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Test-and-setIn computer science, the test-and-set CPU instruction is a special instruction that atomically tests and modifies the contents of a memory location. It is used to implement semaphores in multiprocessor systems. In uniprocessor systems, it is sufficient to disable interrupts before accessing a semaphore. However, in multiprocessor systems, it is impossible and undesirable to disable interrupts on all processors at the same time; and even with interrupts disabled, two or more processors could be attempting to access the same semaphore's memory at the same time. The test-and-set instruction, allows any processor to atomically test and modify a memory location, preventing such multiple processor collisions. See also External links
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