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

From: Natalia Portillo
Subject: RE: [Qemu-devel] BIOS behaves different as real BIOS
Date: Sat, 17 Jul 2004 09:20:05 +0100

Well the PC-DOS 1.0 does not have that signature, and also does not create
it in any disk you format. Nor the SCO Xenix has it.

I know this signature is present on DOS 3.0 and upper versions in all disks
you create. I not know about DOS 2.0 (I lost my copy of this version).

> -----Mensaje original-----
> De: address@hidden
> [mailto:address@hidden
> En nombre de Antony T Curtis
> Enviado el: sábado, 17 de julio de 2004 8:13
> Para: address@hidden
> Asunto: Re: [Qemu-devel] BIOS behaves different as real BIOS
> 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)
> <snip>
> --
> Antony T Curtis <address@hidden>
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