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From: daveb@cbmvax.cbm.UUCP (Dave Berezowski)
Newsgroups: net.micro.amiga
Subject: Re: Who is stealing my cycles?
Message-ID: <800@cbmvax.cbmvax.cbm.UUCP>
Date: Sun, 28-Sep-86 12:03:37 EDT
Article-I.D.: cbmvax.800
Posted: Sun Sep 28 12:03:37 1986
Date-Received: Tue, 30-Sep-86 13:12:22 EDT
References: <858@navajo.STANFORD.EDU>
Reply-To: daveb@cbmvax.UUCP (Dave Berezowski)
Organization: Commodore Technology, West Chester, PA
Lines: 47

In article <858@navajo.STANFORD.EDU> rokicki@navajo.STANFORD.EDU (Tomas Rokicki) writes:
>[   / | \    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


	During your test did you disable/enable multitasking with
Forbid()/Permit() or disable/enable interupts with Disable()/Enable()?

	If not, this could account for some of the overhead.