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 those introduced in the 1960s.
[3] These operating systems run only on a few processor models and are suitable only for scientific and engineering calculations.
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.
[4] 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.
[5][6] 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.
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.
[25] IBM concluded that these factors were increasing its design and production costs for both hardware and software to a level that was unsustainable, and were reducing sales by deterring customers from upgrading.
There are at least two accounts of why IBM later decided it should also produce a simpler batch-oriented operating system, DOS/360: System/360's operating systems were more complex than previous IBM operating systems for several reasons, including:[28] This made the development of OS/360 and other System/360 software one of the largest software projects anyone had attempted, and IBM soon ran into trouble, with huge time and cost overruns and large numbers of bugs.
But DOS/360 had significant limitations compared with OS/360, which was used to control most larger System/360 machines: IBM expected that DOS/360 users would soon upgrade to OS/360, but despite its limitations, DOS/360 became the world's most widely used operating system because: DOS/360 ran well on the System/360 processors which medium-sized organizations could afford, and it was better than the "operating systems" these customers had before.
When installing MFT, customers would specify up to four partitions of memory with fixed boundaries, in which application programs could be run simultaneously.
It treated all memory not used by the operating system as a single pool from which contiguous "regions" could be allocated as required by an indefinite number of simultaneous application programs.
System/360's hardware and operating systems were designed for processing batch jobs which in extreme cases might run for hours.
As a result, they were unsuitable for transaction processing, in which there are thousands of units of work per day and each takes between 30 seconds and a very few minutes.
[42][43] In the 1950s airlines were expanding rapidly but this growth was held back by the difficulty of handling thousands of bookings manually (using card files).
In the early 1960s IBM undertook similar projects for other airlines and soon decided to produce a single standard booking system, PARS, to run on System/360 computers.
IBM developed ACP and its successors because: in the mid-1960s IBM's standard operating systems (DOS/360 and OS/360) were batch-oriented and could not handle large numbers of short transactions quickly enough; even its transaction monitors IMS and CICS, which run under the control of standard general-purpose operating systems, are not fast enough for handling reservations on hundreds of flights from thousands of travel agents.
ACP had by then incorporated a hypervisor module (CHYR) which supported a virtual OS (usually VS1, but possibly also VS2) as a guest, with which program development or file maintenance could be accomplished concurrently with the online functions.
[46] Then in 1972 IBM announced "System/370 Advanced Functions", of which the main item was that future sales of System/370 would include virtual memory capability and this could also be retro-fitted to existing System/370 CPUs.
Hence IBM also committed to delivering enhanced operating systems which could support the use of virtual memory.
The Special Real Time Operating System (SRTOS), Programming RPQ Z06751, is a variant of OS/VS1 extended to support real-time computing.
[54] VM/370 combines a virtual machine facility with a single-user system called Conversational Monitor System (CMS); this combination provides time-sharing by allowing each user to run a copy of CMS on a virtual machine.
As part of 370/Extended Architecture, IBM added the Start Interpretive Execution (SIE) instruction[57] to allow a further speedup of the CP hypervisor.