Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!att!alberta!uqv-mts!ualtavm.bitnet!ECULHAM
From: ECULHAM@UALTAVM.BITNET (Earl Culham)
Newsgroups: comp.arch
Subject: Re: Understanding variations in Dhrystone performance
Message-ID: <483@UALTAVM.BITNET>
Date: 17 May 89 18:00:04 GMT
References: <474@estevax.UUCP> <1989May15.173631.3029@utzoo.uucp>
Reply-To: ECULHAM@UALTAVM.BITNET
Organization: University of Alberta VM/CMS
Lines: 17
Disclaimer: Author bears full responsibility for contents of this article

In article <1989May15.173631.3029@utzoo.uucp>, henry@utzoo.uucp (Henry Spencer) writes:
 
<In article <474@estevax.UUCP> wck353@estevax.UUCP (HrDr Weicker Reinhold ) writes:
<>... Note that
<>processors with an instruction that checks a word for a null byte (such
<>as AMD's 29000 and Intel's 80960) have an advantage here...
<
<Only a small one; you can do the same check on a machine without the
<fancy instruction by being clever.  Consider:
<
<        (((x & ~0x80808080) - 0x01010101) & 0x80808080)
<
<The result is nonzero if, and only if, there was a NUL byte in x.
                       EEEEEEEEEEEEEEE
 
Actually, this also causes a false trigger if any byte contains
X'80'.