the session has been left to timeout through inactivity. When a user logs in, the OS4000 JCL command interpreter SYS.COMM is loaded into the user's COMM process and started. This reads commands from the terminal. A number of system commands are built into SYS.COMM. In the case of a command which isn't built in, executable binary files are loaded into the USER process and run, and text JCL files are opened and processed directly by SYS.COMM itself. A user normally also gets an AIDA process which is privileged and used to load only trusted debugging programs. Main Applications Real-time Process Control accounts for over half of all the OS4000 systems deployed. Of these systems, steel production accounts for a significant proportion. The earlier of these Real-time Process Control systems were upgraded from DOS to OS4000. X.25 Packet Switches account for a significant proportion of systems (although earlier GEC X.25 Packet Switches ran a special operating system called NOS which was a cut down operating system halfway between DOS and OS4000). Civil Command and Control systems, e.g. Fire Service control systems interfacing the emergency telephone operator with the Fire Stations. Prestel (UK) and the public Videotex systems used in many other countries, and many private Viewdata systems. Multi-User Minicomputers, used in many Education and Research establishments. Ports OS4000 was ported to the GEC Series 63 minicomputer where it was known as OS6000. This required the addition of a software Nucleus emulation, as this was not a feature of the GEC Series 63 hardware. GEC Computers dropped OS6000, and the source code was given to Daresbury Laboratory who was the main user of it, and they continued to keep it in step with OS4000 releases for the lifetime of their two GEC Series 63 systems. See also GEC 4000 series minicomputers Babbage (programming language) GEC Computers Limited References Further reading External links GEC 4000 family, Which Computer?, May 1979 The Centre for Computing History Bullet III - A Part of UK Network History Proprietary operating systems Real-time operating systems Time-sharing operating systems GEC Computers 1977 software