Computers are an essential tool of information technology (IT). They are multi-purpose machines that are used to solve a variety of problems in different fields. The basic working principle of a modern computer is based on the analytical engine designed by Charles Babbage in the 19th century. Computers have changed our daily routine as the entire task performed by us in our daily routine is automated.

Our lives are directly or indirectly affected by the computers. It was in the past era where computers were used in industries. In this era of information, we are dependent on the storage, flow, and processing of data and information which can only be possible with the help of computers. This is the reason a computer is called a multi-purpose machine.

Computer System

The term ‘computer’ is derived from the word ‘compute’, meaning ‘to calculate’. A computer is a programmable electronic machine that accepts data from the user, processes it by performing calculations and operations on it, and generates the desired output results. Computer performs both simple and complex operations, with speed and accuracy.


Computing is not restricted to only mathematical computing but to a variety of logic-based tasks. Computer, process the data as per the given set of instructions. It can perform operations like generating bills, reserving tickets, printing mark-sheets, printing business reports, or communicating messages. Data can be text, number, audio, video, graphs, or animations.

Characteristics and Limitations of a Computer

As we know, computers can execute millions of instructions per second. The computer gives a correct result only if the data and instructions given are correct. The term garbage in, garbage out (GIGO) is used to refer to this feature. Computers are very useful because of their characteristics like speed, accuracy, automation, programming ability, storage, diligence, versatility, and ability to retain data.

Characteristics of Computer


The speed of computer data processing is measured in terms of instructions per second. Modern computers can process the data very fast that is at the rate of millions of instructions per second. It means the calculations which otherwise require hours and days to compute manually, can be completed in a few seconds using computers.


Computers can perform complex arithmetic and logical computations with 100% accuracy. For example, the computer can accurately give the result of division of any two numbers up to ten decimal places.


Computers can automatically perform a given task based on the given instructions. Once data and necessary instructions are stored in the computer memory, human intervention is not required.


Computers can be programmed to execute predefined set of instructions. The program once written can be modified later for the revised task.


Computers can store large volumes of data permanently in secondary storage for a long time. The stored content can be retrieved whenever required. A limited amount of data can be stored, temporarily, in the primary memory.


Computers are machines and hence they can operate consistently for long hours without any break. They can perform long and complex calculations with the same speed and accuracy from the start till the end.


Computers are versatile in nature. They can perform different types of tasks with the same ease. At one moment, you can use the computer to prepare a letter document and the next moment you may play music or print a document.

Limitations of Computer

Computers also have some limitations. Computers do not have emotions, knowledge, experience, and feelings. They can only perform tasks as per the instructions given by the user and do not take their own decisions.

Hardware and Software

A computer needs both hardware and software to function properly.


It consists of mechanical and electronic devices which we can see and touch. CPU, keyboard, mouse, and monitor are examples of hardware.



It consists of programs, operating systems, and the data that resides in the memory and storage devices. Windows, Microsoft Office, LibreOffice, and Photoshop are some examples of software. A computer system is useful only if it consists of both hardware and software.

The Input-Process-Output Concept

A computer is an electronic device that accepts data, processes data, generates output, and stores data. A computer mainly performs the following four functions:


The computer accepts input data from the user via an input device like keyboard. The input data can be characters, word, text, sound, images, document, etc.


The computer processes the input data. It performs arithmetic or logic calculation, editing, modifying a document, etc. During processing, the data, instructions, and the output are stored temporarily in the computer’s main memory.


It is the result generated after the processing of data. The output may be in the form of text, sound, image, document, etc. The computer may display the output on a monitor and send the output to the printer
for printing.


The input data, instructions, and output are stored permanently in the secondary storage devices like hard disk. The stored data can be retrieved, whenever needed.

Main Components of the Computer System

The internal design of a computer differs from one model to another but the basic components of all computers remain the same. The basic working model of a computer is based on the John von Neumann architecture.

John von Neumann proposed the first usable draft of a working computer. It consists of some functional units namely input/output unit, central processing unit (CPU), and memory. A computer has the following three main components — (i) input/output unit (ii) central processing unit (iii) memory unit

Input and output unit

The user interacts with the computer via the I/O unit. The input unit accepts data from the user and the output unit provides the processed data that is the information to the user. The input unit accepts data from the user, converts it into computer understandable form. Similarly, the output unit provides the output in a form that is understandable by the user. The input is provided to the computer using input devices like keyboard and mouse. The commonly used output devices are monitor and printer.

