NEC and Toshiba announced ACOS-6, a large general-purpose operating system that realized multi-dimensional processing, in November 1974, and started shipping it in December. In 1978, the two companies started shipping a version that used a full-scale virtual storage system. 　ACOS-6 maintained the succession of the software assets of the ACOS Series 77, which was the basis of ACOS-6, but strengthened flexibility, expandability and reliability by adopting multi-dimensional processing and virtual storage systems to boost multi-user support. It was installed in large machines of the ACOS Series, systems 600, 600S, 700, 800, 900 and 1000, and in sub-models of the series, systems 650 and 850. 　The multi-dimensional processing and virtual storage systems adopted in these machines were also inherited by successors of ACOS-6 as basic technologies.