📝 Posted:

2023-03-05 12:56 UTC

🚚 Summary of:

P0229, P0230, P0231, P0232, P0233, P0234

⌨ Commits:

6370f96...d535d87, d535d87...ca523b4, ca523b4...05a49b9, f7ef7f8...abeaf85, abeaf85...dbc5b51, dd2265c...12f29c6

💰 Funded by:

Ember2528, [Anonymous]

🏷 Tags:

th01

-

rec98

+

pc98

-

tcc

+

debloating

+

file-format

+

anniversary-edition

+

master.lib

+

bullet

+

128 commits! Who would have thought that the ideal first release of the TH01 Anniversary Edition would involve so much maintenance, and raise so many research questions? It's almost as if the real work only starts after the 100% finalization mark… Once again, I had to steal some funding from the reserved JIS trail word pushes to cover everything I liked to research, which means that the next towards the anything goal will repay this debt. Luckily, this doesn't affect any immediate plans, as I'll be spending March with tasks that are already fully funded.

So, how did this end up so massive? The list of things I originally set out to do was pretty short:

1. Build entire game into single executable
2. Fix rendering issues in the one or two most important parts of the game for a good initial impression

But even the first point already started with tons of little cleanup commits. A part of them can definitely be blamed on the rush to hit the 100% decompilation mark before the 25^th anniversary last August. However, all the structural changes that I can't commit to master reveal how much of a mess the TH01 codebase actually is.
Merging the executables is mainly difficult because of all the inconsistencies between REIIDEN.EXE and FUUIN.EXE. The worst parts can be found in the REYHI*.DAT format code and the High Score menu, but the little things are just as annoying, like how the current score is an unsigned variable in REIIDEN.EXE, but a signed one in FUUIN.EXE. If it takes me this long and this many commits just to sort out all of these issues, it's no wonder that the only thing I've seen being done with this codebase since TH01's 100% decompilation was a single porting attempt that ended in a rather quick ragequit.
So why are we merging the executables in preparation for the Anniversary Edition, and not waiting with it until we start doing ports?

• Distributing and updating one executable is cleaner than doing the same with three, especially as long as installation will still involve manually dropping the new binary into the game directory.
• The Anniversary Edition won't be the only fork binary. We are already going to start out with a separate DEBLOAT.EXE that contains only the bloat removal changes without any bug fixes, and spaztron64 will probably redo his seizure-less edition. We don't want to clutter the game directory with three binaries for each of these fork builds, and we especially don't want to remember things like oh, but this fork only modifies REIIDEN.EXE…
• All forks should run side-by-side with the original game. During the time I was maintaining thcrap, I've had countless bug reports of people assuming that thcrap was responsible for bugs that were present in the original game, and the same is certain to happen with the Anniversary Edition. Separate binaries will make it easier for everyone to check where these bugs came from.
• Also, I'd like to make a point about how bloated the original three-executable structure really is, since I've heard people defending it as neat software architecture. Really, even in Real Mode where you typically want to use as little of the 640 KiB of conventional memory as possible, you don't want to split your game up like this.

The game actually is so bloated that the combined binary ended up smaller than the original REIIDEN.EXE. If all you see are the file sizes of the original three executables, this might look like a pretty impressive feat. Like, how can we possibly get 407,812 bytes into less than 238,612 bytes, without using compression?
If you've ever looked at the linker map though, it's not at all surprising. Excluding the aforementioned inconsistencies that are hard to quantify, OP.EXE and FUUIN.EXE only feature 5,767 and 6,475 bytes of unique code and data, respectively. All other code in these binaries is already part of REIIDEN.EXE, with more than half of the size coming from the Borland C++ runtime. The single worst offender here is the C++ exception handler that Borland forces onto every non-.COM binary by default, which alone adds 20,512 bytes even if your binary doesn't use C++ exceptions.
On a more hilarious note, this single line is responsible for pulling another unnecessary 14,242 bytes into OP.EXE and FUUIN.EXE. This floating-point multiplication is completely unnecessary in this context because all possible parameters are integers, but it's enough for Turbo C++ and TLINK to pull in the entire x87 FPU emulation machinery. These two binaries don't even draw lines, but since this function is part of the general graphics code translation unit and contains other functions that these binaries do need, TLINK links in the entire thing. Maybe, multiple executables aren't the best choice either if you use a linker that can't do dead code elimination…

Since the 📝 Orb's physics do turn the entire precision of a double variable into gameplay effects, it's not feasible to ever get rid of all FPU code in TH01. The exception handler, however, can be removed, which easily brings the combined binary below the size of the original REIIDEN.EXE. Compiling all code with a single set of compiler optimization flags, including the more x86-friendly pascal calling convention, then gets us a few more KB on top. As does, of course, removing unused code: The only remaining purpose of features such as 📝 resident palettes is to potentially make porting more difficult for anyone who doesn't immediately realize that nothing in the game uses these functions.
Technically, all unused code would be bloat, but for now, I'm keeping the parts that may tell stories about the game's development history (such as unused effects or the 📝 mouse cursor), or that might help with debugging. Even with that in mind, I've only scratched the surface when it comes to bloat removal, and the binary is only going to get smaller from here. A lot smaller.

If only we now could start MDRV98 from this new combined binary, we wouldn't need a second batch file either…

Which brings us to the first big research question of this delivery. Using the C spawn() function works fine on this compiler, so spawn("MDRV98.COM") would be all we need to do, right? Except that the game crashes very soon after that subprocess returned.
So it's not going to be that easy if the spawned process is a TSR. But why should this be a problem? Let's take a look at the DOS heap, and how DOS lays out processes in conventional memory if we launch the game regularly through GAME.BAT:

The batch file starts MDRV98 first, which will therefore end up below the game in conventional memory. This is perfect for a TSR: The program can resize itself arbitrarily before returning to DOS, and the rest of memory will be left over for the game. If we assume such a layout, a DOS program can implement a custom memory allocator in a very simple way, as it only has to search for free memory in one direction – and this is exactly how Borland implemented the C heap for functions like malloc() and free(), and the C++ new and delete operators.
But if we spawn MDRV98 after starting TH01, well…

MDRV98 will spawn in the next free memory location, allocate itself, return to TH01… which suddenly finds its C heap blocked from growing. As a result, the next big allocation will immediately fail with a rather misleading "out of memory" error.

So, what can we do about this? Still in a bloat removal mindset, my gut reaction was to just throw out Borland's C heap implementation, and replace it with a very thin wrapper around the DOS heap as managed by INT 21h, AH=48h/49h/4Ah. Like, why did these DOS compilers even bother with a custom allocator in the first place if DOS already comes with a perfectly fine native one? Using the native allocator would completely erase the distinction between TSR memory and game memory, and inherently allow the game to allocate beyond MDRV98.
I did in fact implement this, and noticed even more benefits:

• While DOS uses 16 bytes rather than Borland's 4 bytes for the control structure of each memory block, this larger size automatically aligns all allocations to 16-byte boundaries. Therefore, all allocation addresses would fit into 16-bit segment-only pointers rather than needing 32-bit far ones. On the Borland heap, the 4-byte header further limits regular far pointers to 65,532 bytes, forcing you into expensive huge pointers for bigger allocations.
• Debuggers in DOS emulators typically have features to show and manage the DOS heap. No need for custom debugging code.
• You can change the memory placement strategy to allocate from the top of conventional memory down to the bottom. This is how the games allocate their resident structures.

Ultimately though, the drawbacks became too significant. Most of them are related to the PC-98 Touhou games only ever creating a single DOS process, even though they contain multiple executables. Switching executables is done via exec(), which resizes a program's main allocation to match the new binary and then overwrites the old program image with the new one. If you've ever wondered why DOSBox-X only ever shows OP as the active process name in the title bar, you now know why. As far as DOS is concerned, it's still the same OP.EXE process rooted at the same segment, and exec() doesn't bother rewriting the name either. Most importantly though, this is how REIIDEN.EXE can launch into another REIIDEN.EXE process even if there are less than 238,612 bytes free when exec() is called, and without consuming more memory for every successive binary.
For now, ANNIV.EXE still re-exec()s itself at every point where the original game did, as ZUN's original code really depends on being reinitialized at boss and scene boundaries. The resulting accidental semi-hot reloading is also a useful property to retain during development.
So why is the DOS heap a bad idea for regular game allocation after all?

• Even DOS automatically releases all memory associated with a process during its termination. But since we keep running the same process until the player quits out of the main menu, we lose the C heap's implicit cleanup on exec(), and have to manually free all memory ourselves.
• Since the binary can be larger after hot reloading, we in fact have to allocate all regular memory using the last fit strategy. Otherwise, exec() fails to resize the program's main block for the same reason that crashed the game on our initial attempt to spawn("MDRV98.COM").
• Just like Borland's heap implementation, the DOS heap stores its control structures immediately before each allocation, forming a singly linked list. But since the entire OS shares this single list, corruptions from heap overflows also affect the whole system, and become much more disastrous. Theoretically, it might be possible to recover from them by forcibly releasing all blocks after the last correct one, or even by doing a brute-force search for valid memory control blocks, but in reality, DOS will likely just throw error code #7 (ERROR_ARENA_TRASHED) on the next memory management syscall, forcing a reboot.
  With a custom allocator, small corruptions remain isolated to the process. They can be even further limited if the process adds some padding between its last internal allocation and the end of the allocated DOS memory block; Borland's heap sort of does this as well by always rounding up the DOS block to a full KiB. All this might not make a difference in today's emulated and single-tasked usage, but would have back then when software was still developed inside IDEs running on the same system.
• TH01's debug mode uses heapcheck() and heapchecknode(), and reimplementing these on top of the DOS heap is not trivial. On the contrary, it would be the most complicated part of such a wrapper, by far.
• Finally, and most importantly for TH01 in particular: The observable effects of the 📝 test/debug mode HP bar heap corruption glitches are a direct result of Borland's C heap implementation.

I could release this DOS heap wrapper in unused form for another push if anyone's interested, but for now, I'm pretty happy with not actually using it in the games. Instead, let's stay with the Borland C heap, and find a way to push MDRV98 to the very top of conventional RAM. Like this:

Which is much easier said than done. It would be nice if we could just use the last fit allocation strategy here, but .COM executables always receive all free memory by default anyway, which eliminates any difference between the strategies.
But we can still change memory itself. So let's temporarily claim all remaining free memory, minus the exact amount we need for MDRV98, for our process. Then, the only remaining free space to spawn MDRV98 is at the exact place where we want it to be:

Now we only need to know how much memory to not temporarily allocate. First, we need to replicate the assumption that MDRV98's -M7 command-line parameter corresponds to a resident size of 23,552 bytes. This is not as bad as it seems, because the -M parameter explicitly has a KiB unit, and we can nicely abstract it away for the API.
The (env.) block though? Its minimum size equals the combined length of all environment variables passed to the process, but its maximum size is… not limited at all?! As in, DOS implementations can add and have historically added more free space because some programs insisted on storing their own new environment variables in this exact segment. DOSBox and DOSBox-X follow this tradition by providing a configuration option for the additional amount of environment space, with the latter adding 1024 additional bytes by default, y'know, just in case someone wants to compile FreeDOS on a slow emulator. It's not even worth sending a bug report for this specific case, because it's only a symptom of the fact that unexpectedly large program environment blocks can and will happen, and are to be expected in DOS land.
So thanks to this cruel joke, it's technically impossible to achieve what we want to do there. Hooray! The only thing we can kind of do here is an educated guess: Sum up the length of all environment variables in our environment block, compare that length against the allocated size of the block, and assume that the MDRV98 process will get as much additional memory as our process got. 🤷

The remaining hurdles came courtesy of some Borland C runtime implementation details. You would think that the temporary reallocation could even be done in pure C using the sbrk(), coreleft(), and brk() functions, but all values passed to or returned from these functions are inaccurate because they don't factor in the aforementioned KiB padding to the underlying DOS memory block. So we have to directly use the DOS syscalls after all. Which at least means that learning about them wasn't completely useless…
The final issue is caused inside Borland's spawn() implementation. The environment block for the child process is built out of all the strings reachable from C's environ pointer, which is what that FreeDOS build process should have used. Coalescing them into a single buffer involves yet another C heap allocation… and since we didn't report our DOS memory block manipulation back to the C heap, the malloc() call might think it needs to request more memory from DOS. This resets the DOS memory block back to its intended level, undoing our manipulation right before the actual INT 21h, AH=4Bh EXEC syscall. Or in short:

Manipulate DOS heap ➜ spawn() call ➜ _LoadProg() ➜ allocate and prepare environment block ➜ _spawn() ➜ DOS EXEC syscall

The obvious solution: Replace _LoadProg(), implement the coalescing ourselves, and do it before the heap manipulation. Fortunately, Borland's internal low-level _spawn() function is not static, so we can call it ourselves whenever we want to:

Allocate and prepare environment block ➜ manipulate DOS heap ➜ _spawn() call ➜ EXEC syscall

So yes, launching MDRV98 from C can be done, but it involves advanced witchcraft and is completely ridiculous. Launching external sound drivers from a batch file is the right way of doing things.
Fortunately, you don't have to rely on this auto-launching feature. You can still launch DEBLOAT.EXE or ANNIV.EXE from a batch file that launched MDRV98.COM before, and the binaries will detect this case and skip the attempt of launching MDRV98 from C. It's unlikely that my heuristic will ever break, but I definitely recommend replicating GAME.BAT just to be completely sure – especially for user-friendly repacks that don't want to include the original game anyway.
This is also why ANNIV.EXE doesn't launch ZUNSOFT.COM: The "correct" and stable way to launch ANNIV.EXE still involves a batch file, and I would say that expecting people to remove ZUNSOFT.COM from that file is worse than not playing the animation. It's certainly a debate we can have, though.

