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From: nfs@notecnirp.Princeton.EDU (Norbert Schlenker)
Newsgroups: comp.arch
Subject: Re: Integer Multiply/Divide
Message-ID: <22705@princeton.Princeton.EDU>
Date: 3 Jan 90 05:34:01 GMT
References: <84768@linus.UUCP> <4057@brazos.Rice.edu> <9193@cbmvax.commodore.com>
Sender: news@princeton.Princeton.EDU
Reply-To: nfs@notecnirp.UUCP (Norbert Schlenker)
Organization: Dept. of Computer Science, Princeton University
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In article <9193@cbmvax.commodore.com> jesup@cbmvax.commodore.com (Randell Jesup) writes:
>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).
>
>	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).

This doesn't work.  Consider -10/2^3, which results in -2.

>Randell Jesup, Keeper of AmigaDos, Commodore Engineering.