Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!sharkey!edsews!uunet!mcvax!dik
From: dik@cwi.nl (Dik T. Winter)
Newsgroups: comp.arch
Subject: Re: Endian wars
Message-ID: <7858@boring.cwi.nl>
Date: 27 Jan 89 23:28:44 GMT
References: <6133@columbia.edu> <5124@aldebaran.UUCP>
Organization: CWI, Amsterdam
Lines: 22

In article <5124@aldebaran.UUCP> jimp@cognos.UUCP (Jim Patterson) writes:
 > word-oriented algorithm to be effective here, you need an algorithm
 > which detects which byte of a word (if any) contains a NUL.  
 > 
 > I contend that there's no simple way to do this with integer
 > instructions; it's more effective to use byte-oriented instructions.
 > The only word-oriented approaches I can think of are along these
 > lines.
 > 
 >    has_NUL = ! (i & 0xff && i & 0xff00 && i & 0xff0000 && i & 0xff000000);
 > 
On a ones complement machine (because of end-around carry):
      has_NUL = (~i ^ (i - 0x01010101)) & 0x01010101
On a twos complement machine you have to look at the carry bit too.
However you might also try:
      j = i - 0x01010101;
      has_NUL = ((~i ^ j) & 0x01010101) | (~j & i & 0x80000000)
modulo some typos of course.
-- 
dik t. winter, cwi, amsterdam, nederland
INTERNET   : dik@cwi.nl
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