via assembly (#pragma inline) would work around
this bug and generate the correct instructions. But that would incur yet
another dependency on a 16-bit TASM, for something honestly quite
insignificant.
What we can always do, however, is using __emit__() to simply
output x86 opcodes anywhere in a function. Unlike spelled-out inline
assembly, that can even be used in helper functions that are supposed to
inline… which does in fact allow us to fully abstract away this compiler
bug. Regular if() comparisons with pseudo-registers
wouldn't inline, but "converting" them into C++ template function
specializations does. All that's left is some C preprocessor abuse
to turn the pseudo-registers into types, and then we do retain a
normal-looking poke() call in the blitting functions in the
end. 🤯
Yeah… the result is
batshitinsane.
I may have gone too far in a few places…
One might certainly argue that all these ridiculous decompilations
actually hurt the preservation angle of this project. "Clearly, ZUN
couldn't have possibly written such unreasonable C++ code.
So why pretend he did, and not just keep it all in its more natural ASM
form?" Well, there are several reasons:
Future port authors will merely have to translate all the
pseudo-registers and inline assembly to C++. For the former, this is
typically as easy as replacing them with newly declared local variables. No
need to bother with function prolog and epilog code, calling conventions, or
the build system.
No duplication of constants and structures in ASM land.
As a more expressive language, C++ can document the code much better.
Meticulous documentation seems to have become the main attraction of ReC98
these days – I've seen it appreciated quite a number of times, and the
continued financial support of all the backers speaks volumes. Mods, on the
other hand, are still a rather rare sight.
Having as few .ASM files in the source tree as possible looks better to
casual visitors who just look at GitHub's repo language breakdown. This way,
ReC98 will also turn from an "Assembly project" to its rightful state
of "C++ project" much sooner.
And finally, it's not like the ASM versions are
gone – they're still part of the Git history.
Unfortunately, these pushes also demonstrated a second disadvantage in
trying to decompile everything possible: Since Turbo C++ lacks TASM's
fine-grained ability to enforce code alignment on certain multiples of
bytes, it might actually be unfeasible to link in a C-compiled object file
at its intended original position in some of the .EXE files it's used in.
Which… you're only going to notice once you encounter such a case. Due to
the slightly jumbled order of functions in the
📝 second, shared code segment, that might
be long after you decompiled and successfully linked in the function
everywhere else.
And then you'll have to throw away that decompilation after all 😕 Oh
well. In this specific case (the lookup table generator for horizontally
flipping images), that decompilation was a mess anyway, and probably
helped nobody. I could have added a dummy .OBJ that does nothing but
enforce the needed 2-byte alignment before the function if I
really insisted on keeping the C version, but it really wasn't
worth it.
Now that I've also described yet another meta-issue, maybe there'll
really be nothing to say about the next technical debt pushes?
Next up though: Back to actual progress
again, with TH01. Which maybe even ends up pushing that game over the 50%
RE mark?
So, TH05 OP.EXE. The first half of this push started out
nicely, with an easy decompilation of the entire player character
selection menu. Typical ZUN quality, with not much to say about it. While
the overall function structure is identical to its TH04 counterpart, the
two games only really share small snippets inside these functions, and do
need to be RE'd separately.
The high score viewing (not registration) menu would have been next.
Unfortunately, it calls one of the GENSOU.SCR loading
functions… which are all a complete mess that still needed to be sorted
out first. 5 distinct functions in 6 binaries, and of course TH05 also
micro-optimized its MAIN.EXE version to directly use the DOS
INT 21h file loading API instead of master.lib's wrappers.
Could have all been avoided with a single method on the score data
structure, taking a player character ID and a difficulty level as
parameters…
So, no score menu in this push then. Looking at the other end of the ASM
code though, we find the starting functions for the main game, the Extra
Stage, and the demo replays, which did fit perfectly to round out
this push.
Which is where we find an easter egg! 🥚 If you've ever looked into
怪綺談2.DAT, you might have noticed 6 .REC files
with replays for the Demo Play mode. However, the game only ever seems to
cycle between 4 replays. So what's in the other two, and why are they
40 KB instead of just 10 KB like the others? Turns out that they
combine into a full Extra Stage Clear replay with Mima, with 3 bombs and 1
death, obviously recorded by ZUN himself. The split into two files for the
stage (DEMO4.REC) and boss (DEMO5.REC) portion is
merely an attempt to limit the amount of simultaneously allocated heap
memory.
To watch this replay without modding the game, unlock the Extra Stage with
all 4 characters, then hold both the ⬅️ left and ➡️ right arrow keys in the
main menu while waiting for the usual demo replay.
I can't possibly be the first one to discover this, but I couldn't find
any other mention of it. Edit (2021-03-15): ZUN did in fact document this replay
in Section 6 of TH05's OMAKE.TXT, along with the exact method
to view it.
Thanks
to Popfan for the discovery!
Here's a recording of the whole replay:
Note how the boss dialogue is skipped. MAIN.EXE actually
contains no less than 6 if() branches just to distinguish
this overly long replay from the regular ones.
I'd really like to do the TH04 and TH05 main menus in parallel, since we
can expect a bit more shared code after all the initial differences.
Therefore, I'm going to put the next "anything" push towards covering the
TH04 version of those functions. Next up though, it's back to TH01, with
more redundant image format code…
🎉 TH05 is finally fully position-independent! 🎉 To celebrate this
milestone, -Tom- coded a little demo, which we recorded on
both an emulator and on real PC-98 hardware:
You can now freely add or remove both data and code anywhere in TH05, by
editing the ReC98 codebase, writing your mod in ASM or C/C++, and
recompiling the code. Since all absolute memory addresses have now been
converted to labels, this will work without causing any instability. See
the position independence section in the FAQ
for a more thorough explanation about why this was a problem.
By extension, this also means that it's now theoretically possible
to use a different compiler on the source code. But:
What does this not mean?
The original ZUN code hasn't been completely reverse-engineered yet, let
alone decompiled. As the final PC-98 Touhou game, TH05 also happens to
have the largest amount of actual ZUN-written ASM that can't ever
be decompiled within ReC98's constraints of a legit source code
reconstruction. But a lot of the originally-in-C code is also still in
ASM, which might make modding a bit inconvenient right now. And while I
have decompiled a bunch of functions, I selected them largely
because they would help with PI (as requested by the backers), and not
because they are particularly relevant to typical modding interests.
As a result, the code might also be a bit confusingly organized. There's
quite a conflict between various goals there: On the one hand, I'd like to
only have a single instance of every function shared with earlier games,
as well as reduce ZUN's code duplication within a single game. On the
other hand, this leads to quite a lot of code being scattered all over the
place and then #include-pasted back together, except for the
places where
📝 this doesn't work, and you'd have to use multiple translation units anyway…
I'm only beginning to figure out the best structure here, and some more
reverse-engineering attention surely won't hurt.
Also, keep in mind that the code still targets x86 Real Mode. To work
effectively in this codebase, you'd need some familiarity with
memory
segmentation, and how to express it all in code. This tends to make
even regular C++ development about an order of magnitude harder,
especially once you want to interface with the remaining ASM code. That
part made -Tom- struggle quite a bit with implementing his
custom scripting language for the demo above. For now, he built that demo
on quite a limited foundation – which is why he also chose to release
neither the build nor the source publically for the time being.
So yeah, you're definitely going to need the TASM and Borland C++ manuals
there.
tl;dr: We now know everything about this game's data, but not quite
as much about this game's code.
So, how long until source ports become a realistic project?
You probably want to wait for 100% RE, which is when everything
that can be decompiled has been decompiled.
Unless your target system is 16-bit Windows, in which case you could
theoretically start right away. 📝 Again,
this would be the ideal first system to port PC-98 Touhou to: It would
require all the generic portability work to remove the dependency on PC-98
hardware, thus paving the way for a subsequent port to modern systems,
yet you could still just drop in any undecompiled ASM.
