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

Guillem Jover guillem at hadrons.org
Fri Mar 9 02:44:39 CET 2012

On Thu, 2012-03-08 at 22:57:30 +0000, Julian Pidancet wrote:
> 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.

Certainly! And sorry, somehow missed the fact this is exclusively
real-mode being emulated.

> Also, I think the right thing to do is to decrement BP instead of EBP
> when SYSMODE_PREFIX_DATA is set:
>                 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

Regardless of the mode, if the instructions are being modified by the
prefixes to use 32-bit operands or addresses, they are in my book
32-bit instructions. In any case what I meant was that ebp can have
any value because the register can be assigned directly, for example,
and wrap-around might vary depending on what part of it it's being
operated on.


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