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Showing all posts tagged pc98-

📝 Posted:
🚚 Summary of:
P0140, P0141, P0142
Commits:
d985811...d856f7d, d856f7d...5afee78, 5afee78...08bc188
💰 Funded by:
[Anonymous], rosenrose, Yanga
🏷 Tags:
rec98+ th01+ 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. :tannedcirno:

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:
🚚 Summary of:
P0130, P0131
Commits:
6d69ea8...576def5, 576def5...dc9e3ee
💰 Funded by:
Yanga
🏷 Tags:
rec98+ th01+ 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 :tannedcirno: 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. :zunpet: 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! 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 :onricdennat:


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:
🚚 Summary of:
P0126, P0127
Commits:
6c22af7...8b01657, 8b01657...dc65b59
💰 Funded by:
Blue Bolt, [Anonymous]
🏷 Tags:
rec98+ th03+ th04+ th05+ pc98- micro-optimization+ tcc+ tasm+ meta+

Alright, back to continuing the master.hpp transition started in P0124, and repaying technical debt. The last blog post already announced some ridiculous decompilations… and in fact, not a single one of the functions in these two pushes was decompilable into idiomatic C/C++ code.

As usual, that didn't keep me from trying though. The TH04 and TH05 version of the infamous 16-pixel-aligned, EGC-accelerated rectangle blitting function from page 1 to page 0 was fairly average as far as unreasonable decompilations are concerned.
The big blocker in TH03's MAIN.EXE, however, turned out to be the .MRS functions, used to render the gauge attack portraits and bomb backgrounds. The blitting code there uses the additional FS and GS segment registers provided by the Intel 386… which

  1. are not supported by Turbo C++'s inline assembler, and
  2. can't be turned into pointers, due to a compiler bug in Turbo C++ that generates wrong segment prefix opcodes for the _FS and _GS pseudo-registers.

Apparently I'm the first one to even try doing that with this compiler? I haven't found any other mention of this bug…
Compiling via assembly (#pragma inline) would work around this bug and generate the correct instructions. But that would incur yet another dependency on a 16-bit TASM, for something honestly quite insignificant.

What we can always do, however, is using __emit__() to simply output x86 opcodes anywhere in a function. Unlike spelled-out inline assembly, that can even be used in helper functions that are supposed to inline… which does in fact allow us to fully abstract away this compiler bug. Regular if() comparisons with pseudo-registers wouldn't inline, but "converting" them into C++ template function specializations does. All that's left is some C preprocessor abuse to turn the pseudo-registers into types, and then we do retain a normal-looking poke() call in the blitting functions in the end. 🤯

Yeah… the result is batshit insane. I may have gone too far in a few places…


One might certainly argue that all these ridiculous decompilations actually hurt the preservation angle of this project. "Clearly, ZUN couldn't have possibly written such unreasonable C++ code. So why pretend he did, and not just keep it all in its more natural ASM form?" Well, there are several reasons:

  • Future port authors will merely have to translate all the pseudo-registers and inline assembly to C++. For the former, this is typically as easy as replacing them with newly declared local variables. No need to bother with function prolog and epilog code, calling conventions, or the build system.
  • No duplication of constants and structures in ASM land.
  • As a more expressive language, C++ can document the code much better. Meticulous documentation seems to have become the main attraction of ReC98 these days – I've seen it appreciated quite a number of times, and the continued financial support of all the backers speaks volumes. Mods, on the other hand, are still a rather rare sight.
  • Having as few .ASM files in the source tree as possible looks better to casual visitors who just look at GitHub's repo language breakdown. This way, ReC98 will also turn from an "Assembly project" to its rightful state of "C++ project" much sooner.
  • And finally, it's not like the ASM versions are gone – they're still part of the Git history.

Unfortunately, these pushes also demonstrated a second disadvantage in trying to decompile everything possible: Since Turbo C++ lacks TASM's fine-grained ability to enforce code alignment on certain multiples of bytes, it might actually be unfeasible to link in a C-compiled object file at its intended original position in some of the .EXE files it's used in. Which… you're only going to notice once you encounter such a case. Due to the slightly jumbled order of functions in the 📝 second, shared code segment, that might be long after you decompiled and successfully linked in the function everywhere else.

And then you'll have to throw away that decompilation after all 😕 Oh well. In this specific case (the lookup table generator for horizontally flipping images), that decompilation was a mess anyway, and probably helped nobody. I could have added a dummy .OBJ that does nothing but enforce the needed 2-byte alignment before the function if I really insisted on keeping the C version, but it really wasn't worth it.


Now that I've also described yet another meta-issue, maybe there'll really be nothing to say about the next technical debt pushes? :onricdennat: Next up though: Back to actual progress again, with TH01. Which maybe even ends up pushing that game over the 50% RE mark?

