From djh@ccl.net Tue Jun 25 14:40:49 1991
Date: Tue, 25 Jun 91 13:34:57 -0400
From: djh@ccl.net
Subject: floating point precision
To: chemistry@ccl.net
Status: R

Kahan's "paranoia" is an interesting program that determines epsilon,
as well as a host of other parameters, and makes a judgement on the
quality of the machine's fp design and implementation.  [It thoroughly
pans Crays!]
 
It's available from netlib.  Send mail to
 
    netlib@ornl.gov
    Subject: send index from paranoia
 
to get an overview of paranoia and directions on how to get various
language versions of the program.
 
You have to be careful in determining epsilon with a simple program.
The Intel coprocessors, for example, work in 80 bit extended format,
so depending on your compiler, you may get an epsilon of 2 ** (-52)
if values get stored to memory, or 2 ** (-63) if they do not!  The
latter value isn't going to be achieved in most real programs.
 
I think for all IEEE fp, appropriate values are 2 ** (-23) for single
                                                2 ** (-52) for double
--
David J. Heisterberg		djh@ccl.net		We are NOT all
The Ohio Supercomputer Center	djh@ohstpy.bitnet	Keynesians now.
Columbus, Ohio  43212		ohstpy::djh