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From: preston@titan.rice.edu (Preston Briggs)
Newsgroups: comp.arch
Subject: Re: Integer Multiply/Divide
Message-ID: <4081@brazos.Rice.edu>
Date: 3 Jan 90 05:23:21 GMT
References: <84768@linus.UUCP> <4057@brazos.Rice.edu> <9193@cbmvax.commodore.com>
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Reply-To: preston@titan.rice.edu (Preston Briggs)
Organization: Rice University, Houston
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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"

But what about -6/4 ?
 6   = 0...0110
-6   = 1...1010

x>>2 = 1...1110 + (1...1010  & 1 & (x >> 31))
     = 1...1110 + 0
     = 1...1110
     = -2

On the other hand, perhaps I've misinterpreted your intent.

Preston Briggs