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📝 Posted:
🚚 Summary of:
P0214, P0215
Commits:
158a91e...414770c, 414770c...3123c9d
💰 Funded by:
Ember2528, Yanga
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Last blog post before the 100% completion of TH01! The final parts of REIIDEN.EXE would feel rather out of place in a celebratory blog post, after all. They provided quite a neat summary of the typical technical details that are wrong with this game, and that I now get to mention for one final time:

But hey, there's an error message if you start REIIDEN.EXE without a resident MDRV2 or a correctly prepared resident structure! And even a good, user-friendly one, asking the user to launch the batch file instead. For some reason, this convenience went out of fashion in the later games.


The Game Over animation (how fitting) gives us TH01's final piece of weird sprite blitting code, which seriously manages to include 2 bugs and 3 quirks in under 50 lines of code. In test mode (game t or game d), you can trigger this effect by pressing the ⬇️ down arrow key, which certainly explains why I encountered seemingly random Game Over events during all the tests I did with this game…
The animation appears to have changed quite a bit during development, to the point that probably even ZUN himself didn't know what he wanted it to look like in the end:

The original version unblits a 32×32 rectangle around Reimu that only grows on the X axis… for the first 5 frames. The unblitting call is only run if the corresponding sprite wasn't clipped at the edges of the playfield in the frame before, and ZUN uses the animation's frame number rather than the sprite loop variable to index the per-sprite clip flag array. The resulting out-of-bounds access then reads the sprite coordinates instead, which are never 0, thus interpreting all 5 sprites as clipped.
This variant would interpret the declared 5 effect coordinates as distinct sprites and unblit them correctly every frame. The end result is rather wimpy though… hardly appropriate for a Game Over, especially with the original animation in mind.
This variant would not unblit anything, and is probably closest to what the final animation should have been.

Finally, we get to the big main() function, serving as the duct tape that holds this game together. It may read rather disorganized with all the (actually necessary) assignments and function calls, but the only actual minor issue I've seen there is that you're robbed of any pellet destroy bonus collected on the final frame of the final boss. There is a certain charm in directly nesting the infinite main gameplay loop within the infinite per-life loop within the infinite stage loop. But come on, why is there no fourth scene loop? :zunpet: Instead, the game just starts a new REIIDEN.EXE process before and after a boss fight. With all the wildly mutated global state, that was probably a much saner choice.

The final secrets can be found in the debug stage selection. ZUN implemented the prompts using the C standard library's scanf() function, which is the natural choice for quick-and-dirty testing features like this one. However, the C standard library is also complete and utter trash, and so it's not surprising that both of the scanf() calls do… well, probably not what ZUN intended. The guaranteed out-of-bounds memory access in the select_flag route prompt thankfully has no real effect on the game, but it gets really interesting with the 面数 stage prompt.
Back in 2020, I already wrote about 📝 stages 21-24, and how they're loaded from actual data that ZUN shipped with the game. As it now turns out, the code that maps stage IDs to STAGE?.DAT scene numbers contains an explicit branch that maps any (1-based) stage number ≥21 to scene 7. Does this mean that an Extra Stage was indeed planned at some point? That branch seems way too specific to just be meant as a fallback. Maybe Asprey was on to something after all…

However, since ZUN passed the stage ID as a signed integer to scanf(), you can also enter negative numbers. The only place that kind of accidentally checks for them is the aforementioned stage ID → scene mapping, which ensures that (1-based) stages < 5 use the shrine's background image and BGM. With no checks anywhere else, we get a new set of "glitch stages":

TH01's stage -1
Stage -1
TH01's stage -2
Stage -2
TH01's stage -3
Stage -3
TH01's stage -4
Stage -4
TH01's stage -5
Stage -5

