METHOD AND APPARATUS FOR A COMPARE AND SWAP INSTRUCTION

In a data processing system having linked lists it is useful to be able to add and delete items from such lists while maintaining the integrity of the linked nature of such lists. A new compare and swap instruction provides for effectively simultaneously swapping 2 values which is useful for safely adding and deleting items from linked lists. Prior to the instruction the status of the two value are read at the locations to be swapped. During the instruction these locations are checked again to ensure that no change has occurred at these locations before the instruction performs the swap of the two new values. The instruction then performs the proposed 2 value swap but only if no change has occurred at these two locations where the swap is to be performed.