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From: jkenton@pinocchio.encore.com (Jeff Kenton)
Newsgroups: comp.sys.m88k
Subject: Re: Fixed point multiply overflow detection
Message-ID: <12439@encore.Encore.COM>
Date: 9 Aug 90 19:23:41 GMT
References: <12425@encore.Encore.COM>
Sender: news@Encore.COM
Lines: 26

From article <12425@encore.Encore.COM>, I wrote:
> 
> 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.
> 

Two more things come to mind on this subject:

	* If you use "ff1" instructions to see how big your arguments
	  are, you discover that there is an indeterminate case where
	  the answer is at least 2^31 but might be > 2^32.  You can't
	  tell for sure without doing as much work as the multiply.

	* Depending on the details of your problem, there may be a
	  way to condition your values so that overflow can't occur.

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