Path: utzoo!utdoe!generic!pnet91!ericmcg
From: ericmcg@pnet91.cts.com (Eric Mcgillicuddy)
Newsgroups: comp.sys.apple2
Subject: Re: Multitasking on a II
Message-ID: <313@generic.UUCP>
Date: 23 Dec 90 00:50:04 GMT
Sender: root@generic.UUCP
Organization: People-Net [pnet91], Etobicoke, ON
Lines: 55

>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

That will not work since you do not test the flag before modifying it. Suppose
the Flag were 0, meaning that the resource is locked. You now decrement it and
it is not zero meaning that it is unlocked. If you only grab a resource when
it is free ($01 in flag) not just when is not locked (big difference
logically), then it could work, but requires much more protection coding.
Another problem is that your test loop will eventually DEC FLAG to $01 and on
the next pass tell you that the resource is free. Even on a 65816 where FLAG
is a 2 byte value, you will eventually luck out and grab a locked resource. 

Multiprocesses will compound the problem. Regardless of how you test a flag,
you must not alter it unless you are claiming it, this is where TSB comes in.

        lda     flag_mask
w8_4_free tsb   global_flag
        bne     w8_4_free
        ......your code
        lda     flag_mask
        trb     global_flag     ;clear busy bit when done with resource

This should protect whatever resources capable of being claimed in a
pre-emptive multitasking emvironment.

The limited addressing modes available will cause problems in a 65816 system
unless this is in a shared memory region of the system since the global
semaphores must be in the same bank as the code that tests and claims them. 

An advantage is that a single byte/word can hold 8/16 flags as opposed to just
a single flag using ASL or INC/DEC.

Of course it is dead simple in a Toolbox based system such as the GS.

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