A model for a real-time minicomputer oporating system is described in this paper. The approach taken here was to use the GPSS simulation language to test the efficiency of the operating system model. The model allows the system analyst to experiment with different configuration of priority task and priority devices mixture that can run on the minicomputer operating system. The main objective of this simulation is to identify the devices that may bottleneck the system and to find out how many tasks can be efficiently run on the system at the same time.