Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site bonnie.UUCP Path: utzoo!watmath!clyde!bonnie!jww 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