Path: utzoo!utgpu!watmath!att!tut.cis.ohio-state.edu!cica!ctrsol!uakari.primate.wisc.edu!indri!caesar!blake!uw-beaver!ubc-cs!alberta!calgary!enme3!deraadt
From: deraadt@enme3.ucalgary.ca (Theo Deraadt)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Denise/Paula/Agnus/Gary/Portia
Summary: TI 340x0
Message-ID: <1671@cs-spool.calgary.UUCP>
Date: 5 Aug 89 07:10:46 GMT
References: <1388@bnr-fos.UUCP>
Sender: news@calgary.UUCP
Reply-To: deraadt@enme3.UUCP (Theo Deraadt)
Organization: U. of Calgary, Calgary, Alberta, Canada
Lines: 46

In article <1388@bnr-fos.UUCP> protcoop@leibniz.uucp () writes:
>Something has been on my mind for the last while and I want to bring it
>to the net community to see if my theory is plausible.  Let us *suppose*
>that the Amiga 3000 did not use Commodore's custom chips but used some
>'off the line' parts by other companies, for example the TI 34000 (?)

I thought about what would be involved in using a TI 340x0 in an Amiga
system before. The format of the video memory leads me to say it's not
practical. The Amiga is bitplane oriented, as all the graphics.library
structures indicate, while the TI chipsets are pixelpacked.

Thus, on the Amiga a pixel is formed by grabbing bits from a bunch of
different bitplanes, with the number of bitplanes being completely
variable [up to a maximum that is always too small :-)]. The Custom chips
do one dma cycle for each bitplane then get 16 pixels out of it
[16 is memory bit width]. A 32bit bitplane solution would be able to
display twice as much 16bit bitplane solution. Greater pipelining or
extensive fifo usage could make it even better.

On the 340x0, it's a little different. If you have one bit/pixel it works
the same. If you have two bits/pixel, then two consecutive bits in memory
generate a pixel. If you have 16 colors, you have 4 consecutive bits
to a pixel. Because the shifting gets icky, you don't get non power-of-two
bits/pixel. Shifting 3 bits at a time out of a 16bit word gets messy
when you get to the end of the word :-) It is easier to build video
shifting circuitry for pixel packing.

There is a National graphics chip set [not sure about availability] that
has a super blitter/display chip for each bitplane, the entire architecture
is 32bit, and has a full graphics processor. I looked at a 10 or so page
pamphlet on it, and it certainly was impressive. THIS THING HAD EVERYTHING,
but it probably takes up a ton of board space.. the main processor had
50ns cycles for some stuff...

I certainly would much rather stay with a bitplaned approach. Everytime
I think of displaying text on a nonbitplaned multi bit/pixel screen it
scares the heck out of me. Of course these systems do text for you,
but I still think bitplaning is better.

I suspect the difficulty in switching to a pixel packed chip such as
the 340x0 using just a new graphics.library is probably prohibitive.
Too much bitplaned data in structures would have to be repeatedly
converted to and from pixel packed format.