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📝 Posted:
🚚 Summary of:
P0264, P0265
⌨ Commits:
46cd6e7...78728f6, 78728f6...ff19bed
💰 Funded by:
Blue Bolt, [Anonymous], iruleatgames
🏷 Tags:

Oh, it's 2024 already and I didn't even have a delivery for December or January? Yeah… I can only repeat what I said at the end of November, although the finish line is actually in sight now. With 10 pushes across 4 repositories and a blog post that has already reached a word count of 9,240, the Shuusou Gyoku SC-88Pro BGM release is going to break 📝 both the push record set by TH01 Sariel two years ago, and 📝 the blog post length record set by the last Shuusou Gyoku delivery. Until that's done though, let's clear some more PC-98 Touhou pushes out of the backlog, and continue the preparation work for the non-ASCII translation project starting later this year.

But first, we got another free bugfix according to my policy! 📝 Back in April 2022 when I researched the Divide Error crash that can occur in TH04's Stage 4 Marisa fight, I proposed and implemented four possible workarounds and let the community pick one of them for the generally recommended small bugfix mod. I still pushed the others onto individual branches in case the gameplay community ever wants to look more closely into them and maybe pick a different one… except that I accidentally pushed the wrong code for the warp workaround, probably because I got confused with the second warp variant I developed later on.
Fortunately, I still had the intended code for both variants lying around, and used the occasion to merge the current master branch into all of these mod branches. Thanks to wyatt8740 for spotting and reporting this oversight!

  1. The Music Room background masking effect
  2. The GRCG's plane disabling flags
  3. Text color restrictions
  4. The entire messy rest of the Music Room code
  5. TH04's partially consistent congratulation picture on Easy Mode
  6. TH02's boss position and damage variables

As the final piece of code shared in largely identical form between 4 of the 5 games, the Music Rooms were the biggest remaining piece of low-hanging fruit that guaranteed big finalization% gains for comparatively little effort. They seemed to be especially easy because I already decompiled TH02's Music Room together with the rest of that game's OP.EXE back in early 2015, when this project focused on just raw decompilation with little to no research. 9 years of increased standards later though, it turns out that I missed a lot of details, and ended up renaming most variables and functions. Combined with larger-than-expected changes in later games and the usual quality level of ZUN's menu code, this ended up taking noticeably longer than the single push I expected.

The undoubtedly most interesting part about this screen is the animation in the background, with the spinning and falling polygons cutting into a single-color background to reveal a spacey image below. However, the only background image loaded in the Music Room is OP3.PI (TH02/TH03) or MUSIC3.PI (TH04/TH05), which looks like this in a .PI viewer or when converted into another image format with the usual tools:

TH02's Music Room background in its on-disk state TH03's Music Room background in its on-disk state TH04's Music Room background in its on-disk state TH05's Music Room background in its on-disk state
Let's call this "the blank image".

That is definitely the color that appears on top of the polygons, but where is the spacey background? If there is no other .PI file where it could come from, it has to be somewhere in that same file, right? :thonk:
And indeed: This effect is another bitplane/color palette trick, exactly like the 📝 three falling stars in the background of TH04's Stage 5. If we set every bit on the first bitplane and thus change any of the resulting even hardware palette color indices to odd ones, we reveal a full second 8-color sub-image hiding in the same .PI file:

TH02's Music Room background, with all bits in the first bitplane set to reveal the spacey background image, and the full color palette at the bottom TH03's Music Room background, with all bits in the first bitplane set to reveal the spacey background image, and the full color palette at the bottom TH04's Music Room background, with all bits in the first bitplane set to reveal the spacey background image, and the full color palette at the bottom TH05's Music Room background, with all bits in the first bitplane set to reveal the spacey background image, and the full color palette at the bottom
The spacey sub-image. Never before seen!1!! …OK, touhou-memories beat me by a month. Let's add each image's full 16-color palette to deliver some additional value.

On a high level, the first bitplane therefore acts as a stencil buffer that selects between the blank and spacey sub-image for every pixel. The important part here, however, is that the first bitplane of the blank sub-images does not consist entirely of 0 bits, but does have 1 bits at the pixels that represent the caption that's supposed to be overlaid on top of the animation. Since there now are some pixels that should always be taken from the spacey sub-image regardless of whether they're covered by a polygon, the game can no longer just clear the first bitplane at the start of every frame. Instead, it has to keep a separate copy of the first bitplane's original state (called nopoly_B in the code), captured right after it blitted the .PI image to VRAM. Turns out that this copy also comes in quite handy with the text, but more on that later.

