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Showing all posts tagged blitting-

📝 Posted:
🚚 Summary of:
P0214, P0215
Commits:
158a91e...414770c, 414770c...3123c9d
💰 Funded by:
Ember2528, Yanga
🏷 Tags:
rec98+ th01+ animation+ blitting- score+ card-flipping+ glitch+ rng+ debug+

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 debug mode, 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:


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 mode 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:
P0207, P0208, P0209, P0210, P0211
Commits:
454c105...c26ef4b, c26ef4b...239a3ec, 239a3ec...5030867, 5030867...149fbca, 149fbca...d398a94
💰 Funded by:
GhostPhanom, Yanga, Arandui, Lmocinemod
🏷 Tags:
rec98+ th01+ gameplay+ boss+ yuugenmagan+ danmaku-pattern+ unused+ blitting- palette+ tcc+

Whew, TH01's boss code just had to end with another beast of a boss, taking way longer than it should have and leaving uncomfortably little time for the rest of the game. Let's get right into the overview of YuugenMagan, the most sequential and scripted battle in this game:


At a pixel-perfect 81×61 pixels, the Orb hitboxes are laid out rather generously this time, reaching quite a bit outside the 64×48 eye sprites:

TH01 YuugenMagan's hitboxes.

And that's about the only positive thing I can say about a position calculation in this fight. Phase 0 already starts with the lasers being off by 1 pixel from the center of the iris. Sure, 28 may be a nicer number to add than 29, but the result won't be byte-aligned either way? This is followed by the eastern laser's hitbox somehow being 24 pixels larger than the others, stretching a rather unexpected 70 pixels compared to the 46 of every other laser.
On a more hilarious note, the eye closing keyframe contains the following (pseudo-)code, comprising the only real accidentally "unused" danmaku subpattern in TH01:

// Did you mean ">= RANK_HARD"?
if(rank == RANK_HARD) {
	eye_north.fire_aimed_wide_5_spread();
	eye_southeast.fire_aimed_wide_5_spread();
	eye_southwest.fire_aimed_wide_5_spread();

	// Because this condition can never be true otherwise.
	// As a result, no pellets will be spawned on Lunatic mode.
	// (There is another Lunatic-exclusive subpattern later, though.)
	if(rank == RANK_LUNATIC) {
		eye_west.fire_aimed_wide_5_spread();
		eye_east.fire_aimed_wide_5_spread();
	}
}

Featuring the weirdly extended hitbox for the eastern laser, as well as an initial Reimu position that points out the disparity between byte-aligned rendering and the internal coordinates one final time.

After a few utility functions that look more like a quickly abandoned refactoring attempt, we quickly get to the main attraction: YuugenMagan combines the entire boss script and most of the pattern code into a single 2,634-instruction function, totaling 9,677 bytes inside REIIDEN.EXE. For comparison, ReC98's version of this code consists of at least 49 functions, excluding those I had to add to work around ZUN's little inconsistencies, or the ones I added for stylistic reasons.
In fact, this function is so large that Turbo C++ 4.0J refuses to generate assembly output for it via the -S command-line option, aborting with a Compiler table limit exceeded in function error. Contrary to what the Borland C++ 4.0 User Guide suggests, this instance of the error is not at all related to the number of function bodies or any metric of algorithmic complexity, but is simply a result of the compiler's internal text representation for a single function overflowing a 64 KiB memory segment. Merely shortening the names of enough identifiers within the function can help to get that representation down below 64 KiB. If you encounter this error during regular software development, you might interpret it as the compiler's roundabout way of telling you that it inlined way more function calls than you probably wanted to have inlined. Because you definitely won't explicitly spell out such a long function in newly-written code, right? :tannedcirno:
At least it wasn't the worst copy-pasting job in this game; that trophy still goes to 📝 Elis. And while the tracking code for adjusting an eye's sprite according to the player's relative position is one of the main causes behind all the bloat, it's also 100% consistent, and might have been an inlined class method in ZUN's original code as well.

The clear highlight in this fight though? Almost no coordinate is precisely calculated where you'd expect it to be. In particular, all bullet spawn positions completely ignore the direction the eyes are facing to:

Pellets unexpectedly spawned at the exact
	bottom center of an eye
Combining the bottom of the pupil with the exact horizontal center of the sprite as a whole might sound like a good idea, but looks especially wrong if the eye is facing right.
Missile spawn positions in the TH01
	YuugenMagan fight
Here it's the other way round: OK for a right-facing eye, really wrong for a left-facing one.
Spawn position of the 3-pixel laser in the
	TH01 YuugenMagan fight
Dude, the eye is even supposed to track the laser in this one!
The final center position of the regular
	pentagram in the TH01 YuugenMagan fight
Hint: That's not the center of the playfield. At least the pellets spawned from the corners are sort of correct, but with the corner calculates precomputed, you could only get them wrong on purpose.

Due to their effect on gameplay, these inaccuracies can't even be called "bugs", and made me devise a new "quirk" category instead. More on that in the TH01 100% blog post, though.


While we did see an accidentally unused bullet pattern earlier, I can now say with certainty that there are no truly unused danmaku patterns in TH01, i.e., pattern code that exists but is never called. However, the code for YuugenMagan's phase 5 reveals another small piece of danmaku design intention that never shows up within the parameters of the original game.
By default, pellets are clipped when they fly past the top of the playfield, which we can clearly observe for the first few pellets of this pattern. Interestingly though, the second subpattern actually configures its pellets to fall straight down from the top of the playfield instead. You never see this happening in-game because ZUN limited that subpattern to a downwards angle range of 0x73 or 162°, resulting in none of its pellets ever getting close to the top of the playfield. If we extend that range to a full 360° though, we can see how ZUN might have originally planned the pattern to end:

YuugenMagan's phase 5 patterns, on , with the second subpattern extended to reveal the different pellet behavior that remained in the final game code. In the original game, the eyes would stop spawning bullets on this frame.

If we also disregard everything else about YuugenMagan that fits the upcoming definition of quirk, we're left with 6 "fixable" bugs, all of which are a symptom of general blitting and unblitting laziness. Funnily enough, they can all be demonstrated within a short 9-second part of the fight, from the end of phase 9 up until the pentagram starts spinning in phase 13:

  1. General flickering whenever any sprite overlaps an eye. This is caused by only reblitting each eye every 3 frames, and is an issue all throughout the fight. You might have already spotted it in the videos above.
  2. Each of the two lasers is unblitted and blitted individually instead of each operation being done for both lasers together. Remember how 📝 ZUN unblits 32 horizontal pixels for every row of a line regardless of its width? That's why the top part of the left, right-moving laser is never visible, because it's blitted before the other laser is unblitted.
  3. ZUN forgot to unblit the lasers when phase 9 ends. This footage was recorded by pressing ↵ Return in debug mode, and it's probably impossible to achieve this during actual gameplay without TAS techniques. You would have to deal the required 6 points of damage within 491 frames, with the eye being invincible during 240 of them. Simply shooting up an Orb with a horizontal velocity of 0 would also only work a single time, as boss entities always repel the Orb with a horizontal velocity of ±4.
  4. The shrinking pentagram is unblitted after the eyes were blitted, adding another guaranteed frame of flicker on top of the ones in 1). Like in 2), the blockiness of the holes is another result of unblitting 32 pixels per row at a time.
  5. Another missing unblitting call in a phase transition, as the pentagram switches from its not quite correctly interpolated shrunk form to a regular star polygon with a radius of 64 pixels. Indirectly caused by the massively bloated coordinate calculation for the shrink animation being done separately for the unblitting and blitting calls. Instead of, y'know, just doing it once and storing the result in variables that can later be reused.
  6. The pentagram is not reblitted at all during the first 100 frames of phase 13. During that rather long time, it's easily possible to remove it from VRAM completely by covering its area with player shots. Or HARRY UP pellets.

Definitely an appropriate end for this game's entity blitting code. :onricdennat: I'm really looking forward to writing a proper sprite system for the Anniversary Edition…

And just in case you were wondering about the hitboxes of these pentagrams as they slam themselves into Reimu:

62 pixels on the X axis, centered around each corner point of the star, 16 pixels below, and extending infinitely far up. The latter part becomes especially devious because the game always collision-detects all 5 corners, regardless of whether they've already clipped through the bottom of the playfield. The simultaneously occurring shape distortions are simply a result of the line drawing function's rather poor re-interpolation of any line that runs past the 640×400 VRAM boundaries; 📝 I described that in detail back when I debugged the shootout laser crash. Ironically, using fixed-size hitboxes for a variable-sized pentagram means that the larger one is easier to dodge.


The final puzzle in TH01's boss code comes 📝 once again in the form of weird hardware palette changes. The kanji on the background image goes through various colors throughout the fight, which ZUN implemented by gradually incrementing and decrementing either a single one or none of the color's three 4-bit components at the beginning of each even-numbered phase. The resulting color sequence, however, doesn't quite seem to follow these simple rules:

Adding some debug output sheds light on what's going on there:

Since each iteration of phase 12 adds 63 to the red component, integer overflow will cause the color to infinitely alternate between dark-blue and red colors on every 2.03 iterations of the pentagram phase loop. The 65th iteration will therefore be the first one with a dark-blue color for a third iteration in a row – just in case you manage to stall the fight for that long.

Yup, ZUN had so much trust in the color clamping done by his hardware palette functions that he did not clamp the increment operation on the stage_palette itself. :zunpet: Therefore, the 邪 colors and even the timing of their changes from Phase 6 onwards are "defined" by wildly incrementing color components beyond their intended domain, so much that even the underlying signed 8-bit integer ends up overflowing. Given that the decrement operation on the stage_palette is clamped though, this might be another one of those accidents that ZUN deliberately left in the game, 📝 similar to the conclusion I reached with infinite bumper loops.
But guess what, that's also the last time we're going to encounter this type of palette component domain quirk! Later games use master.lib's 8-bit palette system, which keeps the comfort of using a single byte per component, but shifts the actual hardware color into the top 4 bits, leaving the bottom 4 bits for added precision during fades.

OK, but now we're done with TH01's bosses! 🎉That was the 8th PC-98 Touhou boss in total, leaving 23 to go.


With all the necessary research into these quirks going well into a fifth push, I spent the remaining time in that one with transferring most of the data between YuugenMagan and the upcoming rest of REIIDEN.EXE into C land. This included the one piece of technical debt in TH01 we've been carrying around since March 2015, as well as the final piece of the ending sequence in FUUIN.EXE. Decompiling that executable's main() function in a meaningful way requires pretty much all remaining data from REIIDEN.EXE to also be moved into C land, just in case you were wondering why we're stuck at 99.46% there.
On a more disappointing note, the static initialization code for the 📝 5 boss entity slots ultimately revealed why YuugenMagan's code is as bloated and redundant as it is: The 5 slots really are 5 distinct variables rather than a single 5-element array. That's why ZUN explicitly spells out all 5 eyes every time, because the array he could have just looped over simply didn't exist. 😕 And while these slot variables are stored in a contiguous area of memory that I could just have taken the address of and then indexed it as if it were an array, I didn't want to annoy future port authors with what would technically be out-of-bounds array accesses for purely stylistic reasons. At least it wasn't that big of a deal to rewrite all boss code to use these distinct variables, although I certainly had to get a bit creative with Elis.

