Computers are available in different sizes and types. Some computers are designed to be used by a single user only, whereas some computers can handle the needs of many users simultaneously. Computers also differ based on their data-processing abilities. In general, the computers can be classified according to purpose, data handling and functionality.


Classification According to Purpose

Computers are designed for different purposes. They can be used for either general or specific purposes.


General-purpose Computers

A general-purpose computer, as the name suggests, is designed to perform a range of tasks.

These computers have the ability to store numerous programs. These machines can be used for various applications, ranging from scientific to business-purpose applications. Even though such computers are versatile, they generally lack speed and efficiency. The computers used in schools and homes are general-purpose computers.


Specific-purpose Computers

These computers are designed to handle a specific problem or to perform a single specific task. A set of instructions for the specific task is built into the machine. Hence, they cannot be used for other applications unless their circuits are redesigned, that is, they lack versatility. However, being designed for specific tasks, they can provide the result very quickly and efficiently. These computers are used for airline reservations, satellite tracking and air traffic control.


Classification According to Type of Data-handling Techniques

Different types of computers process the data in different manner. According to the basic data-handling techniques, computers can be classified into three categories: analog, digital and hybrid.

1) Analog Computers: A computing machine that operates on data in the form of continuously variable physical quantities is known as analog computer. These are different from a digital computer because an analog computer can perform several mathematical operations simultaneously. It uses continuous variables for mathematical operations and utilizes mechanical or electrical energy.

These computers do not deal directly with the numbers. They measure continuous physical magnitudes (e.g. temperature, pressure and voltage), which are analogous to the numbers under consideration. For example, the petrol pump may have an analog computer that converts the flow of pumped petrol into two measurements: the quantity of petrol and the price of that quantity.

Analog computers are used for scientific and engineering purposes. The main feature of analog computers is that they are very fast in operation as all the calculations are done in ‘parallel mode’. It is very easy to get graphical results directly using analog computer.

However, the accuracy of analog computers is less.


2) Digital Computers: They use digital circuits and are designed to operate on two states, namely bits 0 and 1. They are analogous to states ON and OFF. Data on these computers is represented as a series of Os and 1s. Digital computers are suitable for complex computation and have higher processing speeds. They are programmable. Digital computers are either general purpose computers or special purpose ones. General purpose computers, as their name suggests, are designed for general use while specific purpose computers are meant for specific use.


A computer that operates with information, numerical or otherwise, represented in a digital form is known as digital computer. Such computers process data (including text, sound, graphics and video) into a digital value (in Os and 1s). In digital computers, analog quantities must be converted into digital quantity before processing. In this case, the output will also be digital. If analog output is desired, the digital output has to be converted into analog quantity. The components, which perform these conversions, are the essential parts or peripherals of the digital computer.

Digital computers can give the results with more accuracy and at a faster rate. The accuracy of such computers is limited only by the size of their registers and memory. The desktop PC is a classic example of digital computer.


3) Hybrid Computers: These computers are a combination of both digital and analog computers. In this type of computers, the digital segments perform process control by conversion of analog signals to digital ones. Hybrid computer incorporates the measuring feature of an analog computer and counting feature of a digital computer. For computational purposes, these computers use the analog components and for the storage of intermediate results, digital memories are used. To bind the powers of analog and digital techniques, that is, analog to digital and digital to analog, the hybrid computers comprehensively use converters. Such computers are broadly used in scientific applications, various fields of engineering and industrial control processes.


Classification According to Functionality

Based on physical size, performance and application areas, the computers are generally classified into four major categories: micro, mini, mainframe and super computers.

1) Micro Computers: A micro computer is a small, low-cost digital computer, which usually consists of a micro processor, a storage unit, an input channel and an output channel, all of which may be on one chip inserted into one or several PC boards.

A computer with a microprocessor and its central processing unit is known as a microcomputer. They do not occupy space as much as mainframes. When supplemented with a keyboard and a mouse, microcomputers can be called personal computers. A monitor, a keyboard and other similar input-output devices, computer memory in the form of RAM and a power supply unit come packaged in a microcomputer. These computers can fit on desks or tables and prove to be the best choice for single-user tasks. IBM-PCPentium 100, IBM-PC Pentium 200 and Apple Macintosh are some of the examples of micro computers. Micro computers include desktop, laptop and hand-held models such as personal digital assistants PDAS).


a) Desktops: A desktop computer or personal computer (PC) is the most common type of micro computer.

