Exchange Instruction

operating-systems hardware concurrency

Definition

Exchange Instruction

The exchange instruction (or swap) is an atomic hardware operation that interchanges the contents of a register and a memory location in a single, uninterruptible step. Like Test-and-Set, it implements mutual exclusion.

Mechanism

exchange(Key, Lock)
    temp = Key
    Key = Lock
    Lock = temp

Mutual Exclusion Protocol

int lock = 0

Void process()
    int key = 1
    do
        exchange(key, lock)
    while (key != 0)          // busy wait for lock
    
    /* --- Critical Section --- */
    
    exchange(key, lock)       // release lock
    
    /* --- Remainder Section --- */

Comparison with Test-and-Set

Functionally similar; exchange is often more flexible on architectures with multiple registers. Both rely on busy waiting, making them less efficient for long waits than kernel-level semaphores.