Porting to IBM-compatible DOS would only be a harder and less universally
useful version of that. You'd then simply exchange one architecture, with
its idiosyncrasies and limits, for another, with its own set of
idiosyncrasies and limits. (Unless, of course, you already happen to be
intimately familiar with that architecture.) The fact that master.lib
provides DOS/V support would have only mattered if ZUN consistently used
it to abstract away PC-98 hardware at every single place in the code,
which is definitely not the case.
The list of actually interesting findings in this push is,
📝 again, very short. Probably the most
notable discovery: The low-level part of the code that renders Marisa's
laser from her TH04 Illusion Laser shot type is still present in
TH05. Insert wild mass guessing about potential beta version shot types…
Oh, and did you know that the order of background images in the Extra
Stage staff roll differs by character?
Next up: Finally driving up the RE% bar again, by decompiling some TH05
main menu code.
Wouldn't it be a bit disappointing to have TH05 completely
position-independent, but have it still require hex-editing of the
original ZUN.COM to mod its gaiji characters? As in, these
custom "text" glyphs, available to the PC-98 text RAM:
Especially since we now even have a sprite converter… the lack of which
was exactly 📝 what made rebuilding ZUN.COM not that worthwhile before.
So, before the big release, let's get all the remaining
ZUN.COM sub-binaries of TH04 and TH05 dumped into .ASM files,
and re-assembled and linked during the build process.
This is also the moment in which Egor's 2018
reimplementation of O. Morikawa's comcstm finally gets
to shine. Back then, I considered it too early to even bother with
ZUN.COM and reimplementing the .COM wrapper that ZUN
originally used to bundle multiple smaller executables into that single
binary. But now that the time is right, it is nice to have that
code, as it allowed me to get these rebuilds done in half a push.
Otherwise, it would have surely required one or two dedicated ones.
Since we like correctness here, newly dumped ZUN code means that it also
has to be included in the RE%
baseline calculation. This is why TH04's and TH05's overall RE% bars
have gone back a tiny bit… in case you remember how they previously looked
like After all, I would like to figure
out where all that memory allocated during TH04's and TH05's memory check
is freed, if at all.
Alright, one half of a push left… Y'know, getting rid of those last few PI
false positives is actually one of the most annoying chores in this
project, and quite stressful as well: I have to convince myself that the
remaining false positives are, in fact, not memory references, but with
way too little time for in-depth RE and to denote what they are
instead. In that situation, everyone (including myself!)
is anticipating that PI goal, and no one is really interested in RE.
(Well… that is, until they actually get to developing their mod. But more
on that tomorrow. ) Which means that it boils
down to quite some hasty, dumb, and superficial RE around those remaining
numbers.
So, in the hope of making it less annoying for the other 4 games in the
future, let's systematically cover the sources of those remaining false
positives in TH05, over all games. I/O port accesses with either the port
or the value in registers (and thus, no longer as an immediate argument to
the IN or OUT instructions, which the PI counter
can clearly ingore), palette color arithmetic, or heck, 0xFF constants that
obviously just mean "-1" and are not a reference to offset 0xFF in
the data segment. All of this, of course, once again had a way bigger
effect on everything but an almost position-independent TH05… but
hey, that's the sort of thing you reserve the "anything" pushes for. And
that's also how we get some of the single biggest PI% gains we have seen
so far, and will be seeing before the 100% PI mark. And yes, those will
continue in the next push.
Finally, after a long while, we've got two pushes with barely anything to
talk about! Continuing the road towards 100% PI for TH05, these were
exactly the two pushes that TH05 MAINE.EXE PI was estimated
to additionally cost, relative to TH04's. Consequently, they mostly went
to TH05's unique data structures in the ending cutscenes, the score name
registration menu, and the
staff roll.
A unique feature in there is TH05's support for automatic text color
changes in its ending scripts, based on the first full-width Shift-JIS
codepoint in a line. The \c=codepoint,color
commands at the top of the _ED??.TXT set up exactly this
codepoint→color mapping. As far as I can tell, TH05 is the only Touhou
game with a feature like this – even the Windows Touhou games went back to
manually spelling out each color change.
The orb particles in TH05's staff roll also try to be a bit unique by
using 32-bit X and Y subpixel variables for their current position. With
still just 4 fractional bits, I can't really tell yet whether the extended
range was actually necessary. Maybe due to how the "camera scrolling"
through "space" was implemented? All other entities were pretty much the
usual fare, though.
12.4, 4.4, and now a 28.4 fixed-point format… yup,
📝 C++ templates were
definitely the right choice.
At the end of its staff roll, TH05 not only displays
the usual performance
verdict, but then scrolls in the scores at the end of each stage
before switching to the high score menu. The simplest way to smoothly
scroll between two full screens on a PC-98 involves a separate bitmap…
which is exactly what TH05 does here, reserving 28,160 bytes of its global
data segment for just one overly large monochrome 320×704 bitmap where
both the screens are rendered to. That's… one benefit of splitting your
game into multiple executables, I guess?
Not sure if it's common knowledge that you can actually scroll back and
forth between the two screens with the Up and Down keys before moving to
the score menu. I surely didn't know that before. But it makes sense –
might as well get the most out of that memory.
The necessary groundwork for all of this may have actually made
TH04's (yes, TH04's) MAINE.EXE technically
position-independent. Didn't quite reach the same goal for TH05's – but
what we did reach is ⅔ of all PC-98 Touhou code now being
position-independent! Next up: Celebrating even more milestones, as
-Tom- is about to finish development on his TH05
MAIN.EXE PI demo…
Only one newly ordered push since I've reopened the store? Great, that's
all the justification I needed for the extended maintenance delay that was
part of these two pushes 😛
Having to write comments to explain whether coordinates are relative to
the top-left corner of the screen or the top-left corner of the playfield
has finally become old. So, I introduced
distinct
types for all the coordinate systems we typically encounter, applying
them to all code decompiled so far. Note how the planar nature of PC-98
VRAM meant that X and Y coordinates also had to be different from each
other. On the X side, there's mainly the distinction between the
[0; 640] screen space and the corresponding [0; 80] VRAM byte
space. On the Y side, we also have the [0; 400] screen space, but
the visible area of VRAM might be limited to [0; 200] when running in
the PC-98's line-doubled 640×200 mode. A VRAM Y coordinate also always
implies an added offset for vertical scrolling.
During all of the code reconstruction, these types can only have a
documenting purpose. Turning them into anything more than just
typedefs to int, in order to define conversion
operators between them, simply won't recompile into identical binaries.
Modding and porting projects, however, now have a nice foundation for
doing just that, and can entirely lift coordinate system transformations
into the type system, without having to proofread all the meaningless
int declarations themselves.
So, what was left in terms of memory references? EX-Alice's fire waves
were our final unknown entity that can collide with the player. Decently
implemented, with little to say about them.
That left the bomb animation structures as the one big remaining PI
blocker. They started out nice and simple in TH04, with a small 6-byte
star animation structure used for both Reimu and Marisa. TH05, however,
gave each character her own animation… and what the hell is going
on with Reimu's blue stars there? Nope, not going to figure this out on
ASM level.
A decompilation first required some more bomb-related variables to be
named though. Since this was part of a generic RE push, it made sense to
do this in all 5 games… which then led to nice PI gains in anything
but TH05. Most notably, we now got the
"pulling all items to player" flag in TH04 and TH05, which is
actually separate from bombing. The obvious cheat mod is left as an
exercise to the reader.
So, TH05 bomb animations. Just like the
📝 custom entity types of this game, all 4
characters share the same memory, with the superficially same 10-byte
structure.
