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From: roy@phri.nyu.edu (Roy Smith)
Newsgroups: comp.sys.mac.hardware
Subject: Re: Why no VM on a 68K? (was: Re: Why 68000?)
Keywords: MMU virtual memory Sys. 7.0 low-end
Message-ID: <1990Feb19.150030.4221@phri.nyu.edu>
Date: 19 Feb 90 15:00:30 GMT
References: <1990Feb11.154304.19943@smsc.sony.com> <3919@hub.UUCP> <10223@hoptoad.uucp> <1990Feb15.155556.5319@uncecs.edu> <19472@dartvax.Dartmouth.EDU> <1672@aurora.AthabascaU.CA>
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Organization: Public Health Research Institute, New York City
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In <1672@aurora.AthabascaU.CA> rwa@cs.AthabascaU.CA (Ross Alexander) writes:
> 7th edition requires protection and relocation, nothing more.
> Restartable instructions aren't needed; PDP-11's didn't have them.

	I fear we're going to degenerate into historical nit-picking, but I
don't think that's quite right.  Some 11's did indeed have restartable
instructions.  If you had the full-blown 18-bit memory management unit
(like an 11/45) there was a register which kept track of which GP registers
had been incremented (or decremented), and by how much, before an
instruction faulted.  With that information, you could back out any state
changes the partially-executed instruction might have caused and restart.

	I remember mucking around in the 6th edition kernel to get it to
run on an 11/34 (which was sort of, but not quite, an 11/45 when it came to
memory management).  In the code that dealt with growing the stack, there
are some comments to the effect that lacking the inc/dec status information
(which the 11/34 indeed lacked), you could almost always intuit what
registers had been changed by examining the contents of the stack and
looking at the instruction that had just been executed.  There were some
pathalogical cases, however, where it was not possible, and in that case
you had no choice but to kill the faulting process.
--
Roy Smith, Public Health Research Institute
455 First Avenue, New York, NY 10016
roy@alanine.phri.nyu.edu -OR- {att,philabs,cmcl2,rutgers,hombre}!phri!roy
"My karma ran over my dogma"