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From: robison@brazil (Arch D. Robison)
Newsgroups: comp.arch
Subject: Re: Implementing Interval Arithmetic with IEEE rounding modes
Message-ID: <1991Jun26.025738.18876@shell.shell.com>
Date: 26 Jun 91 02:57:38 GMT
Sender: Arch D. Robison
Organization: Shell Development
Lines: 15

There have been comments that changing the rounding mode on some
machines can be slow.  But does it really need to be changed?
IEEE arithmetic is symmetric with regard to +/-, and we have the identity:

	ceiling(x) = -floor(-x)
	
So by flipping signs, we can stay in round-down mode all the way.
Better yet, instead of storing an interval as (lower,upper),
store it as (lower,-upper) and avoid flipping signs.  Then two 
intervals can be added with a single setting of the rounding mode 
to ``round-down'' and two addition instructions.

Arch D. Robison				robison@shell.com
Bellaire Research Center 		(713)-245-7240
Shell Development Company