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From: mziober@trocadero.ics.uci.edu (Michael A. Ziober)
Newsgroups: comp.sys.apple2
Subject: Re: Multitasking on a II
Message-ID: <2772A18A.3472@ics.uci.edu>
Date: 21 Dec 90 23:58:02 GMT
References: <289@generic.UUCP> <1990Dec20.143649.11598@world.std.com> <1990Dec20.184254.16292@nntp-server.caltech.edu> <10739@ucrmath.ucr.edu>
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In <10739@ucrmath.ucr.edu> rhyde@ucrmath.ucr.edu (randy hyde) writes:

>                                                       If you can come
>up with some code to protect a critical region using only INC or DEC as
>a test and set instruction I'd be interested in seeing it (I'm not being
>facetious, I really don't know if it can be done, although I tend to doubt
>it).
>*** Randy Hyde

Why not?

When the resource is initialized by the system:
resource_init	...		;blah, blah, blah
		lda #1		;could be $ff if you prefer to
		sta flag	; grab resource with inc
		...		;blah, blah, blah

Then for each process that wants a piece of the action:
your_code	...		;blah, blah, blah
loop		dec flag	;try to grab resource
		beq locked	;got it!
		inc flag	;undo attempt
		jmp loop	;block until available
locked		...		;Here you get exclusive use of the resource.
release		inc flag	;undo resource lock
		...		;blah, blah, blah

I think this should work for about 254 processes.  With a slight
variation on this theme, you can also implement non-blocking resource
contention.

Michael Ziober