Subject:
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Re: RCX Programming Questions and Sensor Ideas
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Newsgroups:
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lugnet.robotics
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Date:
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Wed, 11 Nov 1998 10:02:47 GMT
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Original-From:
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Ben Laurie <ben@algroup.co.+SayNoToSpam+uk>
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Viewed:
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3089 times
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Paul Haas wrote:
> 2. You can use a counter, because += 1 and -= 1 are atomic operations.
>
> int r31;
> int done;
> r31 = 0;
>
> // stuff happens.
> // Heres our semaphore stuff.
> done = 0; // We're not done yet.
>
> while ( ! done ) {
> while ( r31 != 0 ) {
> } ; // busy wait.
> r31 += 1; // This operation is atomic.
> if ( r31 == 1 ) { // Were we the only process to touch the counter?
> // Do critical section stuff here.
> done = 1;
> }
> r31 -= 1; // Remove our reservation.
> }
>
> Note, this isn't fair, fifo, lifo, or any other of those spiffy
> attributes. You can use this sort of semaphore to wrap around the fancy
> stuff.
It isn't guaranteed to work, either, though it is likely to in the end.
This is because two processes can conspire to always have r31 == 2 when
you test for r31 == 1.
Cheers,
Ben.
--
Ben Laurie |Phone: +44 (181) 735 0686| Apache Group member
Freelance Consultant |Fax: +44 (181) 735 0689|http://www.apache.org/
and Technical Director|Email: ben@algroup.co.uk |
A.L. Digital Ltd, |Apache-SSL author http://www.apache-ssl.org/
London, England. |"Apache: TDG" http://www.ora.com/catalog/apache/
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Message has 1 Reply:
 | | Re: RCX Programming Questions and Sensor Ideas
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| (...) Oops. I don't know how the scheduler in the RCX works. It might execute 1 opcode from each task in order. If 2 tasks get in sync, then they will never leave the semaphore code as written. Here's a fixed version. I added a random backoff. (...) (...) (26 years ago, 11-Nov-98, to lugnet.robotics)
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Message is in Reply To:
| | Re: RCX Programming Questions and Sensor Ideas
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| (...) Check and set do not have to be atomic, they have to be ordered. The sequence is: Check all flags, when clear, set our flag, check all flags again, if only ours is set, do stuff. 1. You can dedicate a task to managing semaphores. I described (...) (26 years ago, 11-Nov-98, to lugnet.robotics)
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