Path: utzoo!attcan!uunet!samsung!sdd.hp.com!decwrl!nsc!taux01!amos
From: amos@taux01.nsc.com (Amos Shapir)
Newsgroups: comp.arch
Subject: F.P. vs. arbitrary-precision (was: Killer Micro II)
Message-ID: <4513@taux01.nsc.com>
Date: 6 Sep 90 15:08:31 GMT
References: <3755@osc.COM>
Organization: National Semiconductor (IC) Ltd, Israel, Home of the Series 32000
Lines: 40
X-Hdate: 16 Elul 5750

[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.

>Our current programming languages have a strong influence.  C has `float' and
>`double' types, and most machines have single-precision and double-precision
>floating point numbers.  Coincidence?  I think not.

Certainly not, but you got it backwards - C has it because PDP-11 has it
because FORTRAN has it because the early IBM machines had it...


>I don't care if you have 65536-bit floating point, it's still floating point.
>That means underflow, overflow, round-off error, loss of precision, need i
>continue?
...
>Another interesting area is hardware support for arbitrary-precision real
>numbers.

But even with arbitrary precision, at some point you have to start disposing
of insignificant bits - and suffer the same side-effects.

-- 
	Amos Shapir		amos@taux01.nsc.com, amos@nsc.nsc.com
National Semiconductor (Israel) P.O.B. 3007, Herzlia 46104, Israel
Tel. +972 52 522255  TWX: 33691, fax: +972-52-558322 GEO: 34 48 E / 32 10 N