This deep dive into memory allocation revealed another previously undocumented bug in the original game. The RLE decompression code for the 東方靈異.伝 packfile contains two heap overflows, which are actually triggered by SinGyoku's BOSS1_3.BOS and Konngara's BOSS8_1.BOS. They only do not immediately crash the game when loading these bosses thanks to two implementation details of Borland's C heap.
Obviously, this is a bug we should fix, but according to the definition of bugs, that fix would be exclusive to the anniversary branch. Isn't that too restrictive for something this critical? This code is guaranteed to blow up with a different heap implementation, if only in a Debug build. And besides, nobody would notice a fix just by looking at the game's rendered output…

Looks like we have to introduce a fourth category of weird code, in addition to the previous bloat, bug, and quirk categories, for invisible internal issues like these. Let's call it landmine, and fix them on the debloated branch as well. Thanks to Clerish for the naming inspiration!
With this new category, the full definitions for all categories have become quite extensive. Thus, they now live in CONTRIBUTING.md inside the ReC98 repository.

With the new discoveries and the new landmine category, TH01 is now at 67 bugs and 20 landmines. And the solution for the landmine in question? Simplifying the 61 lines of the original code down to 16. And yes, I'm including comments in these numbers – if the interactions of the code are complex enough to require multi-paragraph comments, these are a necessary and valid part of the code.

While we're on the topic of weird code and its visible or invisible effects, there's one thing you might be concerned about. With all the rearchitecting and data shifting we're doing on the debloated branch, what will happen to the 📝 negative glitch stages? These are the result of a clearly observable bug that, by definition, must not be fixed on the debloated branch. But given that the observable layout of the glitch stages is defined by the memory surrounding the scene stage variable, won't the debloated branch inherently alter their appearance (= ⚠️ fanfiction ⚠️), or even remove them completely?

Well, yes, it will. But we can still preserve their layout by hardcoding the exact original data that the game would originally read, and even emulate the original segment relocations and other pieces of global data.
Doing this is feasible thanks to the fact that there are only 4 glitch stages. Unfortunately, the same can't be said for the timer values, which are determined by an array lookup with the un-modulo'd stage ID. If we wanted to preserve those as well, we'd have to bundle an exact copy of the original REIIDEN.EXE data segment to preserve the values of all 32,768 negative stages you could possibly enter, together with a map of all relocations in this segment. 😵 Which I've decided against for now, since this has been going on for far too long already. Let's first see if anyone ever actually complains about details like this…

Alright, time to start the anniversary branch by rendering everything at its correct internal unaligned X position? Eh… maybe not quite yet. If we just hacked all the necessary bit-shifting code into all the format-specific blitting functions, we'd still retain all this largely redundant, bad, and slow code, and would make no progress in terms of portability. It'd be much better to first write a single generic blitter that's decently optimized, but supports all kinds of sprites to make this optimization actually worth something.
So, next research question: How would such a blitter look like? After I learned during my 📝 first foray into cycle counting that port I/O is slow on 486 CPUs, it became clear that TH04's 📝 GRCG batching for pellets was one of the more useful optimizations that probably contributed a big deal towards achieving the high bullet counts of that game. This leads to two conclusions:

• master.lib's super_*() sprite functions are slow, and not worth looking at for inspiration. Even the 📝 tiny format reinitializes the GRCG on every color change, wasting 80 cycles.
• Hence, our low-level blitting API should not even care about colors. It should only concern itself with blitting a given 1bpp sprite to a single VRAM segment. This way, it can work for both 4-plane sprites and single-plane sprites, and just assume that the GRCG is active.

Maybe we should also start by not even doing these unaligned bit shifts ourselves, and instead expect the call site to 📝 always deliver a byte-aligned sprite that is correctly preshifted, if necessary? Some day, we definitely should measure how slow runtime shifting would really be…

What we should do, however, are some further general optimizations that I would have expected from master.lib: Unrolling the vertical loop, and baking a single function for every sprite width to eliminate the horizontal loop. We can then use the widest possible x86 MOV instruction for the lowest possible number of cycles per row – for example, we'd blit a 56-wide sprite with three MOVs (32-bit + 16-bit + 8-bit), and a 64-wide one with two 32-bit MOVs.
Or maybe not? There's a lot of blitting code in both master.lib and PC-98 Touhou that checks for empty bytes within sprites to skip needlessly writing them to VRAM:

uint8_t left_half = ((uint8_t *)(sprite))[0];
uint8_t right_half = ((uint8_t *)(sprite))[1];
if(right_half != 0x00) {
	pokeb(VRAM_SEGMENT, (vram_offset + 0), left_half);
}
if(right_half != 0x00) {
	pokeb(VRAM_SEGMENT, (vram_offset + 1), right_half);
}

Which goes against everything you seem to know about computers. We aren't running on an 8-bit CPU here, so wouldn't it be faster to always write both halves of a sprite in a single operation?

uint16_t both_halves = ((uint16_t *)(sprite))[0];
pokew(VRAM_SEGMENT, vram_offset, both_halves);

That's a single CPU instruction, compared to two instructions and two branches. The only possible explanation for this would be that VRAM writes are so slow on PC-98 that you'd want to avoid them at all costs, even if that means additional branching on the CPU to do so. Or maybe that was something you would want to do on certain models with slow VRAM, but not on others?

So I wrote a benchmark to answer all these questions, and to compare my new blitter against typical TH01 blitting code:

2023-03-05-blitperf.zip And here are the real-hardware results I've got from the PC-9800 Central Discord server:

The key takeaways:

• Checking for empty bytes has never been a good idea.
• Preshifting sprites made a slight difference on the 286. Starting with the 386 though, that difference got smaller and smaller, until it completely vanished on Pentium models. The memory tradeoff is especially not worth it for 4-plane sprites, given that you would have to preshift each of the 4 planes and possibly even a fifth alpha plane. Ironically, ZUN only ever preshifted monochrome single-bitplane sprites with a width of 8 pixels. That's the smallest possible amount of memory a sprite can possibly take, and where preshifting consequently has the smallest effect on performance. Shifting 8-wide sprites on the fly literally takes a single ROL or ROR instruction per row.
• You might want to use MOVS instead of MOV when targeting the 286 and 386, but the performance gains are barely worth the resulting mess you would make out of your blitting code. On Pentium models, there is no difference.
• Use the GRCG whenever you have to render lots of things that share a static 8×1 pattern.
• These are the PC-98 models that the people who are willing to test your newly written PC-98 code actually use.

Since this won't be the only piece of game-independent and explicitly PC-98-specific custom code involved in this delivery, it makes sense to start a dedicated PC-98 platform layer. This code will gradually eliminate the dependency on master.lib and replace it with better optimized and more readable C++ code. The blitting benchmark, for example, is already implemented completely without master.lib.
While this platform layer is mainly written to generate optimal code within Turbo C++ 4.0J, it can also serve as general PC-98 documentation for everyone who prefers code over machine-translating old Japanese books. Not to mention the immediacy of having all actual relevant information in one place, which might otherwise be pretty well hidden in these books, or some obscure old text file. For example, did you know that uploading gaiji via INT 18h might end up disabling the VSync interrupt trigger, deadlocking the process on the next frame delay loop? This nuisance is not replicated by any emulators, and it's quite frustrating to encounter it when trying to run your code on real hardware. master.lib works around it by simply hooking INT 18h and unconditionally reenabling the VSync interrupt trigger after the original handler returns, and so does our platform layer.

So, with the pellet draw calls batched and routed through the new renderer, we should have gained enough free CPU cycles to disable 📝 interlaced pellet rendering without any impact on frame rates?

Well, kinda. We do get 56.4 FPS, but only together with noticeable and reproducible tearing in the top part of the playfield, suggesting exactly why ZUN interlaced the rendering in the first place. 😕 So have we already reached the limit of single-buffered PC-98 games here, or can we still do something about it?
As it turns out, the main bottleneck actually lies in the pellet unblitting code. Every EGC-"accelerated" unblitting call in TH01 is as unbatched as the pellet blitting calls were, spending an additional 17 I/O port writes per call to completely set up and shut down the EGC, every time. And since this is TH01, the two-instruction operation of changing the active PC-98 VRAM page isn't inlined either, but instead done via a function call to a faraway segment. On the 486, that's:

• >341 cycles for EGC setup and teardown, plus
• >72 cycles for each 16-pixel chunk to be unblitted.

This sums up to

• >917 cycles of completely unnecessary work for every active pellet, in the optimal 50% of cases where it lies on an even VRAM byte, or
• >1493 cycles if it lies on an odd VRAM byte, because ZUN's code extends the unblitted rectangle to a gargantuan 32×8 pixels in this case

And this calculation even ignores the lack of small micro-optimizations that could further optimize the blitting loop. Multiply that by the game's pellet cap of 100, and we get a 6-digit number of wasted CPU cycles. On paper, that's roughly ¹/₆ of the time we have for each of our target 56.423 FPS on the game's target 33 MHz systems. Might not sound all too critical, but the single-buffered nature of the game means that we're effectively racing the beam on every frame. In turn, we have to be even more serious about performance.

So, time to also add a batched EGC API to our PC-98 platform layer? Writing our own EGC code presents a nice opportunity to finally look deeper into all its registers and configuration options, and see what exactly we can do about ZUN's enforced 16-pixel alignment.
To nobody's surprise, this alignment is completely unnecessary, and only displays a lack of knowledge about the chip. While it is true that the EGC wants VRAM to be exclusively addressed in 16-bit chunks at 16-bit-aligned addresses, it specifically provides

• an address register (0x4AC) for shifting the horizontal start offsets of the source and destination to any pixel within the 16 pixels of such a chunk, and
• a bit length register (0x4AE) for specifying the total width of pixels to be transferred, which also implies the correct end offsets.

And it gets even better: After ⌈bitlength ÷ 16⌉ write instructions, the EGC's internal shifter state automatically reinitializes itself in preparation for blitting another row of pixels with the same initially configured bit addresses and length. This is perfect for blitting rectangles, as two I/O port writes before the start of your blitting loop are enough to define your entire rectangle.

The manual nature of reading and writing in 16-pixel chunks does come with a slight pitfall though. If the source bit address is larger than the destination bit address, the first 16-bit read won't fill the EGC's internal shift register with all pixels that should appear in the first 16-pixel destination chunk. In this case, the EGC simply won't write anything and leave the first chunk unchanged. In a 📝 regular blitting loop, however, you expect that memory to be written and immediately move on to the next chunks within the row. As a result, the actual blitting process for such a rectangle will no longer be aligned to the configured address and bit length. The first row of the rectangle will appear 16 pixels to the right of the destination address, and the second one will start at bit offset 0 with pixels from the rightmost byte of the first line, which weren't blitted and remained in the tile register.
There is an easy solution though: Before the horizontal loop on each line of the rectangle, simply read one additional 16-pixel chunk from the source location to prefill the shift register. Thankfully, it's large enough to also fit the second read of the then full 16 pixels, without dropping any pixels along the way.

