linux-kernel - Re: Matching hard disks to BIOS boot order

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Message-ID: <4A32D8E0.5090900@zytor.com>
Date:	Fri, 12 Jun 2009 15:38:24 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	Harvey Chapman <hchapman-linux-kernel@...p.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Matching hard disks to BIOS boot order

Alan Cox wrote:
> On Fri, 12 Jun 2009 00:26:16 -0400
> Harvey Chapman <hchapman-linux-kernel@...p.com> wrote:
> 
>> Is there a way to figure out which hard disks match the BIOS boot order?
>>
>> I'm trying to tell a Linux program which disk to use based on the disk 
>> numbers (0,1,...) used by Windows. The best solution I've found so far 
>> is disk serial number, but that hasn't been terribly reliable for other 
>> reasons.
> 
> There is a BIOS interface for this on newer systems, although the kernel
> doesn't capture enough data to make it completely reliable below EDD 3.0
> (in theory if we grabbed a few more bits we could do EDD 1.x as well)
> 
> Most PCs today do EDD 3.0 however
> 

Actually, the best option is to completely forget about BIOS boot order
-- on some systems it can literally change from one boot to the next --
and instead rely on MBR signatures.  As far as I know, Windows requires
all hard drives to have unique signatures and will "rebrand" the disks
to make it so if necessary.

The MBR signature is 4 bytes starting at offset 440 decimal.

	-hpa
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