Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!bpa!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.atari.st
Subject: Re: Blitter disappointment (was: Time to create comp.sys.atari.flames)
Message-ID: <8698@cbmvax.UUCP>
Date: 23 Nov 89 05:46:21 GMT
References: <5312@sdcc6.ucsd.edu>
Distribution: na
Organization: Commodore Technology, West Chester, PA
Lines: 29

in article <5312@sdcc6.ucsd.edu>, pa1329@sdcc13.ucsd.edu (pa1329) says:

> Hmmm,.  I wonder why is the Atari blitter so ineffective.  Its
> speed is no where comparable to the Amiga's blitter.   Why is the
> difference?

I know of two differences.  The primary one is that the Atari blitter
shares the main system bus with the 68000 in the ST.  So you have to
alternate between blitter access and CPU access.  The Amiga blitter,
on the other hand, is on a separate bus, and when the video load isn't
outrageous, or the CPU has work to do outside of video memory, the
CPU and blitter can operate in parallel.  And since the Amiga blitter
only takes two clocks for a memory cycle, it can run twice the speed
of the 68000 if it uses both halves of the cycle (thus kicking out the
68000, but of course, the Atari always has to do that).

The other obvious difference is that the Atari blitter handles only 
two sources into one destination, with 16 possible operations, while 
the Amiga blitter handler three sources into one destination, with 
256 possible operations.  There are some clever additions to plain bit 
blitting.  The Atari blitter apparently has a little internal chunk 
of memory which is used to support pattern fills without having to 
constantly fetch the pattern from main memory.  The Amiga blitter has 
hardware line drawing.

-- 
Dave Haynie Commodore-Amiga (Systems Engineering) "The Crew That Never Rests"
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                    Too much of everything is just enough