From:	Jeff Weeks <pweeks AT execulink DOT com>
Newsgroups:	comp.os.msdos.djgpp
Subject:	16.16 fixed point math multiply (overflow problems)
Date:	Wed, 30 Jul 1997 14:59:13 -0400
Organization:	Code X Software
Lines:	28
Message-ID:	<33DF8F01.58BCC37C@execulink.com>
NNTP-Posting-Host:	ppp35.saturn.execulink.com
Mime-Version:	1.0
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

Yeah, yeah, I know... Fixed point math is easy.  And I've used it many
times... but 24.8, not 16.16.  I just recently wrote a tiny 16.16 fixed
point math class which doesn't work well at all.

My multiply function overflows everytime, and it's obvious why.  Here's
what I do:

(x * y) >> 16;

Obviously anything over 1 * 1 (65536 * 65536) will result in an
overflow, even in a 32-bit register.  I'm wondering what the best
technique is to multiplying in a 16.16 fixed point format?

Secondly... Michael Abrash's multiply code seems to do EXACTLY what I
do!?!  How can he publish this code (Zen of Graphics Programming) when
it obviosly doesn't work in all (most) cases!  Or am I missing
something?

Thanks a lot,
Jeff

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