a) Mainframe computer: Mainframe computers are very large computers with a very high capacity of storage. It is a high performance computer used for large scale computing purposes that require greater availability and security than a smaller scale machine. Mainframe computers are large and expensive computers used mainly by government institutions, examination department of examinations and large companies to process data of complex nature. It can response several hundred requests very quickly and uses several CPU for data processing. It is very powerful multi-user computer where more than 100 users can use at a time because it works on time sharing mode. It supports a wide range of I/O terminals with high speed. IBM built the first mainframe computer, IBM S/360 in 1964.Minicomputer: Minicomputer is midsized computers used in universities, research labs and smaller corporations. They are small and general purpose system. It is also a multi-user computer and supports more than dozens of people at a time. It is costlier than micro computer. They support a lesser number of terminals than mainframe computers, but their speed is faster than microcomputers. Such computers are used in scientific research, engineering analysis, and industrial process monitoring and control. Examples of minicomputer are IBM 9375 and Motorola 68040.