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If lock is not true (!lock), then make lock true, and return the value true to the calling program. Otherwise, return the value false to the calling program. Some boolean variable called lock is used to lock or unlock the resource. If the resource is not locked at the time of the call, the instruction will lock it and return true meaning, It s OK to proceed. If the resource is locked at the time of the call, the instruction will return false meaning, You must wait. If such an instruction is part of the machine instruction set, code like this can take advantage of it: boolean thisLock; . . . while( !testAndSet( thisLock ) ) { /* Do nothing */ } /* Now do whatever needs to be done in isolation */ . . . /* Now free the lock */ thisLock = false; . . . /* Carry on */ . . . This code says: While testAndSet returns false (!testAndSet), do nothing. When testAndSet returns true, do whatever needs to be done in the critical section. (Note that the locking variable is now set true, and competing processes using the same testAndSet routine will have to wait.) When the critical section is complete, indicate that by changing thisLock, the locking variable, to false. Because the test-and-set instruction requires a busy wait, it often is not the best choice of synchronization mechanism. However, this approach is quite useful for coordinating threads in an operating system on a multiprocessor machine. The effect of the busy wait is much less onerous when the waiting thread has its own processor, and the wait is not likely to be long. There are other variations on the test-and-set instruction idea. A swap or exchange instruction that atomically (without interruption) exchanges the values of two arguments can be used the same way. Here is pseudocode for a swap instruction: void swap(boolean a, boolean b) { boolean temp = a; a = b; b = temp; } This code says: Given two boolean values (a and b can each be either true or false), exchange the two values, using the variable temp for temporary storage. (The void means that this routine does not return any value. It simply swaps a and b.) Application code such as the following uses the swap instruction:
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boolean lock;
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// shared data; initialized to 'false'
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boolean key = true; // not shared; used for this thread only . . . //Wait until key becomes false, which means lock was false while ( key ) swap(lock, key); /* critical section */ ... / Now free the lock / lock = false; ... /* remainder section */ . . . This code says: While the variable key is true, swap the values of key and lock. (When key becomes false, it will mean that lock was false, and because key was true before, lock will now be true.) When key becomes false, execute the critical section. When the critical section is complete, release the lock by setting it back to false. The swap instruction has the same applications and limitations as the test-and-set.
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SEMAPHORES In 1965 one of the most prolific contributors to computer science proposed the construct of a semaphore which could be used to provide mutual exclusion and to avoid the requirement for busy waiting. Edgar Dijkstra proposed that the operating system execute a short series of instructions atomically to provide such a service. The operating system can execute several instructions atomically, because the operating system can turn the interrupt system off while the code for a semaphore is being executed. Basically, the semaphore is a short section of code that operates on an integer variable. A semaphore offers two operations. Dijkstra (from the Netherlands) called these operations proberen (P, or test) and verhogen (V, or increment). As time has passed, other names for these methods have included wait, down, and sleep for P, and signal, up, and wake for V. Multiple names for the same operations can make it difficult to read different accounts of semaphores, but the idea is simple. When a process or thread invokes the P operation, the semaphore P method decrements the integer, and if the value is now less than 0, the operating system blocks the process. Later, some other process will invoke the V operation using the same semaphore. The V operation will increment the value of the integer, and if the integer value is now equal to or greater than zero, the process that was blocked will be unblocked. The semaphore construct is very flexible. Its integer value can be initialized to allow for a variety of uses. Most often it is initialized to 1, which allows the first process access via the P operation, and blocks any other process until the first process completes its critical section and executes the V operation. A semaphore also may be initialized to 0, which will cause the first process that executes a P operation to block. Some other process will have to execute a V operation on the same semaphore before the first process will be unblocked. Programmers can use this mechanism to insure that one of two processes always executes first. A semaphore also may be initialized to some number greater than 1, say 3. That would allow the first three attempts to execute the P operation to succeed without blocking. To show how this might be useful, here is an example of two cooperating programs where one produces a series of n items to be consumed by the other (this is the producer consumer problem, much like the server thread worker thread example we discussed earlier).
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