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Wired-logic control is efficient but inflexible, whereas stored-program control is flexible but less efficient. The DS 4 system is a judicious attempt to combine the best features of both methods of control. This is the most evident in the telegraph line control unit, via which messages are received and transmitted, for this unit has the ability to absorb unexpected traffic peaks as a normal operating feature. System coordination, message analysis, and processing are handled by a stored-program computer. Simple and repetitive character assembly and disassembly and transfer to the core-store functions are performed by a wired-logic processor, as IS the function of information transfer between bulk storage and core store. The computer used in the system was designed for telecommunications applications. The instructions are simple, whereas addressing is sophisticated. These techniques reduce program storage occupation and hence hardware, and also increase the capacity of the system, which can handle up to ten terminating messages per second. Up to 256 low-speed and 32 medium-speed telegraph lines can be served. Transistor-transistor logic integrated circuits are used for high-speed operation and diode-transistor logic circuits are used for logic elements outside the critical path of high-speed operation. This use of integrated circuits, modular construction, wire-wrapped connections, and clamp-type connectors makes the DS 4 an extremely reliable equipment. A two-stage system of synchronization with complete frequency and phase locking of telegraph bit clocks permits duplex operation with no switchover problems and gives almost perfect reliability.