But let's just look at the very first field. Seen from a low level, it's a
simple struct { int x, y; } pos, storing the current position
of the character-specific bomb animation entity. But all 4 characters use
this field differently:
For Reimu's blue stars, it's the top-left position of each star, in the
12.4 fixed-point format. But unlike the vast majority of these values in
TH04 and TH05, it's relative to the top-left corner of the
screen, not the playfield. Much better represented as
struct { Subpixel screen_x, screen_y; } topleft.
For Marisa's lasers, it's the center of each circle, as a regular 12.4
fixed-point coordinate, relative to the top-left corner of the playfield.
Much better represented as
struct { Subpixel x, y; } center.
For Mima's shrinking circles, it's the center of each circle in regular
pixel coordinates. Much better represented as
struct { screen_x_t x; screen_y_t y; } center.
For Yuuka's spinning heart, it's the top-left corner in regular pixel
coordinates. Much better represented as
struct { screen_x_t x; screen_y_t y; } topleft.
And yes, singular. The game is actually smart enough to only store a single
heart, and then create the rest of the circle on the fly. (If it were even
smarter, it wouldn't even use this structure member, but oh well.)
Therefore, I decompiled it as 4 separate structures once again, bundled
into an union of arrays.
As for Reimu… yup, that's some pointer arithmetic straight out of
Jigoku* for setting and updating the positions of the falling star
trails. While that certainly required several
comments to wrap my head around the current array positions, the one "bug"
in all this arithmetic luckily has no effect on the game.
There is a small glitch with the growing circles, though. They are
spawned at the end of the loop, with their position taken from the star
pointer… but after that pointer has already been incremented. On
the last loop iteration, this leads to an out-of-bounds structure access,
with the position taken from some unknown EX-Alice data, which is 0 during
most of the game. If you look at the animation, you can easily spot these
bugged circles, consistently growing from the top-left corner (0, 0)
of the playfield:
After all that, there was barely enough remaining time to filter out and
label the final few memory references. But now, TH05's
MAIN.EXE is technically position-independent! 🎉
-Tom- is going to work on a pretty extensive demo of this
unprecedented level of efficient Touhou game modding. For a more impactful
effect of both the 100% PI mark and that demo, I'll be delaying the push
covering the remaining false positives in that binary until that demo is
done. I've accumulated a pretty huge backlog of minor maintenance issues
by now…
Next up though: The first part of the long-awaited build system
improvements. I've finally come up with a way of sanely accelerating the
32-bit build part on most setups you could possibly want to build ReC98
on, without making the building experience worse for the other few setups.
… and just as I explained 📝 in the last post
how decompilation is typically more sensible and efficient than ASM-level
reverse-engineering, we have this push demonstrating a counter-example.
The reason why the background particles and lines in the Shinki and
EX-Alice battles contributed so much to position dependence was simply
because they're accessed in a relatively large amount of functions, one
for each different animation. Too many to spend the remaining precious
crowdfunded time on reverse-engineering or even decompiling them all,
especially now that everyone anticipates 100% PI for TH05's
MAIN.EXE.
Therefore, I only decompiled the two functions of the line structure that
also demonstrate best how it works, which in turn also helped with RE.
Sadly, this revealed that we actually can't📝 overload operator =() to get
that nice assignment syntax for 12.4 fixed-point values, because one of
those new functions relies on Turbo C++'s built-in optimizations for
trivially copyable structures. Still, impressive that this abstraction
caused no other issues for almost one year.
As for the structures themselves… nope, nothing to criticize this time!
Sure, one good particle system would have been awesome, instead of having
separate structures for the Stage 2 "starfield" particles and the one used
in Shinki's battle, with hardcoded animations for both. But given the
game's short development time, that was quite an acceptable compromise,
I'd say.
And as for the lines, there just has to be a reason why the game
reserves 20 lines per set, but only renders lines #0, #6, #12, and #18.
We'll probably see once we get to look at those animation functions more
closely.
This was quite a 📝 TH03-style RE push,
which yielded way more PI% than RE%. But now that that's done, I can
finally not get distracted by all that stuff when looking at the
list of remaining memory references. Next up: The last few missing
structures in TH05's MAIN.EXE!
Back to TH05! Thanks to the good funding situation, I can strike a nice
balance between getting TH05 position-independent as quickly as possible,
and properly reverse-engineering some missing important parts of the game.
Once 100% PI will get the attention of modders, the code will then be in
better shape, and a bit more usable than if I just rushed that goal.
By now, I'm apparently also pretty spoiled by TH01's immediate
decompilability, after having worked on that game for so long.
Reverse-engineering in ASM land is pretty annoying, after all,
since it basically boils down to meticulously editing a piece of ASM into
something I can confidently call "reverse-engineered". Most of the
time, simply decompiling that piece of code would take just a little bit
longer, but be massively more useful. So, I immediately tried decompiling
with TH05… and it just worked, at every place I tried!? Whatever the issue
was that made 📝 segment splitting so
annoying at my first attempt, I seem to have completely solved it in the
meantime. 🤷 So yeah, backers can now request pretty much any part of TH04
and TH05 to be decompiled immediately, with no additional segment
splitting cost.
(Protip for everyone interested in starting their own ReC project: Just
declare one segment per function, right from the start, then group them
together to restore the original code segmentation…)
Except that TH05 then just throws more of its infamous micro-optimized and
undecompilable ASM at you. 🙄 This push covered the function that adjusts
the bullet group template based on rank and the selected difficulty,
called every time such a group is configured. Which, just like pretty
much all of TH05's bullet spawning code, is one of those undecompilable
functions. If C allowed labels of other functions as goto
targets, it might have been decompilable into something useful to
modders… maybe. But like this, there's no point in even trying.
This is such a terrible idea from a software architecture point of view, I
can't even. Because now, you suddenly have to mirror your C++
declarations in ASM land, and keep them in sync with each other. I'm
always happy when I get to delete an ASM declaration from the codebase
once I've decompiled all the instances where it was referenced. But for
TH05, we now have to keep those declarations around forever. 😕 And all
that for a performance increase you probably couldn't even measure. Oh
well, pulling off Galaxy Brain-level ASM optimizations is kind of
fun if you don't have portability plans… I guess?
If I started a full fangame mod of a PC-98 Touhou game, I'd base it on
TH04 rather than TH05, and backport selected features from TH05 as
needed. Just because it was released later doesn't make it better, and
this is by far not the only one of ZUN's micro-optimizations that just
went way too far.
Dropping down to ASM also makes it easier to introduce weird quirks.
Decompiled, one of TH05's tuning conditions for
stack
groups on Easy Mode would look something like:
case BP_STACK:
// […]
if(spread_angle_delta >= 2) {
stack_bullet_count--;
}
The fields of the bullet group template aren't typically reset when
setting up a new group. So, spread_angle_delta in the context
of a stack group effectively refers to "the delta angle of the last
spread group that was fired before this stack – whenever that was".
uth05win also spotted this quirk, considered it a bug, and wrote
fanfiction by changing spread_angle_delta to
stack_bullet_count.
As usual for functions that occur in more than one game, I also decompiled
the TH04 bullet group tuning function, and it's perfectly sane, with no
such quirks.
In the more PI-focused parts of this push, we got the TH05-exclusive
smooth boss movement functions, for flying randomly or towards a given
point. Pretty unspectacular for the most part, but we've got yet another
uth05win inconsistency in the latter one. Once the Y coordinate gets close
enough to the target point, it actually speeds up twice as much as the
X coordinate would, whereas uth05win used the same speedup factors for
both. This might make uth05win a couple of frames slower in all boss
fights from Stage 3 on. Hard to measure though – and boss movement partly
depends on RNG anyway.
Next up: Shinki's background animations – which are actually the single
biggest source of position dependence left in TH05.
