[coreboot] SimNOW VGA int 1a

Myles Watson mylesgw at gmail.com
Thu Oct 16 01:27:32 CEST 2008

> 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:
> -mregparm=3
> A nice optimization that utterly destroys the bios interrupt support.
> Myles, try setting -mregparm=0 and see if life is better.

I'll try it when I get back to that machine.  My laptop isn't AMD64, so I
can't run SimNOW on it.

Thanks for finding that.

> 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.

I'll be interested to see how much faster v3 is when it's not set to SPEW.
Right now it seems a lot slower than v2dsr on SimNOW.


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