Implementation of BEDSOCS: An Interactive Simulation Language

Bradford EDucational Simulation language fOr Continuous Systems (BEDSOCS), is an interactive digital computer language designed to allow the easy solution of problems described in terms of ordinary differential equations. The language uses Dartmouth BASIC as the procedural language and, like most BASIC systems, operates interpretively. BEDSOCS has such advanced features as automatic equation sorting, variable-step integration and graphic displays. The equations describing the simulation need not be represented in terms of block diagrams, but rather are entered much like ordinary differential equations. Above all, because BEDSOCS uses an interpreter, there is no compilation time. This allows users to write and debug programs quickly at the cost of longer execution time. BEDSOCS, written by Geoffrey Brown (1973) at the University of Bradford, can be implemented on an HP2100 computer having 16k words of memory. The system uses HP BASIC as a subset and supports floating point firmware and a Tektronix storage display. Many options are available for implementing BEDSOCS. However, the authors used a single user version distributed on paper tape. Using BASIC as a subset, BEDSOCS partitions a program into a control region, which contains BASIC statements, and a dynamic region, which describes the representation of the system to be simulated. In the control region the user can program in BASIC to establish initial conditions, set control variables and perform general computation and input/output. The dynamic region contains the representation of the set of differential equations (see Fig. 1). The control region is divided into the initial region and the terminal region. The initial region precedes the dynamic portion of the BEDSOCS program and is used to set initial conditions and control variables to be used in the dynamic region. The terminal region is executed when the simulation is completed. This region is generally used to perform final calculations, print results, or loop back for iterative differential equation solving runs. The dynamic region of a BEDSOCS program contains the 'representation set'. This is a set of equations and procedure blocks representing the simulation to be performed. Fig. 2 shows a typical procedure block, enclosed by the 'PROCED' and 'PROEND' statements. The 'PROCED' statement has the form: