NEC ACOS-4

NEC announced ACOS-4 (Advanced Comprehensive Operating System-4) in May 1974 and started shipping it in October 1975. 　ACOS-4 was developed with a focus on flexibility and expandability for responding to the latest technologies and advancing architecture and credibility in increasing the availability of the system, while maintaining the legacy and compatibility of software assets with NEAC-series 2200 users. It was installed in small-to-medium machines: ACOS System 300 and System 350, and medium-to-large machines: System 400, System 450 and System 500.