Path: utzoo!utgpu!watserv1!watmath!att!pacbell!pacbell.com!decwrl!bacchus.pa.dec.com!deccrl!decvax.dec.com!zinn!wgc386!slum!slumlord
From: slumlord@slum.MV.COM (Laird Heal)
Newsgroups: comp.sys.mac.system
Subject: Re: Protected-mode snake oil
Summary: the F line
Keywords: MMU, coprocessor, MPU
Message-ID: <1990Sep6.031748.6404@slum.MV.COM>
Date: 6 Sep 90 03:17:48 GMT
References: <33164@cup.portal.com> <1204.26c2fb48@waikato.ac.nz> <8919@ur-cc.UUCP> <1990Aug24.180757.3439@Neon.Stanford.EDU> <1990Sep5.201244.3447@slum.MV.COM>
Organization: dis
Lines: 39

>In article <33164@cup.portal.com>, ts@cup.portal.com (Tim W Smith) writes:
>> The Callan Data Systems Unistar 200 I used to have had only
>> a 68000 and supported an MMU quite well.  A whole bunch of
>> other 68000 systems of the same vintage also supported
>> MMUs quite well.
>> 
>There was a discussion of this on comp.arch a while back. I don't remember
>the details, but it wasn't trivial. The problem is the 68000 doesn't
>keep enough state to restart some instructions that may be interrupted
>by a page fault. Some solutions included tricks like a second 68000 -
>it didn't sound to me as if one could simply tack on an MMU and get paging,
>the way you can with a 68020 (well, if assuming getting the software side
>to work is simple...).
>
>Philip Machanick
>philip@pescadero.stanford.edu

The second 68000 could have been any processor to hold the first 68000 in 
a wait state while it obtained memory referenced by the particular address 
and read it from disk if necessary.  You cannot restart a 68000 instruction 
due to the preincrement-postdecrement addressing modes, which make it well 
nigh impossible to determine what the value of the registers were when the 
instruction began.  The 68010 solved this by pushing a few more bytes on 
the stack when servicing an interrupt.

The 68020 and later began to utilize the previously-illegal "F-line" op
codes, which are reserved for Motorola coprocessors.  There are some FPU
(68881) emulations of the 68020 for the 68000, and no reason that an MMU
emulation could not be written - I would guess that it is little more
than cleaning up from the ILLEGAL trap and writing to the address of the
coprocessor.  However, why bother?  From a hardware standpoint it is just
as easy to add a 68030 as a 68551, and from a software standpoint that is
MUCH easier.

Laird Heal	laird@slum.MV.COM
(Salem, NH)	(603) 898-1406	
-- 
Laird J. Heal   {root, slumlord, laird}@slum.MV.COM
(Salem, NH)     (603) 898-1406