Sonic CD decompilation

That would indeed be neat, which is why merging all the code is exactly what I've been doing this past month! That effort has a separate Git repo.

 

The decompilation of the ELF files that have debug symbols was finished mid-July last year. After that, I moved on to decompiling the PC version's executable, as I couldn't make anything of the PS2 version's control program due to lack of debug symbols.

While I made great progress decompiling SONICCD.EXE, all of its graphics operations were abstracted by an Intel RDX layer, and I got discouraged. I then used the obtained experience to successfully decompile Sonic & Knuckles Collection's C++ wrapper over the summer.

In November last year, Clownacy showed off his attempt at creating a control program for Sonic CD, and this reinvigorated my courage. I proceeded to decompile the rest of SONICCD.EXE, then tried making my own control program for Palmtree Panic.

When I succeeded, I decided to take it one step further: do away with the moduled approach and merge all the code. I did so one round at a time during January, making corrections to the decompilation along the way.

With all that said, thanks for the notes! I had attempted to look up the differences myself, but didn't find a detailed list, so this is a big help!

 

"respinned" is not grammatically correct? Damn, I thought I had found a cool name. "respun" doesn't have the same ring to it.

I've already applied some bug fixes to make it closer to the Sega Mega CD version (most notably, there's a chunk missing in the background of R11D in the C version). I'm open to fixing bugs present in the original version, but preferably with a toggle for people preferring the authentic experience.

Speaking of Wacky Workbench, I wonder if it was intentional that Sonic never accelerates to his fast run animation in that round. If you use the Super Peel Out, he does use his fastest running animation.

I've been tackling the title screen's graphics logic, and I need to rant. The files that store tiles and sprites contain metadata that, among others, specify which palette line is being used. Yet, for reasons that escape me, the data on palette line usage in the title screen's files is wrong. It is, instead, defined in arrays in a source file. Why?!

 

Worked in earnest on the title screen today, now that everything was clear in my head. It was partially successful:

 

It's looking much better now:



Not only the palette line data is wrong. The dimensions of the sprites are also wrong! That's why the game has a custom sprite loading function for this screen.

This custom function also loads two external bitmaps into two slots. One to replace the 1995 copyright notice with a 1996 one, and one with the "EXIT" text.

Spent a lot of time trying to find out why Sonic wasn't wagging his finger. First, I found out that thanks to Ghidra I filled in the struct containing the timing data incorrectly. After fixing that, and confirming that the correct sprites were being selected, I noticed that the sprite counter wasn't reset to zero in subsequent executions of OEDraw. Always remember to initialise your variables!

Little Planet even bobs up and down, which for some reason it doesn't do in Sonic Gems Collection.

No idea how the scaled clouds are implemented, as it's hidden behind a hmx_ddagrid_set_scan_module call.

Next up: finding out why the menu options don't display, and animating the water.

 

Today's progress report: made the menu options show up (they didn't because of a bug I introduced in the graphics refactoring, oops), fixed the left and right arrows (decompilation error), and I just managed to animate the water!

The "Exit" option is still missing because I'm not loading external bitmaps yet, but I'll add it later.

That leaves the clouds, which will require me to implement what Clownacy termed "coordinate conversion". What's odd is that the cloud sprites have a lower priority than Little Planet's sprite, yet in practice you can clearly see that they display in front of it.

I guess I'll have to skip those sprites when queueing sprites for blitting, and add code for queueing coordinate conversed graphics.

Getting a bit worried about the mess I might be making, because ideally I could consolidate all drawing code, but it's better that I focus on getting all of this working, first.

 

Can you tell me how to get started? I tried reading Devon's explanation, but for someone not familiar with the Mega CD's internals, it's not detailed enough for me. The demo refers to Tonc's explanation of Mode 7, but I don't understand it, either. Finally, I tried looking at Clownmdemu's code, but need some context.

The title screen's clouds have to use some form of coordinate conversion, because the PC version uses the SMORPH effect on them, just like it does for the Special Stages and D.A. Garden. But the input consists of two 160x88 images instead of a big 4096x4096 one, which is not a size that is made up of 16x16 and 32x32 stamps.

Honestly, I'm lost.

 

Thanks, I think I get it now, now.

Now I have to figure out how to get the right coordinates from the trace table. For example, the first StartX is 4160000. If I assume that's 13.3 fixed point, I get a value of 520000 (right shift 3, or divide by 8), which is still way too high. Or am I misunderstanding the notation?

If I'm not, what's the point of using 11 decimals for the delta (5.11 fixed point) when the start values have only 3 decimals?

 

"Hmm, maybe the values are half integer and half fractional. Let's try that. OSHI-"



I was so surprised to see it suddenly working!

Like I mentioned before, Little Planet bobs up and down here, unlike on Sonic Gems Collection. But that's not all! I restored the animated left and right arrows, as their animation was missing. There was code for them, but it was broken, and only accounted for one arrow showing up at a time.

 

The past week, I've been refactoring my code, and I've glued the title screen and the stages together. Now, you can choose "New Game" and start playing. Things are coming together!

Today, I've added the 1996 copyright and the Exit option bitmaps:



However, I'm not entirely happy with how I've added them to the bitmap loading function.

Currently, I use the metadata of the compressed sprites file to calculate how much memory I need to allocate for the sprite bitmap data. To add the two aforementioned bitmaps, I had to load them and add their sizes to the count. Next, I extract the sprites as normal, then copy the data of the two bitmaps behind the rest of the data.

In my opinion, it took a bit too much code. But I don't think I can improve it unless I change the logic to allocate memory per sprite.

Finally, earlier today, I was curious how big the executable had become. The debug version is almost 4 MB. I compiled a release version with O2 optimisation for the Pentium MMX: 891 KB. If I turn off optimisation for a fairer comparison with the original code, it's 1065 KB.

The original executable is 113 KB. But that's just the core program. There are 70 DLLs for the levels, which on average are 130 KB a piece. That means there's about 9100 KB, or 8,89 MB, of level code.

That's a lot of duplicate code and data I removed.

 

I've been thinking of releasing a preview version of the executable for everyone to try out, but as it's made for SDL1, the result would be either a postage stamp size window, or full-screen with bilinear filtering. Not ideal.

That's why I've added compatibility with SDL2. It was pretty easy, save for figuring out how to get 8-bit pixel data to a 32-bit texture. Thankfully, someone wanted to do the same thing in the past, and documented how to do it.

Now you can enjoy it full-screen, integer-scaled to your resolution. I've attached the Windows executable to this post.

Next, you need to put the following in the same folder as the executable:

• SDL2.
• The high quality sound effects file from Sonic CD PC 1995, PCM.CMP, which has been archived on Google Drive and MediaFire.
• The BGM file BGM_US.AFS from Sonic Gems Collection. Yes, I'm using the US version for now.
• The R1, R3, R4, R5, R6, R7, R8, and TITLE folders (and their contents, though the .ELF files aren't necessary) from Sonic Gems Collection.

For the uninitiated, Sonic CD's files on the Sonic Gems Collection DVD are contained in an archive called SONICCD.CVM in the SONICCD folder. Use 7-Zip or another program that can open archives. It's not a ZIP archive, so changing the extension to .zip won't work.

You can only play through the levels, there's no Special Stage, and save data isn't implemented. Use the arrow keys to move, and space bar to jump. This comes with no warranty, yada yada.

Enjoy!