[SeaBIOS] ipxe/seabios: segment register initialization

Michael S. Tsirkin mst at redhat.com
Sun Feb 14 20:52:04 CET 2016

On Sun, Feb 14, 2016 at 05:36:38PM +0000, Michael Brown wrote:
> On 14/02/16 15:53, Michael S. Tsirkin wrote:
> >>That's for execution of a PXE NBP, rather than a (SAN-booted) disk boot
> >>sector.
> >
> >I actually thought the initial post was about iPXE calling NBP (starting
> >at location 07C00h) with a non-zero DS, not about booting from disk.
> The initial post mentioned a "boot sector which works just fine when loaded
> directly from BIOS, but doesn't work when loaded by iPXE".  That can only be
> referring to a disk boot sector, not a PXE NBP.  (For a start, the BIOS has
> no way to directly load a PXE NBP.)
> >Rephrasing the original question,
> It's not a rephrasing; it's an entirely different question.

True - I was just curious since there are similarities between
NBP and boot sector (e.g. both are loaded at 7c00) so sometimes
the same code can work as both a boot sector and an NBP.

> >do you think %DS should be zeroed when NBP is called then?
> Not according to the PXE spec.
> As far as I can tell, the initial value of %ds is undefined for both a PXE
> NBP and a BIOS boot sector.
> Michael



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