Article View: comp.arch
Article #21918Re: Integer Multiply/Divide
From: Randell Jesup
Date: Tue, 02 Jan 1990 23:26
Date: Tue, 02 Jan 1990 23:26
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1256 bytes
In article <4057@brazos.Rice.edu> preston@titan.rice.edu (Preston Briggs) writes:
>Division is slightly more complex, due to sign handling.
>For most languages, it isn't safe to replace a divide by 2^N
>with a shift right (doesn't work for negative numbers).
>The same problem arises when replacing I % 4 with I & 3.
Easy solution: x/2^n = (x >> n) + (x & 1 & (bit 31 of x)). Of course,
the >> operation must be "arithmetic" and not "logical" (i.e. sign-extending).
For a machine with slow divides and a barrel shifter (many risc chips),
this may be faster than a divide subroutine/instruction. Also note it avoids
any branches.
--
Randell Jesup, Keeper of AmigaDos, Commodore Engineering.
{uunet|rutgers}!cbmvax!jesup, jesup@cbmvax.cbm.commodore.com BIX: rjesup
Common phrase heard at Amiga Devcon '89: "It's in there!"
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Path: arc1-ca.pugleaf.net!utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cbmvax!jesup From: jesup@cbmvax.commodore.com (Randell Jesup) Newsgroups: comp.arch Subject: Re: Integer Multiply/Divide Message-ID: <9193@cbmvax.commodore.com> Date: 2 Jan 90 23:26:50 GMT References: <84768@linus.UUCP> <4057@brazos.Rice.edu> Reply-To: jesup@cbmvax.commodore.com (Randell Jesup) Organization: Commodore, West Chester, PA Lines: 16