Next up: Finding out how many points we got in totle, and hoping that ZUN didn't hide more unexpected complexities in the remaining 45 functions of this game. If you have to spare, there are two ways in which that amount of money would help right now:

📝 Posted:
🚚 Summary of:
P0201, P0202
Commits:
9342665...ff49e9e, ff49e9e...4568bf7
💰 Funded by:
Ember2528, Yanga, [Anonymous]
🏷 Tags:
rec98+ th01+ gameplay+ boss+ singyoku+ blitting- glitch+ animation+ bullet+

The positive:

The negative:

The overview:


This time, we're back to the Orb hitbox being a logical 49×49 pixels in SinGyoku's center, and the shot hitbox being the weird one. What happens if you want the shot hitbox to be both offset to the left a bit and stretch the entire width of SinGyoku's sprite? You get a hitbox that ends in mid-air, far away from the right edge of the sprite:

Due to VRAM byte alignment, all player shots fired between gx = 376 and gx = 383 inclusive appear at the same visual X position, but are internally already partly outside the hitbox and therefore won't hit SinGyoku – compare gx = 376 to gx = 380. So much for precisely visualizing hitboxes in this game…

Since the female and male forms also use the sphere entity's coordinates, they share the same hitbox.


Onto the rendering glitches then, which can – you guessed it – all be found in the sphere form's slam movement:

By having the sphere move from the right edge of the playfield to the left, this video demonstrates both the lazy reblitting and broken unblitting at the right edge for negative X velocities. Also, isn't it funny how Reimu can partly disappear from all the sloppy SinGyoku-related unblitting going on after her sprite was blitted?

Due to the low contrast of the sphere against the background, you typically don't notice these glitches, but the white invincibility flashing after a hit really does draw attention to them. This time, all of these glitches aren't even directly caused by ZUN having never learned about the EGC's bit length register – if he just wrote correct code for SinGyoku, none of this would have been an issue. Sigh… I wonder how many more glitches will be caused by improper use of this one function in the last 18% of REIIDEN.EXE.

There's even another bug here, with ZUN hardcoding a horizontal delta of 8 pixels rather than just passing the actual X velocity. Luckily, the maximum movement speed is 6 pixels on Lunatic, and this would have only turned into an additional observable glitch if the X velocity were to exceed 24 pixels. But that just means it's the kind of bug that still drains RE attention to prove that you can't actually observe it in-game under some circumstances.


The 5 pellet patterns are all pretty straightforward, with nothing to talk about. The code architecture during phase 2 does hint towards ZUN having had more creative patterns in mind – especially for the male form, which uses the transformation function's three pattern callback slots for three repetitions of the same pellet group.
There is one more oddity to be found at the very end of the fight:

The first frame of TH01 SinGyoku's defeat animation, showing the sphere blitted on top of a potentially active person form

Right before the defeat white-out animation, the sphere form is explicitly reblitted for no reason, on top of the form that was blitted to VRAM in the previous frame, and regardless of which form is currently active. If SinGyoku was meant to immediately transform back to the sphere form before being defeated, why isn't the person form unblitted before then? Therefore, the visibility of both forms is undeniably canon, and there is some lore meaning to be found here… :thonk:
In any case, that's SinGyoku done! 6th PC-98 Touhou boss fully decompiled, 25 remaining.


No FUUIN.EXE code rounding out the last push for a change, as the 📝 remaining missile code has been waiting in front of SinGyoku for a while. It already looked bad in November, but the angle-based sprite selection function definitely takes the cake when it comes to unnecessary and decadent floating-point abuse in this game.
The algorithm itself is very trivial: Even with 📝 .PTN requiring an additional quarter parameter to access 16×16 sprites, it's essentially just one bit shift, one addition, and one binary AND. For whatever reason though, ZUN casts the 8-bit missile angle into a 64-bit double, which turns the following explicit comparisons (!) against all possible 4 + 16 boundary angles (!!) into FPU operations. :zunpet: Even with naive and readable division and modulo operations, and the whole existence of this function not playing well with Turbo C++ 4.0J's terrible code generation at all, this could have been 3 lines of code and 35 un-inlined constant-time instructions. Instead, we've got this 207-instruction monster… but hey, at least it works. 🤷
The remaining time then went to YuugenMagan's initialization code, which allowed me to immediately remove more declarations from ASM land, but more on that once we get to the rest of that boss fight.

That leaves 76 functions until we're done with TH01! Next up: Card-flipping stage obstacles.

📝 Posted:
🚚 Summary of:
P0198, P0199, P0200
Commits:
48db0b7...440637e, 440637e...5af2048, 5af2048...67e46b5
💰 Funded by:
Ember2528, Lmocinemod, Yanga
🏷 Tags:
rec98+ th01+ gameplay+ boss+ kikuri+ blitting- glitch+ danmaku-pattern+ cutscene+ pc98+ performance+ tcc+

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

So yeah, there's your new timeout challenge. :godzun:


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

TODO TH01 Kikuri's intended hitboxTH01 Kikuri's actual hitbox
Kikuri's actual hitbox. Since the Orb sprite doesn't change its shape, we can visualize the hitbox in a pixel-perfect way here. The Orb must be completely within the red area for a hit to be registered.

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

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

Due to the deterministic nature of this part of the fight, it's always 5 pixels for this first set of ripples. These visualizations are obviously not pixel-perfect due to the different potential shapes of Reimu's sprite, so they instead relate to her 32×32 bounding box, which needs to be entirely inside the red area.

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


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

Kikuri's phase 4 patterns, on .


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

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

Picture #2 from ED2A.GRP.

Which is rendered by the following code:

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

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

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

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

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

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

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

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

(Note how the background of the ドカーン image is shifted 1 pixel to the left compared to pic #1.)

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

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

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

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

📝 Posted:
🚚 Summary of:
P0193, P0194, P0195, P0196, P0197
Commits:
e1f3f9f...183d7a2, 183d7a2...5d93a50, 5d93a50...e18c53d, e18c53d...57c9ac5, 57c9ac5...48db0b7
💰 Funded by:
Ember2528, Yanga
🏷 Tags:
rec98+ th01+ gameplay+ boss+ elis+ mima-th01+ rng+ danmaku-pattern+ bug+ jank+ glitch+ unused+ pc98+ blitting- mod+ score+

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.

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 . 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 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:
P0189
Commits:
22abdd1...b4876b6
💰 Funded by:
Arandui, Lmocinemod
🏷 Tags:
rec98+ th04+ th05+ gameplay+ boss+ kurumi+ marisa-4+ bug+ danmaku-pattern+ mod+ sara+ blitting- animation+

(Before we start: Make sure you've read the current version of the FAQ section on a potential takedown of this project, updated in light of the recent DMCA claims against PC-98 Touhou game downloads.)


Slight change of plans, because we got instructions for reliably reproducing the TH04 Kurumi Divide Error crash! Major thanks to Colin Douglas Howell. With those, it also made sense to immediately look at the crash in the Stage 4 Marisa fight as well. This way, I could release both of the obligatory bugfix mods at the same time.
Especially since it turned out that I was wrong: Both crashes are entirely unrelated to the custom entity structure that would have required PI-centric progress. They are completely specific to Kurumi's and Marisa's danmaku-pattern code, and really are two separate bugs with no connection to each other. All of the necessary research nicely fit into Arandui's 0.5 pushes, with no further deep understanding required here.

But why were there still three weeks between Colin's message and this blog post? DMCA distractions aside: There are no easy fixes this time, unlike 📝 back when I looked at the Stage 5 Yuuka crash. Just like how division by zero is undefined in mathematics, it's also, literally, undefined what should happen instead of these two Divide error crashes. This means that any possible "fix" can only ever be a fanfiction interpretation of the intentions behind ZUN's code. The gameplay community should be aware of this, and might decide to handle these cases differently. And if we have to go into fanfiction territory to work around crashes in the canon games, we'd better document what exactly we're fixing here and how, as comprehensible as possible.


With that out of the way, let's look at Kurumi's crash first, since it's way easier to grasp. This one is known to primarily happen to new players, and it's easy to see why:


The pattern that causes the crash in Kurumi's fight. Also demonstrates how the number of bullets in a ring is always halved on Easy Mode after the rank-based tuning, leading to just a 3-ring on playperf = 16.

So, what should the workaround look like? Obviously, we want to modify neither the default number of ring bullets nor the tuning algorithm – that would change all other non-crashing variations of this pattern on other difficulties and ranks, creating a fork of the original gameplay. Instead, I came up with four possible workarounds that all seemed somewhat logical to me:

  1. Firing no bullet, i.e., interpreting 0-ring literally. This would create the only constellation in which a call to the bullet group spawn functions would not spawn at least one new bullet.
  2. Firing a "1-ring", i.e., a single bullet. This would be consistent with how the bullet spawn functions behave for "0-way" stack and spread groups.
  3. Firing a "∞-ring", i.e., 200 bullets, which is as much as the game's cap on 16×16 bullets would allow. This would poke fun at the whole "division by zero" idea… but given that we're still talking about Easy Mode (and especially new players) here, it might be a tad too cruel. Certainly the most trollish interpretation.
  4. Triggering an immediate Game Over, exchanging the hard crash for a softer and more controlled shutdown. Certainly the option that would be closest to the behavior of the original games, and perhaps the only one to be accepted in Serious, High-Level Play™.

As I was writing this post, it felt increasingly wrong for me to make this decision. So I once again went to Twitter, where 56.3% voted in favor of the 1-bullet option. Good that I asked! I myself was more leaning towards the 0-bullet interpretation, which only got 28.7% of the vote. Also interesting are the 2.3% in favor of the Game Over option but I get it, low-rank Easy Mode isn't exactly the most competitive mode of playing TH04.
There are reports of Kurumi crashing on higher difficulties as well, but I could verify none of them. If they aren't fixed by this workaround, they're caused by an entirely different bug that we have yet to discover.


Onto the Stage 4 Marisa crash then, which does in fact apply to all difficulty levels. I was also wrong on this one – it's a hell of a lot more intricate than being just a division by the number of on-screen bits. Without having decompiled the entire fight, I can't give a completely accurate picture of what happens there yet, but here's the rough idea:

Reference points for Marisa's point-reflected movement. Cyan: Marisa's position, green: (192, 112), yellow: the intended end point.

One of the two patterns in TH04's Stage 4 Marisa boss fight that feature frame number-dependent point-reflected movement. The bits were hacked to self-destruct on .

tl;dr: "Game crashes if last bit destroyed within 4-frame window near end of two patterns". For an informed decision on a new movement behavior for these last 8 frames, we definitely need to know all the details behind the crash though. Here's what I would interpret into the code:

  1. Not moving at all, i.e., interpreting 0 as the middle ground between positive and negative movement. This would also make sense because a 12-frame duration implies 100% of the movement to consist of the braking phase – and Marisa wasn't moving before, after all.
  2. Move at maximum speed, i.e., dividing by 1 rather than 0. Since the movement duration is still 12 in this case, Marisa will immediately start braking. In total, she will move exactly ¾ of the way from her initial position to (192, 112) within the 8 frames before the pattern ends.
  3. Directly warping to (192, 112) on frame 0, and to the point-reflected target on 4, respectively. This "emulates" the division by zero by moving Marisa at infinite speed to the exact two points indicated by the velocity formula. It also fits nicely into the 8 frames we have to fill here. Sure, Marisa can't reach these points at any other duration, but why shouldn't she be able to, with infinite speed? Then again, if Marisa is far away enough from (192, 112), this workaround would warp her across the entire playfield. Can Marisa teleport according to lore? I have no idea… :tannedcirno:
  4. Triggering an immediate Game O– hell no, this is the Stage 4 boss, people already hate losing runs to this bug!

