Path: utzoo!attcan!uunet!aplcen!uakari.primate.wisc.edu!zaphod.mps.ohio-state.edu!usc!apple!apple.com!esmith
From: esmith@apple.com (Eric Smith)
Newsgroups: comp.sys.mac.system
Subject: Re: Which ROMs are 32-bit clean?
Message-ID: <9054@goofy.Apple.COM>
Date: 9 Jul 90 18:54:55 GMT
Sender: usenet@Apple.COM
Organization: Amalgamated Conglomerates
Lines: 38
References:<1935@runxtsa.runx.oz.au> <9048@goofy.Apple.COM>

In article <9048@goofy.Apple.COM> alexr@ucscb.ucsc.edu (Alexander M. 
Rosenberg) writes:
> In article <1935@runxtsa.runx.oz.au> johnr@runxtsa.runx.oz.au (John 
> Rotenstein) writes:
> > System 7.0 requires a 32-bit clean ROM to operate virtual memory above 
> 14MB.
> > With such a ROM, it can go to 1 gigabyte.
> 
> Don't you mean 4 gigabytes? :-)

Well, he does and he doesn't.  There is only 1 gigabyte of address space
allocated contiguously from addresses $00000000 through $3FFFFFFF for RAM.
It is possible to have RAM on NuBus cards from $60000000 through $EFFFFFFF
(superslot space for slots $6 through $E) and from $F1000000 through
$FEFFFFFF (slot space for slots $1 through $E).  Note that not all of these
slots exist on any given Mac, although to my knowledge nothing but
courtesy prevents a card in one slot from responding to addresses reserved
for another slot.  The total slot and superslot space is therefore 2318
(decimal) megabytes, although some of the slot space is taken up by
declaration ROMs.

To use the slot memory for main RAM (as opposed to a RAM disk or 
something),
it is necessary to either have an PMMU (either as part of the processor or
separate), or make the Memory Manager understand non-contiguous memory.  If
you have a PMMU, you do not necessarily have to have virtual memory to 
remap
the slot memory to be contiguous with main RAM.

Virtual memory in System 7.0 does handle noncontiguous memory in 24-bit
mode in order to allow memory beyond 8 megabytes to be mapped above the
ROMs and IO.  I don't know whether VM supports noncontiguous memory in
32-bit mode.

----
Eric Smith    esmith@apple.com
Opinions expressed herein are not necessarily those of my
employer, family, friends, computer, or even me!  :-)