[SeaBIOS] vgabios testing

[Julian Pidancet]

On Sun, Mar 4, 2012 at 7:04 PM, Kevin O'Connor <kevin at koconnor.net> wrote:
> On Sun, Mar 04, 2012 at 03:03:49AM +0000, Julian Pidancet wrote:
>> I've done further debugging, and you can ignore all of the crap above.
>> x86emu badly handles the retl instruction and only pops a 16bit wide
>> value from the stack, whereas the corresponding calll pushes a 32bit
>> return address. leavel suffers from the same problem.
>
> Well, that's unfortunate.  The system SeaBIOS uses to get gcc compiled
> code working in 16bit mode is going to generate "retl" instructions.
>
> It may be possible to post-process the resulting assembler code, but
> (assuming we could) it would be quite ugly.
>
>> I've applied the following patch to x86emu and it seems to work better:
>
> So, I guess the question is, how important is support for
> current/legacy x86emu versions?
>

I've tried to replace .code16gcc with .code16 in src/code16gcc.s to
see if gcc would be able to generate code which doesn't use 32bit
version of the call/ret instructions. The result was quite
disappointing, it generates functions like this:

push   %ebp
mov    %esp,%ebp
[...]
call 123 <func>
[...]
leave
ret

Which seems correct, except that the prologue pushes 32bit on the
stack, whereas the epilogue pops 16bit. (Could it be a GCC bug ?)

I am going to propose a patch to xorg-devel in the next few days, but
in the meantime, it would be nice to find a solution in SeaBIOS so the
code can work with older versions of Xorg.

I've done my testing with a 64bit machine though, I am wondering if
there is a way to force Xorg to use vm86 at least on 32bit machines.
Anybody knows anything about that ?

-- 
Julian