In 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

• Fetch-and-add
• Test and Test-and-set
• Load-Link/Store-Conditional

External links

• Description from Encyclopaedia of Delay-Insensitive Systems
• "Wait-free Test-and-Set" by Yehuda Afek
• int testandset(int *lock) - C-callable routine written in Sun Sparc assembly language
• Techniques for mutual exclusion