As expected, we've now got the TH04 and TH05 stage enemy structure,
finishing position independence for all big entity types. This one was
quite straightfoward, as the .STD scripting system is pretty simple.
Its most interesting aspect can be found in the way timing is handled. In
Windows Touhou, all .ECL script instructions come with a frame field that
defines when they are executed. In TH04's and TH05's .STD scripts, on the
other hand, it's up to each individual instruction to add a frame time
parameter, anywhere in its parameter list. This frame time defines for how
long this instruction should be repeatedly executed, before it manually
advances the instruction pointer to the next one. From what I've seen so
far, these instruction typically apply their effect on the first frame
they run on, and then do nothing for the remaining frames.
Oh, and you can't nest the LOOP instruction, since the enemy
structure only stores one single counter for the current loop iteration.
Just from the structure, the only innovation introduced by TH05 seems to
have been enemy subtypes. These can be used to parametrize scripts via
conditional jumps based on this value, as a first attempt at cutting down
the need to duplicate entire scripts for similar enemy behavior. And
thanks to TH05's favorable segment layout, this game's version of the
.STD enemy script interpreter is even immediately ready for decompilation,
in one single future push.
As far as I can tell, that now only leaves
.MPN file loading
player bomb animations
some structures specific to the Shinki and EX-Alice battles
plus some smaller things I've missed over the years
until TH05's MAIN.EXE is completely position-independent.
Which, however, won't be all it needs for that 100% PI rating on the front
page. And with that many false positives, it's quite easy to get lost with
immediately reverse-engineering everything around them. This time, the
rendering of the text dissolve circles, used for the stage and BGM title
popups, caught my eye… and since the high-level code to handle all of
that was near the end of a segment in both TH04 and TH05, I just decided
to immediately decompile it all. Like, how hard could it possibly be?
Sure, it needed another segment split, which was a bit harder due
to all the existing ASM referencing code in that segment, but certainly
not impossible…
Oh wait, this code depends on 9 other sets of identifiers that haven't
been declared in C land before, some of which require vast reorganizations
to bring them up to current consistency standards. Whoops! Good thing that
this is the part of the project I'm still offering for free…
Among the referenced functions was tiles_invalidate_around(),
which marks the stage background tiles within a rectangular area to be
redrawn this frame. And this one must have had the hardest function
signature to figure out in all of PC-98 Touhou, because it actually
seems impossible. Looking at all the ways the game passes the center
coordinate to this function, we have
X and Y as 16-bit integer literals, merged into a single
PUSH of a 32-bit immediate
X and Y calculated and pushed independently from each other
by-value copies of entire Point instances
Any single declaration would only lead to at most two of the three cases
generating the original instructions. No way around separately declaring
the function in every translation unit then, with the correct parameter
list for the respective calls. That's how ZUN must have also written it.
Oh well, we would have needed to do all of this some time. At least
there were quite a bit of insights to be gained from the actual
decompilation, where using const references actually made it
possible to turn quite a number of potentially ugly macros into wholesome
inline functions.
But still, TH04 and TH05 will come out of ReC98's decompilation as one big
mess. A lot of further manual decompilation and refactoring, beyond the
limits of the original binary, would be needed to make these games
portable to any non-PC-98, non-x86 architecture.
And yes, that includes IBM-compatible DOS – which, for some reason, a
number of people see as the obvious choice for a first system to port
PC-98 Touhou to. This will barely be easier. Sure, you'll save the effort
of decompiling all the remaining original ASM. But even with
master.lib's MASTER_DOSV setting, these games still very much
rely on PC-98 hardware, with corresponding assumptions all over ZUN's
code. You will need to provide abstractions for the PC-98's
superimposed text mode, the gaiji, and planar 4-bit color access in
general, exchanging the use of the PC-98's GRCG and EGC blitter chips with
something else. At that point, you might as well port the game to one
generic 640×400 framebuffer and away from the constraints of DOS,
resulting in that Doom source code-like situation which made that
game easily portable to every architecture to begin with. But ZUN just
wasn't a John Carmack, sorry.
Or what do I know. I've never programmed for IBM-compatible DOS, but maybe
ReC98's audience does include someone who is intimately familiar
with IBM-compatible DOS so that the constraints aren't much of an issue
for them? But even then, 16-bit Windows would make much more sense
as a first porting target if you don't want to bother with that
undecompilable ASM.
At least I won't have to look at TH04 and TH05 for quite a while now.
The delivery delays have made it obvious that
my life has become pretty busy again, probably until September. With a
total of 9 TH01 pushes from monthly subscriptions now waiting in the
backlog, the shop will stay closed until I've caught up with most of
these. Which I'm quite hyped for!
Alright, the score popup numbers shown when collecting items or defeating
(mid)bosses. The second-to-last remaining big entity type in TH05… with
quite some PI false positives in the memory range occupied by its data.
Good thing I still got some outstanding generic RE pushes that haven't
been claimed for anything more specific in over a month! These
conveniently allowed me to RE most of these functions right away, the
right way.
Most of the false positives were boss HP values, passed to a "boss phase
end" function which sets the HP value at which the next phase should end.
Stage 6 Yuuka, Mugetsu, and EX-Alice have their own copies of this
function, in which they also reset certain boss-specific global variables.
Since I always like to cover all varieties of such duplicated functions at
once, it made sense to reverse-engineer all the involved variables while I
was at it… and that's why this was exactly the right time to cover the
implementation details of Stage 6 Yuuka's parasol and vanishing animations
in TH04.
With still a bit of time left in that RE push afterwards, I could also
start looking into some of the smaller functions that didn't quite fit
into other pushes. The most notable one there was a simple function that
aims from any point to the current player position. Which actually only
became a separate function in TH05, probably since it's called 27 times in
total. That's 27 places no longer being blocked from further RE progress.
WindowsTiger already
did most of the work for the score popup numbers in January, which meant
that I only had to review it and bring it up to ReC98's current coding
styles and standards. This one turned out to be one of those rare features
whose TH05 implementation is significantly less insane than the
TH04 one. Both games lazily redraw only the tiles of the stage background
that were drawn over in the previous frame, and try their best to minimize
the amount of tiles to be redrawn in this way. For these popup numbers,
this involves calculating the on-screen width, based on the exact number
of digits in the point value. TH04 calculates this width every frame
during the rendering function, and even resorts to setting that field
through the digit iteration pointer via self-modifying code… yup. TH05, on
the other hand, simply calculates the width once when spawning a new popup
number, during the conversion of the point value to
binary-coded
decimal. The "×2" multiplier suffix being removed in TH05 certainly
also helped in simplifying that feature in this game.
And that's ⅓ of TH05 reverse-engineered! Next up, one more TH05 PI push,
in which the stage enemies hopefully finish all the big entity types.
Maybe it will also be accompanied by another RE push? In any case, that
will be the last piece of TH05 progress for quite some time. The next TH01
stretch will consist of 6 pushes at the very least, and I currently have
no idea of how much time I can spend on ReC98 a month from now…
Wait, PI for FUUIN.EXE is mainly blocked by the high score
menu? That one should really be properly decompiled in a separate
RE push, since it's also present in largely identical form in
REIIDEN.EXE… but I currently lack the explicit funding to do
that.
And as it turns out, I shouldn't really capture any of the existing generic
RE contributions for it either. Back in 2018 when I ran the crowdfunding
on the Touhou Patch Center Discord server, I said that generic RE
contributions would never go towards TH01. No one was interested in that
game back then, and as it's significantly different from all the other
games, it made sense to only cover it if explicitly requested.
As Touhou Patch Center still remains one of the biggest supporters and
advertisers for ReC98, someone recently believed that this rule was still
in effect, despite not being mentioned anywhere on this website.
Fast forward to today, and TH01 has become the single most supported game
lately, with plenty of incomplete pushes still open to be completed.
