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From: jss@sjuvax.UUCP (J. Shapiro)
Newsgroups: net.arch,net.micro.16k,net.micro.68k
Subject: Re: 24 bit vs. 32 bit address space
Message-ID: <927@sjuvax.UUCP>
Date: Thu, 7-Mar-85 20:41:44 EST
Article-I.D.: sjuvax.927
Posted: Thu Mar  7 20:41:44 1985
Date-Received: Mon, 11-Mar-85 11:13:21 EST
References: <983@watdcsu.UUCP> <2385@nsc.UUCP> <730@amdcad.UUCP>
Organization: Saint Josephs Univ. Phila., Pa.
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[Pacman's revenge...]
	It has been suggested that with the MMU, the 320xx chips allow 32
Mbytes of process space.  This is true, but misleading.  The A24 line is
determined by  a bit set in the MMU, and unless the operating system
provides you with the ability to set that bit (it is intended for
supervisor address space protection), you are out of luck... Also, unless
you do some funky stuff with the page tables, setting that bit will
suddenly cause the processor to have trouble finding your code...

	The right answer to 32 bits is to use the 68020 or 68012 if your need
is immediate.  Also, check out the NCR/32 series. If your need is less
immediate, hold off until the 32532 appears.  It looks as though that will
prove a very interesting chip.

	In the meantime, I can think of many things I can do 24 bits that I
can't do on a Macintosh.  I can also think of things I can't do on a VAX
unless it is in single user mode....


Jon Shapiro
Haverford College