CentralNotice Compare-and-swap From Wikipedia, the free encyclopedia Jump to: navigation , search In computer science , compare-and-swap ( CAS ) is an atomic instruction used in multithreading to achieve synchronization . It compares the contents of a memory location to a given value and, only if they are the same, modifies the contents of that memory location to a given new value. This is done as a single atomic operation. The atomicity guarantees that the new value is calculated based on up-to-date information; if the value had been updated by another thread in the meantime, the write would fail. The result of the operation must indicate whether it performed the substitution; this can be done either with a simple Boolean response (this variant is often called compare-and-set ), or by returning the value read from the memory location ( not the value written to it). ...