[ipxe-devel] ipxe/seabios: segment register initialization
mcb30 at ipxe.org
Sun Feb 14 12:41:31 UTC 2016
On 14/02/16 10:52, Victor Kaplansky wrote:
> I've created a boot-sector which works just fine when loaded
> directly from BIOS, but doesn't work when loaded by ipxe:
> 0: b8 ad de mov ax,0xdead
> 3: a3 10 7c mov ds:0x7c10,ax
> 0xdead is not written to 0000:7c10.
> The reason is that BIOS have data segment register initialized to
> zero, while ipxe leaves it dirty before the jump to boot sector
> After adding to boot sector some code to initialize DS register,
> my code works well:
> 0: b8 00 00 mov ax,0x0
> 3: 8e d8 mov ds,ax
> 5: b8 ad de mov ax,0xdead
> 8: a3 10 7c mov ds:0x7c10,ax
> Is such behaviour intentional, or it is a bug?
> Who is responsible for segment registers initialization?
I'm not aware of any standard describing the content of %ds when jumping
to a boot sector at 0000:7c00. Every boot sector that I have
encountered explicitly initialises %ds (and %es, and %ss:%sp) to known
Your boot sector should probably do likewise.
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