A review of the capabilities of the prototype Extendable Computer System Simulator (ECSS) conceptually described in RM-6132, pointing out strengths and weaknesses of this approach to computer system simulation, and indicating general design principles for computer system simulation language. ECSS complies with these principles in being powerful, flexible, readily extended, and easily used, and in producing models that are economical to rerun. However, the user should be given more leeway for unusual cases, including greater freedom to skip undesired operations via a more extensive set of procedural statements. A computer system simulation language should also include automatic collection of statistics, with output summaries or trace output available on demand. A representative ECSS program is appended. 36 pp. Ref.