Then, the game simply draws polygons onto only the reblitted first bitplane to conditionally set the respective bits. ZUN used master.lib's grcg_polygon_c() function for this, which means that we can entirely thank the uncredited master.lib developers for this iconic animation – if they hadn't included such a function, the Music Rooms would most certainly look completely different.
This is where we get to complete the series on the PC-98 GRCG chip with the last remaining four bits of its mode register. So far, we only needed the highest bit (0x80) to either activate or deactivate it, and the bit below (0x40) to choose between the 📝 RMW and 📝 TCR/📝 TDW modes. But you can also use the lowest four bits to restrict the GRCG's operations to any subset of the four bitplanes, leaving the other ones untouched: 

// Enable the GRCG (0x80) in regular RMW mode (0x40). All bitplanes are
// enabled and written according to the contents of the tile register.
outportb(0x7C, 0xC0);

// The same, but limiting writes to the first bitplane by disabling the
// second (0x02), third (0x04), and fourth (0x08) one, as done in the
// PC-98 Touhou Music Rooms.
outportb(0x7C, 0xCE);

// Regular GRCG blitting code to any VRAM segment…
pokeb(0xA8000, offset, …);

// We're done, turn off the GRCG.
outportb(0x7C, 0x00);

This could be used for some unusual effects when writing to two or three of the four planes, but it seems rather pointless for this specific case at first. If we only want to write to a single plane, why not just do so directly, without the GRCG? Using that chip only involves more hardware and is therefore slower by definition, and the blitting code would be the same, right?
This is another one of these questions that would be interesting to benchmark one day, but in this case, the reason is purely practical: All of master.lib's polygon drawing functions expect the GRCG to be running in RMW mode. They write their pixels as bitmasks where 1 and 0 represent pixels that should or should not change, and leave it to the GRCG to combine these masks with its tile register and OR the result into the bitplanes instead of doing so themselves. Since GRCG writes are done via MOV instructions, not using the GRCG would turn these bitmasks into actual dot patterns, overwriting any previous contents of each VRAM byte that gets modified.
Technically, you'd only have to replace a few MOV instructions with OR to build a non-GRCG version of such a function, but why would you do that if you haven't measured polygon drawing to be an actual bottleneck.

Three overlapping Music Room polygons rendered using master.lib's grcg_polygon_c() function with a disabled GRCGThree overlapping Music Room polygons rendered as in the original game, with the GRCG enabled
An example with three polygons drawn from top to bottom. Without the GRCG, edges of later polygons overwrite any previously drawn pixels within the same VRAM byte. Note how treating bitmasks as dot patterns corrupts even those areas where the background image had nonzero bits in its first bitplane.

As far as complexity is concerned though, the worst part is the implicit logic that allows all this text to show up on top of the polygons in the first place. If every single piece of text is only rendered a single time, how can it appear on top of the polygons if those are drawn every frame?
Depending on the game (because of course it's game-specific), the answer involves either the individual bits of the text color index or the actual contents of the palette:

The contents of nopoly_B with each game's first track selected.

Finally, here's a list of all the smaller details that turn the Music Rooms into such a mess:

And that's all the Music Rooms! The OP.EXE binaries of TH04 and especially TH05 are now very close to being 100% RE'd, with only the respective High Score menus and TH04's title animation still missing. As for actual completion though, the finalization% metric is more relevant as it also includes the ZUN Soft logo, which I RE'd on paper but haven't decompiled. I'm 📝 still hoping that this will be the final piece of code I decompile for these two games, and that no one pays to get it done earlier… :onricdennat:

For the rest of the second push, there was a specific goal I wanted to reach for the remaining anything budget, which was blocked by a few functions at the beginning of TH04's and TH05's MAINE.EXE. In another anticlimactic development, this involved yet another way too early decompilation of a main() function…
Generally, this main() function just calls the top-level functions of all other ending-related screens in sequence, but it also handles the TH04-exclusive congratulating All Clear images within itself. After a 1CC, these are an additional reward on top of the Good Ending, showing the player character wearing a different outfit depending on the selected difficulty. On Easy Mode, however, the Good Ending is unattainable because the game always ends after Stage 5 with a Bad Ending, but ZUN still chose to show the EASY ALL CLEAR!! image in this case, regardless of how many continues you used.
While this might seem inconsistent with the other difficulties, it is consistent within Easy Mode itself, as the enforced Bad Ending after Stage 5 also doesn't distinguish between the number of continues. Also, Try to Normal Rank!! could very well be ZUN's roundabout way of implying "because this is how you avoid the Bad Ending".