Asking Twitter worked great for the Kurumi workaround, so let's do it again! Gotta attach a screenshot of an earlier draft of this blog post though, since this stuff is impossible to explain in tweets…

…and it went through the roof, becoming the most successful ReC98 tweet so far?! Apparently, y'all really like to just look at descriptions of overly complex bugs that I'd consider way beyond the typical attention span that can be expected from Twitter. Unfortunately, all those tweet impressions didn't quite translate into poll turnout. The results were pretty evenly split between 1) and 2), with option 1) just coming out slightly ahead at 49.1%, compared to 41.5% of option 2).

(And yes, I only noticed after creating the poll that warping to both the green and yellow points made more sense than warping to just one of the two. Let's hope that this additional variant wouldn't have shifted the results too much. Both warp options only got 9.4% of the vote after all, and no one else came up with the idea either. :onricdennat: In the end, you can always merge together your preferred combination of workarounds from the Git branches linked below.)


So here you go: The new definitive version of TH04, containing not only the community-chosen Kurumi and Stage 4 Marisa workaround variant, but also the 📝 No-EMS bugfix from last year. Edit (2022-05-31): This package is outdated, 📝 the current version is here! 2022-04-18-community-choice-fixes.zip Oh, and let's also add spaztron64's TH03 GDC clock fix from 2019 because why not. This binary was built from the community_choice_fixes branch, and you can find the code for all the individual workarounds on these branches:

Again, because it can't be stated often enough: These fixes are fanfiction. The gameplay community should be aware of this, and might decide to handle these cases differently.


With all of that taking way more time to evaluate and document, this research really had to become part of a proper push, instead of just being covered in the quick non-push blog post I initially intended. With ½ of a push left at the end, TH05's Stage 1-5 boss background rendering functions fit in perfectly there. If you wonder how these static backdrop images even need any boss-specific code to begin with, you're right – it's basically the same function copy-pasted 4 times, differing only in the backdrop image coordinates and some other inconsequential details.
Only Sara receives a nice variation of the typical 📝 blocky entrance animation: The usually opaque bitmap data from ST00.BB is instead used as a transition mask from stage tiles to the backdrop image, by making clever use of the tile invalidation system:

TH04 uses the same effect a bit more frequently, for its first three bosses.

Next up: Shinki, for real this time! I've already managed to decompile 10 of her 11 danmaku patterns within a little more than one push – and yes, that one is included in there. Looks like I've slightly overestimated the amount of work required for TH04's and TH05's bosses…

📝 Posted:
🚚 Summary of:
P0184, P0185
Commits:
f9d983e...f918298, f918298...a21ab3d
💰 Funded by:
-Tom-, Blue Bolt, [Anonymous]
🏷 Tags:
rec98+ th04+ th05+ gameplay+ louise+ shinki+ bullet+ danmaku-pattern+ uth05win+ blitting-

Two years after 📝 the first look at TH04's and TH05's bullets, we finally get to finish their logic code by looking at the special motion types. Bullets as a whole still aren't completely finished as the rendering code is still waiting to be RE'd, but now we've got everything about them that's required for decompiling the midboss and boss fights of these games.

Just like the motion types of TH01's pellets, the ones we've got here really are special enough to warrant an enum, despite all the overlap in the "slow down and turn" and "bounce at certain edges of the playfield" types. Sure, including them in the bitfield I proposed two years ago would have allowed greater variety, but it wouldn't have saved any memory. On the contrary: These types use a single global state variable for the maximum turn count and delta speed, which a proper customizable architecture would have to integrate into the bullet structure. Maybe it is possible to stuff everything into the same amount of bytes, but not without first completely rearchitecting the bullet structure and removing every single piece of redundancy in there. Simply extending the system by adding a new enum value for a new motion type would be way more straightforward for modders.

Speaking about memory, TH05 already extends the bullet structure by 6 bytes for the "exact linear movement" type exclusive to that game. This type is particularly interesting for all the prospective PC-98 game developers out there, as it nicely points out the precision limits of Q12.4 subpixels.
Regular bullet movement works by adding a Q12.4 velocity to a Q12.4 position every frame, with the velocity typically being calculated only once on spawn time from an 8-bit angle and a Q12.4 speed. Quantization errors from this initial calculation can quickly compound over all the frames a bullet spends moving across the playfield. If a bullet is only supposed to move on a straight line though, there is a more precise way of calculating its position: By storing the origin point, movement angle, and total distance traveled, you can perform a full polar→Cartesian transformation every frame. Out of the 10 danmaku patterns in TH05 that use this motion type, the difference to regular bullet movement can be best seen in Louise's final pattern:

Louise's final pattern in its original form, demonstrating exact linear bullet movement. Note how each bullet spawns slightly behind the delay cloud: ZUN simply forgot to shift the fixed origin point along with it.
The same pattern with standard bullet movement, corrupting its intended appearance. No delay cloud-related oversights here though, at least.

Not far away from the regular bullet code, we've also got the movement function for the infamous curve / "cheeto" bullets. I would have almost called them "cheetos" in the code as well, which surely fits more nicely into 8.3 filenames than "curve bullets" does, but eh, trademarks…

As for hitboxes, we got a 16×16 one on the head node, and a 12×12 one on the 16 trail nodes. The latter simply store the position of the head node during the last 16 frames, Snake style. But what you're all here for is probably the turning and homing algorithm, right? Boiled down to its essence, it works like this:

// [head] points to the controlled "head" part of a curve bullet entity.
// Angles are stored with 8 bits representing a full circle, providing free
// normalization on arithmetic overflow.
// The directions are ordered as you would expect:
// • 0x00: right	(sin(0x00) =  0, cos(0x00) = +1)
// • 0x40: down 	(sin(0x40) = +1, cos(0x40) =  0)
// • 0x80: left 	(sin(0x80) =  0, cos(0x80) = -1)
// • 0xC0: up   	(sin(0xC0) = -1, cos(0xC0) =  0)
uint8_t angle_delta = (head->angle - player_angle_from(
	head->pos.cur.x, head->pos.cur.y
));

// Stop turning if the player is 1/128ths of a circle away from this bullet
const uint8_t SNAP = 0x02;

// Else, turn either clockwise or counterclockwise by 1/256th of a circle,
// depending on what would reach the player the fastest.
if((angle_delta > SNAP) && (angle_delta < static_cast<uint8_t>(-SNAP))) {
	angle_delta = (angle_delta >= 0x80) ? -0x01 : +0x01;
}
head_p->angle -= angle_delta;

5 lines of code, and not all too difficult to follow once you are familiar with 8-bit angles… unlike what ZUN actually wrote. Which is 26 lines, and includes an unused "friction" variable that is never set to any value that makes a difference in the formula. :zunpet: uth05win correctly saw through that all and simplified this code to something equivalent to my explanation. Redoing that work certainly wasted a bit of my time, and means that I now definitely need to spend another push on RE'ing all the shared boss functions before I can start with Shinki.

So while a curve bullet's speed does get faster over time, its angular velocity is always limited to 1/256th of a circle per frame. This reveals the optimal strategy for dodging them: Maximize this delta angle by staying as close to 180° away from their current direction as possible, and let their acceleration do the rest.

At least that's the theory for dodging a single one. As a danmaku designer, you can now of course place other bullets at these technically optimal places to prevent a curve bullet pattern from being cheesed like that. I certainly didn't record the video above in a single take either… :tannedcirno:


After another bunch of boring entity spawn and update functions, the playfield shaking feature turned out as the most notable (and tricky) one to round out these two pushes. It's actually implemented quite well in how it simply "un-shakes" the screen by just marking every stage tile to be redrawn. In the context of all the other tile invalidation that can take place during a frame, that's definitely more performant than 📝 doing another EGC-accelerated memmove(). Due to these two games being double-buffered via page flipping, this invalidation only really needs to happen for the frame after the next one though. The immediately next frame will show the regular, un-shaken playfield on the other VRAM page first, except during the multi-frame shake animation when defeating a midboss, where it will also appear shifted in a different direction… 😵 Yeah, no wonder why ZUN just always invalidates all stage tiles for the next two frames after every shaking animation, which is guaranteed to handle both sporadic single-frame shakes and continuous ones. So close to good-code here.

Finally, this delivery was delayed a bit because -Tom- requested his round-up amount to be limited to the cap in the future. Since that makes it kind of hard to explain on a static page how much money he will exactly provide, I now properly modeled these discounts in the website code. The exact round-up amount is now included in both the pre-purchase breakdown, as well as the cap bar on the main page.
With that in place, the system is now also set up for round-up offers from other patrons. If you'd also like to support certain goals in this way, with any amount of money, now's the time for getting in touch with me about that. Known contributors only, though! 😛

Next up: The final bunch of shared boring boss functions. Which certainly will give me a break from all the maintenance and research work, and speed up delivery progress again… right?

📝 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:
rec98+ th01+ gameplay+ boss+ sariel+ danmaku-pattern+ rng+ waste+ pc98+ blitting- good-code+ glitch+ tcc+ unused+ jank+

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

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

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 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.

📝 Posted:
🚚 Summary of:
P0172, P0173
Commits:
49e6789...2d5491e, 2d5491e...27f901c
💰 Funded by:
Blue Bolt, [Anonymous]
🏷 Tags:
rec98+ th03+ file-format+ menu+ score+ blitting- glitch+

TH03 finally passed 20% RE, and the newly decompiled code contains no serious ZUN bugs! What a nice way to end the year.

There's only a single unlockable feature in TH03: Chiyuri and Yumemi as playable characters, unlocked after a 1CC on any difficulty. Just like the Extra Stages in TH04 and TH05, YUME.NEM contains a single designated variable for this unlocked feature, making it trivial to craft a fully unlocked score file without recording any high scores that others would have to compete against. So, we can now put together a complete set for all PC-98 Touhou games: 2021-12-27-Fully-unlocked-clean-score-files.zip It would have been cool to set the randomly generated encryption keys in these files to a fixed value so that they cancel out and end up not actually encrypting the file. Too bad that TH03 also started feeding each encrypted byte back into its stream cipher, which makes this impossible.

