From: Paul de Weerd (weerdweirdnet.nl)
Date: Thu Mar 09 2006 - 08:04:43 CST

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On Thu, Mar 09, 2006 at 02:44:10PM +0100, Stefan Drexleri wrote:
| Hi,
|
| when installing OpenBSD fdisk utility does extra show selected disk's MBR
| signature 0xAA55.
|
| Why does it do? Do more MBR signatures exist? Or is this only for
| verification of disk's mbr health?

The 0xAA55 signature is the last two bytes of the first sector of your
bootdisk (bootsector/Master Boot Record/MBR). If it is 0xAA55, then
the BIOS will try booting the system.
If it's not found (it garbled or 0x0000), you'll get an error message
from your BIOS that it didn't find a bootable disk (or the system
tries booting the next disk).

This signature is represented (in binary) as 0b1010101001010101. The
alternating bit pattern was thought to be a protection against certain
failures (drive or controller).

Of course, this is an i386ism (also present on amd64 I believe). Lots
of other architectures may take different approaches.

Cheers,

Paul 'WEiRD' de Weerd

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