Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!oakhill!tomj
From: tomj@oakhill.UUCP (Tom Johnson)
Newsgroups: comp.sys.mac.hardware
Subject: Re: Why no VM on a 68K? (was: Re: Why 68000?)
Message-ID: <2983@oakhill.UUCP>
Date: 19 Feb 90 20:52:56 GMT
References: <1990Feb11.154304.19943@smsc.sony.com> <3919@hub.UUCP> <10223@hoptoad.uucp> <1990Feb15.155556.5319@uncecs.edu> <19472@dartvax.Dartmouth.EDU> <22145@pasteur.Berkeley.EDU> <1990Feb16.164414.6377@intercon.com>
Reply-To: tomj@oakhill.UUCP (Tom Johnson)
Organization: Motorola Inc., Austin, Texas
Lines: 41

Amanda Walker:
>Robert Menke:
>> What you do is have
>> TWO processors, with one running just behind the other.  When the
>> first one bus-faults, the secondary one is halted...
>[a moment while I search through my brain's backup tapes...]
>
>I think it was Motorola themselves who first suggested this...
>...One of these was an example of doing virtual memory with a two-processor
>system.  When a page fault occured, processor A just went into a wait state,
>processor B woke up, swapped in the page, and then woke up processor A
>again.
>
Very Close...actually, when a memory fault occurs due to the "primary" 68K
making a memory access, the bus error was returned to the "secondary" 68K
as an interrupt (and no DTACK to the primary).  The interrupt handler on 
the secondary 68K then issues (via external logic) a RELINQUISH & RETRY 
request (i.e. HALT, BERR, and BR) to the primary processor.  The primary 
would 1) terminate the current bus cycle and prepare to run it over, and 
2) give up the bus.  The secondary processor then fixed up memory, did an 
RTE followed by a STOP instruction, and awaited another "interrupt" to wake
it up again.  This caused the secondary processor to give up the bus, and
allowed the primary processor to have the bus back again.  It proceeded to
run the "faulted" cycle over again, using the same address, data, etc.

>Trivia quiz: anyone remember the 68012?
>
Of course, but as someone pointed out, lifetime buys have already been
accepted.  The part was originally made especially for a "large customer"
who wanted the virtual memory capability AND a large address space.  A
more interesting trivia question:  What address line was missing on the '012,
and WHY?
>--
>Amanda Walker
>InterCon Systems Corporation

Tom Johnson
Motorola, Inc.

Disclaimer (as much as I hate them):  Don't take anything I write, speak,
dream, or moan as in any way representing anything Motorola might believe.