The main loading and saving code turned out to be the second-cleanest implementation of a score file format in PC-98 Touhou, just behind TH02. Only two of the YUME.NEM functions come with nonsensical differences between OP.EXE and MAINL.EXE, rather than 📝 all of them, as in TH01 or 📝 too many of them, as in TH04 and TH05. As for the rest of the per-difficulty structure though… well, it quickly becomes clear why this was the final score file format to be RE'd. The name, score, and stage fields are directly stored in terms of the internal REGI*.BFT sprite IDs used on the high score screen. TH03 also stores 10 score digits for each place rather than the 9 possible ones, keeps any leading 0 digits, and stores the letters of entered names in reverse order… yeah, let's decompile the high score screen as well, for a full understanding of why ZUN might have done all that. (Answer: For no reason at all. :zunpet:)


And wow, what a breath of fresh air. It's surely not good-code: The overlapping shadows resulting from using a 24-pixel letterspacing with 32-pixel glyphs in the name column led ZUN to do quite a lot of unnecessary and slightly confusing rendering work when moving the cursor back and forth, and he even forgot about the EGC there. But it's nowhere close to the level of jank we saw in 📝 TH01's high score menu last year. Good to see that ZUN had learned a thing or two by his third game – especially when it comes to storing the character map cursor in terms of a character ID, and improving the layout of the character map:

The alphabet available for TH03 high score names.

That's almost a nicely regular grid there. With the question mark and the double-wide SP, BS, and END options, the cursor movement code only comes with a reasonable two exceptions, which are easily handled. And while I didn't get this screen completely decompiled, one additional push was enough to cover all important code there.

The only potential glitch on this screen is a result of ZUN's continued use of binary-coded decimal digits without any bounds check or cap. Like the in-game HUD score display in TH04 and TH05, TH03's high score screen simply uses the next glyph in the character set for the most significant digit of any score above 1,000,000,000 points – in this case, the period. Still, it only really gets bad at 8,000,000,000 points: Once the glyphs are exhausted, the blitting function ends up accessing garbage data and filling the entire screen with garbage pixels. For comparison though, the current world record is 133,650,710 points, so good luck getting 8 billion in the first place.

Next up: Starting 2022 with the long-awaited decompilation of TH01's Sariel fight! Due to the 📝 recent price increase, we now got a window in the cap that is going to remain open until tomorrow, providing an early opportunity to set a new priority after Sariel is done.

📝 Posted:
🚚 Summary of:
P0168, P0169
Commits:
c2de6ab...8b046da, 8b046da...479b766
💰 Funded by:
rosenrose, Blue Bolt
🏷 Tags:
rec98+ th04+ th05+ boss+ yuuka-5+ blitting- bug+ master.lib+ waste+ mod+

EMS memory! The infamous stopgap measure between the 640 KiB ("ought to be enough for everyone") of conventional memory offered by DOS from the very beginning, and the later XMS standard for accessing all the rest of memory up to 4 GiB in the x86 Protected Mode. With an optionally active EMS driver, TH04 and TH05 will make use of EMS memory to preload a bunch of situational .CDG images at the beginning of MAIN.EXE:

  1. The "eye catch" game title image, shown while stages are loaded
  2. The character-specific background image, shown while bombing
  3. The player character dialog portraits
  4. TH05 additionally stores the boss portraits there, preloading them at the beginning of each stage. (TH04 instead keeps them in conventional memory during the entire stage.)

Once these images are needed, they can then be copied into conventional memory and accessed as usual.

Uh… wait, copied? It certainly would have been possible to map EMS memory to a regular 16-bit Real Mode segment for direct access, bank-switching out rarely used system or peripheral memory in exchange for the EMS data. However, master.lib doesn't expose this functionality, and only provides functions for copying data from EMS to regular memory and vice versa.
But even that still makes EMS an excellent fit for the large image files it's used for, as it's possible to directly copy their pixel data from EMS to VRAM. (Yes, I tried!) Well… would, because ZUN doesn't do that either, and always naively copies the images to newly allocated conventional memory first. In essence, this dumbs down EMS into just another layer of the memory hierarchy, inserted between conventional memory and disk: Not quite as slow as disk, but still requiring that memcpy() to retrieve the data. Most importantly though: Using EMS in this way does not increase the total amount of memory simultaneously accessible to the game. After all, some other data will have to be freed from conventional memory to make room for the newly loaded data.


The most idiomatic way to define the game-specific layout of the EMS area would be either a struct or an enum. Unfortunately, the total size of all these images exceeds the range of a 16-bit value, and Turbo C++ 4.0J supports neither 32-bit enums (which are silently degraded to 16-bit) nor 32-bit structs (which simply don't compile). That still leaves raw compile-time constants though, you only have to manually define the offset to each image in terms of the size of its predecessor. But instead of doing that, ZUN just placed each image at a nice round decimal offset, each slightly larger than the actual memory required by the previous image, just to make sure that everything fits. :tannedcirno: This results not only in quite a bit of unnecessary padding, but also in technically the single biggest amount of "wasted" memory in PC-98 Touhou: Out of the 180,000 (TH04) and 320,000 (TH05) EMS bytes requested, the game only uses 135,552 (TH04) and 175,904 (TH05) bytes. But hey, it's EMS, so who cares, right? Out of all the opportunities to take shortcuts during development, this is among the most acceptable ones. Any actual PC-98 model that could run these two games comes with plenty of memory for this to not turn into an actual issue.

On to the EMS-using functions themselves, which are the definition of "cross-cutting concerns". Most of these have a fallback path for the non-EMS case, and keep the loaded .CDG images in memory if they are immediately needed. Which totally makes sense, but also makes it difficult to find names that reflect all the global state changed by these functions. Every one of these is also just called from a single place, so inlining them would have saved me a lot of naming and documentation trouble there.
The TH04 version of the EMS allocation code was actually displayed on ZUN's monitor in the 2010 MAG・ネット documentary; WindowsTiger already transcribed the low-quality video image in 2019. By 2015 ReC98 standards, I would have just run with that, but the current project goal is to write better code than ZUN, so I didn't. 😛 We sure ain't going to use magic numbers for EMS offsets.

The dialog init and exit code then is completely different in both games, yet equally cross-cutting. TH05 goes even further in saving conventional memory, loading each individual player or boss portrait into a single .CDG slot immediately before blitting it to VRAM and freeing the pixel data again. People who play TH05 without an active EMS driver are surely going to enjoy the hard drive access lag between each portrait change… :godzun: TH04, on the other hand, also abuses the dialog exit function to preload the Mugetsu defeat / Gengetsu entrance and Gengetsu defeat portraits, using a static variable to track how often the function has been called during the Extra Stage… who needs function parameters anyway, right? :zunpet:

This is also the function in which TH04 infamously crashes after the Stage 5 pre-boss dialog when playing with Reimu and without any active EMS driver. That crash is what motivated this look into the games' EMS usage… but the code looks perfectly fine? Oh well, guess the crash is not related to EMS then. Next u–

OK, of course I can't leave it like that. Everyone is expecting a fix now, and I still got half of a push left over after decompiling the regular EMS code. Also, I've now RE'd every function that could possibly be involved in the crash, and this is very likely to be the last time I'll be looking at them.


Turns out that the bug has little to do with EMS, and everything to do with ZUN limiting the amount of conventional RAM that TH04's MAIN.EXE is allowed to use, and then slightly miscalculating this upper limit. Playing Stage 5 with Reimu is the most asset-intensive configuration in this game, due to the combination of

The star image used in TH04's Stage 5.
The star image used in TH04's Stage 5.

Remove any single one of the above points, and this crash would have never occurred. But with all of them combined, the total amount of memory consumed by TH04's MAIN.EXE just barely exceeds ZUN's limit of 320,000 bytes, by no more than 3,840 bytes, the size of the star image.

But wait: As we established earlier, EMS does nothing to reduce the amount of conventional memory used by the game. In fact, if you disabled TH04's EMS handling, you'd still get this crash even if you are running an EMS driver and loaded DOS into the High Memory Area to free up as much conventional RAM as possible. How can EMS then prevent this crash in the first place?

The answer: It's only because ZUN's usage of EMS bypasses the need to load the cached images back out of the XOR-encrypted 東方幻想.郷 packfile. Leaving aside the general stupidity of any game data file encryption*, master.lib's decryption implementation is also quite wasteful: It uses a separate buffer that receives fixed-size chunks of the file, before decrypting every individual byte and copying it to its intended destination buffer. That really resembles the typical slowness of a C fread() implementation more than it does the highly optimized ASM code that master.lib purports to be… And how large is this well-hidden decryption buffer? 4 KiB. :onricdennat:

So, looking back at the game, here is what happens once the Stage 5 pre-battle dialog ends:

  1. Reimu's bomb background image, which was previously freed to make space for her dialog portraits, has to be loaded back into conventional memory from disk
  2. BB0.CDG is found inside the 東方幻想.郷 packfile
  3. file_ropen() ends up allocating a 4 KiB buffer for the encrypted packfile data, getting us the decisive ~4 KiB closer to the memory limit
  4. The .CDG loader tries to allocate 52 608 contiguous bytes for the pixel data of Reimu's bomb image
  5. This would exceed the memory limit, so hmem_allocbyte() fails and returns a nullptr
  6. ZUN doesn't check for this case (as usual)
  7. The pixel data is loaded to address 0000:0000, overwriting the Interrupt Vector Table and whatever comes after
  8. The game crashes
The final frame rendered before the TH04 Stage 5 Reimu No-EMS crash
The final frame rendered by a crashing TH04.

The 4 KiB encryption buffer would only be freed by the corresponding file_close() call, which of course never happens because the game crashes before it gets there. At one point, I really did suspect the cause to be some kind of memory leak or fragmentation inside master.lib, which would have been quite delightful to fix.
Instead, the most straightforward fix here is to bump up that memory limit by at least 4 KiB. Certainly easier than squeezing in a cdg_free() call for the star image before the pre-boss dialog without breaking position dependence.

Or, even better, let's nuke all these memory limits from orbit because they make little sense to begin with, and fix every other potential out-of-memory crash that modders would encounter when adding enough data to any of the 4 games that impose such limits on themselves. Unless you want to launch other binaries (which need to do their own memory allocations) after launching the game, there's really no reason to restrict the amount of memory available to a DOS process. Heck, whenever DOS creates a new one, it assigns all remaining free memory by default anyway.
Removing the memory limits also removes one of ZUN's few error checks, which end up quitting the game if there isn't at least a given maximum amount of conventional RAM available. While it might be tempting to reserve enough memory at the beginning of execution and then never check any allocation for a potential failure, that's exactly where something like TH04's crash comes from.
This game is also still running on DOS, where such an initial allocation failure is very unlikely to happen – no one fills close to half of conventional RAM with TSRs and then tries running one of these games. It might have been useful to detect systems with less than 640 KiB of actual, physical RAM, but none of the PC-98 models with that little amount of memory are fast enough to run these games to begin with. How ironic… a place where ZUN actually added an error check, and then it's mostly pointless.

Here's an archive that contains both fix variants, just in case. These were compiled from the th04_noems_crash_fix and mem_assign_all branches, and contain as little code changes as possible.
Edit (2022-04-18): For TH04, you probably want to download the 📝 community choice fix package instead, which contains this fix along with other workarounds for the Divide error crashes. 2021-11-29-Memory-limit-fixes.zip

So yeah, quite a complex bug, leaving no time for the TH03 scorefile format research after all. Next up: Raising prices.

