The history of IBM mainframe operating systems is significant within the history of mainframe operating systems, because of IBM's long-standing position as the world's largest hardware supplier of mainframe computers. IBM mainframes run operating systems supplied by IBM and by third parties. The operating systems on early IBM mainframes have seldom been very innovative, except for TSS/360 and virtual machine systems beginning with CP-67. The company's prioritization of backward compatibility has allowed users to adopt new IBM systems more quickly. IBM's current mainframe operating systems, z/OS, z/VM, z/VSE, and z/TPF, are backward compatible successors to those introduced in the 1960s.
Before System/360 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. The first operating systems for IBM computers were written in the mid-1950s by IBM customers with very expensive machines at US$2,000,000 (equivalent to about $24,000,000 in 2025), which had sat idle while operators set up jobs manually, and so they wanted a mechanism for maintaining a queue of jobs. These operating systems run only on a few processor models and are suitable only for scientific and engineering calculations. Other IBM computers or other applications function without operating systems. But one of IBM's smaller computers, the IBM 650, introduced a feature which later became part of OS/360, where if processing is interrupted by a "random processing error" (hardware glitch), the machine automatically resumes from the last checkpoint instead of requiring the operators to restart the job manually from the beginning.
From General Motors GM-NAA I/O to IBSYS General Motors Research division produced GM-NAA I/O for its IBM 701 in 1956 (from a prototype, GM Operating System, developed in 1955), and updated it for the 701's successor. In 1960 the IBM user association SHARE took it over and produced an updated version, SHARE Operating System. Finally IBM took over the project and supplied an enhanced version called IBSYS with the IBM 7090 and IBM 7094 computers. IBSYS required 8 tape drives—fewer if one or more disk drives are present. Its main components are a card-based Job Control language, which is the main user interface; compilers for FORTRAN and COBOL; an assembler; and various utilities including a sort program. In 1958, the University of Michigan Executive System adapted GM-NAA I/O to produce UMES, which was better suited to the large number of small jobs created by students. UMES was used until 1967 when it was replaced by the MTS timesharing system.
BESYS Bell Labs produced BESYS (sometimes referred to as BELLMON) and used it until the mid-1960s. Bell also made it available to others without charge or formal technical support.
FORTRAN Monitor System Before IBSYS, IBM produced for its IBM 709, 7090 and 7094 computers a tape-based operating system whose sole purpose was to compile FORTRAN programs. In fact, FMS and the FORTRAN compiler were on the same tape.
Early time-sharing and virtual machine systems
MIT's Fernando Corbató produced the first experimental time-sharing systems, such as CTSS, from 1957 to the early 1960s, using slightly modified IBM 709, IBM 7090, and IBM 7094 mainframes; these systems were based on a proposal by John McCarthy. In the 1960s IBM's own laboratories created experimental time-sharing systems, using standard mainframes with hardware and microcode modifications to support virtual memory: IBM M44/44X in the early 1960s; CP-40 from 1964 to 1967; CP-67 from 1967 to 1972. The company even released CP-67 without warranty or technical support to several large customers from 1968 to 1972. CP-40 and CP-67 used modified System/360 CPUs, but the M44/44X was based on the IBM 7044, an earlier generation of CPU which was very different internally. These experimental systems were too late to be incorporated into the System/360 series which IBM announced in 1964 but encouraged the company to add virtual memory and virtual machine capabilities to its System/370 mainframes and their operating systems in 1972:
The M44/44X showed that a partial approach to virtual machines was not good enough and that thrashing could severely reduce the speed of virtual memory systems. Thrashing is a condition in which the system runs very slowly because it spends a lot of its time shuffling virtual memory pages between physical memory and disk files. IBM learned from CP-40 and CP-67: how to make the thrashing problem manageable; that its other virtual memory and virtual machine technologies were sufficiently fast and reliable to be used in the high-volume commercial systems which were its core business. In particular, IBM's David Sayre convinced the company that automated virtual memory management could consistently perform at least as well as the best programmer-designed overlay schemes. In 1968 a consulting firm called Computer Software Systems used the released version of CP-67 to set up a commercial time-sharing service. The company's technical team included 2 recruits from MIT (see CTSS above), Dick Orenstein and Harold Feinleib. As it grew, the company renamed itself National CSS and modified the software to increase the number of paying users it could support until the system was sufficiently different that it warranted a new name, VP/CSS. VP/CSS was the delivery mechanism for National CSS' services until the early 1980s, when it switched to IBM's VM/370 (see below). Universities produced three other S/360 time-sharing operating systems in the late 1960s:
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