From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.advocacy
Subject: Re: A4000 at WOC
Date: 2 Oct 92 00:40:31 GMT
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Organization: Commodore, West Chester, PA

In article <1992Sep25.175523.20771@klaava.Helsinki.FI> lindblad@klaava.Helsinki.FI (Jarkko Lindblad) writes:
>wallmann@backus.pu.informatik.th-darmstadt.de (Natuerlich!) writes:

>Well, Dave Haynie said that it is/was roughly 25 to 50% of the speed of
>Amiga blitter. Somehow I usually trust him...

>: The truth is: the Blitter does DMA it does share the bus with the

>I never said it does not use DMA - of couse it does, otherwise there
>would not be any use for it. 

The scoop here on the Atari ST blitter is easy to figure out, even if you 
don't recall the technical stuff posted/written when it came out.  The Atari
blitter was an afterthought, that's why there was no place for it in the
original STs.  It basically sits on the 8MHz 68000 bus and, strangely enough,
runs 8MHz 68000 cycles.  When it wants to do something, it requests the bus
from the 68000, not unlike an Amiga hard disk, and when its done, it gives the
bus back.  There was some control over the number of consecutive cycles it 
would take, 64 rings a bell but I'm not certain if that's all.  It could
handle two operand blit operations, and also had some small internal pattern
memory to help, I suppose, with patterned blits without a third source.  

By contrast, the Amiga blitter sat on the Chip bus.  It could hit memory 
twice as fast as the 68000 could, and it could of course run in parallel
with the 68000 as long as there was no contention for resources (eg, the 68000
didn't need Chip RAM).  So that's a best case 4x bandwidth improvement over the
Atari blitter.  The CPU contention issue isn't a big one if you're not 
multitasking, however.  The Amiga blitter handles operations between three 
sources and one destination.  There are other special modes, such as a general
line draw algorithm, flood fills, etc.

>: CPU and can blit concurrently. TOS in its singletasking incarnation didn't

>Hmm, everyone else say it does not do anything concurrently with CPU 

The ST blitter had to share the bus with the 68000 -- only one could be active
at a time.  I don't know anything about other Atari systems, perhaps they have
a new blitter that's local to their equivalent of the chip bus.  

>Blitteri that is slower than CPU is not practical in single bus systems like 
>Atari ST, Falcon and Commodore 64.

That's true, but it doesn't necessarily follow that the ST blitter was useless.
Certainly, if you're dealing with generalized bit image movement on pixel
boundaries, a blitter than can manage even a two operand to one destination
blit with barrel-shifting can out pace a 68000.  In special cases, the 68000
may do better, especially since the ST's chunky pixels in color modes would
greatly complicate the blitting job (I have no idea what they did here).


-- 
Dave Haynie Commodore-Amiga (Cool Advanced High-End Systems You Can't See Yet)
 "The Crew That Never Rests"  {uunet|pyramid|rutgers}!cbmvax!daveh   BIX: hazy
              "I got a pocket full of kryptonite" -Spin Doctors