VM on 680x0? Almost...

[Geoff Kuenning]

In article <142 at redwood.UUCP> rpw3 at 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