IBM mainframe

IBM mainframes are large computer systems produced by IBM since 1952. During the 1960s and 1970s, the term mainframe computer was almost synonymous with IBM products due to their marketshare. Current mainframes in IBM's line of business computers are developments of the basic design of the IBM System/360.

From 1952 into the late 1960s, IBM manufactured and marketed several large computer models, known as the IBM 700/7000 series. The first-generation 700s were based on vacuum tubes, while the later, second-generation 7000s used transistors. These machines established IBM's dominance in electronic data processing ("EDP"). IBM had two model categories: one (701, 704, 709, 7090, 7040) for engineering and scientific use, and one (702, 705, 705-II, 705-III, 7080, 7070, 7010) for commercial or data processing use. The two categories, scientific and commercial, generally used common peripherals but had completely different instruction sets, and there were incompatibilities even within each category.

IBM initially sold its computers without any software, expecting customers to write their own; programs were manually initiated, one at a time. Later, IBM provided compilers for the newly developed higher-level programming languages Fortran and COBOL. The first operating systems for IBM computers were written by IBM customers who did not wish to have their very expensive machines ($2M USD in the mid-1950s) sitting idle while operators set up jobs manually. These first operating systems were essentially scheduled work queues. It is generally thought that the first operating system used for real work was GM-NAA I/O, produced by General Motors' Research division in 1956. IBM enhanced one of GM-NAA I/O's successors, the SHARE Operating System, and provided it to customers under the name IBSYS. As software became more complex and important, the cost of supporting it on so many different designs became burdensome, and this was one of the factors which led IBM to develop System/360 and its operating systems.

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