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FUTURE COMPUTING ENVIRONMENTS FOR MICROSIMULATION MODELING 178 used to permit changes to the parameter and data variable information stored in the CTD. The MICRODOC procedure is used to create printed documentation of the current contents of a TRIM2 microdata file. MICRODOC produces this report by reading the header record information on the microdata file and by accessing the information on TRIM2 data variables stored in the CTD. The MRDOC procedure can be used to create documentation of a simulation module by processing the CTD and the FORTRAN source code. As mentioned above under Auxiliary Models, the TRIM system is required to create or age a TRIM2 microdata file. Operating Environme nt The primary user of the TRIM2 system is the Income and Benefits Policy Center at the Urban Institute. The majority of its TRIM2 work is carried out under a contract from ASPE. The Urban Institute staff also assist CBO in using TRIM2 on CBO's computer facility. More recently, the Urban Institute staff have created the HEALTH master routine for use by the U.S. Department of Labor. The ASPE work is currently run at the time-sharing computer facility maintained by the National Institutes of Health (NIH). The NIH computer center provides 24-hour batch service on a large IBM System 3090 mainframe computer. The Urban Institute programmers use the center's Wylbur interactive system for editing and job submission of TRIM2 model runs. This interactive work is done on remote terminals located at the Urban Institute offices. Actual TRIM2 jobs are run in batch, with larger production jobs submitted for overnight (lower-cost) service. Most TRIM2 jobs require the mounting of one or more tape volumes for the appropriate master or active files being processed.19 The TRIM2 Reference Manual contains a desciption of the procedure for moving TRIM2 to another IBM mainframe installation. The target system would require one to four tape drives and sufficient on-line disk space for the TRIM2 CTD and source code and documentation. The current TRIM2 system requires the IBM FORTRAN VS compiler and the VSAM access method for the TRIM2 CTD. Moving the system to a non-MVS IBM system (e.g., VM/CMS) would require converting all the supplied JCL procedures. Moving TRIM2 to a non-IBM mainframe compatible computer would require significant development resources at least for the conversion of the IBM Assembler language input 19 One of the major advantages of the NIH computing center is its continued support for the mounting of tape volumes during the day and overnight. This permits the Urban Institute staff to continue using the original TRIM2 design, which is predicated on the premise of simultaneous processing of large input and output files. Although there are no technical reasons why TRIM2 files could not reside on magnetic disk, the much higher relative cost of mainframe disk storage versus tape storage prohibits its use except for subsets of the TRIM2 database and for smaller time durations.