And that's how we get arbitrarily unaligned rectangle copies with the EGC! Except for a small register allocation trick to use two-register addressing, there's not much use in further optimizations, as the runtime of these inter-page blit operations is dominated by the VRAM page switches anyway.

Except that T98-Next seems to disagree about the register prefilling issue:

Every other emulator agrees with real hardware in this regard, so we can safely assume this to be a bug in T98-Next. Just in case this old emulator with its last release from June 2010 still has any fans left nowadays… For now though, even they can still enjoy the TH01 Anniversary Edition: The only EGC copy algorithm that TH01 actually needs is the left one during the single-buffered tests, which even that emulator gets right.
That only leaves 📝 my old offer of documenting the EGC raster ops, and we've got the EGC figured out completely!

And that did in fact remove tearing from the pellet rendering function! For the first time, we can now fight Elis, Kikuri, Sariel, and Konngara with a doubled pellet frame rate:

With only pellets and no other animation on screen, this exact pattern presents the optimal demonstration case for the new unblitter. But as you can already tell from the invincibility sprites, we'd also need to route every other kind of sprite through the same new code. This isn't all too trivial: Most sprites are still rendered at byte-aligned positions, and their blitting APIs hide that fact by taking a pixel position regardless. This is why we can't just replace ZUN's original 16-pixel-aligned EGC unblitting function with ours, and always have to replace both the blitter and the unblitter on a per-sprite basis.
To completely remove all flickering, we'd also like to get rid of all the sprite-specific unblit ➜ update ➜ render sequences, and instead gather all unblitting code to the beginning of the game loop, before any update and rendering calls. So yeah, it will take a long time to completely get rid of all flickering. Until we're there, I recommend any backer to tell me their favorite boss, so that I can focus on getting that one rendered without any flickering. Remember that here at ReC98, we can have a Touhou character popularity contest at any time during the year, whenever the store is open!

In the meantime, the consistent use of 8×8 rectangles during pellet unblitting does significantly reduce flickering across the entire game, and shrinks certain holes that pellets tend to rip into lazily reblitted sprites:

To round out the first release, I added all the other bug fixes to achieve parity with my previously released patched REIIDEN.EXE builds:

• I removed the 📝 shootout laser crash by simply leaving the lasers on screen if a boss is defeated,
• prevented the HP bar heap corruption bug in test or debug mode by not letting it display negative HP in the first place, and
• restored 📝 the two animations during the Sariel fight that were lost to type confusion errors in the original game.

So here it is, the first build of TH01's Anniversary Edition: 2023-03-05-th01-anniv.zip Edit (2023-03-12): If you're playing on Neko Project and seeing more flickering than in the original game, make sure you've checked the Screen → Disp vsync option.

Next up: The long overdue extended trip through the depths of TH02's low-level code. From what I've seen of it so far, the work on this project is finally going to become a bit more relaxing. Which is quite welcome after, what, 6 months of stressful research-heavy work?

📝 Posted:

2022-08-11 21:27 UTC

🚚 Summary of:

P0212, P0213

⌨ Commits:

d398a94...363fd54, 363fd54...158a91e

💰 Funded by:

LeyDud, Lmocinemod, GhostRiderCog, Ember2528

🏷 Tags:

th01

-

rec98

+

score

+

pc98

-

performance

+

unused

+

cutscene

+

debug

+

Wow, it's been 3 days and I'm already back with an unexpectedly long post about TH01's bonus point screens? 3 days used to take much longer in my previous projects…

Before I talk about graphics for the rest of this post, let's start with the exact calculations for both bonuses. Touhou Wiki already got these right, but it still makes sense to provide them here, in a format that allows you to cross-reference them with the source code more easily. For the card-flipping stage bonus:

The boss stage bonus is calculated from the exact same metrics, despite half of them being labeled differently. The only actual differences are in the higher multipliers and in the cap for the stage number bonus. Why remove it if raising it high enough also effectively disables it?

The transition between the gameplay and TOTLE screens is one of the more impressive effects showcased in this game, especially due to how wavy it often tends to look. Aside from the palette interpolation (which is, by the way, the first time ZUN wrote a correct interpolation algorithm between two 4-bit palettes), the core of the effect is quite simple. With the TOTLE image blitted to VRAM page 1:

• Shift the contents of a line on VRAM page 0 by 32 pixels, alternating the shift direction between right edge → left edge (even Y values) and the other way round (odd Y values)
• Keep a cursor for the destination pixels on VRAM page 1 for every line, starting at the respective opposite edge
• Blit the 32 pixels at the VRAM page 1 cursor to the newly freed 32 pixels on VRAM page 0, and advance the cursor towards the other edge
• Successive line shifts will then include these newly blitted 32 pixels as well
• Repeat (640 / 32) = 20 times, after which all new pixels will be in their intended place

So it's really more like two interlaced shift effects with opposite directions, starting on different scanlines. No trigonometry involved at all.

Horizontally scrolling pixels on a single VRAM page remains one of the few 📝 appropriate uses of the EGC in a fullscreen 640×400 PC-98 game, regardless of the copied block size. The few inter-page copies in this effect are also reasonable: With 8 new lines starting on each effect frame, up to (8 × 20) = 160 lines are transferred at any given time, resulting in a maximum of (160 × 2 × 2) = 640 VRAM page switches per frame for the newly transferred pixels. Not that frame rate matters in this situation to begin with though, as the game is doing nothing else while playing this effect.
What does sort of matter: Why 32 pixels every 2 frames, instead of 16 pixels on every frame? There's no performance difference between doing one half of the work in one frame, or two halves of the work in two frames. It's not like the overhead of another loop has a serious impact here, especially with the PC-98 VRAM being said to have rather high latencies. 32 pixels over 2 frames is also harder to code, so ZUN must have done it on purpose. Guess he really wanted to go for that 📽 cinematic 30 FPS look 📽 here…

Once all the metrics have been calculated, ZUN animates each value with a rather fancy left-to-right typing effect. As 16×16 images that use a single bright-red color, these numbers would be perfect candidates for gaiji… except that ZUN wanted to render them at the more natural Y positions of the labels inside CLEAR3.GRP that are far from aligned to the 8×16 text RAM grid. Not having been in the mood for hardcoding another set of monochrome sprites as C arrays that day, ZUN made the still reasonable choice of storing the image data for these numbers in the single-color .GRC form– yeah, no, of course he once again chose the .PTN hammer, and its 📝 16×16 "quarter" wrapper functions around nominal 32×32 sprites.

Why do I bring up such a detail? What's actually going on there is that ZUN loops through and blits each digit from 0 to 9, and then continues the loop with "digit" numbers from 10 to 19, stopping before the number whose ones digit equals the one that should stay on screen. No problem with that in theory, and the .PTN sprite selection is correct… but the .PTN quarter selection isn't, as ZUN wrote (digit % 4) instead of the correct ((digit % 10) % 4). Since .PTN quarters are indexed in a row-major way, the 10-19 part of the loop thus ends up blitting 2 → 3 → 0 → 1 → 6 → 7 → 4 → 5 → (nothing):

Seriously though? If the deadline is looming and you've got to rush some part of your game, a standalone screen that doesn't affect anything is the best place to pick. At 4 milliseconds per digit, the animation goes by so fast that this quirk might even add to its perceived fanciness. It's exactly the reason why I've always been rather careful with labeling such quirks as "bugs". And in the end, the code does perform one more blitting call after the loop to make sure that the correct digit remains on screen.

Lovely! With no mouse-related code left in the game otherwise, this cursor sprite provides some great fuel for wild fan theories about TH01's development history:

1. Could ZUN have 📝 stolen the basic PC-98 VSync or text RAM function code from a source that also implemented mouse support?
2. Did he have a mouse-controlled level editor during development? It's highly likely that he had something, given all the 📝 bit twiddling seen in the STAGE?.DAT format.
3. Or was this game actually meant to have mouse-controllable portions at some point during development? Even if it would have just been the menus.

… Actually, you know what, with all shared data moved to C land, I might as well finish FUUIN.EXE right now. The last secret hidden in its main() function: Just like GAME.BAT supports launching the game in various debug modes from the DOS command line, FUUIN.EXE can directly launch one of the game's endings. As long as the MDRV2 driver is installed, you can enter fuuin t1 for the 魔界/Makai Good Ending, or fuuin t for 地獄/Jigoku Good Ending.
Unfortunately, the command-line parameter can only control the route. Choosing between a Good or Bad Ending is still done exclusively through TH01's resident structure, and the continues_per_scene array in particular. But if you pre-allocate that structure somehow and set one of the members to a nonzero value, it would work. Trainers, anyone?

Alright, gotta get back to the code if I want to have any chance of finishing this game before the 15^th… Next up: The final 17 functions in REIIDEN.EXE that tie everything together and add some more debug features on top.

📝 Posted:

2022-06-17 17:03 UTC

🚚 Summary of:

P0198, P0199, P0200

⌨ Commits:

48db0b7...440637e, 440637e...5af2048, 5af2048...67e46b5

💰 Funded by:

Ember2528, Lmocinemod, Yanga

🏷 Tags:

th01

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rec98

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gameplay

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boss

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kikuri

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blitting

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glitch

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danmaku-pattern

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cutscene

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pc98

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performance

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tcc

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What's this? A simple, straightforward, easy-to-decompile TH01 boss with just a few minor quirks and only two rendering-related ZUN bugs? Yup, 2½ pushes, and Kikuri was done. Let's get right into the overview:

• Just like 📝 Elis, Kikuri's fight consists of 5 phases, excluding the entrance animation. For some reason though, they are numbered from 2 to 6 this time, skipping phase 1? For consistency, I'll use the original phase numbers from the source code in this blog post.
• The main phases (2, 5, and 6) also share Elis' HP boundaries of 10, 6, and 0, respectively, and are once again indicated by different colors in the HP bar. They immediately end upon reaching the given number of HP, making Kikuri immune to the 📝 heap corruption in test or debug mode that can happen with Elis and Konngara.
• Phase 2 solely consists of the infamous big symmetric spiral pattern.
• Phase 3 fades Kikuri's ball of light from its default bluish color to bronze over 100 frames. Collision detection is deactivated during this phase.
• In Phase 4, Kikuri activates her two souls while shooting the spinning 8-pellet circles from the previously activated ball. The phase ends shortly after the souls fired their third spread pellet group.
  • Note that this is a timed phase without an HP boundary, which makes it possible to reduce Kikuri's HP below the boundaries of the next phases, effectively skipping them. Take this video for example, where Kikuri has 6 HP by the end of Phase 4, and therefore directly starts Phase 6.
    (Obviously, Kikuri's HP can also be reduced to 0 or below, which will end the fight immediately after this phase.)
• Phase 5 combines the teardrop/ripple "pattern" from the souls with the "two crossed eye laser" pattern, on independent cycles.
• Finally, Kikuri cycles through her remaining 4 patterns in Phase 6, while the souls contribute single aimed pellets every 200 frames.
• Interestingly, all HP-bounded phases come with an additional hidden timeout condition:
  • Phase 2 automatically ends after 6 cycles of the spiral pattern, or 5,400 frames in total.
  • Phase 5 ends after 1,600 frames, or the first frame of the 7^th cycle of the two crossed red lasers.
  • If you manage to keep Kikuri alive for 29 of her Phase 6 patterns, her HP are automatically set to 1. The HP bar isn't redrawn when this happens, so there is no visual indication of this timeout condition even existing – apart from the next Orb hit ending the fight regardless of the displayed HP. Due to the deterministic order of patterns, this always happens on the 8^th cycle of the "symmetric gravity pellet lines from both souls" pattern, or 11,800 frames. If dodging and avoiding orb hits for 3½ minutes sounds tiring, you can always watch the byte at DS:0x1376 in your emulator's memory viewer. Once it's at 0x1E, you've reached this timeout.

So yeah, there's your new timeout challenge.

