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Input and Output Unit in Computer process

Input and Output Unit in Computer process

Input Unit in Computers

The input unit consists of input devices that are attached to the computer. These devices take input and convert it into binary language that the computer understands. Some of the common input devices are keyboard, mouse, joystick, scanner etc.

Central Processing Unit (CPU)

Once the information is entered into the computer by the input device, the processor processes it. The CPU is called the brain of the computer because it is the control centre of the computer. As the CPU is located on a small chip, it is also called the microprocessor. It first fetches instructions from memory and then interprets them so as to know what is to be done. If required, data is fetched from memory or input device. Thereafter CPU executes or performs the required computation and then either stores the output or displays on the output device. The CPU has three main components  which are responsible for different functions – Arithmetic Logic Unit (ALU) , Control Unit (CU) and  Memory registers.

Arithmetic and Logic Unit (ALU)

The ALU, as its name suggests performs mathematical calculations and takes logical decisions. Arithmetic calculations include addition, subtraction, multiplication and division. Logical decisions involve comparison of two data items to see which one is larger or smaller or equal.

Control Unit

The Control unit coordinates and controls the data flow in and out of CPU and also controls all the operations of ALU, memory registers and also input/output units. It is also responsible for carrying out all the instructions stored in the program. It decodes the fetched instruction, interprets (understands) it and sends control signals to input/output devices until the required operation is done properly by ALU and memory.

Memory Registers

A register is a temporary unit of memory in the CPU. These receive data/information and then this data/information is held in them as per the requirement. Registers can be of different sizes(16 bit , 32 bit , 64 bit and so on) and each register inside the CPU has a specific function like storing data, storing an instruction, storing address of a location in memory etc. The user registers can be used by an assembly language programmer for storing operands, intermediate results etc. Accumulator (ACC) is the main register in the ALU and contains one of the operands of an operation to be performed in the ALU.


Memory attached to the CPU is used for storage of data and instructions and is called internal memory. During processing, it is the internal memory that holds the data. The internal memory is divided into many storage locations, each of which can store data or instructions. Each memory location is of the same size and has an address. With the  help of the address, the computer can find any data easily without having to search the entire memory. The internal memory is also called the Primary memory or Main memory. When the task is performed, the CU makes the space available for storing data and instructions, thereafter the memory is cleared and the memory space is then available for the next task. The time of access of data is independent of its location in memory, therefore this memory is also called Random Access memory (RAM). Primary memory is volatile in nature. That means when the power is switched off, the data stored in this memory is permanently erased. That is why secondary memory is needed to store data and information permanently for later use. Some of the examples of secondary storage devices are hard disk, compact disks, pen drives etc.

Output Unit

The output unit consists of output devices that are attached with the computer. It converts the binary data coming from CPU to human understandable from. The common output devices are monitor, printer, plotter etc.

Interconnection between Functional Components

The interconnection between the functional components of a computer can be done in many ways. In microcomputers we generally see a Common Bus Architecture as shown in the figure below. As we have seen before that a computer consists of input unit that takes input, a CPU that processes the input and an output unit that produces output.  All these devices communicate with each other through a common bus. A bus is a transmission path (set of conducting wires) over which data or information in the form of electric signals, is passed from one component to another in a computer. The bus can be of three types – Address bus, Data bus and Control Bus.

The address bus carries the address location of the data or instruction. The data bus carries data from one component to another and the control bus carries the control signals. As shown in the figure above, the system bus is the common communication path that carries signals to/from CPU, main memory and input/output devices. The input/output devices communicate with the system bus through the controller circuit. This controller circuit helps to manage various input/output devices attached to the computer.