Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!rutgers!cs.utexas.edu!usc!snorkelwacker!think!husc6!m2c!wpi!jhallen
From: jhallen@wpi.wpi.edu (Joseph H Allen)
Newsgroups: comp.arch
Subject: Re: IBM PC prehistory
Message-ID: <6727@wpi.wpi.edu>
Date: 15 Jan 90 10:19:02 GMT
References: <1576@aber-cs.UUCP> <2731@odin.SGI.COM> <7413@drilex.UUCP>
Reply-To: jhallen@wpi.wpi.edu (Joseph H Allen)
Organization: Worcester Polytechnic Institute, Worcester ,MA
Lines: 23

In article <7413@drilex.UUCP> dricejb@drilex.UUCP (Craig Jackson drilex1) writes:
>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.)

The Radio Shack model 16 didn't have an MMU but it did have an offset and
limit register.  These were enough to allow it run multiuser xenix and other
multiuser operating systems (RM/COS).  If would certainly be cheap enough to
do this way back when.

The Macintosh doesn't need it- it's a special purpose machine which is only
supposed to run canned apple software :)

The real question is:  Why didn't anyone make a multitasking for the PC?  The
segments provide a simple mechanism for relocation.  This would not be a
terribly great developement system but at least for programs which already
work, it would be fine.

The answer, of course, is that MS-DOS made it easy to port DBASE and WordStar
from CP/M and people didn't care to look very far ahead.