Sonic 2 and Knuckles

Quickman · Jan 24, 2004

I've patched Sonic 2 with Esrael's Knuckles in Sonic 2 IPS patch available at Sonicology, and have found a bug. When you get to the end of EHZ2, the game freezes up. I've taken a screenshot of the 68k debug screen in the hope that I can fix this bug.

UPDATE: I've found the offending command - it's located at $C098. Here's the disassembly.

Code (Text):

0x4EB9 0x0030 0x1068 - JSR 0x301068

This calls a subroutine found at $301068 which of course doesn't exist. The result? The game reads it as lots of zeroes. Here's a disassembly.

Code (Text):

0x0000 0x0000 - ORI.B 0x0,00

Of course, it keeps reading zeroes and keeps spewing out ORI commands, and never exists the subroutine because there's no RTS code. The result is a game which has hung up.

Now I need the location in the modified ROM of the EHZ boss so I can change the JSR command to point to the correct subroutine.

LocalH · Jan 24, 2004

Look at the code at $1068 in S&KUPMEM.BIN (since S&KUPMEM.BIN starts at $300000). Then find the same code in the Esrael-hacked S2&K ROM, and replace $301068 with his location for that code.

And technically, the game didn't freeze up, it just entered a sort of infinite loop. The PC likely increments until it wraps past $3FFFFF, going through loads of ORs, which in this case are effectively NOPs. Then, it starts executing whatever is at $400000 (which, if anything, is definitely not code). Of course, I'm being a bit pedantic, yeah the 68k didn't technically freeze up, but effectively the game is frozen.

Edit: I did a bit if checking, and here's the code at $301068 (not an entire routine, but enough to help):

Code (Text):

00001068 4A38 FFE0                TST.B     $FFE0

0000106C 6606                     BNE       $00001074

0000106E 11C0 FFE0                MOVE.B    D0,$FFE0

00001072 4E75                     RTS

00001074 11C0 FFE4                MOVE.B    D0,$FFE4

00001078 4E75                     RTS

0000107A 11C0 FFE1                MOVE.B    D0,$FFE1

0000107E 4E75                     RTS

00001080 11C0 FFE2                MOVE.B    D0,$FFE2

00001084 4E75                     RTS

I found a match for these bytes in Sonic 2 Final at $135E:

Code (Text):

0000135E 4A38 FFE0                TST.B     $FFE0

00001362 6606                     BNE       $0000136A

00001364 11C0 FFE0                MOVE.B    D0,$FFE0

00001368 4E75                     RTS

0000136A 11C0 FFE4                MOVE.B    D0,$FFE4

0000136E 4E75                     RTS

00001370 11C0 FFE1                MOVE.B    D0,$FFE1

00001374 4E75                     RTS

00001376 11C0 FFE2                MOVE.B    D0,$FFE2

0000137A 4E75                     RTS

And it's the only match. So, change $301068 to $00135E and it should work. Matter of fact, I have that hack on my HD, so I'm going to test it now.

Edit: Scrap that, I did the search for those bytes on the 1MB S2&K and found them at $1068. So just change the $30 in the address to $00. Will try this now.

Son of edit: Changed top description to reference Esrael's hack instead of S2 Final.

LocalH · Jan 24, 2004

Crap. I fixed this, and then it seems to have halted at $301328 according to Gens. Looks like there's more to fix :/

Edit: The JSR $00301328 is at $C0AA in Esrael's hack. Found at $1328 in S&KUPMEM.BIN (with enough surrounding info to make it easier to find in Esrael's hack):

Code (Text):

0000131A 5345                     SUBQ.W    #$1,D5

0000131C 3387 0000                MOVE.W    D7,$00(A1,D0.W)

00001320 5440                     ADDQ.W    #$2,D0

00001322 51CD FFF8                DBRA      D5,$0000131C

00001326 60BA                     BRA       $000012E2

00001328 48E7 0060                MOVEM.L   A1-A2,-(A7)

0000132C 43F9 0033A3FC            LEA       $0033A3FC,A1

00001332 D040                     ADD.W     D0,D0

(I grabbed some before $1328 because of that LEA at $132C, which would have a different argument and thus not be easily found). Found one match in Esrael's hack at $131A. Fixed it, and WTF, the game jumps to $0200, which is the infinite loop before the start vector. Goddamnit...

Meh, I'm through for now, I've gotta get ready for work in a bit. If we can figure out where the stray jump to $0200 is (can't be that hard, it'd probably either be a JMP or a JSR), then we can proceed further.

Edit: Also just noticed that the debug sprite doesn't show up, although I was able to place rings and monitors like normal. This is both Esrael's hack, and the ones I modified above.

Quickman · Jan 24, 2004

Thanks for that. My searches of Esrael's hack have turned up no JMP or JSR commands to $200. I'm at an impasse, unless there's another possibility.

LocalH · Jan 24, 2004

Oh, Goddamnit, I think I fucked that up. I'll check later, but I think I might have changed the $00301328 to $0000131A, which wasn't the start of the routine that was actually being jumped to, but rather a few bytes before. Try changing it to $00001328 instead (I'll do this later).

Quickman · Jan 24, 2004

Okay, changing it to $1328 works, however there is yet another fucked-up subroutine call at $C0D2, calling $313D78. Change this to $013D78.

Esrael · Jan 24, 2004

Thanks. I 'll fix it.
You can fix it chaging the followings bytes:
$C098 to 4EB900001068
$C0AA to 4EB900001328
$C0D2 to 4EB900013D78
$C0F8 to 4EB900001068

Quickman · Jan 24, 2004

You posted that at the exact same moment that I fixed the last one. Heh.

Nonetheless, thank you for making my life easier by putting Knuckles in Sonic 2 first.

Esrael · Jan 25, 2004

I have played the hack and found other bugs.
- Game freezes in ARZ Boss (Invalid Pointers)
- Debug mode does not show objects.

Fixed Now: DOWNLOAD fixed version here -> http://www.projectnemesis.ibforums.com/esr...cks/romhack.htm

Sonic 2K (1 Mb) rev 1 -> http://www.projectnemesis.ibforums.com/esr.../s2k-1mb(1).zip

Use IPS With Sonic 2 Final

Quickman · Jan 25, 2004

Have you fixed the two bugs you have shown there in the fixed IPS patch? I'd like to know before I use it.

Esrael · Jan 25, 2004

Yes. I have fixed all bugs found here.

Sonic 2 and Knuckles

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