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From: pla@zippy.eecs.umich.edu (Paul Anderson)
Newsgroups: comp.sys.amiga
Subject: Re: The Next Generation
Message-ID: <517@zippy.eecs.umich.edu>
Date: Tue, 24-Nov-87 14:16:52 EST
Article-I.D.: zippy.517
Posted: Tue Nov 24 14:16:52 1987
Date-Received: Fri, 27-Nov-87 23:56:34 EST
References: <2785@megaron.arizona.edu> <17218@glacier.STANFORD.EDU> <360@usl-pc.UUCP> <1099@sugar.UUCP>
Sender: usenet@zippy.eecs.umich.edu
Reply-To: pla@zippy.eecs.umich.edu (Paul Anderson)
Organization: University of Michigan EECS Dept. Ann Arbor
Lines: 41
Keywords: MMU paging swapping
UUCP-Path: ihnp4!umich!zippy!pla


>Swap them.
>I know swapping is not as chic as paging, but it should be possible.
certainly is, but you might as well go the rest of the way.
>
>Here's my fantasy for the Amiga:
>68020 with MMU. NO VM.
There is no point in providing an MMU if tasks can still nuke each
other.
>-- Peter da Silva  `-_-'  ...!hoptoad!academ!uhnix1!sugar!peter
>-- Disclaimer: These U aren't mere opinions... these are *values*.

I have lots of respect for your fantasies, but the major thing that
separates a multi-tasking machine like my Amiga from a multi-
tasking machine like my Apollo is interprocess *protection*.

The whole point of a MMU is to allow potentially buggy programs
to run without making other programs crash.  While protected
memory can do this, you might as well go whole hog and add
virtual (address remapping) memory capability.  And, if you
are going to do that, you might as well add paging, too,
since it is a good way of insulating you from memory shortages,
especially ones that might be transient in nature.  Granted,
that isn't going to fix disk space shortages, but those
are generally cheaper to deal with than memory shortages,
anyhow.

The Amiga is a wonderful machine, given its lack of a MMU,
but if it is going to try and compete in functionality with
Suns and Apollos, it is really going to have to take full
advantage of something like the 68851 PMMU.

My feeling is that it is a complete waste of time to talk
about adding an MMU without removing the need for MEMF_PUBLIC,
since no amount of address translation is going to overcome
this "security" hole.  I don't mind having publicly accesible
memory, mind you, but if the OS depends on it, that is
bad news.

Sorry for the length...

 Paul Anderson