Path: utzoo!attcan!uunet!ns-mx!ceres!zaphod.mps.ohio-state.edu!uwm.edu!lll-winken!arisia!sgi!shinobu!odin!maddog!pkr
From: pkr@maddog.sgi.com (Phil Ronzone)
Newsgroups: comp.arch
Subject: Re: IBM PC prehistory
Message-ID: <2731@odin.SGI.COM>
Date: 11 Jan 90 23:39:54 GMT
References: <1576@aber-cs.UUCP>
Sender: news@odin.SGI.COM
Organization: Silicon Graphics, Inc., Mountain View, CA
Lines: 39

In article <1576@aber-cs.UUCP> pcg@cs.aber.ac.uk (Piercarlo Grandi) writes:
>Here I have a small difference. The big problem with *existing*
>68k machines is that they all different MMUs, thus making an
>ABI, or a standardized Unix, that much more difficult. Even
>worse, most 68k machines, even 68020 VME boards, have no MMU,
>because it must be added externally.
>
>This is because Motorola took a long time to get a page fault
>supporting 68K (the 68010), and was too late with a working
>MMU, and then did not put the MMU on chip until the 68030. The
>overriding advantage of the 286 and 386 is that they have an
>on-chip MMU and thus Unix can run on any 286 or 386 machine
>around.
>
>Try for example to port Unix to the MacIntosh, or the Atari ST,
>which would, but for the lack of MMU, perfectly capable of
>running UNIX. The Apple Lisa did have a (third party) UNIX
>because it did have some MMU.


When I worked at UniSoft, I probably spec'ed the porting effort
for over 100 different 68XXXX machines. A different MMU does
NOT mean a different ABI.

The most critical item is the segment size -- since the binary
sections (text, data, shared memory) are aligned on the segment
boundary. Page size tends to be almost of no consequence.

Selecting a segment size of 128K allowed for a 80-90%
common binary among all the various UniSoft ports.

In general, it was only the REALLY grungy discrete MMU
designs that were "incompatible".


------Me and my dyslexic keyboard----------------------------------------------
Phil Ronzone   Manager Secure UNIX           pkr@sgi.COM   {decwrl,sun}!sgi!pkr
Silicon Graphics, Inc.               "I never vote, it only encourages 'em ..."
-----In honor of Minas, no spell checker was run on this posting---------------