FORTRAN Monitor System

The history of operating systems running on IBM mainframes is a notable chapter of history of mainframe operating systems, because of IBM's long-standing position as the world's largest hardware supplier of mainframe computers.

Arguably the operating systems which IBM supplied to customers for use on its early mainframes have seldom been very innovative, except for the virtual machine systems beginning with CP-67. But the company's well-known reputation for preferring proven technology has generally given potential users the confidence to adopt new IBM systems fairly quickly. IBM's current mainframe operating systems, z/OS, z/VM, z/VSE, and z/TPF, are backward compatible successors to operating systems introduced in the 1960s, although of course they have been improved in many ways.

Both IBM-supplied operating systems and those supplied by others are discussed here, if notably used on IBM mainframes.

IBM was slow to introduce operating systems: General Motors produced General Motors OS in 1955 and GM-NAA I/O in 1956 for use on its own IBM computers; and in 1962 Burroughs Corporation released MCP and General Electric introduced GECOS, in both cases for use by their customers.

In fact the first operating systems for IBM computers were written by IBM customers who did not wish to have their very expensive machines (US$2M in the mid-1950s) sitting idle while operators set up jobs manually, and so they wanted a mechanism for maintaining a queue of jobs.

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