With that out of the way, I was finally able to separate the VRAM text renderer of TH04 and TH05 into its own assembly unit, 📝 finishing the technical debt repayment project that I couldn't complete in 2021 due to assembly-time code segment label arithmetic in the data segment. This now allows me to translate this undecompilable self-modifying mess of ASM into C++ for the non-ASCII translation project, and thus unify the text renderers of all games and enhance them with support for Unicode characters loaded from a bitmap font. As the final finalized function in the SHARED segment, it also allowed me to remove 143 lines of particularly ugly segmentation workarounds 🙌

The remaining 1/6th of the second push provided the perfect occasion for some light TH02 PI work. The global boss position and damage variables represented some equally low-hanging fruit, being easily identified global variables that aren't part of a larger structure in this game. In an interesting twist, TH02 is the only game that uses an increasing damage value to track boss health rather than decreasing HP, and also doesn't internally distinguish between bosses and midbosses as far as these variables are concerned. Obviously, there's quite a bit of state left to be RE'd, not least because Marisa is doing her own thing with a bunch of redundant copies of her position, but that was too complex to figure out right now.

Also doing their own thing are the Five Magic Stones, which need five positions rather than a single one. Since they don't move, the game doesn't have to keep 📝 separate position variables for both VRAM pages, and can handle their positions in a much simpler way that made for a nice final commit.
And for the first time in a long while, I quite like what ZUN did there! Not only are their positions stored in an array that is indexed with a consistent ID for every stone, but these IDs also follow the order you fight the stones in: The two inner ones use 0 and 1, the two outer ones use 2 and 3, and the one in the center uses 4. This might look like an odd choice at first because it doesn't match their horizontal order on the playfield. But then you notice that ZUN uses this property in the respective phase control functions to iterate over only the subrange of active stones, and you realize how brilliant it actually is.

Screenshot of TH02's Five Magic Stones, with the first two (both internally and in the order you fight them in) alive and activated Screenshot of TH02's Five Magic Stones, with the second two (both internally and in the order you fight them in) alive and activated Screenshot of TH02's Five Magic Stones, with the last one (both internally and in the order you fight them in) alive and activated

This seems like a really basic thing to get excited about, especially since the rest of their data layout sure isn't perfect. Splitting each piece of state and even the individual X and Y coordinates into separate 5-element arrays is still counter-productive because the game ends up paying more memory and CPU cycles to recalculate the element offsets over and over again than this would have ever saved in cache misses on a 486. But that's a minor issue that could be fixed with a few regex replacements, not a misdesigned architecture that would require a full rewrite to clean it up. Compared to the hardcoded and bloated mess that was 📝 YuugenMagan's five eyes, this is definitely an improvement worthy of the good-code tag. The first actual one in two years, and a welcome change after the Music Room!

These three pieces of data alone yielded a whopping 5% of overall TH02 PI in just 1/6th of a push, bringing that game comfortably over the 60% PI mark. MAINE.EXE is guaranteed to reach 100% PI before I start working on the non-ASCII translations, but at this rate, it might even be realistic to go for 100% PI on MAIN.EXE as well? Or at least technical position independence, without the false positives.

Next up: Shuusou Gyoku SC-88Pro BGM. It's going to be wild.

📝 Posted:
🚚 Summary of:
P0242, P0243
⌨ Commits:
08352a5...dfa758d, dfa758d...ac33bd2
💰 Funded by:
Yanga
🏷 Tags:

OK, let's decompile TH02's HUD code first, gain a solid understanding of how increasing the score works, and then look at the item system of this game. Should be no big deal, no surprises expected, let's go!

…Yeah, right, that's never how things end up in ReC98 land. :godzun: And so, we get the usual host of newly discovered oddities in addition to the expected insights into the item mechanics. Let's start with the latter:

