Control Unit

A computer program is a set of instructions which need to be carried out in order to achieve an intended result. Thus it is necessary to identify each of these instructions separately, understand what needs to be done, and to coordinate and control the other parts of the computer to make the program work. The Control Unit is considered to be the brain of the CPU, which caries out the above role. The Control Unit operates based on the instructions it decodes and decides how other parts of the CPU and the rest of the computer systems should work in order that the instruction gets executed in a correct manner. There are two types of control units, the first type is called hardwired control unit. Hardwired control units are constructed using digital circuits and once formed cannot be changed. The other type of control unit is micro programmed control unit. A micro programmed control unit itself decodes and execute instructions by means of executing micro programs. This article will cover how the control unit functions as the core of the computer and how it handle instructions and manages the other parts of the computer focusing on micro programmed control unit.

Micro Operations and Micro Instructions

Micro operation is the basic operation carried put by the CPU, which is performed during one clock cycle. An instruction released to occur a set of micro-operations in called a micro instruction and a set of such micro instruction will makeup a micro program.

Let’s see how these micro programs are implemented in a control unit. The Control Unit can only generate a set of control signals based on the micro-operations. Therefore, for any micro-operation, the control lines would carry either 1 or 0 (on or off). The composition of the signals carried by each control line would make a Control Word, and each micro operation is represented by a control word. The control words may carry different patterns of 1 and 0 which matches to the micro operation it represents.

Horizontal Micro instruction

Vertical Micro instruction

Micro Programmed Control Unit

When executing micro programs, the micro programmed control unit decodes and execute instructions. A sequence of instructions will create complex operations, and the sequencing of such instructions are called micro-programming or firmware.

The control unit generates a set of control signals, where each control signal indicates ‘on’ or ‘off’, thus represented by a bit. It will have a control word for each micro operation, and a sequence of control words for each machine code instruction.

In order to maintain the flow of the process, or to continue the process the control unit adds an address to specify the next micro instruction, depending on conditions. Following image will illustrate how control unit is structured internally.

Once the Sequencing logic unit issues read command, the word specified in control address register is read into control buffer register. The control buffer register contents then generates control signals and next address information which will be required to maintain the flow. The sequencing logic loads new address into control buffer register based on next address information from control buffer register and ALU flags.