The few issues in this fight all relate to hitboxes, starting with the main one of Kikuri against the Orb. The coordinates in the code clearly describe a hitbox in the upper center of the disc, but then ZUN wrote a < sign instead of a > sign, resulting in an in-game hitbox that's not quite where it was intended to be…

Much worse, however, are the teardrop ripples. It already starts with their rendering routine, which places the sprites from TAMAYEN.PTN at byte-aligned VRAM positions in the ultimate piece of if(…) {…} else if(…) {…} else if(…) {…} meme code. Rather than tracking the position of each of the five ripple sprites, ZUN suddenly went purely functional and manually hardcoded the exact rendering and collision detection calls for each frame of the animation, based on nothing but its total frame counter.
Each of the (up to) 5 columns is also unblitted and blitted individually before moving to the next column, starting at the center and then symmetrically moving out to the left and right edges. This wouldn't be a problem if ZUN's EGC-powered unblitting function didn't word-align its X coordinates to a 16×1 grid. If the ripple sprites happen to start at an odd VRAM byte position, their unblitting coordinates get rounded both down and up to the nearest 16 pixels, thus touching the adjacent 8 pixels of the previously blitted columns and leaving the well-known black vertical bars in their place.

OK, so where's the hitbox issue here? If you just look at the raw calculation, it's a slightly confusingly expressed, but perfectly logical 17 pixels. But this is where byte-aligned blitting has a direct effect on gameplay: These ripples can be spawned at any arbitrary, non-byte-aligned VRAM position, and collisions are calculated relative to this internal position. Therefore, the actual hitbox is shifted up to 7 pixels to the right, compared to where you would expect it from a ripple sprite's on-screen position:

We've previously seen the same issue with the 📝 shot hitbox of Elis' bat form, where pixel-perfect collision detection against a byte-aligned sprite was merely a sidenote compared to the more serious X=Y coordinate bug. So why do I elevate it to bug status here? Because it directly affects dodging: Reimu's regular movement speed is 4 pixels per frame, and with the internal position of an on-screen ripple sprite varying by up to 7 pixels, any micrododging (or "grazing") attempt turns into a coin flip. It's sort of mitigated by the fact that Reimu is also only ever rendered at byte-aligned VRAM positions, but I wouldn't say that these two bugs cancel out each other.
Oh well, another set of rendering issues to be fixed in the hypothetical Anniversary Edition – obviously, the hitboxes should remain unchanged. Until then, you can always memorize the exact internal positions. The sequence of teardrop spawn points is completely deterministic and only controlled by the fixed per-difficulty spawn interval.

Aside from more minor coordinate inaccuracies, there's not much of interest in the rest of the pattern code. In another parallel to Elis though, the first soul pattern in phase 4 is aimed on every difficulty except Lunatic, where the pellets are once again statically fired downwards. This time, however, the pattern's difficulty is much more appropriately distributed across the four levels, with the simultaneous spinning circle pellets adding a constant aimed component to every difficulty level.

That brings us to 5 fully decompiled PC-98 Touhou bosses, with 26 remaining… and another ½ of a push going to the cutscene code in FUUIN.EXE.
You wouldn't expect something as mundane as the boss slideshow code to contain anything interesting, but there is in fact a slight bit of speculation fuel there. The text typing functions take explicit string lengths, which precisely match the corresponding strings… for the most part. For the "Gatekeeper 'SinGyoku'" string though, ZUN passed 23 characters, not 22. Could that have been the "h" from the Hepburn romanization of 神玉?!
Also, come on, if this text is already blitted to VRAM for no reason, you could have gone for perfect centering at unaligned byte positions; the rendering function would have perfectly supported it. Instead, the X coordinates are still rounded up to the nearest byte.

The hardcoded ending cutscene functions should be even less interesting – don't they just show a bunch of images followed by frame delays? Until they don't, and we reach the 地獄/Jigoku Bad Ending with its special shake/"boom" effect, and this picture:

Which is rendered by the following code:

for(int i = 0; i <= boom_duration; i++) { // (yes, off-by-one)
	if((i & 3) == 0) {
		graph_scrollup(8);
	} else {
		graph_scrollup(0);
	}

	end_pic_show(1); // ← different picture is rendered
	frame_delay(2);  // ← blocks until 2 VSync interrupts have occurred

	if(i & 1) {
		end_pic_show(2); // ← picture above is rendered
	} else {
		end_pic_show(1);
	}
}

Notice something? You should never see this picture because it's immediately overwritten before the frame is supposed to end. And yet it's clearly flickering up for about one frame with common emulation settings as well as on my real PC-9821 Nw133, clocked at 133 MHz. master.lib's graph_scrollup() doesn't block until VSync either, and removing these calls doesn't change anything about the blitted images. end_pic_show() uses the EGC to blit the given 320×200 quarter of VRAM from page 1 to the visible page 0, so the bottleneck shouldn't be there either…

…or should it? After setting it up via a few I/O port writes, the common method of EGC-powered blitting works like this:

1. Read 16 bits from the source VRAM position on any single bitplane. This fills the EGC's 4 16-bit tile registers with the VRAM contents at that specific position on every bitplane. You do not care about the value the CPU returns from the read – in optimized code, you would make sure to just read into a register to avoid useless additional stores into local variables.
2. Write any 16 bits to the target VRAM position on any single bitplane. This copies the contents of the EGC's tile registers to that specific position on every bitplane.

To transfer pixels from one VRAM page to another, you insert an additional write to I/O port 0xA6 before 1) and 2) to set your source and destination page… and that's where we find the bottleneck. Taking a look at the i486 CPU and its cycle counts, a single one of these page switches costs 17 cycles – 1 for MOVing the page number into AL, and 16 for the OUT instruction itself. Therefore, the 8,000 page switches required for EGC-copying a 320×200-pixel image require 136,000 cycles in total.

And that's the optimal case of using only those two instructions. 📝 As I implied last time, TH01 uses a function call for VRAM page switches, complete with creating and destroying a useless stack frame and unnecessarily updating a global variable in main memory. I tried optimizing ZUN's code by throwing out unnecessary code and using 📝 pseudo-registers to generate probably optimal assembly code, and that did speed up the blitting to almost exactly 50% of the original version's run time. However, it did little about the flickering itself. Here's a comparison of the first loop with boom_duration = 16, recorded in DOSBox-X with cputype=auto and cycles=max, and with i overlaid using the text chip. Caution, flashing lights:

I pushed the optimized code to the th01_end_pic_optimize branch, to also serve as an example of how to get close to optimal code out of Turbo C++ 4.0J without writing a single ASM instruction.
And if you really want to use the EGC for this, that's the best you can do. It really sucks that it merely expanded the GRCG's 4×8-bit tile register to 4×16 bits. With 32 bits, ≥386 CPUs could have taken advantage of their wider registers and instructions to double the blitting performance. Instead, we now know the reason why 📝 Promisence Soft's EGC-powered sprite driver that ZUN later stole for TH03 is called SPRITE16 and not SPRITE32. What a massive disappointment.

But what's perhaps a bigger surprise: Blitting planar images from main memory is much faster than EGC-powered inter-page VRAM copies, despite the required manual access to all 4 bitplanes. In fact, the blitting functions for the .CDG/.CD2 format, used from TH03 onwards, would later demonstrate the optimal method of using REP MOVSD for blitting every line in 32-pixel chunks. If that was also used for these ending images, the core blitting operation would have taken ((12 + (3 × (320 / 32))) × 200 × 4) = 33,600 cycles, with not much more overhead for the surrounding row and bitplane loops. Sure, this doesn't factor in the whole infamous issue of VRAM being slow on PC-98, but the aforementioned 136,000 cycles don't even include any actual blitting either. And as you move up to later PC-98 models with Pentium CPUs, the gap between OUT and REP MOVSD only becomes larger. (Note that the page I linked above has a typo in the cycle count of REP MOVSD on Pentium CPUs: According to the original Intel Architecture and Programming Manual, it's 13+𝑛, not 3+𝑛.)
This difference explains why later games rarely use EGC-"accelerated" inter-page VRAM copies, and keep all of their larger images in main memory. It especially explains why TH04 and TH05 can get away with naively redrawing boss backdrop images on every frame.

In the end, the whole fact that ZUN did not define how long this image should be visible is enough for me to increment the game's overall bug counter. Who would have thought that looking at endings of all things would teach us a PC-98 performance lesson… Sure, optimizing TH01 already seemed promising just by looking at its bloated code, but I had no idea that its performance issues extended so far past that level.

That only leaves the common beginning part of all endings and a short main() function before we're done with FUUIN.EXE, and 98 functions until all of TH01 is decompiled! Next up: SinGyoku, who not only is the quickest boss to defeat in-game, but also comes with the least amount of code. See you very soon!

📝 Posted:

2022-05-31 23:43 UTC

🚚 Summary of:

P0193, P0194, P0195, P0196, P0197

⌨ Commits:

e1f3f9f...183d7a2, 183d7a2...5d93a50, 5d93a50...e18c53d, e18c53d...57c9ac5, 57c9ac5...48db0b7

💰 Funded by:

Ember2528, Yanga

🏷 Tags:

th01

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rec98

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gameplay

+

boss

+

elis

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mima-th01

+

rng

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danmaku-pattern

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bug

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jank

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glitch

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unused

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pc98

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blitting

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mod

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score

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With Elis, we've not only reached the midway point in TH01's boss code, but also a bunch of other milestones: Both REIIDEN.EXE and TH01 as a whole have crossed the 75% RE mark, and overall position independence has also finally cracked 80%!

And it got done in 4 pushes again? Yup, we're back to 📝 Konngara levels of redundancy and copy-pasta. This time, it didn't even stop at the big copy-pasted code blocks for the rift sprite and 256-pixel circle animations, with the words "redundant" and "unnecessary" ending up a total of 18 times in my source code comments.
But damn is this fight broken. As usual with TH01 bosses, let's start with a high-level overview:

• The Elis fight consists of 5 phases (excluding the entrance animation), which must be completed in order.
• In all odd-numbered phases, Elis uses a random one-shot danmaku pattern from an exclusive per-phase pool before teleporting to a random position.
  • There are 3 exclusive girl-form patterns per phase, plus 4 additional bat-form patterns in phase 5, for a total of 13.
  • Due to a quirk in the selection algorithm in phases 1 and 3, there is a 25% chance of Elis skipping an attack cycle and just teleporting again.
  • In contrast to Konngara, Elis can freely select the same pattern multiple times in a row. There's nothing in the code to prevent that from happening.
• This pattern+teleport cycle is repeated until Elis' HP reach a certain threshold value. The odd-numbered phases correspond to the white (phase 1), red-white (phase 3), and red (phase 5) sections of the health bar. However, the next phase can only start at the end of each cycle, after a teleport.
• Phase 2 simply teleports Elis back to her starting screen position of (320, 144) and then advances to phase 3.
• Phase 4 does the same as phase 2, but adds the initial bat form transformation before advancing to phase 5.
• Phase 5 replaces the teleport with a transformation to the bat form. Rather than teleporting instantly to the target position, the bat gradually flies there, firing a randomly selected looping pattern from the 4-pattern bat pool on the way, before transforming back to the girl form.

This puts the earliest possible end of the fight at the first frame of phase 5. However, nothing prevents Elis' HP from reaching 0 before that point. You can nicely see this in 📝 debug mode: Wait until the HP bar has filled up to avoid heap corruption, hold ↵ Return to reduce her HP to 0, and watch how Elis still goes through a total of two patterns* and four teleport animations before accepting defeat.

But wait, heap corruption? Yup, there's a bug in the HP bar that already affected Konngara as well, and it isn't even just about the graphical glitches generated by negative HP:

• Because 📝 the backgrounds behind the points are kept in main memory, there has to be a cap on the number of hitpoints, which turns out to be 96.
• The initial fill-up animation is drawn to both VRAM pages at a rate of 1 HP per frame… by passing the current frame number as the current_hp number.
• The target_hp is indicated by simply passing the current HP…
• … which, however, can be reduced in debug mode at an equal rate of up to 1 HP per frame.
• The completion condition only checks if ((target_hp - 1) == current_hp). With the right timing, both numbers can therefore run past each other.
• In that case, the function is repeatedly called on every frame, backing up the original VRAM contents for the current HP point before blitting it…
• … until frame ((96 / 2) + 1), where the .PTN slot pointer overflows the heap buffer and overwrites whatever comes after. 📝 Sounds familiar, right?

