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FUTURE COMPUTING ENVIRONMENTS FOR MICROSIMULATION MODELING 175 the user to define which types of parameter names can be used as parameter values. Once a basic parameter definition has been created, the default values for that parameter for each different year or simulation name can be added to the CTD. Although the CTD contains the default parameter values used by TRIM2, users can override the defaults easily in the setup for a run. The contents of the CTD can be listed at any time to provide printed documentation (see Other Software Facilities below). Model Execution TRIM2 is executed in batch by preparing the necessary IBM job control language (JCL) control cards and the TRIM2 input control cards. The simplest TRIM2 job consists of three steps. The first reads the control cards to determine what simulation or master routines are to be used in the job. This information is used to create a set of control cards for the IBM linkage editor, which is used in the second step to create a custom overlaid TRIM2 program that will use the minimum amount of memory. The size of the custom program is minimized by overlaying the three run phases of a TRIM2 job (i.e., initialization, processing, and summary) and by ensuring that the codes for all simulation modules not referenced in the job never come into memory. The third step is the actual execution of the custom overlaid TRIM2 program. The execution of each TRIM2 run is controlled by a part of the system called the supervisor. The supervisor reads the TRIM2 input control cards, which contain either supervisor or general control parameters, the name of a master or simulation module (see Operating Characteristics above) to be executed, or parameter values for either a general control parameter or a simulation module. The set of simulation modules to be executed defines the TRIM2 run sequence. The run sequence must always start with the RDFILE master module, which is responsible for reading the necessary input master and/or active files. The supervisor calls each simulation module at four different phases during the execution of the TRIM2 job. Each simulation module is called one or more times during each phase. During the initialization phase each simulation module is called twice. The first call permits each module to declare what parameters and variables it will require and permits the supervisor to validate their existence in the CTD. The second call determines where the supervisor will store each parameter or variable to be created. Each simulation module is called once for every household processed during the processing phase. Finally, each simulation module is called once during the summary phase to permit each module to write out the standard reports that describe its execution. Some master routines also have optional tables that are calculated and output upon request. The description above assumes that the user's simulation requirements