*BSD News Article 30284


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From: tzs@u.washington.edu (Tim Smith)
Newsgroups: comp.os.os2.programmer.misc,comp.os.linux.development,comp.os.minix,comp.os.mach,comp.periphs,comp.unix.bsd,comp.unix.pc-clone.32bit,comp.os.386bsd.development,comp.sys.ibm.pc.hardware.misc
Subject: Re: [Announcement] 386BSD Release 1.0
Date: 8 May 1994 05:23:50 GMT
Organization: University of Washington School of Law, Class of '95
Lines: 27
Message-ID: <2qht16$msc@news.u.washington.edu>
References: <2q63q2$927@dearg.cuillin.org.uk> <hastyCpCLny.5q8@netcom.com> <newcombe.142.00141E4A@aa.csc.peachnet.edu> <2qdvvp$r@bmerha64.bnr.ca>
NNTP-Posting-Host: stein.u.washington.edu

In article <2qdvvp$r@bmerha64.bnr.ca>, Jean Cyr <jcyr@bnr.ca> wrote:
>Can't be done. The initial boot stage (reading the boot sector and jumping
>to it) is handled by your machine's BIOS rom. Currently all BIOS that I've
>seen look for boot sectors on floppy and hard disk. They never look for CD.
>How could they ? There's no BIOS support for CD.

You seem to think BIOS ROMs come with the machine.  That's only partly
correct.  They also come with adaptor cards.  Early on in system
initialization, the BIOS that comes with the motherboard scans the ROM
address space looking for BIOS ROMs on adaptor cards.  Each such ROM found
is called to initialize itself.

The BIOS ROM on a SCSI card typically at this point scans the SCSI bus,
and if it finds any bootable hard disks, installs INT 13h handlers for
them.  It would be trivial for such a BIOS to make a CD-ROM that contains
a bootable disk image appear to be a read-only hard disk on INT 13h drive
0x80.  The system would then boot from it.  Heck, if the people who wrote
the host adaptor firmware wanted to, they could support booting from a
scanner...

I wrote a SCSI BIOS that doesn't support booting from a CD-ROM, because it
never occurred to me or the people we wrote it for that this would be a
cool thing, but if it had, it would have taken about 5 minutes to modify
the BIOS to support it.  It doesn't surprise me to find that someone did
think of it and do it.  It really is trivial.

--Tim Smith