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Path: utzoo!henry
From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: comp.sys.m68k
Subject: Re: Virtual Machine (was: Apollo (was: MINIX port to A1000))
Message-ID: <8965@utzoo.UUCP>
Date: Fri, 20-Nov-87 13:08:06 EST
Article-I.D.: utzoo.8965
Posted: Fri Nov 20 13:08:06 1987
Date-Received: Fri, 20-Nov-87 13:08:06 EST
References: <8711050534.AA25885@jade.berkeley.edu>, <7862@ism780c.UUCP>
Organization: U of Toronto Zoology
Lines: 16
Keywords: virtual machine, emulation, 68010

> You have to have some way of making sure that long stack frames from
> things like bus errors are valid before allowing them to be used.
> These frames are full of undocumented fields that can can cause all
> kinds of problems if they contain bad data.

The generic solution to this is not to let the user's operating system
meddle with the frames.  Keep the *real* frame somewhere private.  Give
him a fake one, with the documented fields filled in from the real one,
and the rest full of magic numbers and checksums and so on.  When he tries
to do a return using the fake frame, check the checksums etc.  If they're
wrong, give him a simulated hardware failure.  If they're right, give him
a return using the real frame, adjusted to match any legitimate changes
he's made to the fake one.
-- 
Those who do not understand Unix are |  Henry Spencer @ U of Toronto Zoology
condemned to reinvent it, poorly.    | {allegra,ihnp4,decvax,utai}!utzoo!henry