[PATCH] x86emu: Correctly handle 0x66 prefix for some instructions

Julian Pidancet julian.pidancet at gmail.com
Thu Mar 8 14:57:30 PST 2012

On Wed, Mar 7, 2012 at 7:04 PM, Guillem Jover <guillem at hadrons.org> wrote:
> On Wed, 2012-03-07 at 17:54:57 +0000, Julian Pidancet wrote:
>> So according to the manual, it should be BP, not EBP.
> The register being decreased should match the one being used to address
> the stack, and the one to use depends on the descriptor as per above.

There is no "descriptor" in real mode. Default stack size is always 16-bit.
Also, I think the right thing to do is to decrement BP instead of EBP

                M.x86.R_BP -= 4;
                push_long(fetch_data_long_abs(M.x86.R_SS, M.x86.R_BP));

instead of:

                M.x86.R_EBP -= 4;
                push_long(fetch_data_long_abs(M.x86.R_SS, M.x86.R_BP));

...to remain exactly consistent with the manual.

>> In any case, It won't be a problem, because the 16 high bits of EBP
>> will most likely be zero in real-mode code.
> Well, not if the the code is using some 32-bit instructions. :)

They are not "32-bit instructions". The processor is still functioning
in real-mode, therefore, addressing still follows the rules of
real-mode addressing, as mentioned in Volume 1:Basic Architecture :

3.3.5 32-Bit and 16-Bit Address and Operand Sizes


When operating in real-address mode, the default addressing and
operand size is 16 bits. An address-size override can be used in
real-address mode to enable 32-bit addressing. However, the maximum
allowable 32-bit linear address is still 000FFFFFH (2^20-1).

More information about the xorg-devel mailing list