Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!uwm.edu!bionet!agate!volga.Berkeley.EDU!c186bx
From: c186bx@volga.Berkeley.EDU (Dan X. Filner)
Newsgroups: comp.sys.mac.system
Subject: Re: Virtual 32-bit clean ROMs
Message-ID: <1991Apr21.025813.5339@agate.berkeley.edu>
Date: 21 Apr 91 02:58:13 GMT
References: <3336@borg.cs.unc.edu>
Sender: root@agate.berkeley.edu (Charlie Root)
Reply-To: c186bx@volga.Berkeley.EDU.UUCP (Dan X. Filner)
Organization: UC Berkeley Experimental Computing Facility (XCF)
Lines: 32

"danm" wrote :
>The beauty of virtual memory is that ALL of your memory can be virtual, not
>just your RAM.  One possibility is to make your ROM virtual also.  Simply
>load an image of a 32-bit clean ROM from your boot disk into RAM and remap the
>the virtual to physical address translation of the ROMs to the RAM containing
>the ROM image.  I would be more than happy to eat 512K of real RAM in order
>to get large virtual memory.

That is an excellent point - and a very comforting thought! 
No MAC with a PMMU need ever worry about ROM upgrades -
unless... if devices on the NuBus were using hooks to where the
ROMS were physically supposed to be then it wouldn't work too well.
But I don't think that's the case...

The terminology is a little misleading however - you wouldn't really be 
"Virtualizing" the ROMS - you'd only be mapping some RAM to where the ROMS
really are...
You couldn't really make the ROM's "virtual" since the virtual memory
handler is probably going to rely on the code in the ROM's -
you can't page your pager out, that's fer sure...

Problems would seem to arise only in Legal form- if Apple were as 
easygoing with it's ROM upgrades as with the System then we could all rest
happily... as I understand it Apple really, really, really doesn't like
the idea of letting ROM's be copied/publicly available. Letting every 
mid-lifed Mac copy ROMS on a free-ware basis might sort of stick in
their throats. I hope not.

If anyone knows why this scheme wouldn't work, please let me(us all) know!

Dan Filner
c186bx@volga.berkeley.edu