The major task of a computer is to carry out instruction execution. The key questions that can be asked in this respect are: (a) how are the instructions provided to computer? And (b) how are the instructions executed and interpreted?

Let's answer our second question first. All computers have a Unit which performs arithmetic and logical functions. This Unit is called as Arithmetic and Logic Unit (ALU). However how will computer decide what operation is to be carried out by ALU or we can say who will interpret the operation that is to be performed by ALU?

This interpretation is carried out by Control Unit of computer. The control unit recognizes the binary form of instruction as well as interprets instruction to generate control signals. These control signals then direct ALU to carry out a particular arithmetic or logic function on data. Thus by changing control signal desired function can be performed on data. Or on the other hand the operations that require to be performed on the data can be achieved by providing a set of control signals. So for a new operation one just needs to change set of control signals.

The unit which interprets a code (a machine instruction) to produce respective control signals is called as Control Unit (CU). A program comprises a sequence of codes. Every code is, in effect, an instruction for computer. The hardware interprets each one of these instructions and then generates respective control signals such that desired operation is performed on data.

The Control Unit (CU) and Arithmetic Logic Unit (ALU) together are termed as Central Processing Unit (CPU). CPU is the most significant component of a computer's hardware.

All these arithmetic and logical Operations are carried out in CPU in some special storage areas known as registers. Size of the register is one of the significant considerations in signifying processing capabilities of CPU. Register size indicates to the amount of information which can be held in a register at a time for processing. Larger the register size the faster can be the speed of processing.

But how can data and instructions are put into the computers? The data and instructions to a computer are provided by external environment. It determines that input devices are required in computer. The major responsibility of input devices will be to put data in form of signals which can be recognised by system. Correspondingly we require another component that will report results in proper format. This component is known as output device. These components mutually are termed as input/output (I/O) devices.