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Re: [Qemu-devel] BIOS behaves different as real BIOS

From: Antony T Curtis
Subject: Re: [Qemu-devel] BIOS behaves different as real BIOS
Date: Sat, 17 Jul 2004 08:12:44 +0100

On Sat, 2004-07-17 at 07:01, Natalia Portillo wrote:
> Hi!
> 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-).

I remember looking at the IBM PC/XT BIOS source and seeing it, and when
I used to play with putting my own messages in the boot sector, It had
to be there. This was around 15 years ago. The old IBM PC/XT would boot
into ROM Cassette BASIC if it did not find the signature. It is possible
that the original IBM PC did not have the check but by the time the
PC/XT came out, it did. The model of the PC/XT I had was 5150 (the 2
full height floppy drive model)


Antony T Curtis <address@hidden>

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