pochron@cat44.cs.wisc.edu (David Pochron) (11/03/90)
Since the HAM-E product is such a hot topic of discussion these days, I
thought I might ask this of the folks at Commodore, or the very technically
oriented Amiga users out there.
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...)
Okay, and the new HAM-E device takes advantage of this and combines 2 hi-res
pixels into one 8-bitplane lo-res pixel...
So what were the bandwidth limitations that prevented the current set of
chips to output 8 bitplanes in low res? Is there some sort of "hidden
overhead" inside the chip design that does not affect external devices such
as HAM-E?
I am really curious about this!
Thanks in advance! (As usual! :-)
--
-------------------------------------------------------------------------------
David M. Pochron | from Rescue Rangers, _A Fly in the Ointment_
pochron@garfield.cs.wisc.edu| Gadget to Dale: "Keep the hands off the body!"
-------------------------------------------------------------------------------navas@cory.Berkeley.EDU (David C. Navas) (11/04/90)
In article 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? Is there some sort of "hidden >overhead" inside the chip design that does not affect external devices such >as HAM-E? Well, yes. Firstly, Denise lacks 224 of 256 color registers necessary to do the job. Also realize that she is responsible for taking bitplane information and "de-planing" it. There is significant overhead for the CLUT-type stuff (Color LookUp Table), which is why super-hires had to restrict it's palette to only 64 colors. I'm not sure what impact that would have in lo-res, but I wouldn't be suprised if it was significant... Now I'm no techie with inside knowledge about how Denise works, but those are the things I can think up off-hand. David Navas navas@cory.berkeley.edu "Excuse my ignorance, but I've been run over by my train of thought." -me (and Calvin)
raible@cbmvax.commodore.com (Bob Raible - LSI Design) (11/04/90)
FYI non-ECS and the newer ECS DENISE parts are implemented in older NMOS technology. This limited the size of the color lookup table to 32 colors. EHB and HAM were obviously created in an attempt to maximize the usefulness of the "extra" bitplane. The bandwidth indeed exists for 8 bitplanes, however adding the extra color entries to the CLUT would be impractical due to size considerations as stated above. I played some tricks on the original lookup table to get it to process 28MHz pixels in ECS DENISE(8373 as opposed to 8362). I basically split the address and data domains in half and let each half process alternate pixels. This reduced the palette size to 4 out of a possible 64 colors(only 2 bits each of red,green, and blue info). I hope this clarifies things a little.
daveh@cbmvax.commodore.com (Dave Haynie) (11/06/90)
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" {uunet|pyramid|rutgers}!cbmvax!daveh PLINK: hazy BIX: hazy Standing on the shoulders of giants leaves me cold -REM