Advanced Comprehensive Operating System

Advanced Comprehensive Operating System is a family of mainframe computer operating systems developed by NEC for the Japanese market. It consists of three systems, based on the General Comprehensive [Operating System] family developed by General Electric, Honeywell, and Bull. Two of these systems, ACOS-2 and ACOS-4 are still sold, although only ACOS-4 is under active development. ACOS-6 is an obsolete high-end mainframe platform, which ceased active development in the early 2000s.
The first two models in NEC's SX series of supercomputers, the SX-1 and the SX-2, ran an operating system derived from ACOS-4, which was variously called either SX-OS or SXCP. However, subsequent SX supercomputers, starting with the SX-3, instead ran a derivative of Unix.
In late September 2012, NEC announced a return from IA-64 to the previous NOAH line of proprietary mainframe processors for ACOS-4, now produced in a quad-core variant on 40 nm, called NOAH-6.
ACOS-2 runs on Intel Xeon servers.
In June 2022, i-PX AKATSUKI server equipped with NEC's original processor was released.