> Even though it's marked as mechanical, the "weeds" comment made me look.
> It seems that there's a chip that's intercepting certain otherwise unimplemented
> opcodes such as 0f 3c. To the best of my knowledge, this is not an undocumented
> instruction, it simply doesn't exist. The BIOS is attempting to execute this
> instruction, along with two that decode to vmread and vmwrite on modern CPUs, but
> which definitely did not exist at the time. I have no idea what the instructions are
> supposed to do, nor how one would go about finding out.
> Beyond that, it's also waiting on a status from port 0x801 (after the port 0x401
> check). If I bypass that, then it jumps into unmapped memory. I suspect that the BIOS
> was not dumped correctly.
>
> Oh, and there's a crash condition in the debugger for mamepp, but I can't reproduce
> it reliably. Apparently, something to do with the scroll bars, based on the call
> stack.
From Teh Shoutbox at forums.bannister.org:
crazyc said:
Quote:
on the opcode 0f 3c on the MediaGX is CPU_WRITE according to sandpile.org. The docs say CPU_READ and CPU_WRITE access machine specific registers.
Just according to sandpile.org 0f 38-3d are also Cyrix GX1 and AMD Geode opcodes. Doesn't really say which exact chips has which instructions. The AMD GX1 doc at http://support.amd.com/us/Embedded_TechDocs/gx1_ds.pdf does decribe them though on page 97.
- Stiletto
Edited by Stiletto (08/15/13 11:22 PM)
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