Path: utzoo!attcan!uunet!cs.utexas.edu!usc!bbn!drilex!dricejb
From: dricejb@drilex.UUCP (Craig Jackson drilex1)
Newsgroups: comp.arch
Subject: Re: IBM PC prehistory
Message-ID: <7413@drilex.UUCP>
Date: 13 Jan 90 14:55:47 GMT
References: <1576@aber-cs.UUCP> <2731@odin.SGI.COM>
Organization: DRI/McGraw-Hill, Lexington, MA
Lines: 35

All these discussions of issues of MMUs in a possible 68k IBM PC seem moot:
I think it very unlikely that such a machine would have had an MMU at all.
The Macintosh is instructive here: it has no MMU, the ROMs aren't too
far away in the address space (by today's standards) and the I/O bus isn't
fully decoded.  (There are lots of locations above 8MB which access the SIO
chips, etc.)

Any IBM 68k PC would have been designed several years before the Mac, but
with much the same cost & producability goals, so it is unlikely that these
features would have been more sophisticated.

Admittedly, one can code on the raw 68k in a manner which allows relocatibility.
The Mac certainly does; evidently, so does OS/9.  Relocatibility isn't
essential for a multi-tasking OS--look at the Amiga.  But relocatibility
would be essential for almost any Unix-like operating system, and I would
suggest that an MMU is necessary for anything which wants to implement
fork() while allowing two tasks to occupy memory at once.

What's more, once you code a 68k for relocatibility in the absence of an
MMU, it begins to look much more like a segmented architecture.

My conclusions from all this:

If IBM had chosen the 68k, the most common 68k operating system today would
have been some sort of DOS.  Today's high-end 68k-DOS boxes would have an MMU,
 and would be using it to simulate a single large virtual address space.  Unix
boxes would have been made out of both 68ks and 286s, and 286s may
indeed have been touted for their MMUs.  The workstation community would
have grown up slightly sooner, due primarily due to the lower costs of
68k-related parts early on.  The DOS world might actually be stronger than
today, because of less address-space troubles.
-- 
Craig Jackson
dricejb@drilex.dri.mgh.com
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