Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!oliveb!sun!dgh!dgh
From: dgh%dgh@Sun.COM (David Hough)
Newsgroups: comp.arch
Subject: Re: Bandwidth and RISC vs. CISC
Summary: Alliant and IEEE?
Message-ID: <101087@sun.Eng.Sun.COM>
Date: 25 Apr 89 19:38:06 GMT
References: <38853@bbn.COM> <423@bnr-fos.UUCP> <17417@cup.portal.com> <39095@bbn.COM>
Sender: news@sun.Eng.Sun.COM
Lines: 40

In article <39095@bbn.COM>, slackey@bbn.com (Stan Lackey) writes:
> The new micros (at least the i860) trap
> and expect software to fix things up, which includes parsing the
> instructions in the pipe, and fixing up the saved version of the
> internal data pipeline.  I've seen statements in this newsgroup like
> "not usable in a general purpose environment" when referring to the
> i860.  

I agree.  The i860 appears never to have been intended to support
an efficient implementation of IEEE 754.

> In the Alliant we wanted to get the
> design done, and fit it on one board, so we shut denorms off.
... 
> Re: one or two-cycle DP IEEE mul/add exist: Alliant is the only one I

Regardless of how fast you do the arithmetic, if

	(x != y)
and
	(x-y) != 0

are not equivalent for finite x, you don't conform to IEEE 754 or 854.
Subnormal numbers permit this equivalence.  754 committee members
were irked in advance, so to speak, by the prospect that some vendors
would claim conformance for such implementations.

> said) the Cray one is difficult to use.  The VAX one is accurate enough 
> and has enough range for normal use, and if F or G aren't enough, there's
> always H :-)

The VAX standard is D format double precision, not G.  Many people
consider it inadequate because unlike a pocket calculator it won't
accommodate 10+-99.

David Hough

dhough@sun.com   
na.hough@na-net.stanford.edu
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