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From: cdl@mplvax.UUCP (Carl Lowenstein)
Newsgroups: net.micro,net.micro.68k,net.micro.16k
Subject: Re: Floating Point Comparisons
Message-ID: <178@mplvax.UUCP>
Date: Fri, 5-Apr-85 13:06:25 EST
Article-I.D.: mplvax.178
Posted: Fri Apr  5 13:06:25 1985
Date-Received: Sun, 7-Apr-85 04:31:34 EST
References: <133@cfa.UUCP> <6370@boring.UUCP>
Reply-To: cdl@mplvax.UUCP (Carl Lowenstein)
Distribution: net
Organization: Marine Physical Laborator of SIO at UCSD
Lines: 25
Xref: watmath net.micro:9950 net.micro.68k:710 net.micro.16k:314
Summary: 

In article <6370@boring.UUCP> jack@boring.UUCP (Jack Jansen) writes:
>
>In article <133@cfa.UUCP> ward@cfa.UUCP (Steve Ward) writes:
>>     Microprocessor           FADD  FSUB  FMUL  FDIV  DADD  DSUB  DMUL  DDIV
>>NS32016/NS32081 10 MHZ        7.40  7.40  4.80  8.90  7.40  7.40  6.20 11.90
>>
>Is this true? It *does* sound funny to me that additions and
>subtractions take longer than multiplies....
>	Jack Jansen, {decvax|philabs|seismo}!mcvax!jack

If you think about it, floating-point addition and subtraction are much
more difficult than multiplication.  Multiplication involves only
extraction and addition of the exponents, and multiplication of the
fractions, with about a 25% probability of a single-bit renormalization.
Addition requires the same extraction of exponent and fraction, followed
by a variable shift to align binary points, the actual addition of
the fractions, and then possibly a lot of renormalization, since the
sum might overflow by one bit position, or underflow a lot due to
cancellation between positive and negative operands.  All this shifting
for normalizing takes time.


-- 
	carl lowenstein		marine physical lab	u.c. san diego
	{ihnp4|decvax|akgua|dcdwest|ucbvax}	!sdcsvax!mplvax!cdl