[Qemu-devel] Re: [Bochs-developers] [PATCH v2 5/6] Don't use unreservedmemory in BIOS.

[Sebastian Herbszt]

Kevin O'Connor wrote:
> On Mon, Nov 03, 2008 at 11:26:46AM +0200, Gleb Natapov wrote:
> > Use only first page and last page of low memory. OSes assumes that first
> > page is used by bios and last page is reserved in e820 map.
> [...]
> >    ;; init the stack pointer
> > -  mov esp, #0x00080000
> > +  mov esp, #0x9fbf0
>
> Random note - I'm told that some option roms can relocate the EBDA.
> Setting the stack in the EBDA area will prevent that from working.

The option rom for the LSI SCSI controller (8xx_64.rom) relocates the
EBDA. It moves it from 0x9fc0 to 0x9f00.

> I'm not sure if this is really a problem though - a relocated EBDA
> would likely break bochs bios today.  Something to be aware of though.

EBDA relocation breaks BIOS from cvs. It used to work with my patch
"rombios.c - EBDA move fix" available at sf as #1909786.

Since the option rom updates the value at 0040h:000Eh it's possible
to use it to set the correct value of esp.

- Sebastian