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From: pcg@aber-cs.UUCP (Piercarlo Grandi)
Newsgroups: comp.arch
Subject: Re: IBM PC prehistory
Summary: Too bad the 68k did not have a standard MMU
Message-ID: <1576@aber-cs.UUCP>
Date: 10 Jan 90 15:17:16 GMT
Reply-To: pcg@cs.aber.ac.uk (Piercarlo Grandi)
Organization: Dept of CS, UCW Aberystwyth
	(Disclaimer: my statements are purely personal)
Lines: 51

In article <13504@nsc.nsc.com> amos@nsc.nsc.com (Amos Shapir) writes:
    In article <129994@sun.Eng.Sun.COM> kenobi@sun.UUCP (Rick Kwan - Sun Intercon) writes:
    >I have often wondered what would happen if IBM had chosen the 68000
    >instead of 8088/8086, and tailored a their own simple OS to run on
    >it?
    ...
    >    1.	68000/UNIX would have found it very difficult to sell against
    >	a 68000/simple-IBM-OS.  (Not enough perceived product
    >	differentiation.)
    
    Quite the contrary - Unix made its way into the PC market *despite* the fact
    that it needed 68k-like architecture, not *because* of it.  Its main
    advantage over DOS is the sophisticated user interface; the fact that
    it's hard to put UNIX on a 86-like architecture was an undesirable
    side effect.

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. If the MacIntosh or Atari ST had
an MMU third party Unxies for them would have long since been
available.

The idea is that the millions of DOS machines out there have no
need, and no (essentially) use for, an MMU, but the fact that
it is incorporated in the CPU means that it is there, and the
manufacturer cannot omit it.

The more incredible thing is not only that all the 386 running
DOS out there are essentially emulating an 8080 on steroids,
and thus don't ussually bother to use the MMU, but that even
Unix does not really take advantage of the Multics like
features of the 286/386 MMU at all, using the 286/386 as either
a glorified PDP or VAX.
-- 
Piercarlo "Peter" Grandi           | ARPA: pcg%cs.aber.ac.uk@nsfnet-relay.ac.uk
Dept of CS, UCW Aberystwyth        | UUCP: ...!mcvax!ukc!aber-cs!pcg
Penglais, Aberystwyth SY23 3BZ, UK | INET: pcg@cs.aber.ac.uk