From: lindblad@klaava.Helsinki.FI (Jarkko Lindblad)
Newsgroups: comp.sys.amiga.advocacy
Subject: Re: Atari BLiTTER
Date: 1 Oct 92 11:41:54 GMT
Organization: University of Helsinki

gerrymon@eng.umd.edu (Gerry Mon) writes:
: In article <1992Sep24.195307.19082@klaava.Helsinki.FI>
: lindblad@klaava.Helsinki.FI (Jarkko Lindblad) writes:
: 
: >gerrymon@eng.umd.edu (Gerry Mon) writes:
: >
: >: The Atari BLiTTER takes data from the source and destination and
: >: prepares the result to be put back into the destination.  In other
: >: words, it's basically a 2 source blitter.  However, the Atari BLiTTER
: >
: >So, it uses 1 source and 1 source/destination? Ie. One of the sources
: >must change? That is bad, too bad.
: 
: 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.

Most (?) RISC chips have two sources and one destination, and it is an
advantage two one source, one source/destination system.

: 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.

You're asuming that Blitter mostly does simple copies - the CPU (in
Falcon etc.) can do the same thing at the same speed.

: >If I understand correctly, you say that because Ataris blitter only
: >has two sources and therefor only 16 logical operations, the 16
: >operations is equal to Amiga's blitters 256 operations between three
: >sources?
: 
: I'm just saying that they are both complete in a mathematical sense.

And then? This does not change the fact that Atari blitter only has
one source and one source/destination and therefor it does complex
operations slower than it would with 3 sources and separate destination.

: 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

Yes - also if you need some of the original souces for different
blits, what then? You have to use some extra time to copy backups of these.

: should be pointed out that a lot of those 256 operations aren't very
: useful.  As an example, one of the logic operations is a do-nothing.
: It just leaves the destination the same as it was before.

And then - just about all of them are in fact usefull for some
purpouses; youre just trying to say that even as Atari's blitter is
inferior to Amiga's blitter on this area, it still is as good as the
blitter in Amiga even on this area. It's a bit like saying that 6502
is as good as 68000 even as it lacks some of the instructions and
registers etc.

: By the way, I just want to elaborate a little on the Atari BLiTTER's
: logic operations.  As I mentioned earlier, it can perform more logic
: operations if you include the 16x16 halftone ram as one of its
: sources.  Specifically, the BLiTTER can perform 4 logic operations
: between the source1 and 16x16 halftone ram before performing the 16
: logic operations between the source1 and the source2/destination.
: This give a total of 64 logic operations.

And just how usefull is this 16*16 halftone system on large areas? In
what way it operates on, say 200*200 area?

: No, the BLiTTER really does have separate modulos for the source and
: destination.  To be specific, it has 4 registers labeled Src_Xinc,
: Src_Yinc, Dst_Xinc and Dst_Yinc.  These are the modulo registers for
: the source and destination.

Ie. allmost what I said - however this is no better to what I did say:
you can have one modulo for one (or is for both - in that case both
source modulos are forced to be the same) source, one for destination
(or is for destination and one of the sources?).
	In Finglish: you can't have different modulos for both sources
and the destination on Atari blitter.

: >Is the Atari's blitter's shifter a barrell shifter?
: 
: To be honest, I'm not sure.  In the very early days I heard a rumor
: that it might not be a barrel shifter.  I hope this rumor is wrong.

I hope so too.

: scrolling is much faster if it is word aligned).  It is the Desktop
: program that limits window movement to 8 or 16 pixel jumps.  GEM

Ah, then this must be why the magazine mentioned this matter.

: And yes, the Atari BLiTTER does have masking.  To be specific, it has

Hey, I didn't even ask :)

--
Jarkko Lindblad
lindblad@cc.helsinki.fi