Path: 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

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!"