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In computer science, the test-and-set CPU instruction is a special instruction that atomicallyally tests and modifies the contents of a memory location. It is used to implement semaphoress 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. The processor should still disable its interrupts, as in a uniprocessor system, before executing a test-and-set, to prevent a deadlock if another process running on this processor also accesses that semaphore.