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📝 Posted:
🚚 Summary of:
P0139
Commits:
864e864...d985811
💰 Funded by:
[Anonymous]
🏷 Tags:
rec98+ th02+ th03+ th04+ th05+ kaja+ jank+ bug- contribution-ideas-

Technical debt, part 10… in which two of the PMD-related functions came with such complex ramifications that they required one full push after all, leaving no room for the additional decompilations I wanted to do. At least, this did end up being the final one, completing all SHARED segments for the time being.


The first one of these functions determines the BGM and sound effect modes, combining the resident type of the PMD driver with the Option menu setting. The TH04 and TH05 version is apparently coded quite smartly, as PC-98 Touhou only needs to distinguish "OPN- / PC-9801-26K-compatible sound sources handled by PMD.COM" from "everything else", since all other PMD varieties are OPNA- / PC-9801-86-compatible.
Therefore, I only documented those two results returned from PMD's AH=09h function. I'll leave a comprehensive, fully documented enum to interested contributors, since that would involve research into basically the entire history of the PC-9800 series, and even the clearly out-of-scope PC-88VA. After all, distinguishing between more versions of the PMD driver in the Option menu (and adding new sprites for them!) is strictly mod territory.


The honor of being the final decompiled function in any SHARED segment went to TH04's snd_load(). TH04 contains by far the sanest version of this function: Readable C code, no new ZUN bugs (and still missing file I/O error handling, of course)… but wait, what about that actual file read syscall, using the INT 21h, AH=3Fh DOS file read API? Reading up to a hardcoded number of bytes into PMD's or MMD's song or sound effect buffer, 20 KiB in TH02-TH04, 64 KiB in TH05… that's kind of weird. About time we looked closer into this. :thonk:

Turns out that no, KAJA's driver doesn't give you the full 64 KiB of one memory segment for these, as especially TH05's code might suggest to anyone unfamiliar with these drivers. :zunpet: Instead, you can customize the size of these buffers on its command line. In GAME.BAT, ZUN allocates 8 KiB for FM songs, 2 KiB for sound effects, and 12 KiB for MMD files in TH02… which means that the hardcoded sizes in snd_load() are completely wrong, no matter how you look at them. :onricdennat: Consequently, this read syscall will overflow PMD's or MMD's song or sound effect buffer if the given file is larger than the respective buffer size.
Now, ZUN could have simply hardcoded the sizes from GAME.BAT instead, and it would have been fine. As it also turns out though, PMD has an API function (AH=22h) to retrieve the actual buffer sizes, provided for exactly that purpose. There is little excuse not to use it, as it also gives you PMD's default sizes if you don't specify any yourself.
(Unless your build process enumerates all PMD files that are part of the game, and bakes the largest size into both snd_load() and GAME.BAT. That would even work with MMD, which doesn't have an equivalent for AH=22h.)

What'd be the consequence of loading a larger file then? Well, since we don't get a full segment, let's look at the theoretical limit first.
PMD prefers to keep both its driver code and the data buffers in a single memory segment. As a result, the limit for the combined size of the song, instrument, and sound effect buffer is determined by the amount of code in the driver itself. In PMD86 version 4.8o (bundled with TH04 and TH05) for example, the remaining size for these buffers is exactly 45,555 bytes. Being an actually good programmer who doesn't blindly trust user input, KAJA thankfully validates the sizes given via the /M, /V, and /E command-line options before letting the driver reside in memory, and shuts down with an error message if they exceed 40 KiB. Would have been even better if he calculated the exact size – even in the current PMD version 4.8s from January 2020, it's still a hardcoded value (see line 8581).
Either way: If the file is larger than this maximum, the concrete effect is down to the INT 21h, AH=3Fh implementation in the underlying DOS version. DOS 3.3 treats the destination address as linear and reads past the end of the segment, DOS 5.0 and DOSBox-X truncate the number of bytes to not exceed the remaining space in the segment, and maybe there's even a DOS that wraps around and ends up overwriting the PMD driver code. In any case: You will overwrite what's after the driver in memory – typically, the game .EXE and its master.lib functions.

It almost feels like a happy accident that this doesn't cause issues in the original games. The largest PMD file in any of the 4 games, the -86 version of 幽夢 ~ Inanimate Dream, takes up 8,099 bytes, just under the 8,192 byte limit for BGM. For modders, I'd really recommend implementing this properly, with PMD's AH=22h function and error handling, once position independence has been reached.

Whew, didn't think I'd be doing more research into KAJA's drivers during regular ReC98 development! That's probably been the final time though, as all involved functions are now decompiled, and I'm unlikely to iterate over them again.


And that's it! Repaid the biggest chunk of technical debt, time for some actual progress again. Next up: Reopening the store tomorrow, and waiting for new priorities. If we got nothing by Sunday, I'm going to put the pending [Anonymous] pushes towards some work on the website.

