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From: ckp@grebyn.com (Checkpoint Technologies)
Newsgroups: comp.sys.amiga.tech
Subject: Re: addx/subx/movep/Metascope/A68k
Message-ID: <24095@grebyn.com>
Date: 18 Dec 90 04:20:17 GMT
References: <1990Dec17.031120.3658@phoenix.pub.uu.oz.au>
Reply-To: ckp@grebyn.UUCP (Checkpoint Technologies)
Organization: Grebyn Timesharing, Vienna, VA, USA
Lines: 18

In article <1990Dec17.031120.3658@phoenix.pub.uu.oz.au> bytey@phoenix.pub.uu.oz.au (Chris Hames [bytey]) writes:
>   Is it common knowledge that the addx/subx instructions don't set the
>zero flag when the result is zero but clear it if the result is not zero?
>Why didn't someone tell me!!!!!!!!!!!!  hours hours hours debugging.....

This may come under the heading of "obscure feature" - but yes, it's
always been that way.  It lets you find the total zerosity (??) of the
result of a long multi-precision add/sub.  You set Z before starting,
and if any intermediate instr gives a non-zero it's cleared.  This way if
all the intermediate results are zero, Z remains set at the end,
otherwise Z is cleared.

Did we need this feature?  Ah, that's another topic... :-)
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