A set of devices which includes input unit, central processing unit (Primary memory, Arithmetic and Logic Unit, control Unit) and output unit together with secondary storage is known as computer system. In other word, a computer system is a data processing device which following specific instructions, performs a sequence of operations to achieve a desired result. Specially, computer receives input (data or/and instructions) through the input device, processing the data according to the specified instructions in the CPU presents the output result in a readable form on the output device on the paper or CD-ROM or speaker, etc.
The CPU is like the human brain, it has a memory; and just like there is a faculty in the brain, which regulates the functioning of all parts of the body, it has a control unit which regulates all the parts of a computer including its input and output devices. Processing of data is done in the arithmetic and logic unit (ALU). The word “Logic” is used because a computer (Unlike a calculator) has a capacity to compare two numbers, to find out which of the two is greater and then proceed accordingly.
Under the direction of the control unit from the input device (say keyboard) to the primary memory unit, the input is released. The control unit directs the requisite part of input to be sent to the ALU for being processed. After the processing is completed, the control unit directs the ALU to release the processed output to the primary memory. Then, finally the output is released from the memory to the output device, which is directed to accept the output and display or print it. This functioning is described with the help of the direction of arrows in the following diagram. This diagram is called Logical Diagram of Computer System.
