Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uwm.edu!linac!att!princeton!phoenix!bskendig
From: bskendig@phoenix.Princeton.EDU (Brian Kendig)
Newsgroups: comp.sys.mac.programmer
Subject: Re: System 7.0/Supervisor Mode ?
Message-ID: <9432@idunno.Princeton.EDU>
Date: 11 May 91 18:55:30 GMT
References: <1991May10.175654.812@eng.umd.edu> <13455@goofy.Apple.COM>
Sender: news@idunno.Princeton.EDU
Organization: Starfleet Academy: Princeton University
Lines: 41

In article <13455@goofy.Apple.COM> Greg@AppleLink.Apple.Com (Greg Marriott) writes:
>In article <1991May10.175654.812@eng.umd.edu>, macgreg@eng.umd.edu (Scott J. MacGregor) writes:
>> Do applications running under system 7.0 run in supervisory or user mode
>> ofthe 680x0 processor?
>
>If VM is turned on, then applications run in user mode.  If VM is off, then
>applications run in supervisor mode (like they always have...)

Hmm, now, let me get this straight -- is it true that the only visible
difference between supervisor mode and user mode is that a process
running in supervisor mode has the ability to crash in a really bad
way and merrily trounce through all the rest of the machine's memory
and bring the whole machine to a crash, whereas a process running in
user mode is restricted to only having access to a certain range of
memory that it can trash to its heart's content but it is completely
unable to futz with the rest of the machine?

Why do programs run in user mode under VM only?  Is this because VM
does something that makes user mode possible whereas user mode just
isn't feasible when you don't have virtual memory active, or did the
System 7.0 developers just decide for whatever reason that they didn't
feel like having things run in user mode normally?

What I'm getting at: If running things in user mode means my machine
will be a bit more resistant to system crashes, I think it's something
I'd like to have the ability to use -- but my little old SE doesn't do
VM, of course, so right now I guess I have to keep using supervisor
mode.  How difficult a modification would it be to have my SE use user
mode, too?  Is this a matter of a simple INIT that would just toss my
machine into user mode without worrying about VM, or am I talking
about something that would require a major rewrite of the operating
system here?

Any clues are gladly accepted.  ;)

     << Brian >>

| Brian S. Kendig      \ Macintosh |   Engineering,   | bskendig             |
| Computer Engineering |\ Thought  |  USS Enterprise  | @phoenix.Princeton.EDU
| Princeton University |_\ Police  | -= NCC-1701-D =- | @PUCC.BITNET         |
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