Onto score tracking then, which only took a single commit to raise another big research question. It's widely known that TH02 grants extra lives upon reaching a score of 1, 2, 3, 5, or 8 million points. But what hasn't been documented is the fact that the game does not stop at the end of the hardcoded extend score array. ZUN merely ends it with a sentinel value of 999,999,990 points, but if the score ever increased beyond this value, the game will interpret adjacent memory as signed 32-bit score values and continue giving out extra lives based on whatever thresholds it ends up finding there. Since the following bytes happen to turn into a negative number, the next extra life would be awarded right after gaining another 10 points at exactly 1,000,000,000 points, and the threshold after that would be 11,114,905,600 points. Without an explicit counterstop, the number of score-based extra lives is theoretically unlimited, and would even continue after the signed 32-bit value overflowed into the negative range. Although we certainly have bigger problems once scores ever reach that point… :tannedcirno:
That said, it seems impossible that any of this could ever happen legitimately. The current high scores of 42,942,800 points on Lunatic and 42,603,800 points on Extra don't even reach 1/20 of ZUN's sentinel value. Without either a graze or a bullet cancel system, the scoring potential in this game is fairly limited, making it unlikely for high scores to ever increase by that additional order of magnitude to end up anywhere near the 1 billion mark.
But can we really be sure? Is this a landmine because it's impossible to ever reach such high scores, or is it a quirk because these extends could be observed under rare conditions, perhaps as the result of other quirks? And if it's the latter, how many of these adjacent bytes do we need to preserve in cleaned-up versions and ports? We'd pretty much need to know the upper bound of high scores within the original stage and boss scripts to tell. This value should be rather easy to calculate in a game with such a simple scoring system, but doing that only makes sense after we RE'd all scoring-related code and could efficiently run such simulations. It's definitely something we'd need to look at before working on this game's debloated version in the far future, which is when the difference between quirks and landmines will become relevant. Still, all that uncertainty just because ZUN didn't restrict a loop to the size of the extend threshold array…

TH02 marks a pivotal point in how the PC-98 Touhou games handle the current score. It's the last game to use a 32-bit variable before the later games would regrettably start using arrays of binary-coded decimals. More importantly though, TH02 is also the first game to introduce the delayed score counting animation, where the displayed score intentionally lags behind and gradually counts towards the real one over multiple frames. This could be implemented in one of two ways:

  1. Keep the displayed score as a separate variable inside the presentation layer, and let it gradually count up to the real score value passed in from the logic layer
  2. Burden the game logic with this presentation detail, and split the score into two variables: One for the displayed score, and another for the delta between that score and the actual one. Newly gained points are first added to the delta variable, and then gradually subtracted from there and added to the real score before being displayed.

And by now, we can all tell which option ZUN picked for the rest of the PC-98 games, even if you don't remember 📝 me mentioning this system last year. 📝 Once again, TH02 immortalized ZUN's initial attempt at the concept, which lacks the abstraction boundaries you'd want for managing this one piece of state across two variables, and messes up the abstractions it does have. In addition to the regular score transfer/render function, the codebase therefore has

And – you guessed it – I wouldn't have mentioned any of this if it didn't result in one bug and one quirk in TH02. The bug resulting from 1) is pretty minor: The function is called when losing a life, and simply stops any active score-counting animation at the value rendered on the frame where the player got hit. This one is only a rendering issue – no points are lost, and you just need to gain 10 more for the rendered value to jump back up to its actual value. You'll probably never notice this one because you're likely busy collecting the single 5-power spawned around Reimu when losing a life, which always awards at least 10 points.

The quirk resulting from 2) is more intriguing though. Without a separate reset of the score delta, the function effectively awards the current delta value as a one-time point bonus, since the same delta will still be regularly transferred to the score on further game frames.
This function is called at the start of every dialog sequence. However, TH02 stops running the regular game loop between the post-boss dialog and the next stage where the delta is reset, so we can only observe this quirk for the pre-boss sequences and the dialog before Mima's form change. Unfortunately, it's not all too exploitable in either case: Each of the pre-boss dialog sequences is preceded by an ungrazeable pellet pattern and followed by multiple seconds of flying over an empty playfield with zero scoring opportunities. By the time the sequence starts, the game will have long transferred any big score delta from max-valued point items. It's slightly better with Mima since you can at least shoot her and use a bomb to keep the delta at a nonzero value, but without a health bar, there is little indication of when the dialog starts, and it'd be long after Mima gave out her last bonus items in any case.
But two of the bosses – that is, Rika, and the Five Magic Stones – are scrolled onto the playfield as part of the stage script, and can also be hit with player shots and bombs for a few seconds before their dialog starts. While I'll only get to cover shot types and bomb damage within the next few TH02 pushes, there is an obvious initial strategy for maximizing the effect of this quirk: Spreading out the A-Type / Wide / High Mobility shot to land as many hits as possible on all Five Magic Stones, while firing off a bomb.

Turns out that the infamous button-mashing mechanics of the player shot are also more complicated than simply pressing and releasing the Shot key at alternating frames. Even this result took way too many takes.

Wow, a grand total of 1,750 extra points! Totally worth wasting a bomb for… yeah, probably not. :onricdennat: But at the very least, it's something that a TAS score run would want to keep in mind. And all that just because ZUN "forgot" a single score_delta = 0; assignment at the end of one function…

And that brings TH02 over the 30% RE mark! Next up: 100% position independence for TH04. If anyone wants to grab the that have now been freed up in the cap: Any small Touhou-related task would be perfect to round out that upcoming TH04 PI delivery.