GPSS

GPSS, short for General Purpose Simulation System, is a general-purpose discrete-event simulation language first developed in the 1960s. It became one of the most widely used simulation languages of its era and helped shape later tools for system modeling.

Models in GPSS are built from a network of blocks, each representing an operation. A transaction, or entity, passes through blocks in sequence. Typical blocks include GENERATE (to create new transactions with interarrival times), QUEUE (to hold entities awaiting service), SEIZE (to claim a resource), RELEASE (to free a resource), ADVANCE (to delay for a specified time), DEPART (to mark service completion), and TERMINATE (to remove a transaction from the model). Additional blocks handle routing, timing, and statistics collection. The flow is transaction-based: a new transaction is created, may wait in a queue, seize resources, experience delays, and eventually terminate.

GPSS runs as a discrete-event simulation: time advances to the next scheduled event, and an event list controls the sequencing of transactions and block actions. Statistics are collected on resource usage, queue lengths, delays, and other performance measures. The language supports stochastic input and a variety of distributions for arrival and service times.

Applications have included manufacturing systems, logistics and supply chains, telecommunications, and computer-system performance modeling. It is commonly used in education to illustrate queueing theory and discrete-event concepts.

Variants and legacy: Over time, several implementations appeared, including GPSS/H and GPSS World, which extended the language with modern features and graphical interfaces. Today, GPSS remains in use in some industries and academic settings, valued for its concise, block-oriented modeling approach and its historical role in the development of discrete-event simulation.