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Path: utzoo!attcan!uunet!munnari!acp!sns
From: sns@acp.OZ (Stuart Nixon)
Newsgroups: comp.sys.amiga,comp.sys.amiga.tech
Subject: Re: MINTERM_SWAP?
Summary: There is an XOR trick that will swap bits...
Keywords: MINTERM graphics how
Message-ID: <530@acp.OZ>
Date: 8 Dec 88 10:55:02 GMT
References: <1576@nmtsun.nmt.edu>
Organization: Australian Computer Products, Perth, Australia
Lines: 31

In article <1576@nmtsun.nmt.edu>, wbnsnsr@nmtsun.nmt.edu (William Norris) writes:
> Is there a simple way to swap two bitmaps without a third temporary bitmap?
> 
> I've looked over the MINTERMS, but none seem to do the trick.  How come
> the lower nibble in the MINTERM is always 0?  If 0xC0 is a copy, shouldn't
> 0xCC be a swap?
>
> wbnsnsr@nmtsun.nmt.edu                             |    /// Seulement

I seem to remember a trick using three (3) XOR's to swap contents of registers.
There should be no reason why the same can not be carried out on Bitmaps using
the blitter. It will require three blits to carry out the swap.

I have not worked out the MINTERMs to do XORs. However, if my memory serves me
correctly, the logic is something like :

	B = A XOR B		; where A & B are regs/bitmaps/bits/whatever
	A = A XOR B
	B = A XOR B		; tada! bits swapped

There may be a still quicker way of doing this with the blitter, as it has
the ability to accept three inputs. Still, this should give you some ideas...

Hope this helps.

sns

sns      Stuart Nixon Software, via Australian Computer Products
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