OS4000 is a proprietary operating system introduced by GEC Computers Limited in 1977 as the successor to GEC DOS, for its range of GEC 4000 series 16-bit, and later 32-bit, minicomputers. OS4000 was developed through to late 1990s, and has been in a support-only mode since then. History The first operating systems for the GEC 4000 series were COS (Core Operating System) and DOS (Disk Operating System). These were basically single-user multi-tasking operating systems, designed for developing and running Process control type applications. OS4000 was first released around 1977. It reused many of the parts of DOS, but added multi-user access, OS4000 JCL Command-line interpreter, Batch processing, OS4000 hierarchical filesystem (although on-disk format very similar to the non-hierarchical DOS filesystem). OS4000 JCL was based on the Cambridge University Phoenix command interpreter. OS4000 Rel 3 arrived around 1980, and included Linked-OS — support for Linked OS4000 operating systems to enable multi-node systems to be constructed. The main customer for this was the central computing service of University College London (Euclid), where a multi-node system consisting of a Hub file server and multiple Rim multi-access compute server systems provided service for over 100 simultaneous users. Linked-OS was also used to construct fail-over Process control systems with higher resilience. OS4000 Rel 4 arrived around 1983, and upped the maximum number of user modules to 150 (again, mainly for the University College London Euclid system), together with an enhanced Batch processing system. It also included support for the GEC 4090 processor, which introduced a 32-bit addressing mode. OS4000 Rel 5 introduced a modified version of the OS4000 filesystem called CFSX, in order to allow easier use of larger disks. The initial Rel 5 only supported the CFSX filesystem, but support for the original CFS1 filesystem was reintroduced as well quite quickly. OS4000 Rel 6 introduced support for dual processor systems (GEC 4190D). OS4000 was developed in the UK at GEC Computers Borehamwood offices in Elstree Way, and at GEC Computers Dunstable Development Centre in Woodside Estate, Dunstable. Architecture The architecture of OS4000 is very heavily based around the features of the platform it runs on, the GEC 4000 series minicomputers, and these are rather unusual. They include a feature called Nucleus, which is a combination of a hardware- and firmware-based kernel, which cannot be altered under program control. This means that many of the features typically found in operating system kernels do not need