GPSS

General Purpose Simulation System, usually written GPSS, is a simulation language for discrete-event simulation, a style of modelling in which a system is described as a stream of items moving through a set of services. A GPSS model is built from single-purpose commands called blocks. Transactions are generated, queue for or seize resources such as facilities and storages, wait for a period of time, and are finally removed from the model, while the system collects statistics such as queue lengths and facility utilization automatically. The language grew out of the Gordon Simulator, written by Geoffrey Gordon at the Advanced Systems Development Division of IBM in the early 1960s to study teleprocessing systems. After an internal memorandum in October 1960 it was rewritten and released as a supported IBM program on 27 September 1961 with 25 block types, and later versions such as GPSS II, GPSS III, GPSS/360 and GPSS V extended the set of blocks and attributes. Later implementations include GPSS World for Windows and the teaching tool JGPSS.