📝 Posted:
🚚 Summary of:
P0165, P0166, P0167
Commits:
7a0e5d8...f2bca01, f2bca01...e697907, e697907...c2de6ab
💰 Funded by:
Ember2528
🏷 Tags:
rec98+ th01+ gameplay+ animation+ blitting- bullet+ glitch+ waste+ boss+ singyoku+ mima-th01+ elis+ tcc+ menu+

OK, TH01 missile bullets. Can we maybe have a well-behaved entity type, without any weirdness? Just once?

Ehh, kinda. Apart from another 150 bytes wasted on unused structure members, this code is indeed more on the low end in terms of overall jank. It does become very obvious why dodging these missiles in the YuugenMagan, Mima, and Elis fights feels so awful though: An unfair 46×46 pixel hitbox around Reimu's center pixel, combined with the comeback of 📝 interlaced rendering, this time in every stage. ZUN probably did this because missiles are the only 16×16 sprite in TH01 that is blitted to unaligned X positions, which effectively ends up touching a 32×16 area of VRAM per sprite.
But even if we assume VRAM writes to be the bottleneck here, it would have been totally possible to render every missile in every frame at roughly the same amount of CPU time that the original game uses for interlaced rendering:

That's an optimization that would have significantly benefitted the game, in contrast to all of the fake ones introduced in later games. Then again, this optimization is actually something that the later games do, and it might have in fact been necessary to achieve their higher bullet counts without significant slowdown.

Unfortunately, it was only worth decompiling half of the missile code right now, thanks to gratuitous FPU usage in the other half, where 📝 double variables are compared to float literals. That one will have to wait 📝 until after SinGyoku.


After some effectively unused Mima sprite effect code that is so broken that it's impossible to make sense out of it, we get to the final feature I wanted to cover for all bosses in parallel before returning to Sariel: The separate sprite background storage for moving or animated boss sprites in the Mima, Elis, and Sariel fights. But, uh… why is this necessary to begin with? Doesn't TH01 already reserve the other VRAM page for backgrounds?
Well, these sprites are quite big, and ZUN didn't want to blit them from main memory on every frame. After all, TH01 and TH02 had a minimum required clock speed of 33 MHz, half of the speed required for the later three games. So, he simply blitted these boss sprites to both VRAM pages, leading the usual unblitting calls to only remove the other sprites on top of the boss. However, these bosses themselves want to move across the screen… and this makes it necessary to save the stage background behind them in some other way.

Enter .PTN, and its functions to capture a 16×16 or 32×32 square from VRAM into a sprite slot. No problem with that approach in theory, as the size of all these bigger sprites is a multiple of 32×32; splitting a larger sprite into these smaller 32×32 chunks makes the code look just a little bit clumsy (and, of course, slower).
But somewhere during the development of Mima's fight, ZUN apparently forgot that those sprite backgrounds existed. And once Mima's 🚫 casting sprite is blitted on top of her regular sprite, using just regular sprite transparency, she ends up with her infamous third arm:

TH01 Mima's third arm

Ironically, there's an unused code path in Mima's unblit function where ZUN assumes a height of 48 pixels for Mima's animation sprites rather than the actual 64. This leads to even clumsier .PTN function calls for the bottom 128×16 pixels… Failing to unblit the bottom 16 pixels would have also yielded that third arm, although it wouldn't have looked as natural. Still wouldn't say that it was intentional; maybe this casting sprite was just added pretty late in the game's development?


So, mission accomplished, Sariel unblocked… at 2¼ pushes. :thonk: That's quite some time left for some smaller stage initialization code, which bundles a bunch of random function calls in places where they logically really don't belong. The stage opening animation then adds a bunch of VRAM inter-page copies that are not only redundant but can't even be understood without knowing the hidden internal state of the last VRAM page accessed by previous ZUN code…
In better news though: Turbo C++ 4.0 really doesn't seem to have any complexity limit on inlining arithmetic expressions, as long as they only operate on compile-time constants. That's how we get macro-free, compile-time Shift-JIS to JIS X 0208 conversion of the individual code points in the 東方★靈異伝 string, in a compiler from 1994. As long as you don't store any intermediate results in variables, that is… :tannedcirno:

But wait, there's more! With still ¼ of a push left, I also went for the boss defeat animation, which includes the route selection after the SinGyoku fight.
As in all other instances, the 2× scaled font is accomplished by first rendering the text at regular 1× resolution to the other, invisible VRAM page, and then scaled from there to the visible one. However, the route selection is unique in that its scaled text is both drawn transparently on top of the stage background (not onto a black one), and can also change colors depending on the selection. It would have been no problem to unblit and reblit the text by rendering the 1× version to a position on the invisible VRAM page that isn't covered by the 2× version on the visible one, but ZUN (needlessly) clears the invisible page before rendering any text. :zunpet: Instead, he assigned a separate VRAM color for both the 魔界 and 地獄 options, and only changed the palette value for these colors to white or gray, depending on the correct selection. This is another one of the 📝 rare cases where TH01 demonstrates good use of PC-98 hardware, as the 魔界へ and 地獄へ strings don't need to be reblitted during the selection process, only the Orb "cursor" does.

Then, why does this still not count as good-code? When changing palette colors, you kinda need to be aware of everything else that can possibly be on screen, which colors are used there, and which aren't and can therefore be used for such an effect without affecting other sprites. In this case, well… hover over the image below, and notice how Reimu's hair and the bomb sprites in the HUD light up when Makai is selected:

Demonstration of palette changes in TH01's route selection

This push did end on a high note though, with the generic, non-SinGyoku version of the defeat animation being an easily parametrizable copy. And that's how you decompile another 2.58% of TH01 in just slightly over three pushes.


Now, we're not only ready to decompile Sariel, but also Kikuri, Elis, and SinGyoku without needing any more detours into non-boss code. Thanks to the current TH01 funding subscriptions, I can plan to cover most, if not all, of Sariel in a single push series, but the currently 3 pending pushes probably won't suffice for Sariel's 8.10% of all remaining code in TH01. We've got quite a lot of not specifically TH01-related funds in the backlog to pass the time though.

Due to recent developments, it actually makes quite a lot of sense to take a break from TH01: spaztron64 has managed what every Touhou download site so far has failed to do: Bundling all 5 game onto a single .HDI together with pre-configured PC-98 emulators and a nice boot menu, and hosting the resulting package on a proper website. While this first release is already quite good (and much better than my attempt from 2014), there is still a bit of room for improvement to be gained from specific ReC98 research. Next up, therefore:

📝 Posted:
🚚 Summary of:
P0134
Commits:
1d5db71...a6eed55
💰 Funded by:
[Anonymous]
🏷 Tags:
rec98+ th05+ blitting- portability+ micro-optimization+ jank+ tasm+ tcc+

Technical debt, part 5… and we only got TH05's stupidly optimized .PI functions this time?

As far as actual progress is concerned, that is. In maintenance news though, I was really hyped for the #include improvements I've mentioned in 📝 the last post. The result: A new x86real.h file, bundling all the declarations specific to the 16-bit x86 Real Mode in a smaller file than Turbo C++'s own DOS.H. After all, DOS is something else than the underlying CPU. And while it didn't speed up build times quite as much as I had hoped, it now clearly indicates the x86-specific parts of PC-98 Touhou code to future port authors.

After another couple of improvements to parameter declaration in ASM land, we get to TH05's .PI functions… and really, why did ZUN write all of them in ASM? Why (re)declare all the necessary structures and data in ASM land, when all these functions are merely one layer of abstraction above master.lib, which does all the actual work?
I get that ZUN might have wanted masked blitting to be faster, which is used for the fade-in effect seen during TH05's main menu animation and the ending artwork. But, uh… he knew how to modify master.lib. In fact, he did already modify the graph_pack_put_8() function used for rendering a single .PI image row, to ignore master.lib's VRAM clipping region. For this effect though, he first blits each row regularly to the invisible 400th row of VRAM, and then does an EGC-accelerated VRAM-to-VRAM blit of that row to its actual target position with the mask enabled. It would have been way more efficient to add another version of this function that takes a mask pattern. No amount of REP MOVSW is going to change the fact that two VRAM writes per line are slower than a single one. Not to mention that it doesn't justify writing every other .PI function in ASM to go along with it…
This is where we also find the most hilarious aspect about this: For most of ZUN's pointless micro-optimizations, you could have maybe made the argument that they do save some CPU cycles here and there, and therefore did something positive to the final, PC-98-exclusive result. But some of the hand-written ASM here doesn't even constitute a micro-optimization, because it's worse than what you would have got out of even Turbo C++ 4.0J with its 80386 optimization flags! :zunpet:

At least it was possible to "decompile" 6 out of the 10 functions here, making them easy to clean up for future modders and port authors. Could have been 7 functions if I also decided to "decompile" pi_free(), but all the C++ code is already surrounded by ASM, resulting in 2 ASM translation units and 2 C++ translation units. pi_free() would have needed a single translation unit by itself, which wasn't worth it, given that I would have had to spell out every single ASM instruction anyway.

void pascal pi_free(int slot)
{
	if(pi_buffers[slot]) {
		graph_pi_free(&pi_headers[slot], &pi_buffers[slot]);
		pi_buffers[slot] = NULL;
	}
}

There you go. What about this needed to be written in ASM?!?

The function calls between these small translation units even seemed to glitch out TASM and the linker in the end, leading to one CALL offset being weirdly shifted by 32 bytes. Usually, TLINK reports a fixup overflow error when this happens, but this time it didn't, for some reason? Mirroring the segment grouping in the affected translation unit did solve the problem, and I already knew this, but only thought of it after spending quite some RTFM time… during which I discovered the -lE switch, which enables TLINK to use the expanded dictionaries in Borland's .OBJ and .LIB files to speed up linking. That shaved off roughly another second from the build time of the complete ReC98 repository. The more you know… Binary blobs compiled with non-Borland tools would be the only reason not to use this flag.

So, even more slowdown with this 5th dedicated push, since we've still only repaid 41% of the technical debt in the SHARED segment so far. Next up: Part 6, which hopefully manages to decompile the FM and SSG channel animations in TH05's Music Room, and hopefully ends up being the final one of the slow ones.

📝 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

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! (Edit (2022-05-31): Nope, there's a 📝 potential heap corruption after all, which can be triggered in some fights in debug mode.) 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:
P0123
Commits:
4406c3d...72dfa09
💰 Funded by:
Yanga
🏷 Tags:
rec98+ th01+ file-format+ player+ animation+ blitting- waste+ jank+

Done with the .BOS format, at last! While there's still quite a bunch of undecompiled non-format blitting code left, this was in fact the final piece of graphics format loading code in TH01.

📝 Continuing the trend from three pushes ago, we've got yet another class, this time for the 48×48 and 48×32 sprites used in Reimu's gohei, slide, and kick animations. The only reason these had to use the .BOS format at all is simply because Reimu's regular sprites are 32×32, and are therefore loaded from 📝 .PTN files.
Yes, this makes no sense, because why would you split animations for the same character across two file formats and two APIs, just because of a sprite size difference? This necessity for switching blitting APIs might also explain why Reimu vanishes for a few frames at the beginning and the end of the gohei swing animation, but more on that once we get to the high-level rendering code.