The scene loading function takes the entered 0-based stage ID value modulo 5, so these 4 are the only ones that "exist", and lower stage numbers will simply loop around to them. When loading these stages, the function accesses the data in REIIDEN.EXE that lies before the statically allocated 5-element stages-of-scene array, which happens to encompass Borland C++'s locale and exception handling data, as well as a small bit of ZUN's global variables. In particular, the obstacle/card HP on the tile I highlighted in green corresponds to the lowest byte of the 32-bit RNG seed. If it weren't for that and the fact that the obstacles/card HP on the few tiles before are similarly controlled by the x86 segment values of certain initialization function addresses, these glitch stages would be completely deterministic across PC-98 systems, and technically canon… :tannedcirno:
Stage -4 is the only playable one here as it's the only stage to end up below the 📝 heap corruption limit of 102 stage objects. Completing it loads Stage -3, which crashes with a Divide Error just like it does if it's directly selected. Unsurprisingly, this happens because all 50 card bytes at that memory location are 0, so one division (or in this case, modulo operation) by the number of cards is enough to crash the game.
Stage -5 is modulo'd to 0 and thus loads the first regular stage. The only apparent broken element there is the timer, which is handled by a completely different function that still operates with a (0-based) stage ID value of -5. Completing the stage loads Stage -4, which also crashes, but only because its 61 cards naturally cause the 📝 stack overflow in the flip-in animation for any stage with more than 50 cards.

And that's REIIDEN.EXE, the biggest and most bloated PC-98 Touhou executable, fully decompiled! Next up: Finishing this game with the main menu, and hoping I'll actually pull it off within 24 hours. (If I do, we might all have to thank 32th System, who independently decompiled half of the remaining 14 functions…)

📝 Posted:
🚚 Summary of:
P0205, P0206
Commits:
3259190...327730f, 327730f...454c105
💰 Funded by:
[Anonymous], Yanga
🏷 Tags:

Oh look, it's another rather short and straightforward boss with a rather small number of bugs and quirks. Yup, contrary to the character's popularity, Mima's premiere is really not all that special in terms of code, and continues the trend established with 📝 Kikuri and 📝 SinGyoku. I've already covered 📝 the initial sprite-related bugs last November, so this post focuses on the main code of the fight itself. The overview:


And there aren't even any weird hitboxes this time. What is maybe special about Mima, however, is how there's something to cover about all of her patterns. Since this is TH01, it's won't surprise anyone that the rotating square patterns are one giant copy-pasta of unblitting, updating, and rendering code. At least ZUN placed the core polar→Cartesian transformation in a separate function for creating regular polygons with an arbitrary number of sides, which might hint toward some more varied shapes having been planned at one point?
5 of the 6 patterns even follow the exact same steps during square update frames:

  1. Calculate square corner coordinates
  2. Unblit the square
  3. Update the square angle and radius
  4. Use the square corner coordinates for spawning pellets or missiles
  5. Recalculate square corner coordinates
  6. Render the square

Notice something? Bullets are spawned before the corner coordinates are updated. That's why their initial positions seem to be a bit off – they are spawned exactly in the corners of the square, it's just that it's the square from 8 frames ago. :tannedcirno:

Mima's first pattern on Normal difficulty.

Once ZUN reached the final laser pattern though, he must have noticed that there's something wrong there… or maybe he just wanted to fire those lasers independently from the square unblit/update/render timer for a change. Spending an additional 16 bytes of the data segment for conveniently remembering the square corner coordinates across frames was definitely a decent investment.

Mima's laser pattern on Lunatic difficulty, now with correct laser spawn positions. If this pattern reminds you of the game crashing immediately when defeating Mima, 📝 check out the Elis blog post for the details behind this bug, and grab the bugfix patch from there.

When Mima isn't shooting bullets from the corners of a square or hopping across the playfield, she's raising flame pillars from the bottom of the playfield within very specifically calculated random ranges… which are then rendered at byte-aligned VRAM positions, while collision detection still uses their actual pixel position. Since I don't want to sound like a broken record all too much, I'll just direct you to 📝 Kikuri, where we've seen the exact same issue with the teardrop ripple sprites. The conclusions are identical as well.

