[coreboot] SimNOW VGA int 1a

ron minnich rminnich at gmail.com
Thu Oct 16 01:00:23 CEST 2008

On Wed, Oct 15, 2008 at 1:27 PM, Myles Watson <mylesgw at gmail.com> wrote:
> Here's the next part of the log now that I've enabled setup_realmode_idt
> (I'm running it right before real_mode_switch_call_vga.
> Copying VGA ROM image from 0xfe040000 to 0xc0000, 0x8000 bytes
> BREAK HERE run_bios = 0x0000944a
> biosint: INT# 0x18
> biosint: eax 0x2e ebx 0x10000 ecx 0xfe4 edx 0xcf11c
> biosint: ebp 0xc0000000 esp 0xd0000 edi 0x1a esi 0x0
> biosint:  ip 0x1022   cs 0xf  flags 0x2067
> BIOSINT: Unsupport int #0x18

when you're looking for a misaligned stack frame the registers are
always interesting.

Note that edi looks like a 1a.

This code is unchanged for the most part since I wrote it. What you
can do is look via gdb at the biosint function and see where it gets
the int #. It is unlikely that this is a gcc problem. A misguided
directive, on the other hand ...

let's look around:

gdb build/util/x86emu/vm86.o

Dump of assembler code for function biosint:
0x000004f3 <biosint+0>:	push   %esi
0x000004f4 <biosint+1>:	mov    %eax,%esi
0x000004f6 <biosint+3>:	push   %ebx
0x000004f7 <biosint+4>:	sub    $0x4,%esp
0x000004fa <biosint+7>:	movzwl 0x34(%esp),%eax
0x000004ff <biosint+12>:	mov    0x30(%esp),%ebx
0x00000503 <biosint+16>:	mov    %eax,(%esp)
0x00000506 <biosint+19>:	push   %esi
0x00000507 <biosint+20>:	push   $0x86
0x0000050c <biosint+25>:	push   $0x7
0x0000050e <biosint+27>:	call   0x50f <biosint+28>

We are passing arg 1 in eax. How could this be?

Simple. We got Clever in v3:


A nice optimization that utterly destroys the bios interrupt support.

Myles, try setting -mregparm=0 and see if life is better.

I vote we get rid of this type of Cleverness. It's just not
performance critical in a bios. We're not an OS and we should keep it
simple. I don't think we'll live or die on 3 on-stack variables.


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