From: gerrymon@eng.umd.edu (Gerry Mon)
Subject: Re: Atari BLiTTER
Date: Wed, 30 Sep 92 12:45:19 GMT
Organization: College of Engineering, University of Maryland, College Park

In article <1aas74INNd2m@uwm.edu> gblock@csd4.csd.uwm.edu (Gregory R
Block) writes:

>gerrymon@eng.umd.edu (Gerry Mon) writes:
>: Why is this so bad?  Don't most microprocessors have an instruction
>: format like this too?  For example, a lot of microprocessors have an
>: ADD R1,R2 instruction where you add two numbers in R1 and R2 and place
>: the result back in R1.
>: 
>: If you think about it, a typical use for a blitter is to blit a bitmap
>: from memory (the source) to the screen (the destination) thereby
>: changing the destination.  The Atari BLiTTER seems perfectly fine for
>: this.
>
>For that, and that only.  What if you've got a bitmap, a mask, and
>your screen?  Either the bitmap or the mask will be trashed.  Or
>you'll store one on the screen, and that's just plain ugly.  There are
>so many places where having three real sources, or even two real
>sources and a real destination, is useful.

You're right.  I just checked out the Amiga Hardware Reference manual
from my school library and there is one common blitting operation that
really does need those separate sources and destination.  It's called
the "cookie cutter" operation and the Amiga blitter will be much
faster at it than the ST BLiTTER.  I think the ST BLiTTER would need
to do 3 or 4 separate blits to emulate it.

>
>: For example, since the Atari BLiTTER is complete, it can emulate all
>: 256 logic operations of the Amiga blitter by basically doing two
>: blits.  The Atari will be just a little slower at doing it.  Also, it
>
>(a LOT slower...)

Maybe even slower!  I just realized I made a stupid mistake when I
claimed that the Atari BLiTTER could emulate the Amiga blitter by
doing two consecutive blits.  Michael van Elst sent me email that
politely pointed out my error.

Anyway, I started playing around with it and I realized that doing 16
logic operations followed by another 16 logic operation does not
generate all 256 possible logic operations on three operands.  It only
generates a subset of it.

I might be wrong (the analysis got confusing to me) but the smallest
number of blits the Atari BLiTTER would need to emulate _all_ 256 logic
operations of the Amiga blitter seems to be 5!!!  This means that the
Amiga blitter could be as much a 5 times faster than the Atari BLiTTER
in the worst case!  Yikes!  I hope I'm wrong with my analysis.

Is this how Dave Haynie came up with the statement that the Atari
BLiTTER is only about 23% as fast as the Amiga blitter in certain
operations? 

I should point out that the more common operations such as those
listed in Table 6.1 of the Amiga Hardware Reference manual would take
much less than 5 blits to emulate.

I always knew that the Amiga blitter was technically better than the
Atari BLiTTER but I never suspected it could be this much better.

There are rumors that the BLiTTER in the Falcon has some added
functionality along with the increase in clock speed.  I sure hope so.

[stuff deleted]
>
>Greg
>
>--
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>(: 	"Yeah, if improving the system is screwing the user base,     :)
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--Gerry