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From: grr@cbmvax.cbm.UUCP (George Robbins)
Newsgroups: net.micro.atari16,net.micro.amiga,net.micro.68k
Subject: Re: 68000 Memory Managment
Message-ID: <686@cbmvax.cbmvax.cbm.UUCP>
Date: Sun, 31-Aug-86 21:37:00 EDT
Article-I.D.: cbmvax.686
Posted: Sun Aug 31 21:37:00 1986
Date-Received: Mon, 1-Sep-86 02:43:12 EDT
References: <508@elmgate.UUCP> <64@mit-prep.ARPA> <510@elmgate.UUCP> <417@atari.UUcp> <271@dmsd.UUCP>
Reply-To: grr@cbmvax.UUCP (George Robbins)
Organization: Commodore Technology, West Chester, PA
Lines: 40
Keywords: 68000 atari amiga 68k mmu
Xref: mnetor net.micro.atari16:1780 net.micro.amiga:4490 net.micro.68k:1184

In article <271@dmsd.UUCP> bass@dmsd.UUCP (John Bass) writes:
>
[ omitted - description of simplified Sun derivative MMU design ]
>
>MMU's are left out of toy computers because toy engineers don't know how
>to design real computers -- NOT because they take to much time (the above
>solves that problem) or because they cost too much (two mapping rams cost
>less than $10 in volume, and the rest of the circuit could have been dumped
>into the custom parts or pals already on the board -- an adder/comparator
>design could have been placed in the custom parts for nearly zero additional
>production cost).  Ditto for memory parity.
>
>John Bass (DBA:DMS Design)

Gee John, around here we smile when we say 'Toy Computer...'.

I think you are making a mistake in confusing intent with ability.  The
engineers working at the consumer level would love to slip in an MMU function,
but when the time comes to meet to cost criteria, it gets crossed out with a
long sigh.  The cuts are on dime and dollar issues, let alone a ten!

The engineers in Shivji's gang, prior to moving to Atari were working on a
Z8000 based game/graphics/unix type box.  It had a MMU, which coincidentally
did translation in the RAS/CAS delay.  The ST can be viewed as a 68000 based
derivative of this effort.

The Amiga, on the other hand, consists of a high-performance graphics processor
complex, memory and display, with a 68000 added on to tickle the graphics chips
and interface to the real world.  In a sense the software exists to drive the
engine, not the engine existing to run some generic software.

The real obstacle to putting in an MMU is the lack of any real world software
at that level that would make it seem a neccessary item.  Unix tends to make
the minumum machine too expensive (hard disk and license fees).  With two years
of hindsight, OS/9 68K level 2 might look like a good choice, but even now,
Microware doesn't seen too speedy in delivering their Amiga and ST ports.
-- 
George Robbins - now working with,	uucp: {ihnp4|seismo|caip}!cbmvax!grr
but no way officially representing	arpa: cbmvax!grr@seismo.css.GOV
Commodore, Engineering Department	fone: 215-431-9255 (only by moonlite)