Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!clyde!rutgers!sri-spam!ames!cit-vax!amdahl!sjl
From: sjl@amdahl.UUCP
Newsgroups: comp.arch
Subject: Re: Success of IEEE Floating Point Standard (IEEE 754)
Message-ID: <5592@amdahl.UUCP>
Date: Sat, 7-Feb-87 07:10:14 EST
Article-I.D.: amdahl.5592
Posted: Sat Feb  7 07:10:14 1987
Date-Received: Sun, 8-Feb-87 06:05:25 EST
References: <760@orcisi.UUCP> <112@lmi-angel.UUCP> <172@ames.UUCP> <203@ames.UUCP> <5307@amdahl.UUCP> <825@cca.ucsf.edu>
Reply-To: sjl@amdahl.UUCP (Steve Langdon)
Organization: Amdahl Corp, Advanced Systems Planning
Lines: 50
Keywords: IEEE754 supercomputer standards

In article <825@cca.ucsf.edu> root@cca.ucsf.edu (Computer Center)
[actually (thos@cca.ucsf.edu) (Thos Sumner)] replies to my posting which
indicated that my former colleagues at Control Data believed IEEE 754
was difficult to implement in a high end machine.

Thos speculated that design flaws caused by incorrect results from a CAD
program resulted in a catastrophic accident.  The CAD program relied on
the properties of IEEE 754, but produced incorrect results because it
was run on a computer with a different floating point format.  The
computer manufacturer and designers were held responsible and punished.

This is a nice piece of melodrama, but it is about as sensible as the
construction company in Florida who sued Lotus because 1-2-3 failed to
guess which numbers to add in a spreadsheet.  I will leave it to readers
to find the holes in the scenario.  The interesting issue behind the
fiction is that in the past, bad numerical techniques made the results
of many important programs suspect at best.  I can remember a number of
horror stories about nuclear reactor design codes that gave
significantly different answers when moved from a 6600 to a 7600.  My
memory tells me that the least significant bit (these machines used a 60
bit word) of a result was sometimes different on the 7600 from the same
operation on a 6600.

I am unqualified to judge the merits of IEEE 754, but the skill of the
designers makes me confident that it is the best format available.  A
good floating point format will not, however, compensate for badly
chosen algorithms.  IEEE 754 cannot prevent bad programs from giving
the wrong answer, although it will help an informed user to detect many
problems.

I received a couple of good replies to my posting by mail which made me
conclude that I had not clearly explained "*very* fast floating point".
This phrase was intended to imply supercomputer performance.  One reply
mentioned that ELXSI offers IEEE 754 compatibility, but the per
processor performance of the ELXSI machine is well below supercomputer
speeds.  I hesitate to attempt to define a supercomputer, but my
comments were based on the design of machines with 80+ MFLOP peak
rates.

I do not need to be convinced that standards are valuable - the time I
spend at ANSI and ISO meetings is based on this belief.  Standards are
not, however, without their limitations.  One of the important clauses
in an ISO standard is the "Scope and Field of Application".  My comments
on IEEE 754 were intended to remind readers that failure to conform
might be based on technical reasons, rather than a desire to deny users
the advantages offered by the standard.
-- 
Steve Langdon  ...!{decwrl,sun,hplabs,ihnp4,cbosgd}!amdahl!sjl  +1 408 746 6970

[I speak for myself not others.]