Need help debuging i386 in real mode.

Need help debuging i386 in real mode.

[Develop.ReactOS]

Hello:
    I am trying to debug i386 in real mode. How to disassemble 16-bit code?
    gdb appears to always assume a 32-bit segment.Is there a way to coerce gdb into 16-bit disassembly mode?  Is there some other tool or cmd I could use?  

I need 16-bit disassembly instruction,but gdb's output is wrong:
(gdb) x /10i $eip+$cs*16
0xffff0:        ljmp   0x3131,0xf000e05b
0xffff7:        das
0xffff8:        xor    BYTE PTR [ecx],dh
0xffffa:        das
0xffffb:        xor    BYTE PTR [esi],dh
0xffffd:        add    ah,bh
0xfffff:        test   DWORD PTR [eax],0x0
0x100005:       add    BYTE PTR [eax],al
0x100007:       add    BYTE PTR [eax],al
0x100009:       add    BYTE PTR [eax],al

Best regards!
Develop.ReactOS
2007-03-25

Re: Need help debuging i386 in real mode.

[David Daney]

Develop.ReactOS wrote:
> Hello:
>     I am trying to debug i386 in real mode. How to disassemble 16-bit code?
>     gdb appears to always assume a 32-bit segment.Is there a way to coerce gdb into 16-bit disassembly mode?  Is there some other tool or cmd I could use?  
>
> I need 16-bit disassembly instruction,but gdb's output is wrong:
> (gdb) x /10i $eip+$cs*16
> 0xffff0:        ljmp   0x3131,0xf000e05b
> 0xffff7:        das
> 0xffff8:        xor    BYTE PTR [ecx],dh
> 0xffffa:        das
> 0xffffb:        xor    BYTE PTR [esi],dh
> 0xffffd:        add    ah,bh
> 0xfffff:        test   DWORD PTR [eax],0x0
> 0x100005:       add    BYTE PTR [eax],al
> 0x100007:       add    BYTE PTR [eax],al
> 0x100009:       add    BYTE PTR [eax],al
>
>   
I have not tested it, but it looks like the GNU binutils objdump program
can do it. If you can get an image of the memory area you want to
disassemble into a file try something like:

objdump --target=binary --architecture=i8086 --disassemble-all
memory_image_filename

David Daney