Reverse-engineering it has proven to be quite efficient, yielding lots of
completion percentage points per push. This, I suppose, is exactly what
backers that don't give any specific priorities are mainly interested in.
Therefore, I will allocate future partial
contributions to TH01, whenever it makes sense.
So, instead of rushing TH01 PI, let's wait for Ember2528's
April subscription, and get the 25% total RE milestone with some TH05 PI
progress instead. This one primarily focused on the gather circles
(spirals…?), the third-last missing entity type in TH05. These are
rendered using the same 8×8 pellet sprite introduced in TH02… except that
the actual pellets received a darkened bottom part in TH04
.
Which, in turn, is actually rendered quite efficiently – the games first
render the top white part of all pellets, followed by the bottom gray part
of all pellets. The PC-98 GRCG is used throughout the process, doing its
typical job of accelerating monochrome blitting, and by arranging the
rendering like this, only two GRCG color changes are required to draw any
number of pellets. I guess that makes it quite a worthwhile
optimization? Don't ask me for specific performance numbers or even saved
cycles, though
To finish this TH05 stretch, we've got a feature that's exclusive to TH05
for once! As the final memory management innovation in PC-98 Touhou, TH05
provides a single static (64 * 26)-byte array for storing up to 64
entities of a custom type, specific to a stage or boss portion.
(Edit (2023-05-29): This system actually debuted in
📝 TH04, where it was used for much simpler
entities.)
TH05 uses this array for
the Stage 2 star particles,
Alice's puppets,
the tip of curve ("jello") bullets,
Mai's snowballs and Yuki's fireballs,
Yumeko's swords,
and Shinki's 32×32 bullets,
which makes sense, given that only one of those will be active at any
given time.
On the surface, they all appear to share the same 26-byte structure, with
consistently sized fields, merely using its 5 generic fields for different
purposes. Looking closer though, there actually are differences in
the signedness of certain fields across the six types. uth05win chose to
declare them as entirely separate structures, and given all the semantic
differences (pixels vs. subpixels, regular vs. tiny master.lib sprites,
…), it made sense to do the same in ReC98. It quickly turned out to be the
only solution to meet my own standards of code readability.
Which blew this one up to two pushes once again… But now, modders can
trivially resize any of those structures without affecting the other types
within the original (64 * 26)-byte boundary, even without full position
independence. While you'd still have to reduce the type-specific
number of distinct entities if you made any structure larger, you
could also have more entities with fewer structure members.
As for the types themselves, they're full of redundancy once again – as
you might have already expected from seeing #4, #5, and #6 listed as
unrelated to each other. Those could have indeed been merged into a single
32×32 bullet type, supporting all the unique properties of #4
(destructible, with optional revenge bullets), #5 (optional number of
twirl animation frames before they begin to move) and #6 (delay clouds).
The *_add(), *_update(), and *_render()
functions of #5 and #6 could even already be completely
reverse-engineered from just applying the structure onto the ASM, with the
ones of #3 and #4 only needing one more RE push.
But perhaps the most interesting discovery here is in the curve bullets:
TH05 only renders every second one of the 17 nodes in a curve
bullet, yet hit-tests every single one of them. In practice, this is an
acceptable optimization though – you only start to notice jagged edges and
gaps between the fragments once their speed exceeds roughly 11 pixels per
second:
And that brings us to the last 20% of TH05 position independence! But
first, we'll have more cheap and fast TH01 progress.
Well, that took twice as long as I thought, with the two pushes containing
a lot more maintenance than actual new research. Spending some time
improving both field names and types in
32th System's
TH03 resident structure finally gives us all of those
structures. Which means that we can now cover all the remaining
decompilable ZUN.COM parts at once…
Oh wait, their main() functions have stayed largely identical
since TH02? Time to clean up and separate that first, then… and combine
two recent code generation observations into the solution to a
decompilation puzzle from 4½ years ago. Alright, time to decomp-
Oh wait, we'd kinda like to properly RE all the code in TH03-TH05
that deals with loading and saving .CFG files. Almost every outside
contributor wanted to grab this supposedly low-hanging fruit a lot
earlier, but (of course) always just for a single game, while missing how
the format evolved.
So, ZUN.COM. For some reason, people seem to consider it
particularly important, even though it contains neither any game logic nor
any code specific to PC-98 hardware… All that this decompilable part does
is to initialize a game's .CFG file, allocate an empty resident structure
using master.lib functions, release it after you quit the game,
error-check all that, and print some playful messages~ (OK, TH05's also
directly fills the resident structure with all data from
MIKO.CFG, which all the other games do in OP.EXE.)
At least modders can now freely change and extend all the resident
structures, as well as the .CFG files? And translators can translate those
messages that you won't see on a decently fast emulator anyway? Have fun,
I guess 🤷
And you can in fact do this right now – even for TH04 and TH05,
whose ZUN.COM currently isn't rebuilt by ReC98. There is
actually a rather involved reason for this:
One of the missing files is TH05's GJINIT.COM.
Which contains all of TH05's gaiji characters in hardcoded 1bpp form,
together with a bit of ASM for writing them to the PC-98's hardware gaiji
RAM
Which means we'd ideally first like to have a sprite compiler, for
all the hardcoded 1bpp sprites
Which must compile to an ASM slice in the meantime, but should also
output directly to an OMF .OBJ file (for performance now), as well as to C
code (for portability later)
Which I won't put in as long as the backlog contains actual
progress to drive up the percentages on the front page.
So yeah, no meaningful RE and PI progress at any of these levels. Heck,
even as a modder, you can just replace the zun zun_res
(TH02), zun -5 (TH03), or zun -s (TH04/TH05)
calls in GAME.BAT with a direct call to your modified
*RES*.COM. And with the alternative being "manually typing 0 and 1
bits into a text file", editing the sprites in TH05's
GJINIT.COM is way more comfortable in a binary sprite editor
anyway.
For me though, the best part in all of this was that it finally made sense
to throw out the old Borland C++ run-time assembly slices 🗑 This giant
waste of time
became obvious 5 years ago, but any ASM dump of a .COM
file would have needed rather ugly workarounds without those slices. Now
that all .COM binaries that were originally written in C are
compiled from C, we can all enjoy slightly faster grepping over the entire
repository, which now has 229 fewer files. Productivity will skyrocket!
Next up: Three weeks of almost full-time ReC98 work! Two more PI-focused
pushes to finish this TH05 stretch first, before switching priorities to
TH01 again.
Long time no see! And this is exactly why I've been procrastinating
bullets while there was still meaningful progress to be had in other parts
of TH04 and TH05: There was bound to be quite some complexity in this most
central piece of game logic, and so I couldn't possibly get to a
satisfying understanding in just one push.
Or in two, because their rendering involves another bunch of
micro-optimized functions adapted from master.lib.
Or in three, because we'd like to actually name all the bullet sprites,
since there are a number of sprite ID-related conditional branches. And
so, I was refining things I supposedly RE'd in the the commits from the
first push until the very end of the fourth.
When we talk about "bullets" in TH04 and TH05, we mean just two things:
the white 8×8 pellets, with a cap of 240 in TH04 and 180 in TH05, and any
16×16 sprites from MIKO16.BFT, with a cap of 200 in TH04 and
220 in TH05. These are by far the most common types of… err, "things the
player can collide with", and so ZUN provides a whole bunch of pre-made
motion, animation, and
n-way spread / ring / stack group options for those, which can be
selected by simply setting a few fields in the bullet template. All the
other "non-bullets" have to be fired and controlled individually.
