Path: utzoo!attcan!uunet!mcvax!enea!kth!draken!tut!pl
From: pl@tut.fi (Pertti Lehtinen)
Newsgroups: comp.sys.m68k
Subject: Re: Funny MC68000 programs
Message-ID: <5403@korppi.tut.fi>
Date: 23 Nov 88 10:41:28 GMT
References: <17541@santra.UUCP>
Reply-To: pl@tut.fi (Pertti Lehtinen)
Organization: Tampere University of Technology, Finland
Lines: 55


From article <17541@santra.UUCP>, by mat65@tukki.jyu.fi (Toivanen Jari):
> Here are a couple of interesting problems for all those who think they can
> progam the M68000 processor family.  What does the following very interesting
> program do ?
> 
> a	move.l	#b,a0
> b	move.l	(a0),-(a0)
> 	bra	a+2
> 
	This is tiny example of program crawling downward in memory.
	More classic example is same for PDP-11

		mov -(pc),-(pc)

	with single instruction.

	On MC68000 we can do this too by

b:	lea	a(pc),a0
a:	move.l	(a0),-(a0)
	bra	b

	which is not so easy to spot as we do not jump
	into middle of intruction.
	
> Second and perhaps kinkier question: why does the following program fail to do
> the "opposite" of the first program ?
> 
> 	move.l	#a,a0
> a	move.l	(a0)+,(a0)
> 

	Move instruction takes only word, but long word is transferred,
	so instruction at a+2 does not get modified.
	Also prefetch could make us some tricks here.

	We may try something like:

	lea	a(pc),a0
	lea	b(pc),a1
a:	move.l	(a0)+,(a1)+
b:	bra	b

	But this may not do anything useful, (does these anyway do)
	as I did not try.
	I do not have 68000 machine around and
	these forward crawling version do not work on 68020 machines
	as instruction cache does give us some troubles.

------------------------------------------------------------------------
pl@tut.fi				! All opinions expressed above
Pertti Lehtinen				! are preliminary and
Tampere University of Technology	! in subject to change
Software Systems Laboratory		! without any further notice.