Two continuous system modeling programs

Simulation is a well-established tool, with applications ranging from the study of information flow in business organizations to the investigation of the dynamic behavior of complex mechanical systems. The former has often been treated as a discrete process on digital computers through the use of such discrete system simulation programs as the General Purpose System Simulator (GPSS).
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By contrast, those continuous dynamic systems that are the usual concern of engineers and scientists traditionally have been modeled on analog computers.