Which is nothing new, since uth05win covered this part pretty accurately –
I don't think anyone could just make up these structure member
overloads. The interesting insights here all come from applying this
research to TH04, and figuring out its differences compared to TH05. The
most notable one there is in the default groups: TH05 allows you to add
a stack
to any single bullet, n-way spread or ring, but TH04 only lets you create
stacks separately from n-way spreads and rings, and thus gets by with
fewer fields in its bullet template structure. On the other hand, TH04 has
a separate "n-way spread with random angles, yet still aimed at the
player" group? Which seems to be unused, at least as far as
midbosses and bosses are concerned; can't say anything about stage enemies
yet.
In fact, TH05's larger bullet template structure illustrates that these
distinct group types actually are a rather redundant piece of
over-engineering. You can perfectly indicate any permutation of the basic
groups through just the stack bullet count (1 = no stack), spread bullet
count (1 = no spread), and spread delta angle (0 = ring instead of
spread). Add a 4-flag bitfield to cover the rest (aim to player, randomize
angle, randomize speed, force single bullet regardless of difficulty or
rank), and the result would be less redundant and even slightly
more capable.
Even those 4 pushes didn't quite finish all of the bullet-related types,
stopping just shy of the most trivial and consistent enum that defines
special movement. This also left us in a
📝 TH03-like situation, in which we're still
a bit away from actually converting all this research into actual RE%. Oh
well, at least this got us way past 50% in overall position independence.
On to the second half! 🎉
For the next push though, we'll first have a quick detour to the remaining
C code of all the ZUN.COM binaries. Now that the
📝 TH04 and TH05 resident structures no
longer block those, -Tom- has requested TH05's
RES_KSO.COM to be covered in one of his outstanding pushes.
And since 32th System
recently RE'd TH03's resident structure, it makes sense to also review and
merge that, before decompiling all three remaining RES_*.COM
binaries in hopefully a single push. It might even get done faster than
that, in which case I'll then review and merge some more of
WindowsTiger's
research.
A~nd resident structures ended up being exactly
the right thing to start off the new year with.
WindowsTiger and
spaztron64 have already been
pushing for them with their own reverse-engineering, and together with my
own recent GENSOU.SCR RE work, we've clarified just enough
context around the harder-to-explain values to make both TH04's and TH05's
structures fit nicely into the typical time frame of a single push.
With all the apparently obvious and seemingly just duplicated values, it
has always been easy to do a superficial job for most of the structure,
then lose motivation for the last few unknown fields. Pretty glad to got
this finally covered; I've heard that people are going to write trainer
tools now?
Also, where better to slot in a push that, in terms of figures, seems to
deliver 0% RE and only miniscule PI progress, than at the end of
Touhou Patch Center's 5-push order that already had multiple pushes
yielding above-average progress? As usual,
we'll be reaping the rewards of this work in the next few TH04/TH05
pushes…
…whenever they get funded, that is, as for January, the backers have
shifted the priorities towards TH01 and TH03. TH01 especially is something
I'm quite excited about, as we're finally going to see just how fast this
bloated game is really going to progress. Are you excited?
🎉 TH04's and TH05's OP.EXE are now fully
position-independent! 🎉
What does this mean?
You can now add any data or code to the main menus of the two games, by
simply editing the ReC98 source, writing your mod in ASM or C/C++, and
recompiling the code. Since all absolute memory addresses have now been
converted to labels, this will work without causing any instability. See
the position independence section in the FAQ
for a more thorough explanation about why this was a problem.
What does this not mean?
The original ZUN code hasn't been completely reverse-engineered yet, let
alone decompiled. Pretty much all of that is still ASM, which might make
modding a bit inconvenient right now.
Since this push was otherwise pretty unremarkable, I made a video
demonstrating a few basic things you can do with this:
Now, what to do for the last outstanding Touhou Patch Center push?
Bullets, or resident structures?
Just like most of the time, it was more sensible to cover
GENSOU.SCR, the last structure missing in TH05's
OP.EXE,
everywhere it's used, rather than just rushing out OP.EXE
position independence. I did have to look into all of the functions to
fully RE it after all, and to find out whether the unused fields actually
are unused. The only thing that kept this push from yielding even
more above-average progress was the sheer inconsistency in how the games
implemented the operations on this PC-98 equivalent of score*.dat:
OP.EXE declares two structure instances, for simultaneous
access to both Reimu and Marisa scores. TH05 with its 4 playable
characters instead uses a single one, and overwrites it successively for
each character when drawing the high score menu – meaning, you'd only see
Yuuka's scores when looking at the structure inside the rendered high
score menu. However, it still declares the TH04 "Marisa" structure as a
leftover… and also decodes it and verifies its checksum, despite
nothing being ever loaded into it
MAIN.EXE uses a separate ASM implementation of the decoding
and encoding functions
TH05's MAIN.EXE also reimplements the basic loading
functions
in ASM – without the code to regenerate GENSOU.SCR with
default data if the file is missing or corrupted. That actually makes
sense, since any regeneration is already done in OP.EXE, which
always has to load that file anyway to check how much has been cleared
However, there is a regeneration function in TH05's
MAINE.EXE… which actually generates different default
data: OP.EXE consistently sets Extra Stage records to Stage 1,
while MAINE.EXE uses the same place-based stage numbering that
both versions use for the regular ranks
Technically though, TH05's OP.EXEis
position-independent now, and the rest are (should be?
) merely false positives. However, TH04's is
still missing another structure, in addition to its false
positives. So, let's wait with the big announcement until the next push…
which will also come with a demo video of what will be possible then.
Big gains, as expected, but not much to say about this one. With TH05 Reimu
being way too easy to decompile after
📝 the shot control groundwork done in October,
there was enough time to give the comprehensive PI false-positive
treatment to two other sets of functions present in TH04's and TH05's
OP.EXE. One of them, master.lib's super_*()
functions, was used a lot in TH02, more than in any other game… I
wonder how much more that game will progress without even focusing on it
in particular.
Alright then! 100% PI for TH04's and TH05's OP.EXE upcoming…
(Edit: Already got funding to cover this!)
So, where to start? Well, TH04 bullets are hard, so let's
procrastinate start with TH03 instead
The 📝 sprite display functions are the
obvious blocker for any structure describing a sprite, and therefore most
meaningful PI gains in that game… and I actually did manage to fit a
decompilation of those three functions into exactly the amount of time
that the Touhou Patch Center community votes alloted to TH03
reverse-engineering!
And a pretty amazing one at that. The original code was so obviously
written in ASM and was just barely decompilable by exclusively using
register pseudovariables and a bit of goto, but I was able to
abstract most of that away, not least thanks to a few helpful optimization
properties of Turbo C++… seriously, I can't stop marveling at this ancient
compiler. The end result is both readable, clear, and dare I say
portable?! To anyone interested in porting TH03,
take a look. How painful would it be to port that away from 16-bit
x86?
However, this push is also a typical example that the RE/PI priorities can
only control what I look at, and the outcome can actually differ
greatly. Even though the priorities were 65% RE and 35% PI, the progress
outcome was +0.13% RE and +1.35% PI. But hey, we've got one more push with
a focus on TH03 PI, so maybe that one will include more RE than
PI, and then everything will end up just as ordered?
With no feedback to 📝 last week's blog post,
I assume you all are fine with how things are going? Alright then, another
one towards position independence, with the same approach as before…
Since -Tom- wanted to learn something about how the PC-98
EGC is used in TH04 and TH05, I took a look at master.lib's
egc_shift_*() functions. These simply do a hardware-accelerated
memmove() of any VRAM region, and are used for screen shaking
effects. Hover over the image below for the raw effect:
Then, I finally wanted to take a look at the bullet structures, but it
required way too much reverse-engineering to even start within ¾ of
a position independence push. Even with the help of uth05win –
bullet handling was changed quite a bit from TH04 to TH05.