Since Elis starts with 14 HP, which is an even number, this corruption is trivial to cause: Simply hold ↵ Return from the beginning of the fight, and the completion condition will never be true, as the HP and frame numbers run past the off-by-one meeting point.

Regular gameplay, however, entirely prevents this due to the fixed start positions of Reimu and the Orb, the Orb's fixed initial trajectory, and the 50 frames of delay until a bomb deals damage to a boss. These aspects make it impossible to hit Elis within the first 14 frames of phase 1, and ensure that her HP bar is always filled up completely. So ultimately, this bug ends up comparable in seriousness to the 📝 recursion / stack overflow bug in the memory info screen.

These wavy teleport animations point to a quite frustrating architectural issue in this fight. It's not even the fact that unblitting the yellow star sprites rips temporary holes into Elis' sprite; that's almost expected from TH01 at this point. Instead, it's all because of this unused frame of the animation:

With this sprite still being part of BOSS5.BOS, Girl-Elis has a total of 9 animation frames, 1 more than the 📝 8 per-entity sprites allowed by ZUN's architecture. The quick and easy solution would have been to simply bump the sprite array size by 1, but… nah, this would have added another 20 bytes to all 6 of the .BOS image slots. Instead, ZUN wrote the manual position synchronization code I mentioned in that 2020 blog post. Ironically, he then copy-pasted this snippet of code often enough that it ended up taking up more than 120 bytes in the Elis fight alone – with, you guessed it, some of those copies being redundant. Not to mention that just going from 8 to 9 sprites would have allowed ZUN to go down from 6 .BOS image slots to 3. That would have actually saved 420 bytes in addition to the manual synchronization trouble. Looking forward to SinGyoku, that's going to be fun again…

As for the fight itself, it doesn't take long until we reach its most janky danmaku pattern, right in phase 1:

• For whatever reason, the lower-right quarter of the circle isn't animated? This animation works by only drawing the new dots added with every subsequent animation frame, expressed as a tiny arc of a dotted circle. This arc starts at the animation's current 8-bit angle and ends on the sum of that angle and a hardcoded constant. In every other (copy-pasted, and correct) instance of this animation, ZUN uses 0x02 as the constant, but this one uses… 0.05 for the lower-right quarter? As in, a 64-bit double constant that truncates to 0 when added to an 8-bit integer, thus leading to the start and end angles being identical and the game not drawing anything.
• On Easy and Normal, the pattern then spawns 32 bullets along the outline of the circle, no problem there. On Lunatic though, every one of these bullets is instead turned into a narrow-angled 5-spread, resulting in 160 pellets… in a game with a pellet cap of 100. Now, if Elis teleported herself to a position near the top of the playfield, most of the capped pellets would have been clipped at that top edge anyway, since the bullets are spawned in clockwise order starting at Elis' right side with an angle of 0x00. On lower positions though, you can definitely see a difference if the cap were high enough to allow all coded pellets to actually be spawned.
  The Hard version gets dangerously close to the cap by spawning a total of 96 pellets. Since this is the only pattern in phase 1 that fires pellets though, you are guaranteed to see all of the unclipped ones.
• The pellets also aren't spawned exactly on the telegraphed circle, but 4 pixels to the left.

Then again, it might very well be that all of this was intended, or, most likely, just left in the game as a happy accident. The latter interpretation would explain why ZUN didn't just delete the rendering calls for the lower-right quarter of the circle, because seriously, how would you not spot that? The phase 3 patterns continue with more minor graphical glitches that aren't even worth talking about anymore.

And then Elis transforms into her bat form at the beginning of Phase 5, which displays some rather unique hitboxes. The one against the Orb is fine, but the one against player shots…

… uses the bat's X coordinate for both X and Y dimensions. In regular gameplay, it's not too bad as most of the bat patterns fire aimed pellets which typically don't allow you to move below her sprite to begin with. But if you ever tried destroying these pellets while standing near the middle of the playfield, now you know why that didn't work. This video also nicely points out how the bat, like any boss sprite, is only ever blitted at positions on the 8×1-pixel VRAM byte grid, while collision detection uses the actual pixel position.

The bat form patterns are all relatively simple, with little variation depending on the difficulty level, except for the "slow pellet spreads" pattern. This one is almost easiest to dodge on Lunatic, where the 5-spreads are not only always fired downwards, but also at the hardcoded narrow delta angle, leaving plenty of room for the player to move out of the way:

Finally, we've got another potential timesave in the girl form's "safety circle" pattern:

After the circle spawned completely, you lose a life by moving outside it, but doing that immediately advances the pattern past the circle part. This part takes 200 frames, but the defeat animation only takes 82 frames, so you can save up to 118 frames there.

Final funny tidbit: As with all dynamic entities, this circle is only blitted to VRAM page 0 to allow easy unblitting. However, it's also kind of static, and there needs to be some way to keep the Orb, the player shots, and the pellets from ripping holes into it. So, ZUN just re-blits the circle every… 4 frames?! 🤪 The same is true for the Star of David and its surrounding circle, but there you at least get a flash animation to justify it. All the overlap is actually quite a good reason for not even attempting to 📝 mess with the hardware color palette instead.

And that's the 4th PC-98 Touhou boss decompiled, 27 to go… but wait, all these quirks, and I still got nothing about the one actual crash that can appear in regular gameplay? There has even been a recent video about it. The cause has to be in Elis' main function, after entering the defeat branch and before the blocking white-out animation. It can't be anywhere else other than in the 📝 central line blitting and unblitting function, called from 📝 that one broken laser reset+unblit function, because everything else in that branch looks fine… and I think we can rule out a crash in MDRV2's non-blocking fade-out call. That's going to need some extra research, and a 5th push added on top of this delivery.

Reproducing the crash was the whole challenge here. Even after moving Elis and Reimu to the exact positions seen in Pearl's video and setting Elis' HP to 0 on the exact same frame, everything ran fine for me. It's definitely no division by 0 this time, the function perfectly guards against that possibility. The line specified in the function's parameters is always clipped to the VRAM region as well, so we can also rule out illegal memory accesses here…

… or can we? Stepping through it all reminded me of how this function brings unblitting sloppiness to the next level: For each VRAM byte touched, ZUN actually unblits the 4 surrounding bytes, adding one byte to the left and two bytes to the right, and using a single 32-bit read and write per bitplane. So what happens if the function tries to unblit the topmost byte of VRAM, covering the pixel positions from (0, 0) to (7, 0) inclusive? The VRAM offset of 0x0000 is decremented to 0xFFFF to cover the one byte to the left, 4 bytes are written to this address, the CPU's internal offset overflows… and as it turns out, that is illegal even in Real Mode as of the 80286, and will raise a General Protection Fault. Which is… ignored by DOSBox-X, every Neko Project II version in common use, the CSCP emulators, SL9821, and T98-Next. Only Anex86 accurately emulates the behavior of real hardware here.

OK, but no laser fired by Elis ever reaches the top-left corner of the screen. How can such a fault even happen in practice? That's where the broken laser reset+unblit function comes in: Not only does it just flat out pass the wrong parameters to the line unblitting function – describing the line already traveled by the laser and stopping where the laser begins – but it also passes them wrongly, in the form of raw 32-bit fixed-point Q24.8 values, with no conversion other than a truncation to the signed 16-bit pixels expected by the function. What then follows is an attempt at interpolation and clipping to find a line segment between those garbage coordinates that actually falls within the boundaries of VRAM:

1. right/bottom correspond to a laser's origin position, and left/top to the leftmost pixel of its moved-out top line. The bug therefore only occurs with lasers that stopped growing and have started moving.
2. Moreover, it will only happen if either (left % 256) or (right % 256) is ≤ 127 and the other one of the two is ≥ 128. The typecast to signed 16-bit integers then turns the former into a large positive value and the latter into a large negative value, triggering the function's clipping code.
3. The function then follows Bresenham's algorithm: left is ensured to be smaller than right by swapping the two values if necessary. If that happened, top and bottom are also swapped, regardless of their value – the algorithm does not care about their order.
4. The slope in the X dimension is calculated using an integer division of ((bottom - top) / (right - left)). Both subtractions are done on signed 16-bit integers, and overflow accordingly.
5. (-left × slope_x) is added to top, and left is set to 0.
6. If both top and bottom are < 0 or ≥ 640, there's nothing to be unblitted. Otherwise, the final coordinates are clipped to the VRAM range of [(0, 0), (639, 399)].
7. If the function got this far, the line to be unblitted is now very likely to reach from
   1. the top-left to the bottom-right corner, starting out at (0, 0) right away, or
   2. from the bottom-left corner to the top-right corner. In this case, you'd expect unblitting to end at (639, 0), but thanks to an off-by-one error, it actually ends at (640, -1), which is equivalent to (0, 0). Why add clipping to VRAM offset calculations when everything else is clipped already, right?

tl;dr: TH01 has a high chance of freezing at a boss defeat sequence if there are diagonally moving lasers on screen, and if your PC-98 system raises a General Protection Fault on a 4-byte write to offset 0xFFFF, and if you don't run a TSR with an INT 0Dh handler that might handle this fault differently.

The easiest fix option would be to just remove the attempted laser unblitting entirely, but that would also have an impact on this game's… distinctive visual glitches, in addition to touching a whole lot of code bytes. If I ever get funded to work on a hypothetical TH01 Anniversary Edition that completely rearchitects the game to fix all these glitches, it would be appropriate there, but not for something that purports to be the original game.

(Sidenote to further hype up this Anniversary Edition idea for PC-98 hardware owners: With the amount of performance left on the table at every corner of this game, I'm pretty confident that we can get it to work decently on PC-98 models with just an 80286 CPU.)

Since we're in critical infrastructure territory once again, I went for the most conservative fix with the least impact on the binary: Simply changing any VRAM offsets >= 0xFFFD to 0x0000 to avoid the GPF, and leaving all other bugs in place. Sure, it's rather lazy and "incorrect"; the function still unblits a 32-pixel block there, but adding a special case for blitting 24 pixels would add way too much code. And seriously, it's not like anything happens in the 8 pixels between (24, 0) and (31, 0) inclusive during gameplay to begin with. To balance out the additional per-row if() branch, I inlined the VRAM page change I/O, saving two function calls and one memory write per unblitted row.

That means it's time for a new community_choice_fixes build, containing the new definitive bugfixed versions of these games: 2022-05-31-community-choice-fixes.zip Check the th01_critical_fixes branch for the modified TH01 code. It also contains a fix for the HP bar heap corruption in test or debug mode – simply changing the == comparison to <= is enough to avoid it, and negative HP will still create aesthetic glitch art.

Once again, I then was left with ½ of a push, which I finally filled with some FUUIN.EXE code, specifically the verdict screen. The most interesting part here is the player title calculation, which is quite sneaky: There are only 6 skill levels, but three groups of titles for each level, and the title you'll see is picked from a random group. It looks like this is the first time anyone has documented the calculation?
As for the levels, ZUN definitely didn't expect players to do particularly well. With a 1cc being the standard goal for completing a Touhou game, it's especially funny how TH01 expects you to continue a lot: The code has branches for up to 21 continues, and the on-screen table explicitly leaves room for 3 digits worth of continues per 5-stage scene. Heck, these counts are even stored in 32-bit long variables.

