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When IBM announced the System/360 mainframe line in April 1964, the plan was to provide just one operating system for batch-oriented use, known simply as the System/360 Operating System (less formally as OS/360, or OS). OS was big, complex and ambitious for its day, and the resulting system was late, buggy and resource-intensive. Installations that couldn't wait, or couldn't afford the hardware required to run OS, opted for one of the smaller, simpler systems that IBM created as alternatives: BOS/360, TOS/360, DOS/360. Eventually, with most of the bugs shaken out OS/360 became IBM's flagship System/360 operating system.

By the time of the final OS/360 release in 1974, system programmers building an OS system had the choice of two control programs: MFT, Multiprogramming with a Fixed number of partition Tasks, and MVT, Multiprogramming with a Variable number of region Tasks. (A third control program option, PCP, Primary Control Program, had been discontinued. PCP could only run one job at a time, which wasted expensive mainframe hardware resouces. Any machine not big enough to run MFT or MVT generally ran DOS/360 instead.)

MVT offered more flexibility and power than MFT, but the price of MVT's advantages was a higher main storage requirement. Because main storage was probably the single most expensive component of System/360 hardware, adding storage was not trivial, and smaller OS shops made do with MFT.

While MVT provided more capabilities than other System/360 operating system alternatives, it was not complete, and add-on products were developed to fill in the gaps. Many users replaced the primitive spool functionality provided natively by MVT with HASP. A few very large installations used ASP to provide not only spooling but loosely coupled multiprocessing and workload management.

Release 21.8, the final version of OS/360, survives on the Web in several places. Virtual tapes containing the vanilla IBM 2311 OS/360 distribution are available from the IBM Public Domain Software Collection at ibiblio.org. Also at ibiblio.org is a turnkey MVT 21.8 system that I built with ASP and HASP preinstalled.

Some OS/360 publications at bitsavers.org that you might find useful:

No system is complete without programs to run on it, and local modifications to make it run as the user desires. Below you will find links to:

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