Re: x86emu emulation problem

Hi,

>>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.

On a different issue, I think that the emulator may be wrong as it 
sometimes fetches values  from the DATA segment, even if the CS was 
previously selected as the source segment. For exemple,  in 
x86emu/ops.c, see [*]:


           if (M.x86.mode & SYSMODE_PREFIX_DATA) {
                u32 destval,srcval;

                DECODE_PRINTF("TEST\tDWORD PTR ");
                destoffset = decode_rm00_address(rl);
                DECODE_PRINTF(",");
                srcval = fetch_long_imm();
                DECODE_PRINTF2("%x\n", srcval);
                destval = fetch_data_long(destoffset);
                TRACE_AND_STEP();
                test_long(destval, srcval);
            } else {
                u16 destval,srcval;
                DECODE_PRINTF("TEST\tWORD PTR ");
                destoffset = decode_rm00_address(rl);
                DECODE_PRINTF(",");
                srcval = fetch_word_imm();
                DECODE_PRINTF2("%x\n", srcval);
                destval = fetch_data_word(destoffset);   [*]
                TRACE_AND_STEP();
                test_word(destval, srcval);
            }

[*] : shouldn't that be a fetch from the CS segt, since the mode (in 
M.x86.mode) is not of the DATA type?

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.

Thanks,
-jf simon

	

	
		
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