Control Unit (CU)

This Unit is the directing element in any Computer System, having the responsibility for stored program execution per se. It performs this role by repeatedly following a basic instruction execution cycle for each program instruction in turn, first it fetches the instruction from Main Store into a special CU register called the current instruction register (IR), secondly it decodes (separates) the instruction into parts indicating what is to be done (the operation part) and what informaiton is involved (the operand part(s)), and finally it executes (obeys) the instruction by sending appropriate control signals to the ALU/IOUs and Store so as to achieve the desired instruction result. Hence this is called the fetch-decode-execute cycle.

The control unit is identified as having two main components which are

1) instruction decoder and machine cycle encording

2) timing and control

But the essence of a Control Unit is the timing and control module. This module includes a clock and accepts as inputs the current instruction and some external control signals. Its output consists of control signals to the other components of the CPU plus control signal to the external system bus.

The timing of CPU operation is synchronized by the clock and controlled by the control unit with control signals. Each instruction cycle is divided into from one to five machine cycles; each machine cycle is in turn divided into from three to five states. Each state lasts one clock cycle. During a state, the CPU performs one or a set of simultaneous micro - operations as determined by the control signals.

The number of machine cycle is fixed for a given instruction but varies from one instruction to another. Machine cycles are defined to be equivalent to bus accesses. Thus, the number of machine cycles for an instruction depends on the number of times the CPU must communicate with external devices.

Control Unit Inputs and Outputs

The key inputs to the control unit are the instruction register, the clock, flags and control bus signals. First the instruction register. The control unit will make use of the opcade and will perform different instructions. To simplify the control unit logic, there should be a unique logic input for each opcode. This function can be performed by a decoder, which takes an encorded input and produces a single output. In general, a decoder will have n binary inputs and 2 power n binary outputs.

clock : control unit "keeps Time" using this. The conrol unit causes one micro-operation to be performed for each clock pulse. This is sometimes reffered to as thr processor cycle time or the clock cycle time.

Instruction Register : The opcode of the current instruction is used to determine which micro-operations to perform during the execute cycle.

flags :    These are needed by the control unit to determine the status of the CPU and the outcome of previous ALU operations.

Control Signal from Control Bus :    The control bus portion of the system bus provides signals to the control unit, such as interrupt signals and acknowledgements.

And the outputs are:

Control Signals Whithin the CPU :    These are two types : those that cause data to be moved from one register to another, and those that activate specific ALU functions.

Control Signals to Control Bus :    These are also two types : control signals to memory, and control signals to the I / O modules.