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[Qemu-devel] BIOS behaves different as real BIOS

From: Natalia Portillo
Subject: [Qemu-devel] BIOS behaves different as real BIOS
Date: Sat, 17 Jul 2004 07:01:27 +0100


I tried to boot an old operating system from qemu and I get a message from
BIOS saying that tbe floppy is not bootable (literally: "FATAL: Not a
bootable disk").

I think that this is because the BIOS searches for the 0x55AA signature on
last two bytes of first sector of disk.
As far as I remember, this signature was introduced very lately and as far
as I tested on my real systems, this signature is not really searched at
boot time, and every DOS before 3.0 doesn't have it (it is used also by
later DOSes and Windows to identify a FAT filesystem), at least on floppies
(I think that on hard disks it is also ignored, but never tested -0x55AA is
also used to identify Intel partition scheme, aka MBR-).

Natalia Portillo

P.S.: Just have read the BIOS source code from the original IBM PC
documentation, and BIOS doesn't look for that signature, just do INT 13h to
copy the first sector from floppy 0 to 0x7c00 and then jump to it, just as
following (is NOT a copy from the documentation):

Mov ah, 02h
Mov al, 01h
Xor ch, ch
Mov cl, 01h
Xor dx, dx
Mov ex, 0000h
Mov bx, 7c00h
Jmp 7c00h

P.S.2: Maybe this mail should be better sent to bochs mailing list but I'm
not subscrided, so, if so, please resend it.

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