Next up: 📝 Finally finishing the long overdue Touhou Patch Center MediaWiki update work, while continuing with Kikuri in the meantime. Originally I wasn't sure about what to do between Elis and Seihou, but with Ember2528's surprise contribution last week, y'all have demonstrated more than enough interest in the idea of getting TH01 done sooner rather than later. And I agree – after all, we've got the 25th anniversary of its first public release coming up on August 15, and I might still manage to completely decompile this game by that point…

📝 Posted:

2022-01-31 09:17 UTC

🚚 Summary of:

P0174, P0175, P0176, P0177, P0178, P0179, P0180, P0181

⌨ Commits:

27f901c...a0fe812, a0fe812...40ac9a7, 40ac9a7...c5dc45b, c5dc45b...5f0cabc, 5f0cabc...60621f8, 60621f8...9e5b344, 9e5b344...091f19f, 091f19f...313450f

💰 Funded by:

Ember2528, Yanga

🏷 Tags:

th01

-

rec98

+

gameplay

+

boss

+

sariel

+

danmaku-pattern

+

rng

+

waste

+

pc98

-

blitting

+

good-code

+

glitch

+

tcc

+

unused

+

jank

+

Here we go, TH01 Sariel! This is the single biggest boss fight in all of PC-98 Touhou: If we include all custom effect code we previously decompiled, it amounts to a total of 10.31% of all code in TH01 (and 3.14% overall). These 8 pushes cover the final 8.10% (or 2.47% overall), and are likely to be the single biggest delivery this project will ever see. Considering that I only managed to decompile 6.00% across all games in 2021, 2022 is already off to a much better start!

So, how can Sariel's code be that large? Well, we've got:

• 16 danmaku patterns; including the one snowflake detonating into a giant 94×32 hitbox
• Gratuitous usage of floating-point variables, bloating the binary thanks to Turbo C++ 4.0J's particularly horrid code generation
• The hatching birds that shoot pellets
• 3 separate particle systems, sharing the general idea, overall code structure, and blitting algorithm, but differing in every little detail
• The "gust of wind" background transition animation
• 5 sets of custom monochrome sprite animations, loaded from BOSS6GR?.GRC
• A further 3 hardcoded monochrome 8×8 sprites for the "swaying leaves" pattern during the second form

In total, it's just under 3,000 lines of C++ code, containing a total of 8 definite ZUN bugs, 3 of them being subpixel/pixel confusions. That might not look all too bad if you compare it to the 📝 player control function's 8 bugs in 900 lines of code, but given that Konngara had 0… (Edit (2022-07-17): Konngara contains two bugs after all: A 📝 possible heap corruption in test or debug mode, and the infamous 📝 temporary green discoloration.) And no, the code doesn't make it obvious whether ZUN coded Konngara or Sariel first; there's just as much evidence for either.

Some terminology before we start: Sariel's first form is separated into four phases, indicated by different background images, that cycle until Sariel's HP reach 0 and the second, single-phase form starts. The danmaku patterns within each phase are also on a cycle, and the game picks a random but limited number of patterns per phase before transitioning to the next one. The fight always starts at pattern 1 of phase 1 (the random purple lasers), and each new phase also starts at its respective first pattern.

Sariel's bugs already start at the graphics asset level, before any code gets to run. Some of the patterns include a wand raise animation, which is stored in BOSS6_2.BOS:

The "lowered wand" sprite is missing in this file simply because it's captured from the regular background image in VRAM, at the beginning of the fight and after every background transition. What I previously thought to be 📝 background storage code has therefore a different meaning in Sariel's case. Since this captured sprite is fully opaque, it will reset the entire 128×128 wand area… wait, 128×128, rather than 96×96? Yup, this lowered sprite is larger than necessary, wasting 1,967 bytes of conventional memory.
That still doesn't quite explain the second sprite in BOSS6_2.BOS though. Turns out that the black part is indeed meant to unblit the purple reflection (?) in the first sprite. But… that's not how you would correctly unblit that?

Just like Konngara, Sariel continues the approach of using a single function per danmaku pattern or custom entity. While I appreciate that this allows all pattern- and entity-specific state to be scoped locally to that one function, it quickly gets ugly as soon as such a function has to do more than one thing.
The "bird function" is particularly awful here: It's just one if(…) {…} else if(…) {…} else if(…) {…} chain with different branches for the subfunction parameter, with zero shared code between any of these branches. It also uses 64-bit floating-point double as its subpixel type… and since it also takes four of those as parameters (y'know, just in case the "spawn new bird" subfunction is called), every call site has to also push four double values onto the stack. Thanks to Turbo C++ even using the FPU for pushing a 0.0 constant, we have already reached maximum floating-point decadence before even having seen a single danmaku pattern. Why decadence? Every possible spawn position and velocity in both bird patterns just uses pixel resolution, with no fractional component in sight. And there goes another 720 bytes of conventional memory.

Speaking about bird patterns, the red-bird one is where we find the first code-level ZUN bug: The spawn cross circle sprite suddenly disappears after it finished spawning all the bird eggs. How can we tell it's a bug? Because there is code to smoothly fly this sprite off the playfield, that code just suddenly forgets that the sprite's position is stored in Q12.4 subpixels, and treats it as raw screen pixels instead. As a result, the well-intentioned 640×400 screen-space clipping rectangle effectively shrinks to 38×23 pixels in the top-left corner of the screen. Which the sprite is always outside of, and thus never rendered again.
The intended animation is easily restored though:

Also, did you know that birds actually have a quite unfair 14×38-pixel hitbox? Not that you'd ever collide with them in any of the patterns…

Another 3 of the 8 bugs can be found in the symmetric, interlaced spawn rays used in three of the patterns, and the 32×32 debris "sprites" shown at their endpoint, at the edge of the screen. You kinda have to commend ZUN's attention to detail here, and how he wrote a lot of code for those few rapidly animated pixels that you most likely don't even notice, especially with all the other wrong pixels resulting from rendering glitches. One of the bugs in the very final pattern of phase 4 even turns them into the vortex sprites from the second pattern in phase 1 during the first 5 frames of the first time the pattern is active, and I had to single-step the blitting calls to verify it.
It certainly was annoying how much time I spent making sense of these bugs, and all weird blitting offsets, for just a few pixels… Let's look at something more wholesome, shall we?

So far, we've only seen the PC-98 GRCG being used in RMW (read-modify-write) mode, which I previously 📝 explained in the context of TH01's red-white HP pattern. The second of its three modes, TCR (Tile Compare Read), affects VRAM reads rather than writes, and performs "color extraction" across all 4 bitplanes: Instead of returning raw 1bpp data from one plane, a VRAM read will instead return a bitmask, with a 1 bit at every pixel whose full 4-bit color exactly matches the color at that offset in the GRCG's tile register, and 0 everywhere else. Sariel uses this mode to make sure that the 2×2 particles and the wind effect are only blitted on top of "air color" pixels, with other parts of the background behaving like a mask. The algorithm:

1. Set the GRCG to TCR mode, and all 8 tile register dots to the air color
2. Read N bits from the target VRAM position to obtain an N-bit mask where all 1 bits indicate air color pixels at the respective position
3. AND that mask with the alpha plane of the sprite to be drawn, shifted to the correct start bit within the 8-pixel VRAM byte
4. Set the GRCG to RMW mode, and all 8 tile register dots to the color that should be drawn
5. Write the previously obtained bitmask to the same position in VRAM

Quite clever how the extracted colors double as a secondary alpha plane, making for another well-earned good-code tag. The wind effect really doesn't deserve it, though:

• ZUN calculates every intermediate result inside this function over and over and over again… Together with some ugly pointer arithmetic, this function turned into one of the most tedious decompilations in a long while.
• This gradual effect is blitted exclusively to the front page of VRAM, since parts of it need to be unblitted to create the illusion of a gust of wind. Then again, anything that moves on top of air-colored background – most likely the Orb – will also unblit whatever it covered of the effect…

As far as I can tell, ZUN didn't use TCR mode anywhere else in PC-98 Touhou. Tune in again later during a TH04 or TH05 push to learn about TDW, the final GRCG mode!

Speaking about the 2×2 particle systems, why do we need three of them? Their only observable difference lies in the way they move their particles:

1. Up or down in a straight line (used in phases 4 and 2, respectively)
2. Left or right in a straight line (used in the second form)
3. Left and right in a sinusoidal motion (used in phase 3, the "dark orange" one)

Out of all possible formats ZUN could have used for storing the positions and velocities of individual particles, he chose a) 64-bit / double-precision floating-point, and b) raw screen pixels. Want to take a guess at which data type is used for which particle system?

If you picked double for 1) and 2), and raw screen pixels for 3), you are of course correct! Not that I'm implying that it should have been the other way round – screen pixels would have perfectly fit all three systems use cases, as all 16-bit coordinates are extended to 32 bits for trigonometric calculations anyway. That's what, another 1.080 bytes of wasted conventional memory? And that's even calculated while keeping the current architecture, which allocates space for 3×30 particles as part of the game's global data, although only one of the three particle systems is active at any given time.

That's it for the first form, time to put on "Civilization of Magic"! Or "死なばもろとも"? Or "Theme of 地獄めくり"? Or whatever SYUGEN is supposed to mean…

… and the code of these final patterns comes out roughly as exciting as their in-game impact. With the big exception of the very final "swaying leaves" pattern: After 📝 Q4.4, 📝 Q28.4, 📝 Q24.8, and double variables, this pattern uses… decimal subpixels? Like, multiplying the number by 10, and using the decimal one's digit to represent the fractional part? Well, sure, if you really insist on moving the leaves in cleanly represented integer multiples of ⅒, which is infamously impossible in IEEE 754. Aside from aesthetic reasons, it only really combines less precision (10 possible fractions rather than the usual 16) with the inferior performance of having to use integer divisions and multiplications rather than simple bit shifts. And it's surely not because the leaf sprites needed an extended integer value range of [-3276, +3276], compared to Q12.4's [-2047, +2048]: They are clipped to 640×400 screen space anyway, and are removed as soon as they leave this area.

This pattern also contains the second bug in the "subpixel/pixel confusion hiding an entire animation" category, causing all of BOSS6GR4.GRC to effectively become unused:

At least their hitboxes are what you would expect, exactly covering the 30×30 pixels of Reimu's sprite. Both animation fixes are available on the th01_sariel_fixes branch.

After all that, Sariel's main function turned out fairly unspectacular, just putting everything together and adding some shake, transition, and color pulse effects with a bunch of unnecessary hardware palette changes. There is one reference to a missing BOSS6.GRP file during the first→second form transition, suggesting that Sariel originally had a separate "first form defeat" graphic, before it was replaced with just the shaking effect in the final game.
Speaking about the transition code, it is kind of funny how the… um, imperative and concrete nature of TH01 leads to these 2×24 lines of straight-line code. They kind of look like ZUN rattling off a laundry list of subsystems and raw variables to be reinitialized, making damn sure to not forget anything.

Whew! Second PC-98 Touhou boss completely decompiled, 29 to go, and they'll only get easier from here! 🎉 The next one in line, Elis, is somewhere between Konngara and Sariel as far as x86 instruction count is concerned, so that'll need to wait for some additional funding. Next up, therefore: Looking at a thing in TH03's main game code – really, I have little idea what it will be!

Now that the store is open again, also check out the 📝 updated RE progress overview I've posted together with this one. In addition to more RE, you can now also directly order a variety of mods; all of these are further explained in the order form itself.

📝 Posted:

2021-09-12 18:33 UTC

🚚 Summary of:

P0157

⌨ Commits:

4bc6405...bf7bb7e

💰 Funded by:

Yanga

🏷 Tags:

th01

-

rec98

+

pc98

-

animation

+

Yup, there still are features that can be fully covered in a single push and don't lead to sprawling blog posts. The giant STAGE number and HARRY UP messages, as well as the flashing transparent 東方★靈異伝 at the beginning of each scene are drawn by retrieving the glyphs for each letter from font ROM, and then "blitting" them to text RAM by placing a colored fullwidth 16×16 square at every pixel that is set in the font bitmap.
And 📝 once again, ZUN's code there matches the mediocre example code for the related hardware interrupt from the PC-9801 Programmers' Bible. It's not 100% copied this time, but definitely inspired by the code on page 121. Therefore, we can conclude that these letters are probably only displayed as these 16× scaled glyphs because that book had code on how to achieve this effect.

ZUN "improved" on the example code by implementing a write-only cursor over the entire text RAM that fills every 16×16 cell with a differently colored space character, fully clearing the text RAM as a side effect. For once, he even removed some redundancy here by using helper functions! It's all still far from good-code though. For example, there's a function for filling 5 rows worth of cells, which he uses for both the top and bottom margin of these letters. But since the bottom margin starts at the 22nd line, the code writes past the 25th line and into the second TRAM page. Good that this page is not used by either the hardware or the game.

