Subject:
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Re: NQC 2.0 and some math questions
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Newsgroups:
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lugnet.robotics.rcx.nqc
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Date:
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Sat, 2 Oct 1999 10:42:32 GMT
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Viewed:
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5458 times
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Dave Baum wrote:
>
> In article <37F47D52.204EC5C6@algroup.co.uk>, Ben Laurie
> <ben@algroup.co.uk> wrote:
>
> > Dave Baum wrote:
> > > There isn't an atomic test-and-set bytecode, so the only way I know of
> > > implementing sempaphore would be to use bitflags and assign one bit per
> > > task.
> >
> > There is a way to do it even if you have no test and set, though I can't
> > remember it off the top of my head, and it is slow. Don't see how using
> > bitflags helps?
>
>
> I'm going from memory here, so I may miss a detail...
>
> int lock;
> #define TASK_BIT(task_num) (1 << task_num)
>
> void acquire_lock(int task_num)
> {
> while(1)
> {
> // wait for lock to be clear
> while(lock);
>
> // try to own it
> lock |= TASK_BIT(task_num);
>
> // see if we own it
> if (lock == TASK_BIT(task_num))
> return;
> else
> lock &= ~TASK_BIT(task_num);
> }
> }
>
> -----
>
> You try to acquire the lock by setting your task's bit. The lock is only
> successful if after setting the bit, your bit is the only one set. If
> another task started to lock it in between your set and test, then your
> test would fail. Conversely, if the other task sets after you have
> completely locked, then their test will fail.
>
> You release the lock by setting it to 0 (or perhaps just clearing your bit).
This only works if |= is atomic, though. Is it?
Cheers,
Ben.
--
http://www.apache-ssl.org/ben.html
"My grandfather once told me that there are two kinds of people: those
who work and those who take the credit. He told me to try to be in the
first group; there was less competition there."
- Indira Gandhi
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Message has 1 Reply:
Message is in Reply To:
 | | Re: NQC 2.0 and some math questions
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| (...) I'm going from memory here, so I may miss a detail... int lock; #define TASK_BIT(task_num) (1 << task_num) void acquire_lock(int task_num) { while(1) { // wait for lock to be clear while(lock); // try to own it lock |= TASK_BIT(task_num); // (...) (27 years ago, 2-Oct-99, to lugnet.robotics.rcx.nqc)
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