[ipxe-devel] ipxe/seabios: segment register initialization

Victor Kaplansky victork at redhat.com
Sun Feb 14 10:52:16 GMT 2016


Hi,
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
code.

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?

Thanks,
-- 
Victor


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