Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!apple!oliveb!sun!gammara!khb
From: khb@gammara.Sun.COM (gammara)
Newsgroups: comp.arch
Subject: Re: IEEE 754 vs. 854 (floating-point arithmetic)
Keywords: floating-point exceptions
Message-ID: <107183@sun.Eng.Sun.COM>
Date: 30 May 89 17:39:49 GMT
References: <18721@cup.portal.com> <6@pzbaum.uucp>
Sender: news@sun.Eng.Sun.COM
Reply-To: khb@sun.UUCP (gammara)
Organization: Sun Microsystems, Mountain View
Lines: 37

In article <6@pzbaum.uucp> jimv@pzbaum.UUCP (Jim Valerio) writes:
>
>I believe that IEEE 854's permission to return an infinity or zero in the
>case of massive overflow or underflow, rather than IEEE 754's QNaN, was an
>attempt to correct a perceived mistake in the 754 exception handling.  If
>you need to choose a result here, I recommend returning the infinity or
>zero result, and not the QNaN.

754 machines return denormalized numbers for underflow ... not QNaN.
Massive overflow yeilds +- inf not QNaN. 0/0 yeilds NaN.
>
>I understand that there is a review of IEEE 754 coming up in the next few
>years.  Personally, I'm looking forward to arguing for a number of modest
>changes, including:
>   (1)	Replace subnormal/denormal numbers by a single value (call it epsilon
>	or infinitesimal), and define arithmetic underflow in terms of this
>	limiting value.
>   (2)	Remove bias adjusted results for trapped overflow and underflow, and
>	always return either infinity or epsilon when overflow or underflow
>	occurs.
>-- 

for 32-bit machines the subnormization process is a good thing (if one
cares about getting the right answer). For better or worse, most folks
don't care about getting it right; or if they do they use 64-bit math.

Kahan has been pitching a pre-substitution scheme. Those interested
should contact him for details.





Keith H. Bierman      |*My thoughts are my own. Only my work belongs to Sun*
It's Not My Fault     |	Marketing Technical Specialist    ! kbierman@sun.com
I Voted for Bill &    |   Languages and Performance Tools. 
Opus  (* strange as it may seem, I do more engineering now     *)