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From: johnl@ima.UUCP (John R. Levine)
Newsgroups: comp.arch
Subject: Re: IEEE floating point modes
Message-ID: <596@ima.UUCP>
Date: Wed, 1-Jul-87 22:43:25 EDT
Article-I.D.: ima.596
Posted: Wed Jul  1 22:43:25 1987
Date-Received: Fri, 3-Jul-87 04:23:13 EDT
References: <93900007@hcx1>
Reply-To: johnl@ima.UUCP (John R. Levine)
Organization: Javelin Software Corporation
Lines: 21

In article <93900007@hcx1> daver@hcx1.SSD.HARRIS.COM writes:
>Of the 4 rounding modes -- nearest, zero, positive infinity, and negative
>infinity -- which one(s) are you most likely to use?  ...

I always use round to nearest. Round to zero ("chop") is mostly useful for
numerical shenanigans or, I suppose, compatibility with previous machines like
the IBM 360/370 that round that way. In general, round to nearest gives you
the effect of storing an extra bit of precision compared to round to zero. On
some micros, your FP unit is so much more powerful than your regular CPU that
you use it for integer calculations, and chop mode could be useful there.

Round up and round down seem only useful for confidence testing -- you run
your program once in round up mode and once in round down mode and the two
answers you get give you an idea of how solid the answers are. The closer
together they are, the more confident you feel.

(Real numerical analysts are welcome to explain why this is all wrong.)
-- 
John R. Levine, Javelin Software Corp., Cambridge MA +1 617 494 1400
{ ihnp4 | decvax | cbosgd | harvard | yale }!ima!johnl, Levine@YALE.something
U.S. out of New Mexico!