Fetch-and-add

In computer science, the fetch-and-add CPU instruction is a special instruction that atomically modifies the contents of a memory location. It is used to implement mutual exclusion and concurrent algorithms in multiprocessor systems, a generalization of semaphores.

In uniprocessor systems, it is sufficient to disable interrupts before accessing a critical section. However, in multiprocessor systems (even with interrupts disabled) two or more processors could be attempting to access the same memory at the same time. The fetch-and-add instruction allows any processor to atomically increment a value in memory, preventing such multiple processor collisions.

Maurice Herlihy (1991) proved that fetch-and-add has a finite consensus number, in contrast to the compare-and-swap operation. The fetch-and-add operation can solve the wait-free consensus problem for no more than two concurrent processes.