sentences:  The principle feature of stored program control is one or multiple digital processing units (stored-program computers) that execute a set of computer instructions ("program") stored in the memory of the system by which telephone connections are established, maintained, and terminated in associated electronic circuitry.An immediate consequence of stored program control is automation of exchange functions and introduction of a variety of new telephony features to subscribers.A telephone exchange must run continuously without interruption at all times, by implementing a fault-tolerant design.