🚀 go-pugleaf

In article <13822@well.UUCP> nagle@well.UUCP (John Nagle) writes:
[discussion of 680x0 and PMMUs deleted]
>      The MC68020 does not have a memory management unit either, but
>like the 68010, can be provided with an external one.  This is the
>"PMMU" (Paged Memory Management Unit) used in the Mac II.  It's not 
>standard on a Mac II, but is an expensive extra-cost option.  On 
>standard Mac IIs, the MMU socket is filled with a chip which is more
>of a dummy plug than a component; it just passes the addresses through
>unchanged.

The PMMU costs about $350 dollars.  Whether you consider this expensive
in view of what it will get is up to you.

>      The MC68030 has a built-in MMU comparable to (but not
>compatible with) the PMMU.  So all '030-based machines are in theory
>capable of running a virtual memory operating system with protected
>memory.

The 030 MMU implements a subset of the 68851 PMMU (the one you can use
with the 68020). So software written to take advantage of the 030's MMU
should work ok on a 020/851 combo. 

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

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).
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. 
____________________________________________________________________
Have a day. :^|
Murat N. Konar        Honeywell Systems & Research Center, Camden, MN
mnkonar@SRC.honeywell.com (internet) {umn-cs,ems,bthpyd}!srcsip!mnkonar(UUCP)