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 01:06:38 GMT
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Original-From:
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Paul Haas <paulh@hamjudo!avoidspam!.com>
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Viewed:
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2747 times
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On Tue, 10 Nov 1998, Joshua Delahunty wrote:
> > > An example. Say you want to prevent two tasks from fighting over motor
> > > 1. Just insert an instruction in each task before operating the motor
> > > to check that var 31 contains zero. If it does, set var 31 to 1 and
> > > control the motor, then set var 31 to 0 again. If a task sees var 31 is
> > > non-zero then you can program it to wait or abort as you wish.
>
> Kekoa Proudfoot wrote:
> > This assumes that the check and the set are, together, atomic. Does the
> > SDK document describe what if/why this might be atomic? I haven't had a
> > chance to read it in enough detail to find this answer for myself.
> *I* can read it, though <g>, and I can find nothing that indicates a
> "safe" way to implement semaphores.
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 that technique a few weeks ago. Email me if you want it.
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.
I was lazy and wasted the variable "done". It isn't really needed.
--
paulh@hamjudo.com http://www.hamjudo.com
The April 97 WebSight magazine describes me as "(presumably) normal".
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Message has 1 Reply:
Message is in Reply To:
 | | Re: RCX Programming Questions and Sensor Ideas
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| (...) ...and it would be my sincerest recommendation that you NOT read it, Kekoa. It would be safer from the reverse-engineering standpoint that you NOT use this SDK in any manner. *I* can read it, though <g>, and I can find nothing that indicates a (...) (26 years ago, 10-Nov-98, to lugnet.robotics)
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