Mima's flame pillar pattern. This video was recorded on a particularly unlucky seed that resulted in great disparities between a pillar's internal X coordinate and its byte-aligned on-screen appearance, leading to lots of right-shifted hitboxes.
Also note how the change from the meteor animation to the three-arm 🚫 casting sprite doesn't unblit the meteor, and leaves that job to any sprite that happens to fly over those pixels.

However, I'd say that the saddest part about this pattern is how choppy it is, with the circle/pillar entities updating and rendering at a meager 7 FPS. Why go that low on purpose when you can just make the game render ✨ smoothly ✨ instead?

So smooth it's almost uncanny.

The reason quickly becomes obvious: With TH01's lack of optimization, going for the full 56.4 FPS would have significantly slowed down the game on its intended 33 MHz CPUs, requiring more than cheap surface-level ASM optimization for a stable frame rate. That might very well have been ZUN's reason for only ever rendering one circle per frame to VRAM, and designing the pattern with these time offsets in mind. It's always been typical for PC-98 developers to target the lowest-spec models that could possibly still run a game, and implementing dynamic frame rates into such an engine-less game is nothing I would wish on anybody. And it's not like TH01 is particularly unique in its choppiness anyway; low frame rates are actually a rather typical part of the PC-98 game aesthetic.


The final piece of weirdness in this fight can be found in phase 1's hop pattern, and specifically its palette manipulation. Just from looking at the pattern code itself, each of the 4 hops is supposed to darken the hardware palette by subtracting #444 from every color. At the last hop, every color should have therefore been reduced to a pitch-black #000, leaving the player completely blind to the movement of the chasing pellets for 30 frames and making the pattern quite ghostly indeed. However, that's not what we see in the actual game:

Nothing in the pattern's code would cause the hardware palette to get brighter before the end of the pattern, and yet…
The expected version doesn't look all too unfair, even on Lunatic… well, at least at the default rank pellet speed shown in this video. At maximum pellet speed, it is in fact rather brutal.

Looking at the frame counter, it appears that something outside the pattern resets the palette every 40 frames. The only known constant with a value of 40 would be the invincibility frames after hitting a boss with the Orb, but we're not hitting Mima here… :thonk:
But as it turns out, that's exactly where the palette reset comes from: The hop animation darkens the hardware palette directly, while the 📝 infamous 12-parameter boss collision handler function unconditionally resets the hardware palette to the "default boss palette" every 40 frames, regardless of whether the boss was hit or not. I'd classify this as a bug: That function has no business doing periodic hardware palette resets outside the invincibility flash effect, and it completely defies common sense that it does.

That explains one unexpected palette change, but could this function possibly also explain the other infamous one, namely, the temporary green discoloration in the Konngara fight? That glitch comes down to how the game actually uses two global "default" palettes: a default boss palette for undoing the invincibility flash effect, and a default stage palette for returning the colors back to normal at the end of the bomb animation or when leaving the Pause menu. And sure enough, the stage palette is the one with the green color, while the boss palette contains the intended colors used throughout the fight. Sending the latter palette to the graphics chip every 40 frames is what corrects the discoloration, which would otherwise be permanent.

The green color comes from BOSS7_D1.GRP, the scrolling background of the entrance animation. That's what turns this into a clear bug: The stage palette is only set a single time in the entire fight, at the beginning of the entrance animation, to the palette of this image. Apart from consistency reasons, it doesn't even make sense to set the stage palette there, as you can't enter the Pause menu or bomb during a blocking animation function.
And just 3 lines of code later, ZUN loads BOSS8_A1.GRP, the main background image of the fight. Moving the stage palette assignment there would have easily prevented the discoloration.

But yeah, as you can tell, palette manipulation is complete jank in this game. Why differentiate between a stage and a boss palette to begin with? The blocking Pause menu function could have easily copied the original palette to a local variable before darkening it, and then restored it after closing the menu. It's not so easy for bombs as the intended palette could change between the start and end of the animation, but the code could have still been simplified a lot if there was just one global "default palette" variable instead of two. Heck, even the other bosses who manipulate their palettes correctly only do so because they manually synchronize the two after every change. The proper defense against bugs that result from wild mutation of global state is to get rid of global state, and not to put up safety nets hidden in the middle of existing effect code.

