Verification of the Logic Structure of an Experimental Switching System on a Digital Computer

T h e experimental switching system, 1 unlike its predecessors t h e electromechanical switching systems, is a universal machine in the sense t h a t its actions are dictated by an internally stored program. In order to verify whether the system behaves according to t h e designers' intent, it is therefore necessary to check both the program and t h e circuitry. Since by far the larger portion of the logical complexity of the switching system resides in t h e program rather t h a n in t h e hardware, it might be supposed t h a t a corresponding proportion of effort would be required to check the program in the system laboratory and, further, t h a t the program could not be checked until the system circuitry was functioning properly. A strikingly similar problem arises in computer development: when a new computer is in final development, programs must be written and verified by t h e time t h e first model of the computer is ready for use. The solution to this problem is, when feasible, to use an existing computer together with a so-called interpretative program which simulates t h e actions of the new computer. In this type of simulation, internal processing can almost always be simulated exactly, b u t at a sacrifice of processing speed; terminal equipment of the computer usually cannot be simulated exactly. From ou!1 present point of view, t h e experimental switching system is a computer, with its central control corresponding to t h e control and 467