11.12 R4000 Processor Synchronization Support
In other words, a processor can lock out other processors from accessing shared data when the processor is in a critical section, a part of program in which no more than a fixed number of processors is allowed to execute. In the case of test-and-set, only one processor can enter the critical section.
Figure 11-16 illustrates a test-and-set synchronization procedure that uses a semaphore; when the semaphore is set to 0, the shared data is unlocked, and when the semaphore is set to 1, the shared data is locked.
Figure 11-16 Synchronization with Test-and-Set
The processor begins by loading the semaphore and checking to see if it is unlocked (set to 0) in steps 1 and 2. If the semaphore is not 0, the processor loops back to step 1. If the semaphore is 0, indicating the shared data is not locked, the processor next tries to lock out any other access to the shared data (step 3). If not successful, the processor loops back to step 1, and reloads the semaphore.
If the processor is successful at setting the semaphore (step 4), it executes the critical section of code (step 5) and gains access to the shared data, completes its task, unlocks the semaphore (step 6), and continues processing.
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