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From: rcb@rti-sel.UUCP
Newsgroups: net.micro.68k,net.arch
Subject: Re: Nothing New Here / Re: VAX Floating Point
Message-ID: <1195@rti-sel.UUCP>
Date: Fri, 14-Sep-84 09:18:11 EDT
Article-I.D.: rti-sel.1195
Posted: Fri Sep 14 09:18:11 1984
Date-Received: Tue, 25-Sep-84 02:05:02 EDT
References: fortune.4190 <1191@rti-sel.UUCP>
Lines: 25


	The IBM (Itty Bitty Machines) floating point is NOT the
same as the VAX floating point format. The IBM is hex based floating point
and must be normalized such that the highest digit is not zero. The VAX
floating point is binary based, which means that the highest normalized
fraction digit must not be zero which means that it must be one.
VAX takes advantage of this fact by not storing that bit at all.
Also, any number that is a power or factor or 2 (i.e. 1, 2, 4, .5, .25, etc.)
does not have any bits in the exponent and the floating point hardware
can take advantage of this. A benchmark shows that 100 million floating
multiplications takes 45 seconds when these special values are used and
takes 2.5 minutes when any old numbers are used.

	And, for the numerical analysis types that want greater range,
they have 2 options on the VAX. G format floating point uses 64 bits
and gives a range of .56*10**-308 to .9*10**308 with 15 digits precision.
The standard double floating gives .29*10**-38 to 1.7*10**38 with 16 digits
precision. And for you people who really like big numbers, there is H format
floating point which uses 128 bits to give a range of .84*10**-4932 to
.59*10**4932 with a mighty 33 digits of precision. Enough to satisfy
even the most manical numerical analyst.

					Randy Buckland
					Research Triangle Institute
					...!mcnc!rti!rcb