When IBM introduced the AP (attached processor) and MP (multi-processor) systems, they had to add instructions that were guaranteed to be atomic since both proc

When IBM introduced the AP (attached processor) and MP (multi-processor) systems, they had to add instructions that were guaranteed to be atomic since both processors had uniform access to main memory. Interestingly enough, they only added two instructions (compare and swap, compare double and swap) to support these configurations. (I think there might have been one or two other supervisor mode-instructions, but there were weirdos like "stop the other processor", for example).