Central Processing Unit (CPU)

It is the main component of the computer. It performs all the processing of input data and is responsible for activating and controlling the operations of other units of the computer. In microcomputers, the CPU is built on a single chip or integrated circuit (IC) and is called microprocessor. Internal architecture of a CPU consists of the following parts —

(i) Arithmetic logic unit (ALU)
(ii) Registers
(iii) Control unit (CU)
(iv) Buses
(v) Clock

Arithmetic logic unit (ALU)

It consists of two units— arithmetic unit and logic unit. The arithmetic unit performs arithmetic operations such as addition, subtraction, multiplication, and division. Logic unit performs comparisons of numbers, letters, and special characters. Logic operations include testing for greater than, less than or equal to condition. ALU performs arithmetic and logic operations and uses registers to hold the data that is being processed.


They are high speed but have low storage in the CPU. They are referred to as the CPU’s working memory and are directly accessed and manipulated by the CPU during instruction execution. They store data, instructions, addresses, and intermediate results of processing. The data and instructions are brought in the registers processing. For example, if two numbers are to be added, both numbers are brought in the registers and added and the result is also placed in a register. There are different registers for different specific purposes. Some of the important registers in CPU are as follows —

Accumulator (ACC): Stores the result of arithmetic and logic operations.
Instruction register (IR): Contains the most recently fetched instruction.
Program counter (PC): Contains the address of next instruction to be processed.
Memory address register (MAR): Contains the address of next location in the memory to be accessed.
Memory buffer register (MBR): Temporarily stores data from memory or the data to be sent to memory.
Data register (DR): Stores the operands and any other data.

The number of registers and the size (number of bits) of each register in a CPU helps to determine the power and the speed of a CPU. The overall number of registers can vary from about ten to many hundreds, depending on the type and complexity of the processor. The size of the register also called word size, indicates the amount of data with which the computer can work at any given time.

The size of a register may be 8, 16, 32, or 64 bits. In 32-bit CPU, each register is 32 bits wide and it can manipulate 32 bits of data at a time. The modern PCs have 32-bit or 64-bit registers and are referred to as 32-bit processor and 64-bit processor. A 64-bit processor can process the data twice as much as a 32-bit processor.

Control unit (CU)

It controls the input, output, and processing activities inside the computer. It maintains the order and controls the operation of the entire system. The control unit interprets the instructions given to the computer, determines the data to be processed, where to store the results (output), and sends the control signals to the devices required for the execution of the instructions. It directs the computer to carry out stored program instructions by communicating with the ALU and the registers.

CU uses the instructions in the instruction register (IR) to decide which circuit needs to be activated. It also instructs the ALU to perform the arithmetic or logic operations. When a program is run, the program counter (PC) register keeps track of the program instruction to be executed next. CU tells when to fetch the data and instructions, what to do, where to store the results, the sequencing of events during processing, etc. CU also holds the CPU’s instruction set, which is a list of all operations that the CPU can perform.


Data is stored as a unit of 8 bits in a register. Each bit is transferred from one register to another by means of a separate wire. This group of eight wires which is used as a common way to transfer data between registers is known as a bus. Bus is a connection between two components to transmit signals between them. Bus is of three major types namely—data bus, control bus, and address bus.


It is an important component of CPU which measures and allocates a fixed time slot for processing each and every micro operation. CPU executes the instructions in synchronization with the clock pulse. The clock speed of a CPU is measured in terms of mega hertz or millions of cycles per second. The clock speed of a CPU varies from one model to another.

Memory unit

It stores the data, instructions, intermediate results and output, temporarily during the processing of data. The memory unit consists of cache memory and primary memory. Primary memory or main memory of the computer is used to store the data and instructions during execution of the instructions. Random access memory (RAM) and read-only memory (ROM) are the primary memory. The input data that is to be processed is brought into the main memory before processing.

The instructions required for processing of data and any intermediate results are also stored in the main memory. The output is stored in memory before being transferred to the output device. CPU can work with the information stored in the main memory. In addition to the main memory, there is another kind of storage device known as the secondary memory. It is nonvolatile memory and is used for permanent storage of data and programs. A program or data that has to be executed is brought into the RAM from the secondary memory. Magnetic disks, optical disks, and magnetic tapes are examples of secondary memory.

Cache memory

During processing, data and instructions are brought to RAM from secondary storage devices. For processing, the data and instructions are accessed from the RAM and stored in the registers. The time taken to move the data between RAM and CPU registers is large. This affects the speed of processing of a computer and results in decreasing the performance of CPU. Cache memory is a very high speed memory placed in between RAM and CPU.

It increases the speed of processing and it is a storage buffer that stores the data that is used more often temporarily and makes it available to the CPU at a fast rate. During processing, CPU first checks cache for the required data. If data is not found in cache, then it looks in the RAM for data. To access the cache memory, CPU does not have to use the motherboard’s system bus for data transfer.

