[SeaBIOS] FreeBSD VESA console not working with SeaBIOS

Laszlo Ersek lersek at redhat.com
Thu Apr 9 12:47:40 CEST 2015

On 04/09/15 10:59, Jon Doe wrote:
> On Thu, Apr 9, 2015 at 4:32 AM, Kevin O'Connor <kevin at koconnor.net> wrote:
>> On Wed, Apr 08, 2015 at 08:48:11PM +0200, Peter Stuge wrote:
>>> Kevin O'Connor wrote:
>>>> (Specifically, the "leal" instruction is not properly implemented.)
>>>> Unfortunately, there isn't much that can be done about this on the vga
>>>> bios side.
>>> Really? Impossible to save flags, use other opcodes, and restore flags?
>>> lea isn't used in vgasrc/ besides in the trap that triggers the fault.
>>> In src/romlayout.S lea is used in two places to bump esp before calls.
>>> If those code paths are used also by SeaVGABIOS then maybe they could
>>> be rewritten with simpler instructions?
>> The problem is not with leal in hand written assembler - the problem
>> is with leal instructions generated by gcc.  To see the assembler gcc
>> produces for the vgabios one can look at out/vgaccode16.raw.s .  Or,
>> alternatively, one can run:
>>   objdump -m i386 -M i8086 -M suffix -ldr out/vgarom.o
>> We've fixed up gcc assembler in the past (see scripts/vgafixup.py) to
>> work around x86emu.  However, the leal instruction seems painful to
>> patch out - particularly so when %esp is one of the registers read or
>> written in the leal instruction.  If anyone wants to take a stab
>> at a workaround, feel free to submit a patch.
>> -Kevin
> Might be instructive to look at how vmware and virtualbox BIOSes is
> able to work around this problem. Surely their BIOS code is written in
> C?

Not sure about virtualbox, but vmware certainly have full control of the
compiler (and any postprocessors) they build their releases with. I
think users / distributors won't (can't) recompile vmware's C-language
BIOS, possibly giving rise to instructions that trip up NTVDM, x86emu etc.


> Fixing this in x86emu is probably the right thing to do (just checked,
> PCBSD 10.1's x86emu is still broken), but this won't help if you're
> stuck with an old release.
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