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From: baum@apple.UUCP (Allen J. Baum)
Newsgroups: comp.arch
Subject: Re: Multiply as Shift-Add(Subtract)
Message-ID: <761@apple.UUCP>
Date: Wed, 13-May-87 16:33:32 EDT
Article-I.D.: apple.761
Posted: Wed May 13 16:33:32 1987
Date-Received: Sat, 16-May-87 06:57:04 EDT
References: <1270@aw.sei.cmu.edu> <8012@utzoo.UUCP> <16640@amdcad.AMD.COM> <27685@rochester.ARPA> <6651@mimsy.UUCP>
Reply-To: baum@apple.UUCP (Allen Baum)
Organization: Apple Computer, Inc.
Lines: 19

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The HP Spectrum has instructions that shift an index depending on
whether the load/store is byte/halfword/word/doubleword. This
hardware is also used for a set of general 'shift0/1/2/3 & add'
instructions. Each has a variation that traps on overflow, where
overflow is either an addition overflow or a shift overflow. (Sees if
bits shifted out=sign)

These instructions are sufficient to multiply by constants<3800 in
six instructions, and average performance of two 32-bit variables is
under 20 cycles.

Most of this work was done by Dan Magenheimer at HP, and there will be a
publication describing the work.

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