10
Control Unit
Operation
This chapter discusses about control unit operation details of a processor. After completion
of this chapter, the reader would know about:
R Instruction-cycle details of processors.
R Importance and operational details of the control unit.
R Essential features of data path and control signal design.
C HAPTER OBJECTIVES
10.1 INTRODUCTION
From our discussions so far, we know that whenever any computer is switched on, it starts work-
ing, through its processor. This processor undergoes a power-on reset initialization and starts fetching
executable instructions from program memory starting with the reset-initialized address. This instruc-
tion fetching, its decoding and then its execution are repeated as long as the system is not turned off.
Furthermore, during interrupts and such kind, proper branching from this sequential execution is taken
care of by the processor itself. The question is: how the processor implements all these activities?
Which mechanism within the processor forces it to send out the correct memory address and then read
its information in the form of some binary digits? How does it know that the execution of the ongoing
instruction is over and it has to repeat the whole cycle?
To speak the truth, in the case of some processors the power-down mode also stops external
instruction fetch and execution operation of the processor. Interested reader may refer
deeper-sleep mode of Intel Core2Duo in Appendix D, Section D.4
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