MINIMOP was an
operating system An operating system (OS) is system software System software is software designed to provide a platform for other software. Examples of system software include operating systems like macOS macOS (; previously Mac OS X and later ...

operating system
which ran on the
International Computers Limited International Computers Limited (ICL) was a British computer hardware, computer software and computer services company that operated from 1968 until 2002. It was formed through a merger of International Computers and Tabulators (ICT), English Ele ...
(ICL) 1900 series of computers. MINIMOP provided an on-line, time-sharing environment (Multiple Online Programming, or MOP in ICL terminology), and typically ran alongside GEORGE (operating system), George 2 running Batch processing, batch jobs. MINIMOP was named to reflect its role as an alternative to the MOP facilities of GEORGE (operating system), George 3, which required a more powerful machine. MINIMOP would run on all 1900 processors apart from the low-end 1901 and 1902 and required only 16K words of memory and two 4 or 8 million character magnetic disks. Each user was provided with a fixed size file to hold his data, which was subdivided into a number of variable sized ''subfiles''. The command language could be extended with simple macro (computer science), macros.


MINIMOP was implemented as a Multithreading (computer architecture), multithreaded (sub-programmed in ICL terminology) user level program running on the standard ''executive'' (low level operating system) of the ICL 1900. The ''program under control'' facilities of executive were used to run user programs under MINIMOP. All user I/O operations were trapped by MINIMOP and emulated rather than accessing real peripherals. As memory was at a premium user programs would be swapping (memory management), swapped out of memory whenever they needed to wait (for input or output) or when they reached the end of their time slice.


Queen Mary College, London, now Queen Mary, University of London, later developed MAXIMOP, an improved system largely compatible with MINIMOP. The ICL Universities Sales Region started distributing MAXIMOP, and it was used at over 100 sites.


ICL operating systems {{operating-system-stub