Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!gem.mps.ohio-state.edu!apple!chuq
From: chuq@Apple.COM (Chuq Von Rospach)
Newsgroups: comp.sys.mac
Subject: Re: PMMU...
Message-ID: <35041@apple.Apple.COM>
Date: 26 Sep 89 22:30:10 GMT
References: <32679@srcsip.UUCP>
Distribution: usa
Organization: Life is just a Fantasy novel played for keeps
Lines: 41

>>     Unfortunately, none of this excellent memory management hardware
>>does you any good unless you run A/UX.  The regular Mac operating system
>>just turns any memory management unit off and runs in "real mode".

Unless you buy the Virtual INIT.

>>Even
>>release 7, much to the disappointment of people who don't like system
>>crashes, doesn't use the MMU.

Not true. System 7 will use the MMU to implement virtual memory. As has been
mentioned about 4,000,000 times on c.s.m in the last few months.

>>On a machine with memory management and
>>a solid operating system, an application can't crash the system no matter
>>what it does.  Essentially all workstation-class machines (Suns, Apollos,

Um, that's a nice theoretical ideal, but I spent enough time at Sun crashing
machines to know that applications *can* crash the system, even when they're
not supposed to be able to. 

>Sorry dude, you almost got it but not quite.  System 7 implements virtual
>memory which REQUIRES a PMMU (either the 030's own or the 851 for the 020).

Just to make sure it's clear. System 7 does not require a PMMU. To use
virtual memory with System 7 does. (there's enough confusion on that point
that it needs to be reiterated)

>What System 7 does not implement is protected memory where the MMU is used
>to prevent code in one address space from writing directly to another 
>address space. 

As I said above, even *with* protected memory applications can do all sorts
of nasty things. They aren't supposed to be able to, but sometimes they do
anyway. (those are called bugs, of course....)

-- 

Chuq Von Rospach <+> Editor,OtherRealms <+> Member SFWA/ASFA
chuq@apple.com <+> CI$: 73317,635 <+> [This is myself speaking. I am not Appl
Segmentation Fault. Core dumped.