Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!wuarchive!udel!nigel.ee.udel.edu!mccalpin From: mccalpin@perelandra.cms.udel.edu (John D. McCalpin) Newsgroups: comp.arch Subject: Re: IEEE floating point Message-ID: Date: 26 May 91 14:02:57 GMT References: <9105250030.AA08036@ucbvax.Berkeley.EDU> <1991May25.222551.16365@zoo.toronto.edu> Sender: usenet@ee.udel.edu Organization: College of Marine Studies, U. Del. Lines: 28 Nntp-Posting-Host: perelandra.cms.udel.edu In-reply-to: henry@zoo.toronto.edu's message of 25 May 91 22:25:51 GMT >>> On 25 May 91 22:25:51 GMT, henry@zoo.toronto.edu (Henry Spencer) said: Henry> I will confine myself to observing that IBM hex FP is the only Henry> FP format I know of that made half the FP instructions -- the Henry> single-precision ones -- just about useless to most Henry> programmers. I have to point out that the Cyber 205/ETA-10 32-bit FP is worse than the IBM's hex FP --- at least for the cases I tested. I discussed some of this in a paper in Supercomputer in 1987 or 1988 (issue 24, I believe). The more interesting part was an analysis of the roundoff error in the 1000x1000 LINPACK benchmark, which unfortunately did not make it into the paper in its completed form. Without looking up my answers in detail, on the 1000x1000 test case, IEEE machines got 4 significant digits of accuracy, the IBM hex format got about 3, and the Cyber 205 got 1. Since the error is due to rounding modes and truncation, this same loss of accuracy extends to the 64-bit format, which got about 8 digits of accuracy compared to 12 for IEEE. This comparison is not completely fair, since the Cyber 205 64-bit has a bigger exponenent and smaller mantissa, but the difference is nothing like 4 decimal digits.... -- John D. McCalpin mccalpin@perelandra.cms.udel.edu Assistant Professor mccalpin@brahms.udel.edu College of Marine Studies, U. Del. J.MCCALPIN/OMNET