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This paper presents a case study of the development of a realistic, but analytically soluble, model of a very complex Computer system. This modeling project required the development and application of new dements of technology to support the modeling process. These included the use of a hierarchic structuring procedure to provide a fonnal control structure for the modeling process, and the use of perturbation analysis to correct known deficiencies in the faithfulness of the model representation of the actual system. Particular attention was paid to the effects of passive resources, and to contention and interference effects caused by competition for shared resources. The system actually modeled is the Advanced Logistics System of the United States Air Force Logistics Command. The computer configuration of this system includes multiple CDC Cyber 70 mainframes, a large quantity of extended core storage, and approximately 100 disk drives. The analytic model was validated against a simulation which uses the logical queue structure of the software system as its basis. Excellent agreement was obtained. The entire modeling process was completed and documented in two months by a team of six analysts.