[Bug 11643] Gens: Sega Genesis Emulator will not open or run ROMs

[wine-bugs at winehq.org]

http://bugs.winehq.org/show_bug.cgi?id=11643


Alexander Dorofeyev <alexd4 at inbox.lv> changed:

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                 CC|                            |alexd4 at inbox.lv




--- Comment #4 from Alexander Dorofeyev <alexd4 at inbox.lv>  2008-02-28 15:25:35 ---
This was discussed on #winehackers, adding here while I still remember it, so
it doesn't get lost.

After some debugging in IDA disassembler, I believe it crashes in z80_Exec
routine, at z80.asm:5119 (source is available). The reason it crashes is
because it accesses a NULL pointer stored in M_Z80.PC (M_Z80 is a global struct
defined somewhere in C code). Probably, some correct pointer must be in
M_Z80.PC, but for some reason it isn't initialized to anything correct. From
what I understood from a cursory glance over the code, M_Z80.PC is a sort of
instruction pointer for the emulated Z80 processor like IP/EIP on PC.

I also believe that the place where M_Z80.PC should've been initialized to
correct pointer is z80_Reset routine (also z80.asm). It basically does M_Z80.PC
= M_Z80.Fetch[0]. Fetch is an array of pointers to what I guess would be memory
blocks of Z80 code. For some reason, there's NULL there. Looking for where
Fetch array and specifically Fetch[0] is set in the code may show the real
problem's location. Probably, ROM loading goes wrong somewhere.

Hopefully, this may be of help to affect or anybody else who may wish to take
it further. 

2 Affect: would be cool if you let me know if you are continuing to work on
this issue. If that's not so, I may eventually try to finish the job, won't be
immediately though as I've some other stuff planned to do and it may take a
while to figure out what exactly goes wrong with M_Z80.Fetch.


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