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From: alan@pdn.UUCP (Alan Lovejoy)
Newsgroups: comp.sys.atari.st
Subject: Re: question of the day
Message-ID: <1505@pdn.UUCP>
Date: Sun, 11-Oct-87 12:32:48 EDT
Article-I.D.: pdn.1505
Posted: Sun Oct 11 12:32:48 1987
Date-Received: Wed, 14-Oct-87 05:37:21 EDT
References: <36500013@iuvax> <656@rocky.STANFORD.EDU>
Reply-To: alan@pdn.UUCP (0000-Alan Lovejoy)
Organization: Paradyne Corporation, Largo, Florida
Lines: 38

In article <656@rocky.STANFORD.EDU> rokicki@rocky.UUCP (Tomas Rokicki) writes:
/
/>		movea.l	#go1,a0
/>		movea.l	#go2,a1
/>	go1:	move.w	(a0)+,(a1)+
/>	go2:
/
/If you turn to Appendix E of the Motorola MC68000 Programmer's
/Manaul, it describes the prefetch which is breaking that program.
/To quote:
/
/	The MC68000 uses a two-word tightly coupled instruction
/	prefetch mechanism to enhance performance.  ...  When
/	execution of an instruction begins, the operation word
/	and the word following have already been fetched ...
/
/Thus, the instruction at go2: is fetched *before* the instruction
/at go1: is executed, since move.w (a0)+,(a1)+ is a one-word

instruction.  If you inserted another move.w (a0)+,(a1)+ before
/go1, it should work on a 68000, but will break on a 68020 for
/several reasons . . .
/
/It works under RAID because RAID most likely has turned instruction
/tracing on, which does a software interrupt on each instruction,
/thus killing the prefetch with the implicit branch . . .
/
/> We are so interested in understanding the behavior of this program that
/> I have been authorised to award a bust of Abraham Lincoln made from steel
/> to the first person that clears up the mystery.
/
/Thanks!

I don't know about that.  I submitted similar advice Oct 10 (about 3pm
EDT) by mail.  Others may have also.  I hope the prize is awarded based on GMT
submission time, not on arrival time at the University.

--alan@pdn