Message-ID:	<3D8EA8B0.F4EA56@yahoo.com>
From:	CBFalconer <cbfalconer AT yahoo DOT com>
Organization:	Ched Research
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Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Problem with multiplications
References:	<M2%i9.25731$Av4 DOT 488132 AT twister2 DOT libero DOT it>
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Date:	Mon, 23 Sep 2002 14:51:39 GMT
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To:	djgpp AT delorie DOT com
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Carlo Bramini wrote:
> 
> I have a problem with DJGPP, but I'm going to think it's common with all
> i386 target versions of this compiler.
> I have two 32 bit numbers and I want to do:
> 
> c = (a*b) >> 32;
> 
> for getting the upper 32 bit DWORD.
> Unfortunately, it doesn't work!
... snip ...

You need to ensure your numbers never overflow a 32 bit value.  So
break the input up into 16 bit components, so that a = (a1 << 16)
+ a2 and b = (b1 << 16) + b2, and do your arithmetic on the 16 bit
components.  You will have to devise a way of combining them into
a 64 bit value, which will probably be some sort of byte array. 
Watch out for signs.

An easier method is to just use a bignum package.

-- 
Chuck F (cbfalconer AT yahoo DOT com) (cbfalconer AT worldnet DOT att DOT net)
   Available for consulting/temporary embedded and systems.
   <http://cbfalconer.home.att.net>  USE worldnet address!

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