Abstract: An interactive simulation system for simulating an analog or a digital system defined by mathematical model representing the operation of the system. The mathematical model is represented by a network of function blocks specified by an operator, each function block providing an output corresponding to a primitive mathematical or logical operation on an input. The system iteratively steps through the network to simulate the system at sequential points in time. At least some of the function blocks represent delay functions in which their outputs are in response to the inputs during the previous iteration. The simulation system first receives from an operator the identification of the function blocks to be used in the network, the function blocks being entered from the input of the network to the output, except that any delay blocks are entered last.