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From: jww@bonnie.UUCP (Joel West)
Newsgroups: net.micro.68k
Subject: Re: 32 vs. 24 Bit Addresses
Message-ID: <429@bonnie.UUCP>
Date: Fri, 1-Mar-85 12:01:12 EST
Article-I.D.: bonnie.429
Posted: Fri Mar  1 12:01:12 1985
Date-Received: Sat, 2-Mar-85 02:55:15 EST
References: <342@oakhill.UUCP>
Organization: CACI -- La Jolla, Calif.
Lines: 27

>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

Of course, hardware upward compatibility doesn't guarantee software
compatibility.  A number of 32-bit processors have been crippled by 
operating systems writers who, when they designed their software,
saw only physical hardware with 16mb or less and said "we only need
24 bits".  Typically the upper byte is used for the number of arguments
or argument words.

Gould's port of BSD 4.2 suffers from this malady, and IBM's 370-vintage
MVS is riddled with 24-bit usages -- hence the painful transition now
being felt to MVS/XA, which allows a 32-bit address space.  So before
you're done checking on a system, ask if the SOFTWARE supports all
32-bits (virtual or physical.)
-- 
	Joel West				     (619) 457-9681
	CACI, Inc. - Federal 3344 N. Torrey Pines Ct La Jolla 92037
	jww@bonnie.UUCP (ihnp4!bonnie!jww)
	westjw@nosc.ARPA

   "The best is the enemy of the good" - A. Mullarney