For quite sometime computers have been classified under three main classes - Personal, Mainframe and Super. Although with development in technology, the distinction between all these is becoming blurred. Yet it is important to classify them as it is sometimes useful to differentiate the key elements and architecture among the classes. These classes are:

• Personal computer (PC): A small, single-user computer based on a microprocessor. In addition to the microprocessor, a personal computer has a keyboard for entering data, a monitor for displaying information, and a storage device for saving data.

• Workstation: A powerful, single-user computer. A workstation is like a personal computer, but it has a more powerful microprocessor and a high-quality monitor.

• Minicomputer: A multi-user computer capable of supporting from 10 to hundreds of users simultaneously. The term minicomputer originated in 1960s when it was realized that many computing tasks do not require an expensive contemporary mainframe computers but these tasks can be performed by a small, low-priced computer. Initially minicomputers were 8 bit and 12 bit machines but by 1970s almost all minicomputers were 16 bit machines. With the advancement in technology, the speed, memory size and other characteristics developed and the minicomputer was then used for various stand alone or dedicated applications. Gradually, the architectural requirements of minicomputers grew and a 32 bit
. minicomputer which was called supermini was introduced. It had more peripheral devices, larger memory and could support more users working simultaneously" on the computer in comparison to the previous minicomputers.

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• Mainframe: A powerful multi-user computer capable of supporting thousands of users simultaneously. Mainframe computers are generally 32 bit machines or on the higher side. These are suited to big organizations, to manage high volume applications. Few of the popular mainframe series are MEDHA. Sperry, DEC, IBM, HP, HCL, etc. Mainframes are also used as central host computers in distributed systems. Libraries of application programs developed for mainframe computers are much larger than those of the micro or minicomputers because of their evolution over several decades as families of computing.

• Supercomputer: An extremely fast computer that can perform hundreds of mill;ons of instructions per second. These are amongst the fastest machines in terms of processing speed and use multiprocessing techniques, where a number of processors are used to solve a problem. There are a number of manufacturers who dominate the market of supercomputers - CRAY (CRAY YMp, CRAY 2). ETA (CDC _ ETA 10, ETA 20) and IBM 3090(with vector). The supercomputers are reaching upto speeds well over 25000 million arithmetic operators per second. Supercomputers are being used mainly for weather forecasting, computational fluid dynamics, remote sensing, image processing, bio-medical applications, etc.

• Laptop: As the name indicates, Laptop (Fig.I-15) is a very small size computer that you can carry in your hand or that can be put in a briefcase. Now-a-days, it is very much in use since it is very convenient and can be used while travelling. A laptop computer has in-built rechargeable battery, which provides enough power supply for the computer.