Path: utzoo!attcan!uunet!timbuk!cs.umn.edu!msi.umn.edu!src.honeywell.com!sol.ctr.columbia.edu!zaphod.mps.ohio-state.edu!wuarchive!psuvax1!rutgers!cbmvax!daveh
From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: Maximum bitplane depth
Keywords: Bitplanes bandwidth HAM-E
Message-ID: <15630@cbmvax.commodore.com>
Date: 6 Nov 90 01:01:51 GMT
References: <1990Oct22.195001.28782@idt.unit.no> <15343@cbmvax.commodore.com> <1990Nov2.225553.8132@daffy.cs.wisc.edu>
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Organization: Commodore, West Chester, PA
Lines: 29

In article <1990Nov2.225553.8132@daffy.cs.wisc.edu> pochron@cat44.cs.wisc.edu (David Pochron) writes:
>The question is this:

>    The Amiga can output 4 bitplanes of data in hi-res mode, (pixels output
>at twice the rate of lo-res...)

>So what were the bandwidth limitations that prevented the current set of
>chips to output 8 bitplanes in low res?  

There is adequate bus bandwidth for that, but that's hardly the whole picture.
Obviously, Agnus/Denise would need two additional bitplane pointers for this
to work, but that's not the tough part.  The problem is, what do you do with
the 8 bitplanes once you get them.  Notice that Denise only does the normal
bitplane-to-CLUT-on-out operation when you have 5 or fewer bitplanes.  Which
makes sense; there are only 32 CLUT entries.  The 6 bitplane modes are either
HAM or EBH, both of which use tricks and still use 32 or fewer color registers.
To properly support 8 bitplanes, Denise would need room for 224 extra color
registers.  Adding those was apparently considered impossible, at least for
the current ECS chipset.  Denise is very close to being as large as you can
make a chip in that NMOS technology.

>David M. Pochron	    |  from Rescue Rangers, _A Fly in the Ointment_



-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
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	Standing on the shoulders of giants leaves me cold	-REM