In computer science the term "critical section" refers to a section of code for which is not safe to allow muliple processes to concurrently execute, either by time-sharing a single processor or by true simultaneous execution on a multiprocessor system. Such "critical sections" must be protected against concurrent execution by some type of locking mechanism, either a mutex or a spin lock usually, but sometimes by clever coding alone with no locking at all. For example, a concurrently accessible queue or stack may be coded by taking advantage of a special atomic processor instruction that perform a test and increment operation in a single instruction (e.g. the Compaq (nee Digital) Alpha processor has such an instruction), and no locking is required because the critical section has essentially been reduced in length to a single, atomic processor instruction.