NEC started shipping ACOS-4/MVP (Multi Virtual Processor) in July 1979. 　ACOS-4/MVP was an operating system for large and super-large machines that was developed under a concept consistent with the operating system ACOS-4 for medium-sized machines, which had been shipped earlier, and maintained upper compatibility. It achieved higher performance than ACOS-4 in basic system control; for example, by changing the virtual memory method and improving the system parallel processing. It also realized core functions for use by multiple users, such as interactive processing and an online processing package. This OS was installed in medium-to-large machines, System 450 and System 550, and super-large machines, System 800, System 750 and System 950.