No abstract

The processor in an electronic telephone switching system must be designed to efficiently and reliably process telephone calls. Because of the extended life span of a telephone switching system and the nature of the function it provides, it is advantageous to build a great deal of flexibility into the processor design. This paper provides details of a microprogram control design philosophy for the development of a medium-size processor' that provides the required flexibility. The aspects of the microprogram control that make it very suitable for the design of a processor in a faulttolerant system are also described. I. MICROPROGRAMMING BACKGROUNDIn the 22 years since M. V. Wilkes proposed the concept of micro program control, 1 the basic implementation has not varied significantly. Wilkes recognized even then that replacing the complex, irregular structure of the control section with a series of elementary and se quential microinstructions could result in the following advantages :(i) A more regular and systematic approach to the design of the control section of a machine. (ii) The ability to evolve the details of the implementation until late in the design state of the machine. (Hi) The ability to change or add to the instruction set after con struction of the machine has been completed. (iv) A simplified architecture that more readily lends itself to machine maintenance.Because of the flexibility of a microcontrol design, many uses and variations of the design can be made to optimize the particular design * The name of this processor is the 3A cc. It will be used in No. 3 ESS, No. 2B ESS, and other applications where a fault-tolerant system is required. 183