These cursor functions can actually write any fullwidth JIS code point to text RAM… and seem to do that in a rather simplified way, because shouldn't you set the most significant bit to indicate the right half of a fullwidth character? That's what's written in the same book that ZUN copied all functions out of, after all. 🤔 Researching this led me down quite the rabbit hole, where I found an oddity in PC-98 text RAM rendering that no single one of the widely-used PC-98 emulators gets completely right. I'm almost done with the 2-push research into this issue, which will include fixes for DOSBox-X and Neko Project II. The only thing I'm missing to get these fully accurate is a screenshot of the output created by this binary, on any PC-98 model made by EPSON: 2021-09-12-jist0x28.com.zip That's the reason why this push was rather delayed. Thanks in advance to anyone who'd like to help with this!

In maybe more disappointing news: Sariel is going to be delayed for a while longer. 😕 The player- and HUD-related functions, which previously delayed further progress there, turned out to call a lot of not yet RE'd functions themselves. Seems as if we're doing most of the card-flipping code second, after all? Next up: Point and bomb items, which at least are a significant step in terms of position independence.

📝 Posted:

2021-05-27 19:25 UTC

🚚 Summary of:

P0140, P0141, P0142

⌨ Commits:

d985811...d856f7d, d856f7d...5afee78, 5afee78...08bc188

💰 Funded by:

[Anonymous], rosenrose, Yanga

🏷 Tags:

th01

-

rec98

+

pc98

-

dosbox-x

+

gameplay

+

boss

+

konngara

+

danmaku-pattern

+

Alright, onto Konngara! Let's quickly move the escape sequences used later in the battle to C land, and then we can immediately decompile the loading and entrance animation function together with its filenames. Might as well reverse-engineer those escape sequences while I'm at it, though – even if they aren't implemented in DOSBox-X, they're well documented in all those Japanese PDFs, so this should be no big deal…

…wait, ESC )3 switches to "graph mode"? As opposed to the default "kanji mode", which can be re-entered via ESC )0? Let's look up graph mode in the PC-9801 Programmers' Bible then…

> Kanji cannot be handled in this mode.

…and that's apparently all it has to say. Why have it then, on a platform whose main selling point is a kanji ROM, and where Shift-JIS (and, well, 7-bit ASCII) are the only native encodings? No support for graph mode in DOSBox-X either… yeah, let's take a deep dive into NEC's IO.SYS, and get to the bottom of this.

And yes, graph mode pretty much just disables Shift-JIS decoding for characters written via INT 29h, the lowest-level way of "just printing a char" on DOS, which every printf() will ultimately end up calling. Turns out there is a use for it though, which we can spot by looking at the 8×16 half-width section of font ROM:

The half-width glyphs marked in red correspond to the byte ranges from 0x80-0x9F and 0xE0-0xFF… which Shift-JIS defines as lead bytes for two-byte, full-width characters. But if we turn off Shift-JIS decoding…

Jackpot, we get those half-width characters when printing their corresponding bytes.
I've re-implemented all my findings into DOSBox-X, which will include graph mode in the upcoming 0.83.14 release. If P0140 looks a bit empty as a result, that's why – most of the immediate feature work went into DOSBox-X, not into ReC98. That's the beauty of "anything" pushes.

So, after switching to graph mode, TH01 does… one of the slowest possible memset()s over all of text RAM – one printf(" ") call for every single one of its 80×25 half-width cells – before switching back to kanji mode. What a waste of RE time…? Oh well, at least we've now got plenty of proof that these weird escape sequences actually do nothing of interest.

As for the Konngara code itself… well, it's script-like code, what can you say. Maybe minimally sloppy in some places, but ultimately harmless.
One small thing that might not be widely known though: The large, blue-green Siddhaṃ seed syllables are supposed to show up immediately, with no delay between them? Good to know. Clocking your emulator too low tends to roll them down from the top of the screen, and will certainly add a noticeable delay between the four individual images.

… Wait, but this means that ZUN could have intended this "effect". Why else would he not only put those syllables into four individual images (and therefore add at least the latency of disk I/O between them), but also show them on the foreground VRAM page, rather than on the "back buffer"?

Meanwhile, in 📝 another instance of "maybe having gone too far in a few places": Expressing distances on the playfield as fractions of its width and height, just to avoid absolute numbers? Raw numbers are bad because they're in screen space in this game. But we've already been throwing PLAYFIELD_ constants into the mix as a way of explicitly communicating screen space, and keeping raw number literals for the actual playfield coordinates is looking increasingly sloppy… I don't know, fractions really seemed like the most sensible thing to do with what we're given here. 😐

So, 2 pushes in, and we've got the loading code, the entrance animation, facial expression rendering, and the first one out of Konngara's 12 danmaku patterns. Might not sound like much, but since that first pattern involves those blue-green diamond sprites and therefore is one of the more complicated ones, it all amounts to roughly 21.6% of Konngara's code. That's 7 more pushes to get Konngara done, then? Next up though: Two pushes of website improvements.

📝 Posted:

2020-12-18 17:43 UTC

🚚 Summary of:

P0130, P0131

⌨ Commits:

6d69ea8...576def5, 576def5...dc9e3ee

💰 Funded by:

Yanga

🏷 Tags:

th01

-

rec98

+

pc98

-

blitting

+

good-code

+

jank

+

gameplay

+

boss

+

rng

+

50% hype! 🎉 But as usual for TH01, even that final set of functions shared between all bosses had to consume two pushes rather than one…

First up, in the ongoing series "Things that TH01 draws to the PC-98 graphics layer that really should have been drawn to the text layer instead": The boss HP bar. Oh well, using the graphics layer at least made it possible to have this half-red, half-white pattern for the middle section.
This one pattern is drawn by making surprisingly good use of the GRCG. So far, we've only seen it used for fast monochrome drawing:

// Setting up fast drawing using color #9 (1001 in binary)
grcg_setmode(GC_RMW);
outportb(0x7E, 0xFF); // Plane 0: (B): (********)
outportb(0x7E, 0x00); // Plane 1: (R): (        )
outportb(0x7E, 0x00); // Plane 2: (G): (        )
outportb(0x7E, 0xFF); // Plane 3: (E): (********)

// Write a checkerboard pattern (* * * * ) in color #9 to the top-left corner,
// with transparent blanks. Requires only 1 VRAM write to a single bitplane:
// The GRCG automatically writes to the correct bitplanes, as specified above
*(uint8_t *)(MK_FP(0xA800, 0)) = 0xAA;

But since this is actually an 8-pixel tile register, we can set any 8-pixel pattern for any bitplane. This way, we can get different colors for every one of the 8 pixels, with still just a single VRAM write of the alpha mask to a single bitplane:

grcg_setmode(GC_RMW); //  Final color: (A7A7A7A7)
outportb(0x7E, 0x55); // Plane 0: (B): ( * * * *)
outportb(0x7E, 0xFF); // Plane 1: (R): (********)
outportb(0x7E, 0x55); // Plane 2: (G): ( * * * *)
outportb(0x7E, 0xAA); // Plane 3: (E): (* * * * )

And I thought TH01 only suffered the drawbacks of PC-98 hardware, making so little use of its actual features that it's perhaps not fair to even call it "a PC-98 game"… Still, I'd say that "bad PC-98 port of an idea" describes it best.

However, after that tiny flash of brilliance, the surrounding HP rendering code goes right back to being the typical sort of confusing TH01 jank. There's only a single function for the three distinct jobs of

• incrementing HP during the boss entrance animation,
• decrementing HP if hit by the Orb, and
• redrawing the entire bar, because it's still all in VRAM, and Sariel wants different backgrounds,

with magic numbers to select between all of these.

VRAM of course also means that the backgrounds behind the individual hit points have to be stored, so that they can be unblitted later as the boss is losing HP. That's no big deal though, right? Just allocate some memory, copy what's initially in VRAM, then blit it back later using your foundational set of blitting funct– oh, wait, TH01 doesn't have this sort of thing, right The closest thing, 📝 once again, are the .PTN functions. And so, the game ends up handling these 8×16 background sprites with 16×16 wrappers around functions for 32×32 sprites. That's quite the recipe for confusion, especially since ZUN preferred copy-pasting the necessary ridiculous arithmetic expressions for calculating positions, .PTN sprite IDs, and the ID of the 16×16 quarter inside the 32×32 sprite, instead of just writing simple helper functions. He did manage to make the result mostly bug-free this time around, though! (Edit (2022-05-31): Nope, there's a 📝 potential heap corruption after all, which can be triggered in some fights in test mode (game t) or debug mode (game d).) There's one minor hit point discoloration bug if the red-white or white sections start at an odd number of hit points, but that's never the case for any of the original 7 bosses.
The remaining sloppiness is ultimately inconsequential as well: The game always backs up twice the number of hit point backgrounds, and thus uses twice the amount of memory actually required. Also, this self-restriction of only unblitting 16×16 pixels at a time requires any remaining odd hit point at the last position to, of course, be rendered again

After stumbling over the weakest imaginable random number generator, we finally arrive at the shared boss↔orb collision handling function, the final blocker among the final blockers. This function takes a whopping 12 parameters, 3 of them being references to int values, some of which are duplicated for every one of the 7 bosses, with no generic boss struct anywhere. 📝 Previously, I speculated that YuugenMagan might have been the first boss to be programmed for TH01. With all these variables though, there is some new evidence that SinGyoku might have been the first one after all: It's the only boss to use its own HP and phase frame variables, with the other bosses sharing the same two globals.

While this function only handles the response to a boss↔orb collision, it still does way too much to describe it briefly. Took me quite a while to frame it in terms of invincibility (which is the main impact of all of this that can be observed in gameplay code). That made at least some sort of sense, considering the other usages of the variables passed as references to that function. Turns out that YuugenMagan, Kikuri, and Elis abuse what's meant to be the "invincibility frame" variable as a frame counter for some of their animations 🙄
Oh well, the game at least doesn't call the collision handling function during those, so "invincibility frame" is technically still a correct variable name there.

And that's it! We're finally ready to start with Konngara, in 2021. I've been waiting quite a while for this, as all this high-level boss code is very likely to speed up TH01 progress quite a bit. Next up though: Closing out 2020 with more of the technical debt in the other games.

📝 Posted:

2020-09-28 11:52 UTC

🚚 Summary of:

P0120, P0121

⌨ Commits:

453dd3c...3c008b6, 3c008b6...5c42fcd

💰 Funded by:

Yanga

🏷 Tags:

th01

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rec98

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pc98

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blitting

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waste

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jank

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boss

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mima-th01

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Back to TH01, and its boss sprite format… with a separate class for storing animations that only differs minutely from the 📝 regular boss entity class I covered last time? Decompiling this class was almost free, and the main reason why the first of these pushes ended up looking pretty huge.

Next up were the remaining shape drawing functions from the code segment that started with the .GRC functions. P0105 already started these with the (surprisingly sanely implemented) 8×8 diamond, star, and… uh, snowflake (?) sprites , prominently seen in the Konngara, Elis, and Sariel fights, respectively. Now, we've also got:

• ellipse arcs with a customizable angle distance between the individual dots – mostly just used for drawing full circles, though
• line loops – which are only used for the rotating white squares around Mima, meaning that the white star in the YuugenMagan fight got a completely redundant reimplementation
• and the surprisingly weirdest one, drawing the red invincibility sprites.

The weirdness becomes obvious with just a single screenshot:

First, we've got the obvious issue of the sprites not being clipped at the right edge of VRAM, with the rightmost pixels in each row of the sprite extending to the beginning of the next row. Well, that's just what you get if you insist on writing unique low-level blitting code for the majority of the individual sprites in the game… 🤷
More importantly though, the sprite sheet looks like this: So how do we even get these fully filled red diamonds?

Well, turns out that the sprites are never consistently unblitted during their 8 frames of animation. There is a function that looks like it unblits the sprite… except that it starts with by enabling the GRCG and… reading from the first bitplane on the background page? If this was the EGC, such a read would fill some internal registers with the contents of all 4 bitplanes, which can then subsequently be blitted to all 4 bitplanes of any VRAM page with a single memory write. But with the GRCG in RMW mode, reads do nothing special, and simply copy the memory contents of one bitplane to the read destination. Maybe ZUN thought that setting the RMW color to red also sets some internal 4-plane mask register to match that color?
Instead, the rather random pixels read from the first bitplane are then used as a mask for a second blit of the same red sprite. Effectively, this only really "unblits" the invincibility pixels that are drawn on top of Reimu's sprite. Since Reimu is drawn first, the invincibility sprites are overwritten anyway. But due to the palette color layout of Reimu's sprite, its pixels end up fully masking away any invincibility sprite pixels in that second blit, leaving VRAM untouched as a result. Anywhere else though, this animation quickly turns into the union of all animation frames.

