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From: grr@cbmvax.UUCP (George Robbins)
Newsgroups: comp.sys.amiga
Subject: Re: eXtra-Half-Brite: The Full Story Please!
Message-ID: <2886@cbmvax.UUCP>
Date: Wed, 2-Dec-87 19:51:16 EST
Article-I.D.: cbmvax.2886
Posted: Wed Dec  2 19:51:16 1987
Date-Received: Sun, 6-Dec-87 07:38:36 EST
References: <6924@sunybcs.UUCP>
Reply-To: grr@cbmvax.UUCP (George Robbins)
Distribution: na
Organization: Commodore Technology, West Chester, PA
Lines: 34

In article <6924@sunybcs.UUCP> ugfeldmn@joey.UUCP (Jon Feldman) writes:
> 
> 	Can anyone tell me the story behind Extra-Half-Brite, and why some
> machines have it and some don't?  Which Amigas do?  Why?  How?  Where?  When?
> What?  I don't understand!  Where's the tea?

Extra-Half-Brite is a display mode where five bit planes are used to lookup
the color for a pixel in the color lookup table, and the sixth bitplane is
used to modify the resulting color value by shifting the resulting RGB
values right on position.  It is kind of a kludge which takes advantage
of the fact that the logic for handling six bitplanes was already present
to make HAM work, however changing the color lookup table from 32 to 64
entries would have caused an undesirable increase in the size of the
Denise chip.  Chip cost bears an exponential relationship to area, so
you try to avoid major size increases unless better technology is available
to compensate.

The change was incorporated in Denise Rev 6 which was used in production
for about half of the A1000's.  However since this half includes almost
all the European A1000's, the percentage of US machines with half-brite
is somewhat lower than expected.  The mode is also present in Rev 8
chips and all A500's and A2000's use either the Rev 6 or Rev 8 chip.
There are no functional differences between the two revisions.

Commodore didn't make a big fuss about it since it wasn't really a major
improvement and there were massive numbers of perfectly usable older chips
in the production pipeline.  If you don't have one in your machine, you
can order a replacement Denise chip for $65.40 list, or whatever deal you
can get through your dealer's service department.

-- 
George Robbins - now working for,	uucp: {uunet|ihnp4|rutgers}!cbmvax!grr
but no way officially representing	arpa: cbmvax!grr@uunet.uu.net
Commodore, Engineering Department	fone: 215-431-9255 (only by moonlite)