Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!pt.cs.cmu.edu!sei!firth
From: firth@sei.cmu.edu (Robert Firth)
Newsgroups: comp.arch
Subject: Re: Making Rounding Modes Usable
Message-ID: <3893@bd.sei.cmu.edu>
Date: 21 Aug 89 12:39:38 GMT
References: <1989Jul21.035825.27704@cs.rochester.edu> <6421@uklirb.UUCP> <21453@cup.portal.com>
Reply-To: firth@sei.cmu.edu (Robert Firth)
Organization: Software Engineering Institute, Pittsburgh, PA
Lines: 23

In article <21453@cup.portal.com> mmm@cup.portal.com (Mark Robert Thorson) writes:
>This reminds of something I once heard one of the two principal architects
>of the NS16032 microprocessor say (quoted approximately):  "There's only
>one use for rounding!  If you run the program once rounding up, then
>run it again rounding down, you get a value for noise in your application."

A good idea, and one that should be used more often.
However, to do it properly, you need interval arithmetic,
with appropriate rounding.  I'd give a lot for a machine
with good hw support for interval arithmetic.

>This also reminds me of a table I once saw in a research journal for
>economists.  It compared the results of running the same program (doing
>multiple regression analysis) in several different environments.  If I

That brings back memories!  Over a decade ago, a colleague and I
took a subset of an economic model (the so-called 'Treasury model'
of the UK economy), recoded it to use software interval arithmetic,
and ran some calculations.  The general result was that the output
error intervals were large enough to swamp the real numbers, things
like 'if you cut income taxes by 5%, next year's unemployment will
be 7%, plus or minus 20'.  And the government was actually using
this thing to help make policy.