A desktop is intended to be used on a single location. The spare parts of a desktop computer are readily available at relatively lower costs. Power consumption is not as critical as that in laptops. Desktops are widely popular for daily use in the workplace and households.



b) Laptops: A laptop is a portable computer that a user can carry around. Since the laptop resembles a notebook, it is also known as notebooks. Similar in operation to desktops, laptop computers are miniaturized and optimized for mobile use.

Laptops run on a single battery or an external adapter that charges the computer batteries. They are enabled with an inbuilt keyboard, touch pad acting as a mouse and a liquid crystal display. Their portability and capacity to operate on battery power have proven to be of great help to mobile users. The biggest advantage of laptops is that they are lightweight and can be used anywhere and at any time, especially while travelling.

Netbooks: They fall in the category of laptops, but are inexpensive and relatively smaller in size. They had a smaller feature set and lesser capacities in comparison to regular laptops, at the time they came into the market. But with passing time, netbooks too began featuring almost everything that notebooks had.


c) A palmtop or handheld PC: Is a very small microcomputer that also sacrifices power for small size and portability. These devices typically look more like a tiny laptop than a PDA, with a flip-up screen and small keyboard. They may use Windows CE or similar operating system for handheld devices.


d) Personal Digital Assistants (PDAs): It is a handheld computer and popularly known as a palmtop. It has a touch screen and a memory card for storage of data. PDAs can also be used as portable audio players, web browsers and smartphones. Most of them can access the Internet by means of Bluetooth or Wi-Fi communication.

Servers: They are computers designed to provide services to client machines in a computer network. They have larger storage capacities and powerful processors. Running on them are programs that serve client requests and allocate resources like memory and time to client machines. Usually they are very large in size, as they have large processors and many hard drives. They are designed to be fail-safe and resistant to crash.


e) Wearable Computers: A record-setting step in the evolution of computers was the creation of wearable computers. These computers can be worn on the body and are often used in the study of behavior modeling and human health. Military and health professionals have incorporated wearable computers into their daily routine, as part of their studies. When the users’ hands and sensory organs are engaged in other activities, wearable computers are of great help in tracking human actions. Wearable computers do not have to be turned on and off. They remain in operation without user intervention.


f) Tablet Computers: Tablets are mobile computers that are very handy to use. They use the touch screen technology. Tablets come with an onscreen keyboard or use a stylus or a digital pen. Apple’s iPad redefined the class of tablet computers.



2) Mainframe: A mainframe computer is a large, powerful computer that handles the processing for many users simultaneously (up to several hundred users). The name mainframe originated after minicomputers appeared in the 1960’s to distinguish the larger systems from the smaller minicomputers.

Users connect to the mainframe using terminals and submit their tasks for processing by the mainframe. A terminal is a device that has a screen and keyboard for input and output, but it does not do its own processing (they are also called dumb terminals since they can’t process data on their own). The processing power of the mainframe is time-shared between all of the users. (Note that a personal computer may be used to “emulate” a dumb terminal to connect to a mainframe or minicomputer; you run a program on the PC that pretends to be a dumb terminal).

Mainframes typically cost several hundred thousand dollars. They are used in situations where a company wants the processing power and information storage in a centralized location. Mainframes are also now being used as high-capacity server computers for networks with many client workstations or for serving files over the Internet.


3) Mini Computer: A minicomputer is a multi-user computer that is less powerful than a mainframe. This class of computers became available in the 1960’s when large scale integrated circuits made it possible to build a computer much cheaper than the then existing mainframes.


4) Supercomputer: A supercomputer is mainframe computer that has been optimized for speed and processing power. The most famous series of supercomputers were designed by the company founded and named after Seymour Cray. The Cray-1 was built in the 1976 and installed at Los Alamos National Laboratory. Supercomputers are used for extremely calculation-intensive tasks such simulating nuclear bomb detonations, aerodynamic flows, and global weather patterns.

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