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From: peter@sugar.UUCP (Peter da Silva)
Newsgroups: comp.sys.amiga
Subject: Re: The Next Generation
Message-ID: <1131@sugar.UUCP>
Date: Sat, 28-Nov-87 10:13:23 EST
Article-I.D.: sugar.1131
Posted: Sat Nov 28 10:13:23 1987
Date-Received: Tue, 1-Dec-87 01:05:07 EST
References: <2785@megaron.arizona.edu> <17218@glacier.STANFORD.EDU> <517@zippy.eecs.umich.edu>
Organization: Sugar Land UNIX - Houston, TX
Lines: 52
Keywords: MMU paging swapping
Summary: Yet again: VM != protection.

In article <517@zippy.eecs.umich.edu>, pla@zippy.eecs.umich.edu (Paul Anderson) writes:
> >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.

Again, VM != mapped memory or protected memory.

> 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*.

One major thing that seperates Suns & Apollos from my Amiga is they don't
guarantee any degree of binary object portability. We can't go to a new
machine if it means nuking all the software currently out. Not on a consumer
box. Some design flaws are now cast in concrete. It's a pity, but it could be
a lot worse. Look at the design flaws IBM has to deal with.

So: interprocess protection, yes.

> 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.

OK. Let's call what US have been calling VM paged memory. and what Y'ALL
have been calling VM mapped memory. Deal? ALL the current software out there
requires that memory not be remapped, so that's out. Paging, it seems, may
not be.

> 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.

Paging is a LOT more work than memory protection. However, C= seems to have
a good paging scheme in mind.

ModifyIDCMP(MMU, MMU->IDCMPFlags | WAIT_AND_SEE);

> 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.

Well, you're not gonna get rid of MEMF_PUBLIC without breaking all the
software currently out there.
-- 
-- Peter da Silva  `-_-'  ...!hoptoad!academ!uhnix1!sugar!peter
-- Disclaimer: These U aren't mere opinions... these are *values*.