Date   : Wed, 15 Jun 2005 17:01:22 +0100
From   : Richard_Talbot-Watkins@...
Subject: Re: Faster 6502 multiplication

Carlo writes:

> This descends from the fact that a*b is *always* an integer,
> so for the above relationship to hold the right side of the
> equation *must* also be an integer value. From this descends
> that (a+b)^2 - (a-b)^2 is always a multiple of 4.

*bangs forehead against desk*

D'oh, I should've considered that more closely before my last reply.

Of course, since it's a multiple of 4, the bottom two bits will always be
zero.  Which means the bottom two bits of (a+b)^2 and (a-b)^2 have to be
equal.  So we can discard them straight away as we are not expecting
anything other than a result of 00 from them.

Cheers Carlo!


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