What I ultimately settled on was more raw, "boring" PI work based around
an already known set of functions. For this one, I looked at vector
construction… and this time, that actually made the games a little
bit more position-independent, and wasn't just all about removing
false positives from the calculation. This was one of the few sets of
functions that would also apply to TH01, and it revealed just how
chaotically that game was coded. This one commit shows three ways how ZUN
stored regular 2D points in TH01:
"regularly", like in master.lib's Point structure (X
first, Y second)
reversed, (Y first and X second), then obviously with two distinct
variables declared next to each other
… yeah. But in more productive news, this did actually lay the
groundwork for TH04 and TH05 bullet structures. Which might even be coming
up within the next big, 5-push order from Touhou Patch Center? These are
the priorities I got from them, let's see how close I can get!
So, here we have the first two pushes with an explicit focus on position
independence… and they start out looking barely different from regular
reverse-engineering? They even already deduplicate a bunch of item-related
code, which was simple enough that it required little additional work?
Because the actual work, once again, was in comparing uth05win's
interpretations and naming choices with the original PC-98 code? So that
we only ended up removing a handful of memory references there?
(Oh well, you can mod item drops now!)
So, continuing to interpret PI as a mere by-product of reverse-engineering
might ultimately drive up the total PI cost quite a bit. But alright then,
let's systematically clear out some false positives by looking at
master.lib function calls instead… and suddenly we get the PI progress we
were looking for, nicely spread out over all games since TH02. That kinda
makes it sound like useless work, only done because it's dictated by some
counting algorithm on a website. But decompilation will want to convert
all of these values to decimal anyway. We're merely doing that right now,
across all games.
Then again, it doesn't actually make any game more
position-independent, and only proves how position-independent it already
was. So I'm really wondering right now whether I should just rush
actual position independence by simply identifying structures and
their sizes, and not bother with members or false positives until that's
done. That would certainly get the job done for TH04 and TH05 in just a
few more pushes, but then leave all the proving work (and the road
to 100% PI on the front page) to reverse-engineering.
I don't know. Would it be worth it to have a game that's "maybe
fully position-independent", only for there to maybe be rare edge
cases where it isn't?
Or maybe, continuing to strike a balance between identifying false
positives (fast) and reverse-engineering structures (slow) will continue
to work out like it did now, and make us end up close to the current
estimate, which was attractive enough to sell out the crowdfunding for the
first time… 🤔
Please give feedback! If possible, by Friday evening UTC+1, before I start
working on the next PI push, this time with a focus on TH04.
And just in time for zorg's last outstanding pushes, the
TH05 shot type control functions made the speedup happen!
TH05 as a whole is now 20% reverse-engineered, and 50% position
independent,
TH05's MAIN.EXE is now even below TH02's in terms of not
yet RE'd instructions,
and all price estimates have now fallen significantly.
It would have been really nice to also include Reimu's shot
control functions in this last push, but figuring out this entire system,
with its weird bitflags and switch statement
micro-optimizations, was once again taking way longer than it should
have. Especially with my new-found insistence on turning this obvious
copy-pasta into something somewhat readable and terse…
But with such a rather tabular visual structure, things should now be
moddable in hopefully easily consistent way. Of course, since we're
only at 54% position independence for MAIN.EXE,
this isn't possible yet without
crashing the game, but modifying damage would already work.
Deathbombs confirmed, in both TH04 and TH05! On the surface, it's the same
8-frame window as in
most Windows games, but due to the slightly lower PC-98 frame rate of
56.4 Hz, it's actually slightly more lenient in TH04 and TH05.
The last function in front of the TH05 shot type control functions marks
the player's previous position in VRAM to be redrawn. But as it turns out,
"player" not only means "the player's option satellites on shot levels ≥
2", but also "the explosion animation if you lose a life", which required
reverse-engineering both things, ultimately leading to the confirmation of
deathbombs.
It actually was kind of surprising that we then had reverse-engineered
everything related to rendering all three things mentioned above,
and could also cover the player rendering function right now. Luckily,
TH05 didn't decide to also micro-optimize that function into
un-decompilability; in fact, it wasn't changed at all from TH04. Unlike
the one invalidation function whose decompilation would have
actually been the goal here…
But now, we've finally gotten to where we wanted to… and only got 2
outstanding decompilation pushes left. Time to get the website ready for
hosting an actual crowdfunding campaign, I'd say – It'll make a better
impression if people can still see things being delivered after the big
announcement.
The glacial pace continues, with TH05's unnecessarily, inappropriately
micro-optimized, and hence, un-decompilable code for rendering the current
and high score, as well as the enemy health / dream / power bars. While
the latter might still pass as well-written ASM, the former goes to such
ridiculous levels that it ends up being technically buggy. If you
enjoy quality ZUN code, it's
definitely worth a read.
In TH05, this all still is at the end of code segment #1, but in TH04,
the same code lies all over the same segment. And since I really
wanted to move that code into its final form now, I finally did the
research into decompiling from anywhere else in a segment.
Turns out we actually can! It's kinda annoying, though: After splitting
the segment after the function we want to decompile, we then need to group
the two new segments back together into one "virtual segment" matching the
original one. But since all ASM in ReC98 heavily relies on being
assembled in MASM mode, we then start to suffer from MASM's group
addressing quirk. Which then forces us to manually prefix every single
function call
from inside the group
to anywhere else within the newly created segment
with the group name. It's stupidly boring busywork, because of all the
function calls you mustn't prefix. Special tooling might make this
easier, but I don't have it, and I'm not getting crowdfunded for it.
So while you now definitely can request any specific thing in any
of the 5 games to be decompiled right now, it will take slightly
longer, and cost slightly more.
(Except for that one big segment in TH04, of course.)
Only one function away from the TH05 shot type control functions now!
Here we go, new C code! …eh, it will still take a bit to really get
decompilation going at the speeds I was hoping for. Especially with the
sheer amount of stuff that is set in the first few significant
functions we actually can decompile, which now all has to be
correctly declared in the C world. Turns out I spent the last 2 years
screwing up the case of exported functions, and even some of their names,
so that it didn't actually reflect their calling convention… yup. That's
just the stuff you tend to forget while it doesn't matter.
To make up for that, I decided to research whether we can make use of some
C++ features to improve code readability after all. Previously, it seemed
that TH01 was the only game that included any C++ code, whereas TH02 and
later seemed to be 100% C and ASM. However, during the development of the
soon to be released new build system, I noticed that even this old
compiler from the mid-90's, infamous for prioritizing compile speeds over
all but the most trivial optimizations, was capable of quite surprising
levels of automatic inlining with class methods…
…leading the research to culminate in the mindblow that is
9d121c7 – yes, we can use C++ class methods
and operator overloading to make the code more readable, while still
generating the same code than if we had just used C and preprocessor
macros.
Looks like there's now the potential for a few pull requests from outside
devs that apply C++ features to improve the legibility of previously
decompiled and terribly macro-ridden code. So, if anyone wants to help
without spending money…
Back to actual development! Starting off this stretch with something
fairly mechanical, the few remaining generic boss and midboss state
variables. And once we start converting the constant numbers used for and
around those variables into decimal, the estimated position independence
probability immediately jumped by 5.31% for TH04's MAIN.EXE,
and 4.49% for TH05's – despite not having made the game any more position-
independent than it was before. Yup… lots of false positives in there, but
who can really know for sure without having put in the work.
But now, we've RE'd enough to finally decompile something again next,
4 years after the last decompilation of anything!
Boss explosions! And… urgh, I really also had to wade through that overly complicated HUD rendering code. Even though I had to pick -Tom-'s 7th push here as well, the worst of that is still to come. TH04 and TH05 exclusively store the current and high score internally as unpacked little-endian BCD, with some pretty dense ASM code involving the venerable x86 BCD instructions to update it.
