Path: utzoo!mnetor!uunet!husc6!necntc!ima!johnl
From: johnl@ima.ISC.COM (John R. Levine)
Newsgroups: comp.sys.ibm.pc
Subject: Re: PC AT clones, MSDOS and OS2 - request for info.
Message-ID: <1003@ima.ISC.COM>
Date: 5 May 88 15:38:38 GMT
References: <22359@bu-cs.BU.EDU> <973@unccvax.UUCP> <23879@ucbvax.BERKELEY.EDU>
Reply-To: johnl@ima.UUCP (John R. Levine)
Organization: Not much
Lines: 25
Summary: life is awfully complicated

In article <23879@ucbvax.BERKELEY.EDU> brand@janus.UUCP (Graham Brand) writes:
>Isn't OS2 compatibility automatic on clones with MSDOS compatibility?
>If not, why not? Is it similar to MSDOS compatibility in that it is 
>determined primarily by the BIOS implementation and, therefore, should 
>be asked for specifically if needed?

According to PC Week, that well-known source of Nearly Official Information,
the story is like this:  IBM OS/2 does its I/O through the BIOS, while
MS OS/2 that they license to clone vendors doesn't; it does its I/O any way
the cloner chooses to set it up, generally by programming the hardware
directly.  This is much the same situation that originally held with DOS;
machines like the Tandy 2000 ran MSDOS without an IBM-style BIOS.

This means that IBM OS/2 should run on any BIOS compatible machine, while
other OS/2's require much stricter hardware compatibility.  But wait --
there's more.  IBM's PS/2 series has two separate parts of the BIOS, the
real mode CBIOS that DOS uses and the protected mode ABIOS that OS/2 uses.
So your clone has to have an ABIOS.  Except that OS/2 runs on old ATs, which
don't have an ABIOS, so they must either be programming the AT's hardware
directly, or else backflipping into real mode for all I/O on the AT, which
would be extremely slow.   Your guess is as good as anybody else's.
-- 
John R. Levine, IECC, PO Box 349, Cambridge MA 02238-0349, +1 617 492 3869
{ ihnp4 | decvax | cbosgd | harvard | yale }!ima!johnl, Levine@YALE.something
Rome fell, Babylon fell, Scarsdale will have its turn.  -G. B. Shaw