Cache memory is built into the processor and may also be located next to it on a separate chip between the CPU and RAM. Cache built into the CPU is faster than separate cache, running at the speed of the microprocessor itself. However, separate cache is roughly twice as fast as RAM.

Primary memory

It is the main memory of a computer. It is used to store data and instructions during the processing of data. It is semiconductor memory and is of two kinds—random access memory (RAM) and read-only memory (ROM).

Read-only memory (ROM)

It is an essential memory unit in every computer. It holds the boot up program called BIOS (Basic Input Output System) required to boot the computer. This memory is permanent and is not erased when the system is switched off. The memory capacity of ROM varies from 64 KB to 256 KB depending on the model of the computer.

Random access memory (RAM)

It is used to store data and instructions during the execution of programs. CPU accesses the data and the instructions from RAM, as it can access them at a faster speed than the storage devices connected to the input and output unit. The input data that is entered using the input unit is stored in RAM, to be made available during the processing of data.

Similarly, the output data generated after processing is stored in RAM before being sent to the output device. Any intermediate results generated during the processing of program are stored in RAM. RAM is volatile meaning it is temporary and is erased when the computer is switched off. It is a read/write type of memory and thus can be read and written by the user. As it is possible to randomly use any location of this memory, it is known as random access memory. The memory capacity of RAM starts from 640 KB and the present computers have 4 GB to 16 GB RAM.

Secondary memory

It stores data and instructions permanently. The information can be stored in secondary memory for a long time (years), and is generally permanent in nature unless erased by the user. It is a non-volatile memory. It provides backup storage for data and instructions. Hard disk drive and optical disk drives are some examples of storage devices.

The data and instructions that are currently not being used by CPU, but may be required later for processing are stored in secondary memory. It has a higher storage capacity and is also cheaper than the primary memory. But it takes a longer time to access the data and instructions stored in secondary memory than in primary memory.

Magnetic Storage Devices

These use plastic tape or metal or plastic disks coated with magnetic materials. Data is recorded magnetically in these devices. Read/write heads are used to access data from these devices. Hard disk drive is the most popular magnetic storage device now.

Hard disk drive (HDD)

This consists of metal disks coated with magnetic material concealed in dust free containers. Hard disks have a very high storage capacity, high data transfer rates, and low access time. They are more durable, less error prone, and are the most common secondary storage device used
in computers.

Optical storage devices

This is a data storage medium which uses a low power laser beam to read from and write data into it. The laser beam reads the pits and lands (all optical media have pits and lands which are microscopic) as 0s and 1s. It is very cheap to manufacture optical disks in large quantities and it is a popular secondary storage medium. The main types of optical disks are—CD, DVD, and Blu-ray.

Compact disc (CD)

This is an optical storage medium capable of storing up to 700 MB of data. A CD drive uses red laser beams for reading from and writing data into CDs. There are two types of CDs—CD-R and CD-RW. In CD-R (compact disc recordable), data can be written once and read many times whereas in CD-RW (compact disc rewritable) disks, it can be erased and rewritten at any time.

Digital versatile disk (DVD)

This is an optical storage media similar to CD-ROM but with a higher storage capacity. This is achieved by using smaller spots to record data. Recording and reading of data is done using a DVD drive. Here also, red laser beam is used for these operations. The capacity of a DVD varies from 4.37 GB to 15.9 GB. DVDs also come in the form of DVD-ROM and DVD-RW.

Blu-ray DVD

This is an optical disk format developed to enable recording, rewriting, and playback of high definition (HD) video as well as storing huge amounts of data. CD and DVD technologies use red laser to read and write data while Blu-ray format uses a blueviolet laser. The benefit of using a blue-violet laser is that it has a shorter wavelength than a red laser, which makes it possible to focus on the laser spot with greater precision.

This allows data to be packed more tightly. Therefore, it is possible to store more data on the disk even though it is the same size as that of a CD or DVD. The format offers more than five times the storage capacity of traditional DVDs and can hold up to 25 GB on a single layer disc and 50 GB on a dual layer disc.

Semiconductor storage (flash memory)

Flash drives use EEPROM chips for data storage. Flash memory is faster and durable as compared to other types of secondary memory. USB flash drive and flash memory cards are the examples of flash memory. USB flash drive: This is a small external storage device. It is portable and rewritable. The storage capacity of an USB drive currently varies from 2 GB to 128 GB. Flash memory card:

This is of size about 1 inch × 0.75 inches with a thickness of about 2 mm. It has storage capacities in the range of 1 GB–32 GB. It also has a smaller version which is used within cell phones and tablets. These smaller cards are about 6 mm × 3 mm in size and are less than 1 mm thick. These cards have various speed levels as per
their usage.

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