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From: rokicki@navajo.STANFORD.EDU (Tomas Rokicki)
Newsgroups: net.micro.amiga
Subject: Who is stealing my cycles?
Message-ID: <858@navajo.STANFORD.EDU>
Date: Fri, 26-Sep-86 13:14:45 EDT
Article-I.D.: navajo.858
Posted: Fri Sep 26 13:14:45 1986
Date-Received: Tue, 30-Sep-86 04:58:15 EDT
Organization: Stanford University
Lines: 40

[   / | \    cruisin' on down . . . ]

Well, folks, I have some bad news.  I wrote a test routine
to see just exactly how many CPU cycles I was getting from
my Amiga.  This routine was essentially nested dbra's:

loop(i,j)
int i,j
{
#asm
	move.w	8(a5),d1
	move.w	10(a5),d0
loop:	dbra	d0,loop
	dbra	d1,loop
#endasm
}

So, I pass it the appropriate parameters to consume 2,000,000,000
cycles, and time it.  This was with no other programs running, no
workbench loaded, and just a standard CLI open.  I even subtracted
the overhead.  I tried it in chip and in fast memory; I tried it
with my external memory disconnected; I tried it with the workbench
screen moved all the way down so it wasn't visible.  Still,
between 8 and 17 percent of the CPU disappeared somewhere.

So it's not that much, but I'm interested in what the machine could
be possibly doing that eats the CPU.

I have an explanation for the 17% figure; when run out of chip
memory, the processor gets every other 3.6 MHz RAM slot.  Well,
running at 7.2 MHz, occasionally it wants the odd slot (a dbra
does two fetches but requires ten cycles, so it is guilty of
this) so the processor must wait two cycles before it gets the
bus.  But out of fast memory this shouldn't happen . . .

What it comes down to is this; the fastest effective clock speed
I have been able to get is 6.6MHz.  Not to bitch, but I wouldn't
be upset to get more . . .

-tom