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From: cruess@oakhill.UUCP (Michael Cruess)
Newsgroups: net.micro.68k
Subject: 32 vs. 24 Bit Addresses
Message-ID: <342@oakhill.UUCP>
Date: Mon, 25-Feb-85 10:49:36 EST
Article-I.D.: oakhill.342
Posted: Mon Feb 25 10:49:36 1985
Date-Received: Wed, 27-Feb-85 21:14:11 EST
Organization: Motorola Inc. Austin, Tx
Lines: 13

.
My experience has been that  24  bits  is  barely  acceptable  today  as  a
*physical* address. 28 bits is seen as preferable.  However, 32 bits is the
desired *logical* address size, and this is reason that the MC68020 has  32
address pins.

As an aside, having 32 bit addresses designed into the M68000  Family  from
the beginning has made the instruction set upgrade in the MC68020 easy.  If
the original designers of the MC68000 had restricted the register width and
the  instruction set to the number of address pins they had on the part, it
would have been very painful.

Michael Cruess        {ihnp4,seismo,gatech,ctvax}!ut-sally!oakhill!cruess