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To: dherbst@gradient.cis.upenn.edu (Darrel Herbst)
cc: msmith@atrad.adelaide.edu.au, wes@intele.net, questions@freebsd.org
Subject: Re: Booting off SCSI disks when two IDE disks also installed. (fwd) 
In-reply-to: Your message of "Sun, 24 Mar 1996 09:42:57 EST."
             <199603241442.JAA25322@gradin.cis.upenn.edu> 
Date: Sun, 24 Mar 1996 12:15:57 -0500
From: "Jonathan S. Shapiro" <shap@AURORA.CIS.UPENN.EDU>
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Wes:

Here's a slightly more detailed explanation which you may (or may
not) find helpful.

The FreeBSD boot sector uses BIOS calls to load the kernel.  This
imposes a number of limitations, the most important of which are:

	o The entire kernel image (or secondary bootstrap image)
	  must fall within the part of the drive addressable by the
	  BIOS.  On older BIOS's this meant using translation for
	  large drives.

	o Only those disks that are visible to the BIOS (i.e. the
	  first two) can be booted from.

Before you installed the second drive, the adaptec controller was
looking, saying "okay, scsi0 should become BIOS drive 0x81" and
installing the drive in the BIOS table and the adaptec ROM disk
BIOS as the BIOS for that drive.

Once you installed the second IDE drive, though, I bet you didn't
notice that the adaptec board started saying "BIOS not loaded" (or
some such).  Basically, it's deciding that BIOS drives 0x80 and
0x81 were already mapped, and giving up.  Actually, my board
(2940W) says so even if only one IDE drive is installed.

Actually, my guess is that it wasn't installing the BIOS with one
drive either, and you were getting away with this by sheer luck and
register level compatibility.

If your board supports multiple IDE interfaces (many now support
two), you might find that you are able to make things work by
installing your 2nd drive on the 2nd interface.  Even so, be aware
that you're playing with fire here, and not all boards will work.

I don't think that any of the fancy boot loaders will work for you.
Most of them are dependent on the BIOS.


Jonathan