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CONTROL UNIT
The control unit (CU) is the main component that directs the system operations by sending control signals to the datapath. These signals control the flow of data within the CPU and between the CPU and external units such as memory and I/O. There are mainly two different types of control units: hardwired CU and micro programmed CU.
7.1 Hardwired control:
Control buses generally carry signals between the control unit and other computer components in a clock-driven manner.
The system clock produces a continuous sequence of pulses in a specified duration and frequency. A sequence of steps t0 , t1 , t2 , . . . , (t0 , t1 , t2 , . . .) are used to execute a certain instruction. The op-code field of a fetched instruction is decoded to provide the control signal generator with information about the instruction to be executed.

Step information generated by a logic circuit module is used with other inputs to generate control signals. The signal generator can be specified simply by a set of Boolean equations for its output in terms of its inputs. Figure above shows a block diagram that describes how timing is used in generating control signals.
Fixed logic circuits that correspond directly to the Boolean expressions are used to generate the control signals. Let us explain the implementation using a simple example. Assume that the instruction set of a machine has the three instructions: Inst-x, Inst-y, and Inst-z; and A, B, C, D, E, F, G, and H are control lines. The following table shows the control lines that should be activated for the three instructions at the three steps t0 , t1 , and t2 .

The Boolean expressions for control lines A, B, and C can be obtained as follows:

A =Inst-x . t1 ‏ +Inst-z . t1 = (Inst-x +‏ Inst-z) . t1
B = Inst-x . t0 +‏ Inst-y .t2
C = Inst-x . t1 +‏ Inst-x . t2 ‏ +Inst-y . t2 ‏ + Inst-z .t1
= (Inst-x ‏ + Inst-z) .t1 +‏ (Inst-x +‏ Inst-y) . t2
The following figure shows the logic circuits for these control lines. Boolean expressions for the rest of the control lines can be obtained in a similar way.


The following figure shows the state diagram in the execution cycle of these instructions:

Clearly hardwired control is faster than micro programmed control. However, hardwired control could be very expensive and complicated for complex systems. Hardwired control is more economical for small control units. Hardwired control will require a redesign of the entire systems in the case of any change.