Now that we've decompiled all the .BOS implementations in TH01, here's an overview of all of them, together with .PTN to show that there really was no reason for not using the .BOS API for all of Reimu's sprites:

CBossEntity CBossAnim CPlayerAnim ptn_* (32×32)
Format .BOS .BOS .BOS .PTN
Hitbox
Byte-aligned blitting
Byte-aligned unblitting
Unaligned blitting Single-line and wave only
Precise unblitting
Per-file sprite limit 8 8 32 64
Pixels blitted at once 16 16 8 32

And even that last property could simply be handled by branching based on the sprite width, and wouldn't be a reason for switching formats. But well, it just wouldn't be TH01 without all that redundant bloat though, would it?

The basic loading, freeing, and blitting code was yet another variation on the other .BOS code we've seen before. So this should have caused just as little trouble as the CBossAnim code… except that CPlayerAnim did add one slightly difficult function to the mix, which led to it requiring almost a full push after all. Similar to 📝 the unblitting code for moving lasers we've seen in the last push, ZUN tries to minimize the amount of VRAM writes when unblitting Reimu's slide animations. Technically, it's only necessary to restore the pixels that Reimu traveled by, plus the ones that wouldn't be redrawn by the new animation frame at the new X position.
The theoretically arbitrary distance between the two sprites is, of course, modeled by a fixed-size buffer on the stack :onricdennat:, coming with the further assumption that the sprite surely hasn't moved by more than 1 horizontal VRAM byte compared to the last frame. Which, of course, results in glitches if that's not the case, leaving little Reimu parts in VRAM if the slide speed ever exceeded 8 pixels per frame. :tannedcirno: (Which it never does, being hardcoded to 6 pixels, but still.). As it also turns out, all those bit masking operations easily lead to incredibly sloppy C code. Which compiles into incredibly terrible ASM, which in turn might end up wasting way more CPU time than the final VRAM write optimization would have gained? Then again, in-depth profiling is way beyond the scope of this project at this point.

Next up: The TH04 main menu, and some more technical debt.

📝 Posted:
🚚 Summary of:
P0122
Commits:
164591f...4406c3d
💰 Funded by:
Yanga
🏷 Tags:
rec98+ th01+ blitting- waste+ jank+ gameplay+ laser+

This time around, laser is 📝 actually not difficult, with TH01's shootout laser class being simple enough to nicely fit into a single push. All other stationary lasers (as used by YuugenMagan, for example) don't even use a class, and are simply treated as regular lines with collision detection.

But of course, the shootout lasers also come with the typical share of TH01 jank we've all come to expect by now. This time, it already starts with the hardcoded sprite data:

TH01 shootout laser 'sprites'

A shootout laser can have a width from 1 to 8 pixels, so ZUN stored a separate 16×1 sprite with a line for each possible width (left-to-right). Then, he shifted all of these sprites 1 pixel to the right for all of the 8 possible start positions within a planar VRAM byte (top-to-bottom). Because… doing that bit shift programmatically is way too expensive, so let's pre-shift at compile time, and use 16× the memory per sprite? :tannedcirno:

Since a bunch of other sprite sheets need to be pre-shifted as well (this is the 5th one we've found so far), our sprite converter has a feature to automatically generate those pre-shifted variations. This way, we can abstract away that implementation detail and leave modders with .BMP files that still only contain a single version of each sprite. But, uh…, wait, in this sprite sheet, the second row for 1-pixel lasers is accidentally shifted right by one more pixel that it should have been?! Which means that

  1. we can't use the auto-preshift feature here, and have to store this weird-looking (and quite frankly, completely unnecessary) sprite sheet in its entirety
  2. ZUN did, at least during TH01's development, not have a sprite converter, and directly hardcoded these dot patterns in the C++ code :zunpet:

The waste continues with the class itself. 69 bytes, with 22 bytes outright unused, and 11 not really necessary. As for actual innovations though, we've got 📝 another 32-bit fixed-point type, this time actually using 8 bits for the fractional part. Therefore, the ray position is tracked to the 1/256th of a pixel, using the full precision of master.lib's 8-bit sin() and cos() lookup tables.
Unblitting is also remarkably efficient: It's only done once the laser stopped extending and started moving, and only for the exact pixels at the start of the ray that the laser traveled by in a single frame. If only the ray part was also rendered as efficiently – it's fully blitted every frame, right next to the collision detection for each row of the ray.


With a public interface of two functions (spawn, and update / collide / unblit / render), that's superficially all there is to lasers in this game. There's another (apparently inlined) function though, to both reset and, uh, "fully unblit" all lasers at the end of every boss fight… except that it fails hilariously at doing the latter, and ends up effectively unblitting random 32-pixel line segments, due to ZUN confusing both the coordinates and the parameter types for the line unblitting function. :zunpet:
A while ago, I was asked about this crash that tends to happen when defeating Elis. And while you can clearly see the random unblitted line segments that are missing from the sprites, I don't quite think we've found the cause for the crash, since the 📝 line unblitting function used there does clip its coordinates to the VRAM range.

Next up: The final piece of image format code in TH01, covering Reimu's sprites!

📝 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:

The weirdness becomes obvious with just a single screenshot:

TH01 invincibility sprite weirdness

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:


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:
P0118
Commits:
0bb5bc3...cbf14eb
💰 Funded by:
-Tom-, Ember2528
🏷 Tags:
rec98+ th01+ th02+ th04+ th05+ position-independence+ hud+ blitting- unused+

🎉 TH05 is finally fully position-independent! 🎉 To celebrate this milestone, -Tom- coded a little demo, which we recorded on both an emulator and on real PC-98 hardware:

For all the new people who are unfamiliar with PC-98 Touhou internals: Boss behavior is hardcoded into MAIN.EXE, rather than being scriptable via separate .ECL files like in Windows Touhou. That's what makes this kind of a big deal.


What does this mean?

You can now freely add or remove both data and code anywhere in TH05, by editing the ReC98 codebase, writing your mod in ASM or C/C++, and recompiling the code. Since all absolute memory addresses 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.

By extension, this also means that it's now theoretically possible to use a different compiler on the source code. But:

What does this not mean?

The original ZUN code hasn't been completely reverse-engineered yet, let alone decompiled. As the final PC-98 Touhou game, TH05 also happens to have the largest amount of actual ZUN-written ASM that can't ever be decompiled within ReC98's constraints of a legit source code reconstruction. But a lot of the originally-in-C code is also still in ASM, which might make modding a bit inconvenient right now. And while I have decompiled a bunch of functions, I selected them largely because they would help with PI (as requested by the backers), and not because they are particularly relevant to typical modding interests.

As a result, the code might also be a bit confusingly organized. There's quite a conflict between various goals there: On the one hand, I'd like to only have a single instance of every function shared with earlier games, as well as reduce ZUN's code duplication within a single game. On the other hand, this leads to quite a lot of code being scattered all over the place and then #include-pasted back together, except for the places where 📝 this doesn't work, and you'd have to use multiple translation units anyway… I'm only beginning to figure out the best structure here, and some more reverse-engineering attention surely won't hurt.

Also, keep in mind that the code still targets x86 Real Mode. To work effectively in this codebase, you'd need some familiarity with memory segmentation, and how to express it all in code. This tends to make even regular C++ development about an order of magnitude harder, especially once you want to interface with the remaining ASM code. That part made -Tom- struggle quite a bit with implementing his custom scripting language for the demo above. For now, he built that demo on quite a limited foundation – which is why he also chose to release neither the build nor the source publically for the time being.
So yeah, you're definitely going to need the TASM and Borland C++ manuals there.

tl;dr: We now know everything about this game's data, but not quite as much about this game's code.

So, how long until source ports become a realistic project?

You probably want to wait for 100% RE, which is when everything that can be decompiled has been decompiled.

Unless your target system is 16-bit Windows, in which case you could theoretically start right away. 📝 Again, this would be the ideal first system to port PC-98 Touhou to: It would require all the generic portability work to remove the dependency on PC-98 hardware, thus paving the way for a subsequent port to modern systems, yet you could still just drop in any undecompiled ASM.

Porting to IBM-compatible DOS would only be a harder and less universally useful version of that. You'd then simply exchange one architecture, with its idiosyncrasies and limits, for another, with its own set of idiosyncrasies and limits. (Unless, of course, you already happen to be intimately familiar with that architecture.) The fact that master.lib provides DOS/V support would have only mattered if ZUN consistently used it to abstract away PC-98 hardware at every single place in the code, which is definitely not the case.


The list of actually interesting findings in this push is, 📝 again, very short. Probably the most notable discovery: The low-level part of the code that renders Marisa's laser from her TH04 Illusion Laser shot type is still present in TH05. Insert wild mass guessing about potential beta version shot types… Oh, and did you know that the order of background images in the Extra Stage staff roll differs by character?

Next up: Finally driving up the RE% bar again, by decompiling some TH05 main menu code.

📝 Posted:
🚚 Summary of:
P0115, P0116
Commits:
967bb8b...e5328a3, e5328a3...03048c3
💰 Funded by:
Lmocinemod, Blue Bolt, [Anonymous]
🏷 Tags:
rec98+ th03+ th04+ th05+ file-format+ cutscene+ blitting- menu+

Finally, after a long while, we've got two pushes with barely anything to talk about! Continuing the road towards 100% PI for TH05, these were exactly the two pushes that TH05 MAINE.EXE PI was estimated to additionally cost, relative to TH04's. Consequently, they mostly went to TH05's unique data structures in the ending cutscenes, the score name registration menu, and the staff roll.

A unique feature in there is TH05's support for automatic text color changes in its ending scripts, based on the first full-width Shift-JIS codepoint in a line. The \c=codepoint,color commands at the top of the _ED??.TXT set up exactly this codepoint→color mapping. As far as I can tell, TH05 is the only Touhou game with a feature like this – even the Windows Touhou games went back to manually spelling out each color change.

The orb particles in TH05's staff roll also try to be a bit unique by using 32-bit X and Y subpixel variables for their current position. With still just 4 fractional bits, I can't really tell yet whether the extended range was actually necessary. Maybe due to how the "camera scrolling" through "space" was implemented? All other entities were pretty much the usual fare, though.
12.4, 4.4, and now a 28.4 fixed-point format… yup, 📝 C++ templates were definitely the right choice.

At the end of its staff roll, TH05 not only displays the usual performance verdict, but then scrolls in the scores at the end of each stage before switching to the high score menu. The simplest way to smoothly scroll between two full screens on a PC-98 involves a separate bitmap… which is exactly what TH05 does here, reserving 28,160 bytes of its global data segment for just one overly large monochrome 320×704 bitmap where both the screens are rendered to. That's… one benefit of splitting your game into multiple executables, I guess? :tannedcirno:
Not sure if it's common knowledge that you can actually scroll back and forth between the two screens with the Up and Down keys before moving to the score menu. I surely didn't know that before. But it makes sense – might as well get the most out of that memory.


