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Path: utzoo!utcsri!mason
From: mason@utcsri.UUCP (Dave Mason)
Newsgroups: net.micro.16k
Subject: Need word version of test-and-set
Message-ID: <1813@utcsri.UUCP>
Date: Tue, 17-Dec-85 18:33:46 EST
Article-I.D.: utcsri.1813
Posted: Tue Dec 17 18:33:46 1985
Date-Received: Tue, 17-Dec-85 19:21:23 EST
Reply-To: mason@utcsri.UUCP (Dave Mason)
Organization: University of Toronto/Ryerson Polytechnic Institute
Lines: 44
Keywords: ns32000 compare-and-swap multi-processor
Summary: multi-processor synchronization

I am looking for a fast way to exit a critical section and see if anyone
else was waiting to get in.  This is relatively easy (if rather obscure) to
do on other processors (like PDP-11, S/370, VAX, MC68000).  Despite being
a NS32000 booster, I can't think of a way to do this on the NS32000.
I would appreciate help particularly from the friendly people at nsc.
Here is how I would do it on the PDP-11:

flag:	.word	1	; section initially free

	; enter critical section
	decb	flag+1		;say we want in
	asrb	flag		; try & enter
	bcs	wegotit
	;do it the slow way...put us in a queue etc. (don't worry about details

wegotit:	;critical section
	...

	;exit critical section
	add	$0x0101,flag	; say it's free & check for anyone waiting
	bgt	weregone	; no-one was...fast exit
	;do it the slow way, check the queue etc. (more messy details)
(sorry for any non-unix syntax, I've never used unix assemblers)
(I did warn you it was obscure...pretty kludgey too...much cleaner & clearer
with Compare-and-Swap ala S/370)
(Note: the foregoing depends on the read-modify-write cycle for the asrb and
the add to be indivisible across processors)
(Note: test-and-set won't do the trick (at least not anyway I can see it),
because we have to do 2 things: exit the critical section; check for others,
in an indivisible way.  A 2 bit test & set would do, but compare&swap is the
more general solution.)

The reason I'm interested in this is that the slow way is 2-3 decimal orders
of magnitude slower (for other reasons), and I want this to be a cheap,
therefore useful, facility.
If the people at National can't come up with something, could I interest
you in a nice Compare-and-swap instruction?
-- 
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	!utcsri!mason		Dave Mason, U. Toronto CSRI/ Ryerson Polytech
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