Edinburgh Multiple Access System
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The Edinburgh Multi-Access System (EMAS) was a mainframe computer operating system developed at the University of Edinburgh, Scotland, during the 1970s. EMAS was developed because none of the manufacturers' operating systems (nor independent systems such as Multics) came close to satisfying the demanding performance requirements of Edinburgh University.
Originally running on the ICL System 4/75 mainframe (based on the design of the IBM 360) it was later reimplemented  on the ICL 2900 series of mainframes (as EMAS 2900 or EMAS-2) where it ran in service until the mid-1980s. Near the end of its life, the refactored version was back-ported (as EMAS-3) to the Amdahl 470 mainframe clone, and thence to the IBM System/370-XA architecture (the latter with help from the University of Kent, although they never actually ran EMAS-3). The NAS VL80 IBM mainframe clone followed later. EMAS was a powerful and efficient general purpose multi-user system which supplied all the computing needs of Edinburgh University and the University of Kent (the only other site outside Edinburgh to adopt the operating system). The final EMAS system (the Edinburgh VL80) was decommissioned in July 1992.
The University of Kent system went live in December 1979, and ran on the least powerful machine in the ICL 2900 range - an ICL 2960, with 2MB of memory, executing about 290k instructions per second. Despite this, it reliably supported around 30 users. This number increased in 1983 with the addition of an additional 2MB of memory and a second Order Code Processor (OCP) (what is normally known as a CPU) running with symmetric multiprocessing. This system was decommissioned in August 1986.
EMAS had several advanced (for the time) features, including dynamic linking, multi-level storage, an efficient scheduler, a separate user-space kernel ('director'), a user-level shell ('basic command interpreter'), a comprehensive archiving system and a memory-mapped file architecture. Such features lead EMAS supporters to claim that their system was superior to Unix for the first 20 years of the latter's existence.