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This study develops a procedure for modeling computer systems and for analyzing their performance. Here, computer system performance is measured by the actual or potential utilization of its components, irrespective of the effect on any particular job. The utilization of a component is related to the queue length at that component. A know-ledge of the time varying behavior of the queue length is useful in ex-plaining time averaged utilization measures. A computer program based on a queuing model was written to simulate the behavior of computer systems at the device level. The program is flexible enough to allow parametric variation of such things as the data channel structure, the scheduling methods used, and the service time distributions in force. The computer program also keeps a running time average of the utili-zation factor and mean and standard deviation'of the queue length at eac