Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!uakari.primate.wisc.edu!aplcen!haven!grebyn!ckp
From: ckp@grebyn.com (Checkpoint Technologies)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Chip memory bandwidth, graphics modes
Message-ID: <14120@grebyn.com>
Date: 4 Jan 90 16:34:37 GMT
References: <8219@cbnewsm.ATT.COM>
Reply-To: ckp@grebyn.UUCP (Checkpoint Technologies)
Organization: Grebyn Timesharing, Vienna, VA, USA
Lines: 35

In article <8219@cbnewsm.ATT.COM> nsw@cbnewsm.ATT.COM (Neil Weinstock) writes:
>
>Why can the Amiga handle 4 bitplanes at 640 pixels/line, but only 6
>bitplanes at 320 pixels/line?  I understand limitations on number of color
>registers and the like, but that shouldn't have any bearing on the potential
>for future enhanced chipsets to handle 8 bitplane modes.  Nonetheless, we
>hear periodically that the Amiga just *can't* handle more bitplanes/colors
>due primarily to chip memory bandwidth limitations.
>
>I can't seem to make this all add up.  Can someone clarify?

	Don't worry, it really doesn't add up.  The Amiga chip RAM bus
really does have the bandwidth to support 8 bit planes in low res. What
it doesn't have is the bit plane pointers and color palette registers.
There are only 6 bit plane pointers, and 32 color pallette registers,
and that's why it can't do more than 6 bit planes.

	Now, a clever individual might notice, looking at a memory map
of the custom chips, that there are two unused long words in the custom
chips just following the 6 bit plane pointers, and the color
palette registers come last.  In other words, Jay Miner definitely had 8
bit planes in mind when these things were first designed.  He just ran
out of silicon on the chips; 256 color palette registers didn't fit.
And I just *have* to believe that Commodore knows about this.  So maybe
a 256-plane Amiga is in the cards, someday...

	I have this odd theory, though, that when Jay left Amiga he took
a lot of the custom-chip knowledge with him, and that Commodore has only
recently (if at all?!?) figured out how they work.  This would explain
the length of time it took to get the mildly-Enhanced Chip Set
produced, and some of the oddities: Only 64 colors in super-hires? That
could be because they couldn't figure out how to double the speed
of the color palette registers in Denise.  (Another reasonable
explanation would be simply that Commodore didn't have the time and
money to put a reasonable effort into the ECS chips.)