Then again, if that 16-dot-aligned rectangular unblitting function is all you know about the EGC, and you can't be bothered to write a perfect unblitter for 8×8 sprites, it becomes obvious why you wouldn't want to use it:

Because Reimu would barely be visible under all that flicker. In comparison, those fully filled diamonds actually look pretty good.

After all that, the remaining time wouldn't have been enough for the next few essential classes, so I closed out the push with three more VRAM effects instead:

• Single-bitplane pixel inversion inside a 32×32 square – the main effect behind the discoloration seen in the bomb animation, as well as the expanding squares at the end of Kikuri's and Sariel's entrance animation
• EGC-accelerated VRAM row copies – the second half of smooth and fully hardware-accelerated scrolling for backgrounds that are twice the size of VRAM
• And finally, the VRAM page content transition function using meshed 8×8 squares, used for the blocky transition to Sariel's first and second phases. Which is quite ridiculous in just how needlessly bloated it is. I'm positive that this sort of thing could have also been accelerated using the PC-98's EGC… although simply writing better C would have already gone a long way. The function also comes with three unused mesh patterns.

And with that, ReC98, as a whole, is not only ⅓ done, but I've also fully caught up with the feature backlog for the first time in the history of this crowdfunding! Time to go into maintenance mode then, while we wait for the next pushes to be funded. Got a huge backlog of tiny maintenance issues to address at a leisurely pace, and of course there's also the 📝 16-bit build system waiting to be finished.

📝 Posted:

2020-06-13 19:25 UTC

🚚 Summary of:

P0096, P0097, P0098

⌨ Commits:

8ddb778...8283c5e, 8283c5e...600f036, 600f036...ad06748

💰 Funded by:

Ember2528, Yanga

🏷 Tags:

th01

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rec98

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file-format

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pc98

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blitting

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gameplay

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player

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shot

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jank

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mod

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tcc

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So, let's finally look at some TH01 gameplay structures! The obvious choices here are player shots and pellets, which are conveniently located in the last code segment. Covering these would therefore also help in transferring some first bits of data in REIIDEN.EXE from ASM land to C land. (Splitting the data segment would still be quite annoying.) Player shots are immediately at the beginning…

…but wait, these are drawn as transparent sprites loaded from .PTN files. Guess we first have to spend a push on 📝 Part 2 of this format.
Hm, 4 functions for alpha-masked blitting and unblitting of both 16×16 and 32×32 .PTN sprites that align the X coordinate to a multiple of 8 (remember, the PC-98 uses a planar VRAM memory layout, where 8 pixels correspond to a byte), but only one function that supports unaligned blitting to any X coordinate, and only for 16×16 sprites? Which is only called twice? And doesn't come with a corresponding unblitting function?

Yeah, "unblitting". TH01 isn't double-buffered, and uses the PC-98's second VRAM page exclusively to store a stage's background and static sprites. Since the PC-98 has no hardware sprites, all you can do is write pixels into VRAM, and any animated sprite needs to be manually removed from VRAM at the beginning of each frame. Not using double-buffering theoretically allows TH01 to simply copy back all 128 KB of VRAM once per frame to do this. But that would be pretty wasteful, so TH01 just looks at all animated sprites, and selectively copies only their occupied pixels from the second to the first VRAM page.

Alright, player shot class methods… oh, wait, the collision functions directly act on the Yin-Yang Orb, so we first have to spend a push on that one. And that's where the impression we got from the .PTN functions is confirmed: The orb is, in fact, only ever displayed at byte-aligned X coordinates, divisible by 8. It's only thanks to the constant spinning that its movement appears at least somewhat smooth.
This is purely a rendering issue; internally, its position is tracked at pixel precision. Sadly, smooth orb rendering at any unaligned X coordinate wouldn't be that trivial of a mod, because well, the necessary functions for unaligned blitting and unblitting of 32×32 sprites don't exist in TH01's code. Then again, there's so much potential for optimization in this code, so it might be very possible to squeeze those additional two functions into the same C++ translation unit, even without position independence…

More importantly though, this was the right time to decompile the core functions controlling the orb physics – probably the highlight in these three pushes for most people.
Well, "physics". The X velocity is restricted to the 5 discrete states of -8, -4, 0, 4, and 8, and gravity is applied by simply adding 1 to the Y velocity every 5 frames No wonder that this can easily lead to situations in which the orb infinitely bounces from the ground.
At least fangame authors now have a reference of how ZUN did it originally, because really, this bad approximation of physics had to have been written that way on purpose. But hey, it uses 64-bit floating-point variables!

…sometimes at least, and quite randomly. This was also where I had to learn about Turbo C++'s floating-point code generation, and how rigorously it defines the order of instructions when mixing double and float variables in arithmetic or conditional expressions. This meant that I could only get ZUN's original instruction order by using literal constants instead of variables, which is impossible right now without somehow splitting the data segment. In the end, I had to resort to spelling out ⅔ of one function, and one conditional branch of another, in inline ASM. 😕 If ZUN had just written 16.0 instead of 16.0f there, I would have saved quite some hours of my life trying to decompile this correctly…

To sort of make up for the slowdown in progress, here's the TH01 orb physics debug mod I made to properly understand them. Edit (2022-07-12): This mod is outdated, 📝 the current version is here! 2020-06-13-TH01OrbPhysicsDebug.zip To use it, simply replace REIIDEN.EXE, and run the game in debug mode, via game d on the DOS prompt.
Its code might also serve as an example of how to achieve this sort of thing without position independence.

Alright, now it's time for player shots though. Yeah, sure, they don't move horizontally, so it's not too bad that those are also always rendered at byte-aligned positions. But, uh… why does this code only use the 16×16 alpha-masked unblitting function for decaying shots, and just sloppily unblits an entire 16×16 square everywhere else?

The worst part though: Unblitting, moving, and rendering player shots is done in a single function, in that order. And that's exactly where TH01's sprite flickering comes from. Since different types of sprites are free to overlap each other, you'd have to first unblit all types, then move all types, and then render all types, as done in later PC-98 Touhou games. If you do these three steps per-type instead, you will unblit sprites of other types that have been rendered before… and therefore end up with flicker.
Oh, and finally, ZUN also added an additional sloppy 16×16 square unblit call if a shot collides with a pellet or a boss, for some guaranteed flicker. Sigh.

And that's ⅓ of all ZUN code in TH01 decompiled! Next up: Pellets!

📝 Posted:

2020-05-31 15:47 UTC

🚚 Summary of:

P0095

⌨ Commits:

57a8487...8ddb778

💰 Funded by:

Yanga

🏷 Tags:

th01

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rec98

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position-independence

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pc98

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thief

+

🎉 TH01's OP.EXE and FUUIN.EXE are now fully position-independent! 🎉

What does this mean?

You can now add any data or code to TH01's main menu or ending cutscenes, by simply editing the ReC98 source, writing your mod in ASM or C++, and recompiling the code. Since all absolute memory addresses in OP and FUUIN 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.
As an example, the most popular TH01 mod idea, replacing MDRV2 with PMD, could now at least be prototyped and tested in OP.EXE, without having to worry about x86 instruction lengths.
📝 Check the video I made for the TH04/TH05 OP.EXE PI announcement for a basic overview of how to do that.

What does this not mean?

The original ZUN code hasn't been completely decompiled yet. The final high-level parts of both the main menu and the cutscenes are still ASM, which might make modding a bit inconvenient right now.
It's not that much more code though, and could quickly be covered in a few pushes if requested. Due to the plentiful monthly subscriptions, the shop will stay closed for regular orders until the end of June, but backers with outstanding contributions could request that now if they want to – simply drop me a mail. Otherwise, the "generic TH01 RE" money will continue to go towards the main game. That way, we'll have more substance to show once we do decide to decompile the rest of OP.EXE and FUUIN.EXE, and likely get some press coverage as a result.

Then again, we've been building up to this point over the last few pushes, and it only really needed a quick look over the remaining false positives. The majority of the time therefore went towards more PI in REIIDEN.EXE, where the bitplane pointers for .BOS files yielded some quite big gains. Couldn't really find any obvious reason why ZUN used two slighly different variations on loading and blitting those files, though…

As the final function in this rather random push, we got TH01's hardware-powered scrolling function, used for screen shaking effects and the scrolling backgrounds at the start of the Final Boss stages. And while I tried to document all these I/O writes… it turned out that ZUN actually copied the entire function straight from the PC-9801 Programmers' Bible, with no changes. It's the setgsta() example function on page 150. Which is terribly suboptimal and bloated – all those integer divisions are really not how you'd write such code for a 16-bit compiler from the 90's…

And that gives us 60% PI overall, and 50% PI over all of TH01! Next up: More structures… and classes, even?

📝 Posted:

2020-03-03 12:17 UTC

🚚 Summary of:

P0080

⌨ Commits:

cd48aa3...0252da2

💰 Funded by:

Splashman, Ember2528

🏷 Tags:

th01

-

rec98

+

pc98

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palette

+

Last part of TH01's main graphics function segment, and we've got even more code that alternates between being boring and being slightly weird. But at least, "boring" also meant "consistent" for once. And so progress continued to be as fast as expected from the last TH01 pushes, yielding 3.3% in TH01 RE%, and 1% in overall RE%, within a single day. There even was enough time to decompile another full code segment, which bundles all the hardware initialization and cleanup calls into single functions to be run when starting and exiting the game. Which might be interesting for at least one person, I guess

But seriously, trying to access page 2 on a system with only page 0 and 1? Had to get out my real PC-98 to double-check that I wasn't missing anything here, since every emulator only looks at the bottom bit of the page number. But real hardware seems to do the same, and there really is nothing special to it semantically, being equivalent to page 0. 🤷

Next up in TH01, we'll have some file format code!

📝 Posted:

2020-01-14 21:15 UTC

🚚 Summary of:

P0067, P0068, P0069

⌨ Commits:

e55a48b...ebb30ce, ebb30ce...2ac00d4, e0d0dcd...0f18dbc

💰 Funded by:

Splashman, Yanga, [Anonymous]

🏷 Tags:

th01

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rec98

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pc98

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blitting

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glitch

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boss

+

mima-th01

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yuugenmagan

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Now that's more like the speed I was expecting! After a few more unused functions for palette fading and rectangle blitting, we've reached the big line drawing functions. And the biggest one among them, drawing a straight line at any angle between two points using Bresenham's algorithm, actually happens to be the single longest function present in more than one binary in all of PC-98 Touhou, and #23 on the list of individual longest functions.

And it technically has a ZUN bug! If you pass a point outside the (0, 0) - (639, 399) screen range, the function will calculate a new point at the edge of the screen, so that the resulting line will retain the angle intended by the points given. Except that it does so by calculating the line slope using an integer division rather than a floating-point one Doesn't seem like it actually causes any weirdly skewed lines to be drawn in-game, though; that case is only hit in the Mima boss fight, which draws a few lines with a bottom coordinate of 400 rather than the maximum of 399. It might also cause the wrong background pixels to be restored during parts of the YuugenMagan fight, leading to flickering sprites, but seriously, that's an issue everywhere you look in this game.

Together with the rendering-text-to-VRAM function we've mostly already known from TH02, this pushed the total RE percentage well over 20%, and almost doubled the TH01 RE percentage, all within three pushes. And comparatively, it went really smoothly, to the point (ha) where I even had enough time left to also include the single-point functions that come next in that code segment. Since about half of the remaining functions in OP.EXE are present in more than just itself, I'll be able to at least keep up this speed until OP.EXE hits the 70% RE mark. That is, as long as the backers' priorities continue to be generic RE or "giving some love to TH01"… we don't have a precedent for TH01's actual game code yet.

And that's all the TH01 progress funded for January! Next up, we actually do have a focus on TH03's game and scoring mechanics… or at least the foundation for that.