Generations of computers
1. First Generation: 1946-1956
The first generation of computers relied on vacuum tubes to store and process information. The tubes consumed a lot of power, were short-lived and generated a lot of heat.
2. Second Generation: 1957-1963
This generation of computers relied on transistor technology (invented in 1947 at Bell laboratories) and magnetic core memories. Transistors were more stable and reliable than vacuum tubes.
3. Third Generation: 1964-1979
This generation of computers used integrated circuits (ICs) which were made by combining several transistors together (3 to 15 transistors). magnetic disk was developed

during this period for storage purposes.
4. Fourth Generation: 1979-1989
Computers in this generation used large scale integration (LSI which combined hundreds of transistors) and very large-scale integration (VLSI) circuits. Memories used include3d magnetic disks, bubble memories and optical disks.
5. Fifth Generation: 1990-Present
The major thrust of the fifth generation of computers is distributed computing systems and the merging of telecommunications and computing technology. The technologies currently used and under research during this generation, include parallel architectures, three-dimensional circuit design and super conducting materials.

Computers may be classified in various ways which may include the following:
A. According to physical size.
1. Supercomputers- These are the largest and most powerful computers. complex scientific applications like weather forecasting requires a large amount of data to be manipulated within a very short time. Large supercomputers with faster processing using multiple processors and superior technology are used for complex tasks requiring a lot of computational power.
2. Mainframe –These are the second largest computers. They can process large amounts of data very quickly and therefore used by big companies, banks and government departments as network servers. They are usually kept in an air- conditioned environment in a special room. They act as nodes of large national and international communication networks such as the internet. Examples of mainframes are the IBM4381, ICL 39 series and the CDC Cyber series.
3. Minicomputers- they are physically smaller than the mainframe. They also support a number of concurrent users but are usually slower than mainframes. They are easier to manufacture and maintain hence cheaper. Minicomputers can be used in business and commerce and can be used for applications such as payroll, invoicing and stock control. They are mainly used as corporate servers in large organizations and the government or by medium sized organizations. The first really minicomputer was the PDP-8 launched in 1965.
4. Microcomputers- they are smaller than the minicomputers. The ‘heart’ of the microcomputer system is the microprocessor. The new microprocessor is not only cheaper, it is also faster, smaller, more energy efficient, more reliable and more modular. Computing power came onto the desktop and the low cost of microcomputers led to the explosive proliferation of computers into virtually every area of business activity. Individuals are now using computers more and more i n their own offices and homes.

5. Portable computers-they have got the same capability like the microcomputer. the difference is that they are small enough to be carried around unlike the microcomputers. They are basically used by travelling professionals like doctors.
6. Pocket computers-this is the smallest form factor. They have got less features compared to portable computers. They are small enough to fit in one’s pocket. They include the personal digital assistants (PDA).

B. According to data handling.
a. Analogue computers-they are used to process data which is in a continuous form like temperature
b. Digital computers-these are computers uses to represent data on a wave with constant
values to represent either a 0 or a 1 depending on the encoding technique being used.
c. Hybrid computers-these computers combine both features for analogue computers and for digital computers.

C. According to purpose.
a. General purpose computers-these are computers designed to be used in many application areas e.g. graphics design, architectural design, computer gaming etc.
b. Special purpose computer-these computer scan be use in many application areas but they are dedicated to doing only one special task.
c. Dedicated computers-these computers are designed to do only one task.

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