Re: x86emu emulation problem

On Fri, 6 Oct 2006, jf simon wrote:

>>> 2- The same code as seen from ndisasm:

>>> 000068DA  A00080            mov al,[0x8000]
>>> 000068DD  04F5              add al,0xf5
>>> 000068DF  0002              add [bp+si],al
>>> 000068E1  C8008015          enter 0x8000,0x15
>>> 000068E5  0E                push cs
>>> 000068E6  0106C800          add [0xc8],ax
>>> 000068EA  80100E            adc byte [bx+si],0xe
>>> 000068ED  0105              add [di],ax
>>> 000068EF  C800800B          enter 0x8000,0xb
>>> 000068F3  0E                push cs
>>> 000068F4  0104              add [si],ax
>>> 000068F6  C8008006          enter 0x8000,0x6
>>> 000068FA  0E                push cs
>>> 000068FB  0102              add [bp+si],ax
>>> 000068FD  E80080            call 0xe900       <<<<<!!!HERE AGAIN

>> This is probably data -- either font data or VGA register tables.  Can
>> you trace backwards any more and figure out how you got to 68DA?

> You are right. I have found that the problem was on  a JMP SHORT which was 
> incorrectly landing in that part of the VGA BIOS. The relative displacement 
> was negative (was 0xBA), but  the JMP was considering it to be a jump to 
> [PC]+0xBA rather than applying the signed arithmetic. Setting  GCC 
> "-fsigned-char " switch made the signed displacemnt correctly applied....and 
> solved the problem. I didn't know that the "char" type was unsigned by 
> default.

I've just committed a change to insulate x86emu against this.

> Lastly, I have found that the VGA bios i use is doing CF8/CFC PCI 
> configuration style accesses. Which doesn't work on my PowerPC plaftorm. (I 
> think it is only to be seen in the x86 world, but not sure). So they need to 
> be translated to whatever the platform is going to use as PCI configuration 
> access. I just mention this for the record in case others are not aware of 
> this.

The generic int10 modules already intercepts such accesses and emulates them 
using PCI accesses appropriate for the platform.

Marc.

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