The necessary groundwork for all of this may have actually made TH04's (yes, TH04's) MAINE.EXE technically position-independent. Didn't quite reach the same goal for TH05's – but what we did reach is ⅔ of all PC-98 Touhou code now being position-independent! Next up: Celebrating even more milestones, as -Tom- is about to finish development on his TH05 MAIN.EXE PI demo…

📝 Posted:
🚚 Summary of:
P0113, P0114
Commits:
150d2c6...6204fdd, 6204fdd...967bb8b
💰 Funded by:
Lmocinemod
🏷 Tags:
rec98+ th02+ th03+ th04+ build-process+ pipeline+ tasm+ blitting- file-format+ input+

Alright, tooling and technical debt. Shouldn't be really much to talk about… oh, wait, this is still ReC98 :tannedcirno:

For the tooling part, I finished up the remaining ergonomics and error handling for the 📝 sprite converter that Jonathan Campbell contributed two months ago. While I familiarized myself with the tool, I've actually ran into some unreported errors myself, so this was sort of important to me. Still got no command-line help in there, but the error messages can now do that job probably even better, since we would have had to write them anyway.

So, what's up with the technical debt then? Well, by now we've accumulated quite a number of 📝 ASM code slices that need to be either decompiled or clearly marked as undecompilable. Since we define those slices as "already reverse-engineered", that decision won't affect the numbers on the front page at all. But for a complete decompilation, we'd still have to do this someday. So, rather than incorporating this work into pushes that were purchased with the expectation of measurable progress in a certain area, let's take the "anything goes" pushes, and focus entirely on that during them.

The second code segment seemed like the best place to start with this, since it affects the largest number of games simultaneously. Starting with TH02, this segment contains a set of random "core" functions needed by the binary. Image formats, sounds, input, math, it's all there in some capacity. You could maybe call it all "libzun" or something like that? But for the time being, I simply went with the obvious name, seg2. Maybe I'll come up with something more convincing in the future.


Oh, but wait, why were we assembling all the previous undecompilable ASM translation units in the 16-bit build part? By moving those to the 32-bit part, we don't even need a 16-bit TASM in our list of dependencies, as long as our build process is not fully 16-bit.
And with that, ReC98 now also builds on Windows 95, and thus, every 32-bit Windows version. 🎉 Which is certainly the most user-visible improvement in all of these two pushes. :onricdennat:


Back in 2015, I already decompiled all of TH02's seg2 functions. As suggested by the Borland compiler, I tried to follow a "one translation unit per segment" layout, bundling the binary-specific contents via #include. In the end, it required two translation units – and that was even after manually inserting the original padding bytes via #pragma codestring… yuck. But it worked, compiled, and kept the linker's job (and, by extension, segmentation worries) to a minimum. And as long as it all matched the original binaries, it still counted as a valid reconstruction of ZUN's code. :zunpet:

However, that idea ultimately falls apart once TH03 starts mixing undecompilable ASM code inbetween C functions. Now, we officially have no choice but to use multiple C and ASM translation units, with maybe only just one or two #includes in them…

…or we finally start reconstructing the actual seg2 library, turning every sequence of related functions into its own translation unit. This way, we can simply reuse the once-compiled .OBJ files for all the binaries those functions appear in, without requiring that additional layer of translation units mirroring the original segmentation.
The best example for this is TH03's almost undecompilable function that generates a lookup table for horizontally flipping 8 1bpp pixels. It's part of every binary since TH03, but only used in that game. With the previous approach, we would have had to add 9 C translation units, which would all have just #included that one file. Now, we simply put the .OBJ file into the correct place on the linker command line, as soon as we can.

💡 And suddenly, the linker just inserts the correct padding bytes itself.

The most immediate gains there also happened to come from TH03. Which is also where we did get some tiny RE% and PI% gains out of this after all, by reverse-engineering some of its sprite blitting setup code. Sure, I should have done even more RE here, to also cover those 5 functions at the end of code segment #2 in TH03's MAIN.EXE that were in front of a number of library functions I already covered in this push. But let's leave that to an actual RE push 😛


All in all though, I was just getting started with this; the real gains in terms of removed ASM files are still to come. But in the meantime, the funding situation has become even better in terms of allowing me to focus on things nobody asked for. 🙂 So here's a slightly better idea: Instead of spending two more pushes on this, let's shoot for TH05 MAINE.EXE position independence next. If I manage to get it done, we'll have a 100% position-independent TH05 by the time -Tom- finishes his MAIN.EXE PI demo, rather than the 94% we'd get from just MAIN.EXE. That's bound to make a much better impression on all the people who will then (re-)discover the project.

📝 Posted:
🚚 Summary of:
P0105, P0106, P0107, P0108
Commits:
3622eb6...11b776b, 11b776b...1f1829d, 1f1829d...1650241, 1650241...dcf4e2c
💰 Funded by:
Yanga
🏷 Tags:
rec98+ th01+ meta+ file-format+ animation+ blitting- boss+ singyoku+ yuugenmagan+ elis+ kikuri+ konngara+ waste+

And indeed, I got to end my vacation with a lot of image format and blitting code, covering the final two formats, .GRC and .BOS. .GRC was nothing noteworthy – one function for loading, one function for byte-aligned blitting, and one function for freeing memory. That's it – not even a unblitting function for this one. .BOS, on the other hand…

…has no generic (read: single/sane) implementation, and is only implemented as methods of some boss entity class. And then again for Sariel's dress and wand animations, and then again for Reimu's animations, both of which weren't even part of these 4 pushes. Looking forward to decompiling essentially the same algorithms all over again… And that's how TH01 became the largest and most bloated PC-98 Touhou game. So yeah, still not done with image formats, even at 44% RE.

This means I also had to reverse-engineer that "boss entity" class… yeah, what else to call something a boss can have multiple of, that may or may not be part of a larger boss sprite, may or may not be animated, and that may or may not have an orb hitbox?
All bosses except for Kikuri share the same 5 global instances of this class. Since renaming all these variables in ASM land is tedious anyway, I went the extra mile and directly defined separate, meaningful names for the entities of all bosses. These also now document the natural order in which the bosses will ultimately be decompiled. So, unless a backer requests anything else, this order will be:

  1. Konngara
  2. Sariel
  3. Elis
  4. Kikuri
  5. SinGyoku
  6. (code for regular card-flipping stages)
  7. Mima
  8. YuugenMagan

As everyone kind of expects from TH01 by now, this class reveals yet another… um, unique and quirky piece of code architecture. In addition to the position and hitbox members you'd expect from a class like this, the game also stores the .BOS metadata – width, height, animation frame count, and 📝 bitplane pointer slot number – inside the same class. But if each of those still corresponds to one individual on-screen sprite, how can YuugenMagan have 5 eye sprites, or Kikuri have more than one soul and tear sprite? By duplicating that metadata, of course! And copying it from one entity to another :onricdennat:
At this point, I feel like I even have to congratulate the game for not actually loading YuugenMagan's eye sprites 5 times. But then again, 53,760 bytes of waste would have definitely been noticeable in the DOS days. Makes much more sense to waste that amount of space on an unused C++ exception handler, and a bunch of redundant, unoptimized blitting functions :tannedcirno:

(Thinking about it, YuugenMagan fits this entire system perfectly. And together with its position in the game's code – last to be decompiled means first on the linker command line – we might speculate that YuugenMagan was the first boss to be programmed for TH01?)

So if a boss wants to use sprites with different sizes, there's no way around using another entity. And that's why Girl-Elis and Bat-Elis are two distinct entities internally, and have to manually sync their position. Except that there's also a third one for Attacking-Girl-Elis, because Girl-Elis has 9 frames of animation in total, and the global .BOS bitplane pointers are divided into 4 slots of only 8 images each. :zunpet:
Same for SinGyoku, who is split into a sphere entity, a person entity, and a… white flash entity for all three forms, all at the same resolution. Or Konngara's facial expressions, which also require two entities just for themselves.


And once you decompile all this code, you notice just how much of it the game didn't even use. 13 of the 50 bytes of the boss entity class are outright unused, and 10 bytes are used for a movement clamping and lock system that would have been nice if ZUN also used it outside of Kikuri's soul sprites. Instead, all other bosses ignore this system completely, and just party on the X/Y coordinates of the boss entities directly.

As for the rendering functions, 5 out of 10 are unused. And while those definitely make up less than half of the code, I still must have spent at least 1 of those 4 pushes on effectively unused functionality.
Only one of these functions lends itself to some speculation. For Elis' entrance animation, the class provides functions for wavy blitting and unblitting, which use a separate X coordinate for every line of the sprite. But there's also an unused and sort of broken one for unblitting two overlapping wavy sprites, located at the same Y coordinate. This might indicate that Elis could originally split herself into two sprites, similar to TH04 Stage 6 Yuuka? Or it might just have been some other kind of animation effect, who knows.


After over 3 months of TH01 progress though, it's finally time to look at other games, to cover the rest of the crowdfunding backlog. Next up: Going back to TH05, and getting rid of those last PI false positives. And since I can potentially spend the next 7 weeks on almost full-time ReC98 work, I've also re-opened the store until October!

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

Screenshot of the TH01 orb physics debug mod

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:
P0086, P0087
Commits:
54ee99b...24b96cd, 24b96cd...97ce7b7
💰 Funded by:
[Anonymous], Blue Bolt, -Tom-
🏷 Tags:
rec98+ th02+ th04+ th05+ gameplay+ animation+ hud+ blitting- boss+ yuuka-6+

Alright, the score popup numbers shown when collecting items or defeating (mid)bosses. The second-to-last remaining big entity type in TH05… with quite some PI false positives in the memory range occupied by its data. Good thing I still got some outstanding generic RE pushes that haven't been claimed for anything more specific in over a month! These conveniently allowed me to RE most of these functions right away, the right way.

Most of the false positives were boss HP values, passed to a "boss phase end" function which sets the HP value at which the next phase should end. Stage 6 Yuuka, Mugetsu, and EX-Alice have their own copies of this function, in which they also reset certain boss-specific global variables. Since I always like to cover all varieties of such duplicated functions at once, it made sense to reverse-engineer all the involved variables while I was at it… and that's why this was exactly the right time to cover the implementation details of Stage 6 Yuuka's parasol and vanishing animations in TH04. :zunpet:

With still a bit of time left in that RE push afterwards, I could also start looking into some of the smaller functions that didn't quite fit into other pushes. The most notable one there was a simple function that aims from any point to the current player position. Which actually only became a separate function in TH05, probably since it's called 27 times in total. That's 27 places no longer being blocked from further RE progress.

WindowsTiger already did most of the work for the score popup numbers in January, which meant that I only had to review it and bring it up to ReC98's current coding styles and standards. This one turned out to be one of those rare features whose TH05 implementation is significantly less insane than the TH04 one. Both games lazily redraw only the tiles of the stage background that were drawn over in the previous frame, and try their best to minimize the amount of tiles to be redrawn in this way. For these popup numbers, this involves calculating the on-screen width, based on the exact number of digits in the point value. TH04 calculates this width every frame during the rendering function, and even resorts to setting that field through the digit iteration pointer via self-modifying code… yup. TH05, on the other hand, simply calculates the width once when spawning a new popup number, during the conversion of the point value to binary-coded decimal. The "×2" multiplier suffix being removed in TH05 certainly also helped in simplifying that feature in this game.

