OS/360 and Successors - Origin

Origin

See also: History of IBM mainframe operating systems

IBM originally intended that System/360 should have only one batch-oriented operating system, OS/360. It also intended to supply a separate timesharing operating system, TSS/360. There are at least two accounts of why IBM eventually decided to produce other, simpler batch-oriented operating systems: because it found that OS/360 would not fit into the limited memory available on the smaller System/360 models; or because it realized that the development of OS/360 would take much longer than expected. IBM introduced a series of stop-gaps to prevent System/360 hardware sales from collapsing—first BOS/360 (Basic Operating System, for the smallest machines with 8K byte memories), then TOS/360 (Tape Operating System, for machines with at least 16K byte memories and only tape drives), and finally DOS/360 (Disk Operating System), which became a mainstream operating system and is the ancestor of today's widely used z/VSE.

IBM released three variants of OS/360: PCP (Primary Control Program), a short-lived stop-gap which could run only one job at a time, in 1966; MFT (Multiprogramming with Fixed number of Tasks) for the mid-range machines, and MVT (Multiprogramming with Variable number of Tasks) for the top end. MFT and MVT were used until at least 1981, a good five years after their successors had been launched. It is unclear whether the division between MFT and MVT arose because MVT required too much processing power to be usable on mid-range machines or because IBM needed to release a multiprogramming version of OS (MFT) as soon as possible. Initially IBM maintained that MFT and MVT were simply "two configurations of the control program", although later IBM described them as "separate versions of OS/360".

IBM originally wrote OS/360 in assembly language. Later on, IBM wrote some OS/360 code in a new language, Basic Systems Language (BSL), derived from PL/I. A large amount of the TSO code in Release 20 was written in BSL.

TSS/360 was so late and unreliable that IBM canceled it, although IBM later supplied three releases of the TSS/370 PRPQ. By this time CP-67 was running well enough for IBM to offer it without warranty as a timesharing facility for a few large customers.

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