Cache locking controls in a multiprocessor

A lock array is provided with bit positions corresponding to each line entry in an associated cache directory. When a lock bit is on, it inhibits the castout, replacement, or invalidation of the associated cache line, which operations are allowed when the lock bit is off. The lock bit may be in an off state while an associated valid bit is set on, but once the lock bit is set on the valid bit cannot be set off until the lock bit is first set off. Lock array controls operate with a replacement selection circuit (which may be conventional) to eliminate each locked line from being a replacement candidate in its congruence class in a set-associative store-in-cache in a multiprocessor (MP). The lock array enables simultaneous reset of all lock bits at each checkpoint without disturbing the status of the associated cache directory. A special type of IE operand request, called a store-interrogate (SI) request, is used to lock the accessed line, whether or not the SI request hits or misses in the cache. Any locked line can continue to receive any fetch, SI, or store cache request from its own IE. Any line remains unlocked as long as it is not accessed by a SI request; that is a line remains unlocked as long as it only receives fetch requests, and fetch requests are generally much more numerous than SI requests. Line locking enables the castout or invalidation of unlocked cache lines during a checkpoint interval.