The easiest way of reproducing the green discoloration bug in the TH01 Konngara fight, timed to show the maximum amount of time the discoloration can possibly last.

In any case, that's Mima done! 7th PC-98 Touhou boss fully decompiled, 24 bosses remaining, and 59 functions left in all of TH01.


In other thrilling news, my call for secondary funding priorities in new TH01 contributions has given us three different priorities so far. This raises an interesting question though: Which of these contributions should I now put towards TH01 immediately, and which ones should I leave in the backlog for the time being? Since I've never liked deciding on priorities, let's turn this into a popularity contest instead: The contributions with the least popular secondary priorities will go towards TH01 first, giving the most popular priorities a higher chance to still be left over after TH01 is done. As of this delivery, we'd have the following popularity order:

  1. TH05 (1.67 pushes), from T0182
  2. Seihou (1 push), from T0184
  3. TH03 (0.67 pushes), from T0146

Which means that T0146 will be consumed for TH01 next, followed by T0184 and then T0182. I only assign transactions immediately before a delivery though, so you all still have the chance to change up these priorities before the next one.

Next up: The final boss of TH01 decompilation, YuugenMagan… if the current or newly incoming TH01 funds happen to be enough to cover the entire fight. If they don't turn out to be, I will have to pass the time with some Seihou work instead, missing the TH01 anniversary deadline as a result. Edit (2022-07-18): Thanks to Yanga for securing the funding for YuugenMagan after all! That fight will feature slightly more than half of all remaining code in TH01's REIIDEN.EXE and the single biggest function in all of PC-98 Touhou, let's go!

📝 Posted:
🚚 Summary of:
P0193, P0194, P0195, P0196, P0197
Commits:
e1f3f9f...183d7a2, 183d7a2...5d93a50, 5d93a50...e18c53d, e18c53d...57c9ac5, 57c9ac5...48db0b7
💰 Funded by:
Ember2528, Yanga
🏷 Tags:

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:

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:

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.

Edit (2023-07-21): Pressing ↵ Return to reduce HP also works in test mode (game t). There, the game doesn't even check the heap, and consequently won't report any corruption, allowing the HP bar to be glitched even further.

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:

An unused wave animation frame from TH01's BOSS5.BOS

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. :zunpet: 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:

The "pellets along circle" pattern on Lunatic, in its original version and with fanfiction fixes for everything that can potentially be interpreted as a bug.

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. :zunpet: 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:

The "slow pellet spreads" pattern of Elis' bat form, on every difficulty. Which version do you think is the easiest one?

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? :godzun:
Possible laser states that will cause the fault, with some debug output to help understand the cause, and any pellets removed for better readability. This can happen for all bosses that can potentially have shootout lasers on screen when being defeated, so it also applies to Mima. Fixing this is easier than understanding why it happens, but since y'all love reading this stuff…

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:
🚚 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:

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:

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:

TH01 BOSS6_2.BOS
Umm… OK? The same sprite twice, just with slightly different colors? So how is the wand lowered again?

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?

VRAM after blitting the first sprite of TH01's BOSS6_2.BOS VRAM after blitting the second sprite of TH01's BOSS6_2.BOS

The first sprite already eats up part of the red HUD line, and the second one additionally fails to recover the seal pixels underneath, leaving a nice little black hole and some stray purple pixels until the next background transition. :tannedcirno: Quite ironic given that both sprites do include the right part of the seal, which isn't even part of the animation.


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. :zunpet: 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:

Sariel's third pattern, and the first to spawn birds, in its original and fixed versions. Note that I somewhat fixed the bird hatch animation as well: ZUN's code never unblits any frame of animation there, and simply blits every new one on top of the previous one.

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:

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! :godzun: 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:

The "swaying leaves" pattern. ZUN intended a splash animation to be shown once each leaf "spark" reaches the top of the playfield, which is never displayed in the original game.

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.