So, what's actually the goal here. Since I was given no priorities , I still haven't had to (potentially) waste time researching whether we really can decompile from anywhere else inside a segment other than backwards from the end. So, the most efficient place for decompilation right now still is the end of TH05's main_01_TEXT segment. With maybe 1 or 2 more reverse-engineering commits, we'd have everything for an efficient decompilation up to sub_123AD. And that mass of code just happens to include all the shot type control functions, and makes up 3,007 instructions in total, or 12% of the entire remaining unknown code in MAIN.EXE.
So, the most reasonable thing would be to actually put some of the upcoming decompilation pushes towards reverse-engineering that missing part. I don't think that's a bad deal since it will allow us to mod TH05 shot types in C sooner, but zorg and qp might disagree
Next up: thcrap TL notes, followed by finally finishing GhostPhanom's old ReC98 future-proofing pushes. I really don't want to decompile without a proper build system.
Sometimes, "strategically picking things to reverse-engineer" unfortunately also means "having to move seemingly random and utterly uninteresting stuff, which will only make sense later, out of the way". Really, this was so boring. Gonna get a lot more exciting in the next ones though.
So, let's continue with player shots! …eh, or maybe not directly, since they involve two other structure types in TH05, which we'd have to cover first. One of them is a different sort of sprite, and since I like me some context in my reverse-engineering, let's disable every other sprite type first to figure out what it is.
One of those other sprite types were the little sparks flying away from killed stage enemies, midbosses, and grazed bullets; easy enough to also RE right now. Turns out they use the same 8 hardcoded 8×8 sprites in TH02, TH04, and TH05. Except that it's actually 64 16×8 sprites, because ZUN wanted to pre-shift them for all 8 possible start pixels within a planar VRAM byte (rather than, like, just writing a few instructions to shift them programmatically), leading to them taking up 1,024 bytes rather than just 64.
Oh, and the thing I wanted to RE *actually* was the decay animation whenever a shot hits something. Not too complex either, especially since it's exclusive to TH05.
And since there was some time left and I actually have to pick some of the next RE places strategically to best prepare for the upcoming 17 decompilation pushes, here's two more function pointers for good measure.
Stumbled across one more drawing function in the way… which was only a duplicated and seemingly pointlessly micro-optimized copy of master.lib's super_roll_put_tiny() function, used for fast display of 4-color 16×16 sprites.
With this out of the way, we can tackle player shot sprite animation next. This will get rid of a lot of code, since every power level of every character's shot type is implemented in its own function. Which makes up thousands of instructions in both TH04 and TH05 that we can nicely decompile in the future without going through a dedicated reverse-engineering step.
Turns out I had only been about half done with the drawing routines. The rest was all related to redrawing the scrolling stage backgrounds after other sprites were drawn on top. Since the PC-98 does have hardware-accelerated scrolling, but no hardware-accelerated sprites, everything that draws animated sprites into a scrolling VRAM must then also make sure that the background tiles covered by the sprite are redrawn in the next frame, which required a bit of ZUN code. And that are the functions that have been in the way of the expected rapid reverse-engineering progress that uth05win was supposed to bring. So, looks like everything's going to go really fast now?
… yeah, no, we won't get very far without figuring out these drawing routines.
Which process data that comes from the .STD files.
Which has various arrays related to the background… including one to specify the scrolling speed. And wait, setting that to 0 actually is what starts a boss battle?
So, have a TH05 Boss Rush patch: 2018-12-26-TH05BossRush.zip
Theoretically, this should have also worked for TH04, but for some reason,
the Stage 3 boss gets stuck on the first phase if we do this?
Actually, I lied, and lasers ended up coming with everything that makes reverse-engineering ZUN code so difficult: weirdly reused variables, unexpected structures within structures, and those TH05-specific nasty, premature ASM micro-optimizations that will waste a lot of time during decompilation, since the majority of the code actually was C, except for where it wasn't.
Laser… is not difficult. In fact, out of the remaining entity types I checked, it's the easiest one to fully grasp from uth05win alone, as it's only drawn using master.lib's line, circle, and polygon functions. Everything else ends up calling… something sprite-related that needs to be RE'd separately, and which uth05win doesn't help with, at all.
Oh, and since the speed of shoot-out lasers (as used by TH05's Stage 2 boss, for example) always depends on rank, we also got this variable now.
This only covers the structure itself – uth05win's member names for the LASER structure were not only a bit too unclear, but also plain wrong and misleading in one instance. The actual implementation will follow in the next one.
So, after introducing instruction number statistics… let's go for over 2,000 lines that won't show up there immediately That being (mid-)boss HP, position, and sprite ID variables for TH04/TH05. Doesn't sound like much, but it kind of is if you insist on decimal numbers for easier comparison with uth05win's source code.
Let's start this stretch with a pretty simple entity type, the growing and shrinking circles shown during bomb animations and around bosses in TH04 and TH05. Which can be drawn in varying colors… wait, what's all this inlined and duplicated GRCG mode and color setting code? Let's move that out into macros too, it takes up too much space when grepping for constants, and will raise a "wait, what was that I/O port doing again" question for most people reading the code again after a few months.
🎉 With this push, we've also hit a milestone! Less than 200,000 unknown x86 instructions remain until we've completely reverse-engineered all of PC-98 Touhou.
While we're waiting for Bruno to release the next thcrap build with ANM header patching, here are the resulting commits of the ReC98 CDG/CD2 special offer purchased by DTM, reverse-engineering all code that covers these formats.
> OK, let's do a quick ReC98 update before going back to thcrap, shouldn't take long
> Hm, all that input code is kind of in the way, would be nice to cover that first to ease comparisons with uth05win's source code
> What the hell, why does ZUN do this? Need to do more research
> …
> OK, research done, wait, what are those other functions doing?
> Wha, everything about this is just ever so slightly awkward
Which ended up turning this one update into 2/10, 3/10, 4/10 and 5/10 of zorg's reverse-engineering commits. But at least we now got all shared input functions of TH02-TH05 covered and well understood.
What do you do if the TH06 text image feature for thcrap should have been done 3 days™ ago, but keeps getting more and more complex, and you have a ton of other pushes to deliver anyway? Get some distraction with some light ReC98 reverse-engineering work. This is where it becomes very obvious how much uth05win helps us with all the games, not just TH05.
5a5c347 is the most important one in there, this was the missing substructure that now makes every other sprite-like structure trivial to figure out.
This was one of ZUN's better copy-pasting jobs though – and honestly, I can't even imagine how you would mess up a menu that's entirely rendered on the PC-98's text RAM. It almost makes you wonder whether ZUN actually used the same 
and 
labels clearly give it away as copy-pasted:
character, which is simply the PC-98 font ROM's glyph for that
Shift-JIS codepoint.
The rendering function for the big 48×48 explosion sprite, which also
features the same clipping quirk?
The 4 
Stage 4 Reimu's spinning orbs. Note how the game uses two different sets
of sprites just to have two different outline colors. This was probably
better than messing with the palette, which can easily cause unintended
effects if you only have 16 colors to work with. Heck, 
The chasing cross bullets, seen in Phase 14 of the same Stage 6 Yuuka
fight. Featuring some smart sprite work, making use of point symmetry to
achieve a fluid animation in just 4 frames. This is
TH02
for redrawing. In this one, it took just one dumb hand-written ASM
inconsistency in the function's epilog to make the entire function
undecompilable.
) structures showed that the game just
stores the current and previous position of its entities in a slightly
different way compared to the rest of PC-98 Touhou. Instead of having
dedicated structure fields, TH02 uses two-element arrays indexed with the
active VRAM page. Same thing, and such a pattern even helps during RE since
it's easy to spot once you know what to look for.
TH03
, past right edge of current row
gaiji. Not to be confused with 
gaiji.
gaiji.
gaiji.
gaiji.
gaiji.
gaiji.
that Neko Project II auto-generates if you
don't provide either.
is still alive, or all of them have been destroyed.
and all 16×16 bullets loaded from 
