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From: jkenton@pinocchio.encore.com (Jeff Kenton)
Newsgroups: comp.sys.m88k
Subject: Re: Fixed point multiply overflow detection
Message-ID: <12425@encore.Encore.COM>
Date: 9 Aug 90 01:02:18 GMT
References: <1990Aug08.182206.22187@unx.sas.com>
Sender: news@Encore.COM
Lines: 21

From article <1990Aug08.182206.22187@unx.sas.com>, by sasrer@mozart.unx.sas.com (Rodney Radford):
> Does anyone know how to check if an overflow occurs with the MULT instruction?
> Since this check will possibly be done very often, it is desired that the
> method be as fast as possible.

The hardware doesn't provide any direct way (as you've noticed).  Two choices
come to mind:

	* use floating point (double precision for enough accuracy)
	  and check the results.

	* use ff1 instructions to find the magnitude of both operands
	  to see if overflow can occur.

Both of these are clearly slower than "multiply and check a status bit",
but those are your choices.

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      jeff kenton  ---	temporarily at jkenton@pinocchio.encore.com	 
		   ---  always at (617) 894-4508  ---
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