Path: utzoo!attcan!uunet!decwrl!ucbvax!agate!linus!linus!gwr
From: gwr@linus.mitre.org (Gordon W. Ross)
Newsgroups: comp.os.minix
Subject: Re: Shoelace, Boot sector, Partition sector
Summary: See IBM DOS 3.3 Tech. Ref.
Message-ID: <124439@linus.mitre.org>
Date: 25 Oct 90 18:45:32 GMT
References: <34243@nigel.ee.udel.edu> <3275@bruce.cs.monash.OZ.AU>
Reply-To: gwr@linus.mitre.org (Gordon W. Ross)
Organization: The MITRE Corporation, Bedford, MA.
Lines: 39

In article <3275@bruce.cs.monash.OZ.AU> of comp.os.minix,
cechew@bruce.cs.monash.OZ.AU (Earl Chew) writes:

> When I wrote the code, I got the information for this part by
> disassembling the partition table code from a TI machine running
> Dos < 3.3.  Does anyone know if the parameter passing protocol for
> the partition boot is written down somewhere?

Yes.  See the "IBM DOS 3.3 Technical Reference" manual, page 9-9.
Among other details, it specifies:

	"When a partition's boot record is given control, it is passed
	its partition table entry address in the DS:SI registers."

There is no mention of any other registers being initialized by
the primary boot program.  (In article <34243@nigel.ee.udel.edu>,
Michael Temari <TEMARI%ECAMV1.dnet.ge.com@vm1.nodak.edu> mentioned
that bootlace expected "the drive in register dl" to be passed too.)

The partition table is laid out to make it easy to load the cx and dx
registers for the BIOS call to read the secondary boot sector.
Presumably the Minix assembler code should look like this:

	mov	ax,#0x0201	| BIOS read 1 sector
	mov	bx,#0x7c00	| transfer address
	mov	cx,2(si)	| cylinder/sector
	mov	dx,(si)		| drive/head
	int	0x13

By the way, neither MS-DOS nor UNIX (Sys.V/386) make any use of the
address passed in DS:SI but use alternative, less friendly methods to
find the active partition.  For UNIX, this also means it will refuse
to boot unless its partition is marked as active.  Probably the author
of that boot program couldn't find the above information either!
-- 
Gordon W. Ross  (M/S E095)	internet: gwr@linus.mitre.org
The MITRE Corporation    	uucp: {decvax|philabs}!linus!gwr
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