#14839 - animension - Tue Jan 13, 2004 4:16 am
How would one figure out what, exactly, a BIOS function does in assembly? Is there a lookup table that the interrupt uses to reference a particular location in BIOS to process and execute each SWI, and if so where is it?
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#14855 - tepples - Tue Jan 13, 2004 3:05 pm
Yes, there exists such a lookup table. No, I don't have a copy of the disassembled BIOS handy to look it up.
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#14857 - Miked0801 - Tue Jan 13, 2004 6:15 pm
Martin Korth came up with a method for finding out what was in the BIOS using a bug in one of the midi commands to read from the restricted area. I'm not sure of the specifics though. Basically, you set it up in a loop and have it dump the memory somewhere where you can read it. BTW, you won't be impressed with what you see if you do get to look at it ;)
Mike
#14861 - animension - Tue Jan 13, 2004 7:07 pm
I'm just glad that there is a LUT that I can use to reference SWI instead of having to reverse engineer the entire BIOS. That would suck. :)
_________________
"Beer is proof that God loves us and wants us to be happy."
-- Benjamin Franklin
#14883 - sasq - Wed Jan 14, 2004 10:02 am
The SWI-routine is at $140, and it uses the byte in the SWI-opcode to look up an adress in a table at $1C8
An interesting thing is that it doesnt check if the byte is outside the table, so you can find a few SWI-numbers that jumps to ROM or RAM, which means you could use SWI to jump to your own code if you needed to (maybe for breakpoints).
#14926 - tepples - Wed Jan 14, 2004 8:33 pm
| sasq wrote: |
| you can find a few SWI-numbers that jumps to ROM or RAM, which means you could use SWI to jump to your own code if you needed to (maybe for breakpoints). |
Perhaps, but don't use SWIs to RAM in production code. Nintendo reserves the right to change the layout of the BIOS and add more functions in future Game Boy platforms.
_________________
-- Where is he?
-- Who?
-- You know, the human.
-- I think he moved to Tilwick.