AbstractCoumpter models are now being used for computer system evaluation as well as computer tuning. In this respect, the model must be responsive to the hardware/software characteristics and the operating system under control. This paper addresses the problem of developing a model for virtual memory computer systems which fulfills these requirements.A brief description of the 0S/VS2 MVS is presented, discussing in detail, the features which impact the behavior of the model in question. For each class of transactions, three different times are considered: thinking, waiting, and active time. The computer model has two major components or sub-models: the scheduler sub-model and the queueing sub-model. The scheduler submodel involves the mathematical description of the SRM (System Resources Manager) and the task dispatcher. To obtain a simplified version of it (closed-form solution), GPSS has been used as an intermediate tool. The queueing sub-model involves the CPU and peripheral equipment., These components are treated as a network of processors in which a transaction goes from one processor to another, receiving service at each one of them until completion. The network is considered to be closed.To thoroughly validate such a model, an application to evaluate the performance of an IBM/3032 operating under MVS is included. This application is analyzed step by step and comparisons between model results and computer system measurements are discussed. This analysis leads to a systematic approach for computer model development.