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The design of a digital system, especially a CPU, consists of two typical individual parts: the control unit and the data processing unit. The function of a control unit is to issue control signals to the data processing unit at specified times that selects and sequences the desired data processing operations. Control units can be implemented by way of two distinctive fundamental approaches: hardwired control and microprogrammed control, and both these approaches with their salient features and implementation details have been discussed in the text with their individual strengths and drawbacks. Microinstructions are often interpreted by nanoinstructions which directly control the hardware. The primary objective of the nanoprogramming approach is to save costly CM space, but at the cost of indulging considerably slower execution due to a two-level CM arrangement that is operated in sequence and can never be overlapped. Nanoprogramming is best suited and is found to be most effective when the same microinstructions in CM are found in heavy use.
i. Load accumulator
ii. Store accumulator
iii. Add to accumulator
iv. Complement accumulator
6.4 Explain the difference between hardwired control and microprogrammed control. Define:
a. Why these flags are needed?
b. Why they are not needed in a microprogrammed control unit?
Segee, B. and Field, J. Microprogramming and Computer Architecture. New York: Wiley, 1991.
Carter, J. Microprocessor Architecture and Microprogramming. Upper Saddle River, NJ: Prentice-Hall, 1996.
Vassiliadis, S., Wrong, S., and Cotofana, S. "Microcode processing: Positioning and directions." lEEEMicro, vol. 23, no. 4, pp. 21-30, July-August 2003.