the coldfire mutually exclusive

Posted on 2005-03-22
Medium Priority
Last Modified: 2012-06-27
I want to know that ideally there is a need for mutually exclusive access to the console and how this could be achived using the BSET instruction as the basis of a binary semaphore.
Question by:tantoush
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LVL 22

Expert Comment

ID: 13600300
First you need a good algorithm.   I hope your textbook has one, as it is trickierr than it sounds!   Much trickier.  The simplest solutions have race problems.

Please show us that you have an algorithm, then we can help in writing it in asm.  If your book doesnt have an algorithm, please try using google or ask around.


Author Comment

ID: 13601499
I am sorry but realy I dont know what you min by good algorithm.
LVL 22

Expert Comment

ID: 13604104
Whenever you have two or more processes trying to get the use of a resource, you need some reliable method for interlocking their access.  Some method involving flags, counters, or semaphores perhaps.


Author Comment

ID: 13604920
I whant to know more a bout BSET instruction as the basis of a binary semaphore.
if you know any web bage have some information.
LVL 22

Accepted Solution

grg99 earned 1500 total points
ID: 13605510
If you mean the  BSET instruction in the Motorola/Freescale DSP 56xxx series:

BSET is actually a "test and set" instruction.   It indivisibly tests a bit, then sets it.
It returns the value of the bit in the carry bit.

So with BSET you can test to see if a flag is set, and then set it, without any chance another processor or another process or thread can be doing the same thing at the same instant.  In other words, no two processes can see the bit it clear and both set it.

 Example interlock code:

Again:     BSET   #ConsoleLockBit,DeviceInterlockWord     ;; try to get the interlock
              JCS      Again                                                  ;; bit already set, wait for clear

That's it!   Whenyou're done with the resource you just clear the lock bit

     BCLR    #ConsoleLockBit,DeviceInterlockWord


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