📝 Posted:
🚚 Summary of:
P0099, P0100, P0101, P0102
Commits:
1799d67...1b25830, 1b25830...ceb81db, ceb81db...c11a956, c11a956...b60f38d
💰 Funded by:
Ember2528, Yanga
🏷 Tags:
rec98+ th01+ gameplay+ bullet+ jank+ contribution-ideas- bug- boss+ elis+ kikuri+ sariel+ konngara+

Well, make that three days. Trying to figure out all the details behind the sprite flickering was absolutely dreadful…
It started out easy enough, though. Unsurprisingly, TH01 had a quite limited pellet system compared to TH04 and TH05:

As expected from TH01, the code comes with its fair share of smaller, insignificant ZUN bugs and oversights. As you would also expect though, the sprite flickering points to the biggest and most consequential flaw in all of this.


Apparently, it started with ZUN getting the impression that it's only possible to use the PC-98 EGC for fast blitting of all 4 bitplanes in one CPU instruction if you blit 16 horizontal pixels (= 2 bytes) at a time. Consequently, he only wrote one function for EGC-accelerated sprite unblitting, which can only operate on a "grid" of 16×1 tiles in VRAM. But wait, pellets are not only just 8×8, but can also be placed at any unaligned X position…

… yet the game still insists on using this 16-dot-aligned function to unblit pellets, forcing itself into using a super sloppy 16×8 rectangle for the job. 🤦 ZUN then tried to mitigate the resulting flickering in two hilarious ways that just make it worse:

  1. An… "interlaced rendering" mode? This one's activated for all Stage 15 and 20 fights, and separates pellets into two halves that are rendered on alternating frames. Collision detection with the Yin-Yang Orb and the player is only done for the visible half, but collision detection with player shots is still done for all pellets every frame, as are motion updates – so that pellets don't end up moving half as fast as they should.
    So yeah, your eyes weren't deceiving you. The game does effectively drop its perceived frame rate in the Elis, Kikuri, Sariel, and Konngara fights, and it does so deliberately.
  2. 📝 Just like player shots, pellets are also unblitted, moved, and rendered in a single function. Thanks to the 16×8 rectangle, there's now the (completely unnecessary) possibility of accidentally unblitting parts of a sprite that was previously drawn into the 8 pixels right of a pellet. And this is where ZUN went full :tannedcirno: and went "oh, I know, let's test the entire 16 pixels, and in case we got an entity there, we simply make the pellet invisible for this frame! Then we don't even have to unblit it later!" :zunpet:

    Except that this is only done for the first 3 elements of the player shot array…?! Which don't even necessarily have to contain the 3 shots fired last. It's not done for the player sprite, the Orb, or, heck, other pellets that come earlier in the pellet array. (At least we avoided going 𝑂(𝑛²) there?)

    Actually, and I'm only realizing this now as I type this blog post: This test is done even if the shots at those array elements aren't active. So, pellets tend to be made invisible based on comparisons with garbage data. :onricdennat:

    And then you notice that the player shot unblit​/​move​/​render function is actually only ever called from the pellet unblit​/​move​/​render function on the one global instance of the player shot manager class, after pellets were unblitted. So, we end up with a sequence of

    Pellet unblit → Pellet move → Shot unblit → Shot move → Shot render → Pellet render

    which means that we can't ever unblit a previously rendered shot with a pellet. Sure, as terrible as this one function call is from a software architecture perspective, it was enough to fix this issue. Yet we don't even get the intended positive effect, and walk away with pellets that are made temporarily invisible for no reason at all. So, uh, maybe it all just was an attempt at increasing the ramerate on lower spec PC-98 models?

Yup, that's it, we've found the most stupid piece of code in this game, period. It'll be hard to top this.


I'm confident that it's possible to turn TH01 into a well-written, fluid PC-98 game, with no flickering, and no perceived lag, once it's position-independent. With some more in-depth knowledge and documentation on the EGC (remember, there's still 📝 this one TH03 push waiting to be funded), you might even be able to continue using that piece of blitter hardware. And no, you certainly won't need ASM micro-optimizations – just a bit of knowledge about which optimizations Turbo C++ does on its own, and what you'd have to improve in your own code. It'd be very hard to write worse code than what you find in TH01 itself.

(Godbolt for Turbo C++ 4.0J when? Seriously though, that would 📝 also be a great project for outside contributors!)


Oh well. In contrast to TH04 and TH05, where 4 pushes only covered all the involved data types, they were enough to completely cover all of the pellet code in TH01. Everything's already decompiled, and we never have to look at it again. 😌 And with that, TH01 has also gone from by far the least RE'd to the most RE'd game within ReC98, in just half a year! 🎉
Still, that was enough TH01 game logic for a while. :tannedcirno: Next up: Making up for the delay with some more relaxing and easy pieces of TH01 code, that hopefully make just a bit more sense than all this garbage. More image formats, mainly.