📝 Posted:
🚚 Summary of:
P0124, P0125
Commits:
72dfa09...056b1c7, 056b1c7...f6a3246
💰 Funded by:
Blue Bolt, [Anonymous]
🏷 Tags:
rec98+ th02+ th04+ pc98- menu+ waste+ master.lib+ waste+

Turns out that TH04's player selection menu is exactly three times as complicated as TH05's. Two screens for character and shot type rather than one, and a way more intricate implementation for saving and restoring the background behind the raised top and left edges of a character picture when moving the cursor between Reimu and Marisa. TH04 decides to backup precisely only the two 256×8 (top) and 8×244 (left) strips behind the edges, indicated in red in the picture below.

These take up just 4 KB of heap memory… but require custom blitting functions, and expanding this explicitly hardcoded approach to TH05's 4 characters would have been pretty annoying. So, rather than, uh, not explicitly hardcoding it all, ZUN decided to just be lazy with the backup area in TH05, saving the entire 640×400 screen, and thus spending 128 KB of heap memory on this rather simple selection shadow effect. :zunpet:


So, this really wasn't something to quickly get done during the first half of a push, even after already having done TH05's equivalent of this menu. But since life is very busy right now, I also used the occasion to start addressing another code organization annoyance: master.lib's single master.h header file.

  • Now that ReC98 is trying to develop (or at least mimic) a more type-safe C++ foundation to model the PC-98 hardware, a pure C header (with counter-productive C++ extensions) is becoming increasingly unidiomatic. By moving some of the original assumptions about function parameters into the type system, we can also reduce the reliance on its Japanese-only documentation without having to translate it :tannedcirno:
  • It's far from complete with regards to providing compile-time PC-98 hardware constants and helpful types. In fact, we started to add these in our own header files a long time ago.
  • It's quite bloated, with at least 2800 lines of code that currently are #included into the vast majority of files, not counting master.h's recursively included C standard library headers. PC-98 Touhou only makes direct use of a rather small fraction of its contents.
  • And finally, all the DOS/V compatibility definitions are especially useless in the context of ReC98. As I've noted 📝 time and 📝 time again, porting PC-98 Touhou to IBM-compatible DOS won't be easy, and MASTER_DOSV won't be helping much. Therefore, my upstream version of ReC98 will never include all of master.lib. There's no point in lengthening compile times for everyone by default, and those will be getting quite noticeable after moving to a full 16-bit build process.
    (Actually, what retro system ports should rather be doing: Get rid of master.lib's original ASM code, replace it with readable, modern C++, and then simply convert the optimized assembly output of modern compilers to your ISA of choice. Improving the landscape of such assembly or object file converters would benefit everyone!)

So, time to start a new master.hpp header that would contain just the declarations from master.h that PC-98 Touhou actually needs, plus some semantic (yes, semantic) sugar. Comparing just the old master.h to just the new master.hpp after roughly 60% of the transition has been completed, we get median build times of 319 ms for master.h, and 144 ms for master.hpp on my (admittedly rather slow) DOSBox setup. Nice!
As of this push, ReC98 consists of 107 translation units that have to be compiled with Turbo C++ 4.0J. Fully rebuilding all of these currently takes roughly 37.5 seconds in DOSBox. After the transition to master.hpp is done, we could therefore shave some 10 to 15 seconds off this time, simply by switching header files. And that's just the beginning, as this will also pave the way for further #include optimizations. Life in this codebase will be great!


Unfortunately, there wasn't enough time to repay some of the actual technical debt I was looking forward to, after all of this. Oh well, at least we now also have nice identifiers for the three different boldface options that are used when rendering text to VRAM, after procrastinating that issue for almost 11 months. Next up, assuming the existing subscriptions: More ridiculous decompilations of things that definitely weren't originally written in C, and a big blocker in TH03's MAIN.EXE.

📝 Posted:
🚚 Summary of:
P0120, P0121
Commits:
453dd3c...3c008b6, 3c008b6...5c42fcd
💰 Funded by:
Yanga
🏷 Tags:
rec98+ th01+ pc98- blitting+ waste+ jank+ boss+ mima-th01+

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? :zunpet:
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 exploding 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:
🚚 Summary of:
P0096, P0097, P0098
Commits:
8ddb778...8283c5e, 8283c5e...600f036, 600f036...ad06748
💰 Funded by:
Ember2528, Yanga
🏷 Tags:
rec98+ th01+ file-format+ pc98- blitting+ gameplay+ player+ shot+ jank+ mod+ tcc+

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? :thonk:

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. :tannedcirno: 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 :zunpet: 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! :onricdennat:

…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: 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:
🚚 Summary of:
P0095
Commits:
57a8487...8ddb778
💰 Funded by:
Yanga
🏷 Tags:
rec98+ th01+ position-independence+ pc98- 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… :onricdennat:

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. :zunpet: 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:
🚚 Summary of:
P0080
Commits:
cd48aa3...0252da2
💰 Funded by:
Splashman, Ember2528
🏷 Tags:
rec98+ th01+ pc98- 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 :tannedcirno:

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:
🚚 Summary of:
P0067, P0068, P0069
Commits:
e55a48b...ebb30ce, ebb30ce...2ac00d4, e0d0dcd...0f18dbc
💰 Funded by:
Splashman, Yanga, [Anonymous]
🏷 Tags:
rec98+ th01+ pc98- blitting+ glitch+ boss+ mima-th01+ yuugenmagan+

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 :zunpet: 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.

