Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/12/84; site desint.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!allegra!mit-eddie!godot!harvard!talcott!panda!genrad!decvax!ittvax!dcdwest!sdcsvax!sdcrdcf!trwrb!desint!geoff
From: geoff@desint.UUCP (Geoff Kuenning)
Newsgroups: net.unix-wizards,net.micro.68k
Subject: Re: VM on 680x0?  Almost...
Message-ID: <330@desint.UUCP>
Date: Sun, 27-Jan-85 19:37:00 EST
Article-I.D.: desint.330
Posted: Sun Jan 27 19:37:00 1985
Date-Received: Wed, 30-Jan-85 19:06:57 EST
References: <210@wdl1.UUCP> <1966@sun.uucp> <142@redwood.UUCP>
Followup-To: net.micro.68k
Organization: his home computer, Manhattan Beach, CA
Lines: 28
Xref: watmath net.unix-wizards:11778 net.micro.68k:544

In article <142@redwood.UUCP> rpw3@redwood.UUCP (Rob Warnock) writes:

>| The 68010 and 68020 are ALMOST capable of supporting hypervisors.
>| Unfortunately there is one thing you can't simulate: RTE (Return
>| from Exception) into the middle of an instruction as a bus error
>| recovery.

>Actually, John, I think you CAN simulate the RTE, it's just messy. You have
>to [know how to emulate the microcode for the RTE]...

But do we really need to do all this?  The software is permitted to change
only certain limited fields in the saved state;  the format of the other
stuff isn't even documented, and may conceivably vary from rev to rev of the
chip (I seem to remember this was a problem with somebody's multiprocessor
68010 system).

When the VM gets a bus error, save the state vector someplace internally.
Then, when it does an RTE, find the matching state vector and do a comparison,
allowing for changes only in legal places.  If there is a problem, simulate
a format exception in the VM.

P.S.  I am moving this discussion to net.micro.68k, since that seems to be
a more reasonable place than unix-wizards.
-- 

	Geoff Kuenning
	Unix Consultant
	...!ihnp4!trwrb!desint!geoff