Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uwm.edu!zaphod.mps.ohio-state.edu!swrinde!mips!pacbell.com!pacbell!osc!jgk From: jgk@osc.COM (Joe Keane) Newsgroups: comp.arch Subject: Re: Why FP at all? (was: Re: Killer Micro II) Message-ID: <3790@osc.COM> Date: 11 Sep 90 11:00:07 GMT References: <14900015@hpdmd48.boi.hp.com> <3961@bingvaxu.cc.binghamton.edu> Reply-To: jgk@osc.COM (Joe Keane) Organization: Versant Object Technology, Menlo Park, CA Lines: 29 In article <3961@bingvaxu.cc.binghamton.edu> vu0310@bingvaxu.cc.binghamton.edu.cc.binghamton.edu (R. Kym Horsell) writes: >Good G*d! The IEEE std _was_ intended to produce some kind of uniformity >in results across different kind of h/w. (A rather like the idea >of getting results accurate to the _bit_ in FP calcs). You could argue that this is bad not good. Suppose you ran a computation on machine X and it gives the answer -37.69, then you moved to machine Y and it gives the same answer. This might give you some unfounded confidence that the answer is actually -37.69. Actually the right answer could be 2.00 but you don't know that. It used to be that if you wanted to check an answer you would run it on a different machine, but now this doesn't do much for you. In fact you could argue that if you're going to do rounding, the best way to do it is randomly. A couple of free-running oscillators on your FP chip wouldn't take up too much space. If you did this the error in the expected value could actually be much lower than 1 LSB. If you ran the same program 20 times, you'd get 20 different answers. However, if these agreed well you'd have good reason to believe that the answer is right. I'm not arguing that multiple runs is a substitute for good numerical analysis, but it can point out that something is drastically wrong. The floating point on machine X may be much better than that on machine Z, but if you get a terrible answer from machine Z, i wouldn't trust the answer from machine X so much either. I'm sort of playing devil's advocate here. Actually i think the IEEE standard is very good as far as FP goes. However, if you're dealing with inherently inaccurate computations, a little diversity may be a good thing.