Editing Compare-and-swap (section)

{{Short description|Atomic computer processor instruction}}In [[computer science]], '''compare-and-swap''' ('''CAS''') is an [[atomic (computer science)|atomic]] [[instruction (computer science)|instruction]] used in [[thread (computer science)#Multithreading|multithreading]] to achieve [[synchronization (computer science)|synchronization]]. It compares the contents of a [[memory location]] with a given (the previous) value and, only if they are the same, modifies the contents of that memory location to a new given 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 logic|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), thus "swapping" the read and written values.