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From: rokicki@rocky.UUCP
Newsgroups: comp.sys.amiga
Subject: Re: Blitter / Assembly block clear benchmarks
Message-ID: <287@rocky.STANFORD.EDU>
Date: Tue, 5-May-87 00:42:45 EDT
Article-I.D.: rocky.287
Posted: Tue May  5 00:42:45 1987
Date-Received: Thu, 7-May-87 01:14:16 EDT
References: <8705030729.AA23771@cory.Berkeley.EDU>
Organization: Stanford University Computer Science Department
Lines: 41
Summary: Faster than that!

dillon@CORY.BERKELEY.EDU (Matt Dillon) writes:
> for comparison, a 68010 can zero a large block @ 2.04 MBytes/sec

Actually, it's possible to clear memory pretty quickly with a
bare bones 68000.  For block sizes larger than 256, you can do
something like this (after setup):

loop:
	movem.l	d1-d7/a1-a6,-(a0)	; 116
	movem.l	d1-d7/a1-a6,-(a0)	; 116
	movem.l	d1-d7/a1-a6,-(a0)	; 116
	movem.l	d1-d7/a1-a6,-(a0)	; 116
	movem.l	d1-d7/a1-a5,-(a0)	; 108
	dbra	d0,loop			;  10
					_____
					  582 cycles to clear 256 bytes

On a 7.18 MHz processor, that yields a throughput of

	( 7.18 * 10^6 * 256 ) / ( 582 * 1048576 )

or 3.01 MBytes per second.  (Gotta run it to be sure.)

Of course, the bandwidth of the 68000 is:

	( 7.18 * 10^6 ) / ( 2 * 1048576 )

or 3.42 MBytes per second, so we are not too far off.
The blitter can function at twice that:

	( 7.18 * 10^6 ) / 1048576

or 6.85 MBytes per second, if you turn the display off.  And
get rid of all of the other overhead.

Note that in the 68000 calculations above, we are assuming no
contention for the bus for the 68000.  Actually, the 68000 *does*
see some small amount of contention, even with only a high-res 2
bit plane screen like the workbench screen, but it's not much.

Now, what are you going to do with memory you clear so fast?