And that's ⅓ of TH05 reverse-engineered! Next up, one more TH05 PI push, in which the stage enemies hopefully finish all the big entity types. Maybe it will also be accompanied by another RE push? In any case, that will be the last piece of TH05 progress for quite some time. The next TH01 stretch will consist of 6 pushes at the very least, and I currently have no idea of how much time I can spend on ReC98 a month from now…

📝 Posted:
🚚 Summary of:
P0085
Commits:
110d6dd...54ee99b
💰 Funded by:
-Tom-
🏷 Tags:
rec98+ th02+ th04+ th05+ blitting- animation+ waste+ good-code+

Wait, PI for FUUIN.EXE is mainly blocked by the high score menu? That one should really be properly decompiled in a separate RE push, since it's also present in largely identical form in REIIDEN.EXE… but I currently lack the explicit funding to do that.

And as it turns out, I shouldn't really capture any of the existing generic RE contributions for it either. Back in 2018 when I ran the crowdfunding on the Touhou Patch Center Discord server, I said that generic RE contributions would never go towards TH01. No one was interested in that game back then, and as it's significantly different from all the other games, it made sense to only cover it if explicitly requested.
As Touhou Patch Center still remains one of the biggest supporters and advertisers for ReC98, someone recently believed that this rule was still in effect, despite not being mentioned anywhere on this website.

Fast forward to today, and TH01 has become the single most supported game lately, with plenty of incomplete pushes still open to be completed. Reverse-engineering it has proven to be quite efficient, yielding lots of completion percentage points per push. This, I suppose, is exactly what backers that don't give any specific priorities are mainly interested in. Therefore, I will allocate future partial contributions to TH01, whenever it makes sense.

So, instead of rushing TH01 PI, let's wait for Ember2528's April subscription, and get the 25% total RE milestone with some TH05 PI progress instead. This one primarily focused on the gather circles (spirals…?), the third-last missing entity type in TH05. These are rendered using the same 8×8 pellet sprite introduced in TH02… except that the actual pellets received a darkened bottom part in TH04 . Which, in turn, is actually rendered quite efficiently – the games first render the top white part of all pellets, followed by the bottom gray part of all pellets. The PC-98 GRCG is used throughout the process, doing its typical job of accelerating monochrome blitting, and by arranging the rendering like this, only two GRCG color changes are required to draw any number of pellets. I guess that makes it quite a worthwhile optimization? Don't ask me for specific performance numbers or even saved cycles, though :onricdennat:

Next up, one more TH05 PI push!

📝 Posted:
🚚 Summary of:
P0081
Commits:
0252da2...5ac9b30
💰 Funded by:
Ember2528
🏷 Tags:
rec98+ th01+ file-format+ blitting- boss+ konngara+ waste+ contribution-ideas+

Sadly, we've already reached the end of fast triple-speed TH01 progress with 📝 the last push, which decompiled the last segment shared by all three of TH01's executables. There's still a bit of double-speed progress left though, with a small number of code segments that are shared between just two of the three executables.

At the end of the first one of these, we've got all the code for the .GRZ format – which is yet another run-length encoded image format, but this time storing up to 16 full 640×400 16-color images with an alpha bit. This one is exclusively used to wastefully store Konngara's sword slash and kuji-in kill animations. Due to… suboptimal code organization, the code for the format is also present in OP.EXE, despite not being used there. But hey, that brings TH01 to over 20% in RE!

Decoupling the RLE command stream from the pixel data sounds like a nice idea at first, allowing the format to efficiently encode a variety of animation frames displayed all over the screen… if ZUN actually made use of it. The RLE stream also has quite some ridiculous overhead, starting with 1 byte to store the 1-bit command (putting a single 8×1 pixel block, or entering a run of N such blocks). Run commands then store another 1-byte run length, which has to be followed by another command byte to identify the run as putting N blocks, or skipping N blocks. And the pixel data is just a sequence of these blocks for all 4 bitplanes, in uncompressed form…

Also, have some rips of all the images this format is used for:

<code>boss8.grz</code>, image 1/16<code>boss8.grz</code>, image 2/16<code>boss8.grz</code>, image 3/16<code>boss8.grz</code>, image 4/16<code>boss8.grz</code>, image 5/16<code>boss8.grz</code>, image 6/16<code>boss8.grz</code>, image 7/16<code>boss8.grz</code>, image 8/16<code>boss8.grz</code>, image 9/16<code>boss8.grz</code>, image 10/16<code>boss8.grz</code>, image 11/16<code>boss8.grz</code>, image 12/16<code>boss8.grz</code>, image 13/16<code>boss8.grz</code>, image 14/16<code>boss8.grz</code>, image 15/16<code>boss8.grz</code>, image 16/16

To make these, I just wrote a small viewer, calling the same decompiled TH01 code: 2020-03-07-grzview.zip Obviously, this means that it not only must to be run on a PC-98, but also discards the alpha information. If any backers are really interested in having a proper converter to and from PNG, I can implement that in an upcoming push… although that would be the perfect thing for outside contributors to do.

Next up, we got some code for the PI format… oh, wait, the actual files are called "GRP" in TH01.

📝 Posted:
🚚 Summary of:
P0072, P0073, P0074, P0075
Commits:
4bb04ab...cea3ea6, cea3ea6...5286417, 5286417...1807906, 1807906...222fc99
💰 Funded by:
[Anonymous], -Tom-, Myles
🏷 Tags:
rec98+ th04+ th05+ gameplay+ blitting- bullet+ micro-optimization+ uth05win+

Long time no see! And this is exactly why I've been procrastinating bullets while there was still meaningful progress to be had in other parts of TH04 and TH05: There was bound to be quite some complexity in this most central piece of game logic, and so I couldn't possibly get to a satisfying understanding in just one push.

Or in two, because their rendering involves another bunch of micro-optimized functions adapted from master.lib.

Or in three, because we'd like to actually name all the bullet sprites, since there are a number of sprite ID-related conditional branches. And so, I was refining things I supposedly RE'd in the the commits from the first push until the very end of the fourth.

When we talk about "bullets" in TH04 and TH05, we mean just two things: the white 8×8 pellets, with a cap of 240 in TH04 and 180 in TH05, and any 16×16 sprites from MIKO16.BFT, with a cap of 200 in TH04 and 220 in TH05. These are by far the most common types of… err, "things the player can collide with", and so ZUN provides a whole bunch of pre-made motion, animation, and n-way spread / ring / stack group options for those, which can be selected by simply setting a few fields in the bullet template. All the other "non-bullets" have to be fired and controlled individually.

Which is nothing new, since uth05win covered this part pretty accurately – I don't think anyone could just make up these structure member overloads. The interesting insights here all come from applying this research to TH04, and figuring out its differences compared to TH05. The most notable one there is in the default groups: TH05 allows you to add a stack to any single bullet, n-way spread or ring, but TH04 only lets you create stacks separately from n-way spreads and rings, and thus gets by with fewer fields in its bullet template structure. On the other hand, TH04 has a separate "n-way spread with random angles, yet still aimed at the player" group? Which seems to be unused, at least as far as midbosses and bosses are concerned; can't say anything about stage enemies yet.

In fact, TH05's larger bullet template structure illustrates that these distinct group types actually are a rather redundant piece of over-engineering. You can perfectly indicate any permutation of the basic groups through just the stack bullet count (1 = no stack), spread bullet count (1 = no spread), and spread delta angle (0 = ring instead of spread). Add a 4-flag bitfield to cover the rest (aim to player, randomize angle, randomize speed, force single bullet regardless of difficulty or rank), and the result would be less redundant and even slightly more capable.

Even those 4 pushes didn't quite finish all of the bullet-related types, stopping just shy of the most trivial and consistent enum that defines special movement. This also left us in a 📝 TH03-like situation, in which we're still a bit away from actually converting all this research into actual RE%. Oh well, at least this got us way past 50% in overall position independence. On to the second half! 🎉

For the next push though, we'll first have a quick detour to the remaining C code of all the ZUN.COM binaries. Now that the 📝 TH04 and TH05 resident structures no longer block those, -Tom- has requested TH05's RES_KSO.COM to be covered in one of his outstanding pushes. And since 32th System recently RE'd TH03's resident structure, it makes sense to also review and merge that, before decompiling all three remaining RES_*.COM binaries in hopefully a single push. It might even get done faster than that, in which case I'll then review and merge some more of WindowsTiger's research.

📝 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:
P0060
Commits:
29385dd...73f5ae7
💰 Funded by:
Touhou Patch Center
🏷 Tags:
rec98+ th02+ th03+ th04+ th05+ blitting- micro-optimization+ waste+

So, where to start? Well, TH04 bullets are hard, so let's procrastinate start with TH03 instead :tannedcirno: The 📝 sprite display functions are the obvious blocker for any structure describing a sprite, and therefore most meaningful PI gains in that game… and I actually did manage to fit a decompilation of those three functions into exactly the amount of time that the Touhou Patch Center community votes alloted to TH03 reverse-engineering!

And a pretty amazing one at that. The original code was so obviously written in ASM and was just barely decompilable by exclusively using register pseudovariables and a bit of goto, but I was able to abstract most of that away, not least thanks to a few helpful optimization properties of Turbo C++… seriously, I can't stop marveling at this ancient compiler. The end result is both readable, clear, and dare I say portable?! To anyone interested in porting TH03, take a look. How painful would it be to port that away from 16-bit x86?

However, this push is also a typical example that the RE/PI priorities can only control what I look at, and the outcome can actually differ greatly. Even though the priorities were 65% RE and 35% PI, the progress outcome was +0.13% RE and +1.35% PI. But hey, we've got one more push with a focus on TH03 PI, so maybe that one will include more RE than PI, and then everything will end up just as ordered? :onricdennat:

📝 Posted:
🚚 Summary of:
P0029, P0030
Commits:
6ff427a...c7fc4ca, c7fc4ca...dea40ad
💰 Funded by:
zorg
🏷 Tags:
rec98+ th02+ th04+ th05+ blitting- gameplay+ midboss+ boss+ tcc+

Here we go, new C code! …eh, it will still take a bit to really get decompilation going at the speeds I was hoping for. Especially with the sheer amount of stuff that is set in the first few significant functions we actually can decompile, which now all has to be correctly declared in the C world. Turns out I spent the last 2 years screwing up the case of exported functions, and even some of their names, so that it didn't actually reflect their calling convention… yup. That's just the stuff you tend to forget while it doesn't matter.

To make up for that, I decided to research whether we can make use of some C++ features to improve code readability after all. Previously, it seemed that TH01 was the only game that included any C++ code, whereas TH02 and later seemed to be 100% C and ASM. However, during the development of the soon to be released new build system, I noticed that even this old compiler from the mid-90's, infamous for prioritizing compile speeds over all but the most trivial optimizations, was capable of quite surprising levels of automatic inlining with class methods…

…leading the research to culminate in the mindblow that is 9d121c7 – yes, we can use C++ class methods and operator overloading to make the code more readable, while still generating the same code than if we had just used C and preprocessor macros.

Looks like there's now the potential for a few pull requests from outside devs that apply C++ features to improve the legibility of previously decompiled and terribly macro-ridden code. So, if anyone wants to help without spending money…

📝 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.