📝 Posted:
🚚 Summary of:
P0059
Commits:
01de290...8b62780
💰 Funded by:
[Anonymous], -Tom-
🏷 Tags:
rec98+ th04+ th05+ pc98- position-independence+ uth05win+ meta+

With no feedback to 📝 last week's blog post, I assume you all are fine with how things are going? Alright then, another one towards position independence, with the same approach as before…

Since -Tom- wanted to learn something about how the PC-98 EGC is used in TH04 and TH05, I took a look at master.lib's egc_shift_*() functions. These simply do a hardware-accelerated memmove() of any VRAM region, and are used for screen shaking effects. Hover over the image below for the raw effect:

Then, I finally wanted to take a look at the bullet structures, but it required way too much reverse-engineering to even start within ¾ of a position independence push. Even with the help of uth05win – bullet handling was changed quite a bit from TH04 to TH05.

What I ultimately settled on was more raw, "boring" PI work based around an already known set of functions. For this one, I looked at vector construction… and this time, that actually made the games a little bit more position-independent, and wasn't just all about removing false positives from the calculation. This was one of the few sets of functions that would also apply to TH01, and it revealed just how chaotically that game was coded. This one commit shows three ways how ZUN stored regular 2D points in TH01:

  • "regularly", like in master.lib's Point structure (X first, Y second)
  • reversed, (Y first and X second), then obviously with two distinct variables declared next to each other
  • and with multiple points stored in a structure of arrays.

… yeah. But in more productive news, this did actually lay the groundwork for TH04 and TH05 bullet structures. Which might even be coming up within the next big, 5-push order from Touhou Patch Center? These are the priorities I got from them, let's see how close I can get!

📝 Posted:
🚚 Summary of:
P0043, P0044, P0045
Commits:
261d503...612beb8
💰 Funded by:
-Tom-
🏷 Tags:
rec98+ th02+ th03+ th04+ th05+ pc98- blitting+ uth05win+

Turns out I had only been about half done with the drawing routines. The rest was all related to redrawing the scrolling stage backgrounds after other sprites were drawn on top. Since the PC-98 does have hardware-accelerated scrolling, but no hardware-accelerated sprites, everything that draws animated sprites into a scrolling VRAM must then also make sure that the background tiles covered by the sprite are redrawn in the next frame, which required a bit of ZUN code. And that are the functions that have been in the way of the expected rapid reverse-engineering progress that uth05win was supposed to bring. So, looks like everything's going to go really fast now?

📝 Posted:
🚚 Summary of:
P0025, P0026, P0027
Commits:
0cde4b7...261d503
💰 Funded by:
zorg
🏷 Tags:
rec98+ th02+ th03+ th04+ th05+ pc98- blitting+ file-format+ mod+

… yeah, no, we won't get very far without figuring out these drawing routines.
Which process data that comes from the .STD files. Which has various arrays related to the background… including one to specify the scrolling speed. And wait, setting that to 0 actually is what starts a boss battle?

So, have a TH05 Boss Rush patch: 2018-12-26-TH05BossRush.zip Theoretically, this should have also worked for TH04, but for some reason, the Stage 3 boss gets stuck on the first phase if we do this?

Here's the diff for the Boss Rush. Turning it into a thcrap-style Skipgame patch is left as an exercise for the reader.

📝 Posted:
🚚 Summary of:
P0040
Commits:
d7483c0...b03bc91
💰 Funded by:
-Tom-
🏷 Tags:
rec98+ th04+ th05+ pc98- animation+

Let's start this stretch with a pretty simple entity type, the growing and shrinking circles shown during bomb animations and around bosses in TH04 and TH05. Which can be drawn in varying colors… wait, what's all this inlined and duplicated GRCG mode and color setting code? Let's move that out into macros too, it takes up too much space when grepping for constants, and will raise a "wait, what was that I/O port doing again" question for most people reading the code again after a few months.

🎉 With this push, we've also hit a milestone! Less than 200,000 unknown x86 instructions remain until we've completely reverse-engineered all of PC-98 Touhou.

📝 Posted:
🚚 Summary of:
P0019, P0020, P0021, P0022
Commits:
c592464, cbe8a37, 8dfc2cd, 79cc3ed
💰 Funded by:
zorg
🏷 Tags:
rec98+ th03+ th04+ th05+ pc98- input+
> OK, let's do a quick ReC98 update before going back to thcrap, shouldn't take long > Hm, all that input code is kind of in the way, would be nice to cover that first to ease comparisons with uth05win's source code > What the hell, why does ZUN do this? Need to do more research > … > OK, research done, wait, what are those other functions doing? > Wha, everything about this is just ever so slightly awkward

Which ended up turning this one update into 2/10, 3/10, 4/10 and 5/10 of zorg's reverse-engineering commits. But at least we now got all shared input functions of TH02-TH05 covered and well understood.