Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uwm.edu!bionet!agate!linus!linus!bs From: bs@linus.mitre.org (Robert D. Silverman) Newsgroups: comp.arch Subject: Re: F.P. vs. arbitrary-precision (was: Killer Micro II) Message-ID: <119244@linus.mitre.org> Date: 6 Sep 90 16:39:49 GMT References: <3755@osc.COM> <4513@taux01.nsc.com> Reply-To: bs@gauss.UUCP (Robert D. Silverman) Organization: The MITRE Corporation, Bedford MA Lines: 36 In article <4513@taux01.nsc.com> amos@taux01.nsc.com (Amos Shapir) writes: :[Quoted from the referenced article by jgk@osc.COM (Joe Keane)] :>I have to agree that 128-bit floating point isn't really such a hot idea. :>When you get right down to it, floating point is a hack. It's a very useful :>hack; i won't argue with that. We admit 64-bit floating point doesn't work, :>so what do we do? We provide more of the same. : :It works for most uses, and "more of the same" really is an improvement :if it's done right. The point is, there isn't yet any better idea :that works as well. : :>There are a lot of machines out there that can do IEEE 64-bit floating point, :>with all its precise rules and cases, but can't multiply two 32-bit integers :>in a reasonable way. What are we to make of this? It's just dumb. : :If they have a good FPU, using it for integer multiplication *is* a "reasonable :way". Besides, a bad implementation doesn't prove anything about the basic :idea of FP. Having a good FPU just isn't good enough. Even with IEEE 64-bit, there are only 53 bits of a mantissa. So just how does one multiply two 32 bit integers together using floating point? The answer is: one can't without losing bits. (or by doing several multiplies on the low/high halves and combining things) Futhermore, if I should need to multiply integers in this way, they must first be converted to floating point, then the product must be converted back. Can you say expensive??? The four basic operations of arithmetic are +, -, x, /. Any computer that can't perform them on its atomic data units [whatever the word size is] is a joke. -- Bob Silverman #include Mitre Corporation, Bedford, MA 01730 "You can lead a horse's ass to knowledge, but you can't make him think"