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 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 those introduced in the 1960s.
|History of IBM mainframe operating systems|
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 $20,000,000 in 2021), 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.
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.
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.
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.
Main articles: Time-sharing and Virtual machine
Further information: the article History of CP/CMS includes information on early time-sharing and virtual machine systems
For an explanation of time-sharing, virtual memory or virtual machines, see § Technical notes.
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:
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:
Up to the early 1960s, IBM's low-end and high-end systems were incompatible, so programs could not easily be transferred from one to another, and the systems often used completely different peripherals such as disk drives. 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. So in 1964, the company announced System/360, a new range of computers which all used the same peripherals and most of which could run the same programs.
IBM originally intended that System/360 should have only one batch-oriented operating system, OS/360. 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:
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. These problems were only magnified because to develop and test System/360 operating systems on real hardware, IBM first had to develop Basic Programming Support/360 (BPS/360). BPS was used to develop the tools needed to develop DOS/360 and OS/360, as well as the first versions of tools it would supply with these operating systems – compilers for FORTRAN and COBOL, utilities including Sort, and above all the assembler it needed to build all the other software.
IBM's competitors took advantage of the delays in OS/360 and the System/360 to announce systems aimed at what they thought were the most vulnerable parts of IBM's market. To prevent sales of System/360 from collapsing, IBM released four stop-gap operating systems:
When IBM announced the S/360-67 it also announced a timesharing operating system, TSS/360, that would use the new virtual memory capabilities of the 360/67. TSS/360 was late and early releases were slow and unreliable. By this time the alternative operating system CP-67, developed by IBM's Cambridge Scientific Center, was running well enough for IBM to offer it "without warranty" as a timesharing facility for a few large customers. CP-67 would go on to become VM/370 and eventually z/VM. IBM ultimately offered three releases of a TSS/370 PRPQ as a migration path for its TSS/360 customers, and then dropped it.
The traumas of producing the System/360 operating systems gave a boost to the emerging discipline of software engineering, the attempt to apply scientific principles to the development of software, and the management of software projects. Frederick P. Brooks, who was a senior project manager for the whole System/360 project and then was given specific responsibility for OS/360 (which was already long overdue), wrote an acclaimed book, The Mythical Man-Month, based on the problems encountered and lessons learned during the project, two of which were:
Main article: DOS/360 and successors
While OS/360 was the preferred operating system for the higher-end System/360 machines, DOS/360 was the usual operating system for the less powerful machines. It provided a set of utility programs, a macro assembler, and compilers for FORTRAN and COBOL. Support for RPG came later, and eventually a PL/I subset. And it supported a useful range of file organizations with access methods to help in using them:
Sequential and ISAM files could store either fixed-length or variable-length records, and all types could occupy more than one disk volume.
DOS/360 also offered BTAM, a data communications facility that was primitive and hard to use by today's standards. But BTAM could communicate with almost any type of terminal, which was a big advantage at a time when there was hardly any standardization of communications protocols.
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. As a result, its descendant z/VSE is still widely used today, as of 2005.
Main article: OS/360 and successors
OS/360 included multiple levels of support, a single API, and much shared code. PCP was a stop-gap version that could run only one program at a time, but MFT ("Multiprogramming with a Fixed number of Tasks") and MVT ("Multiprogramming with a Variable number of Tasks") were used until at least the late 1970s, a good five years after their successors had been launched. It is unclear whether the divisions among PCP, MFT and MVT arose because MVT required too much memory to be usable on mid-range machines or because IBM needed to release a multiprogramming version of OS (MFT) as soon as possible.
PCP, MFT, and MVT had different approaches to managing memory (see below), but provided very similar facilities:
Experience indicated that it was not advisable to install OS/360 on systems with less than 256 KB of memory, which was a common limitation in the 1960s.
Main article: MFT
When installing MFT, customers would specify up to four partitions of memory with fixed boundaries, in which application programs could be run simultaneously. MFT Version II (MFT-II) raised the limit to 52.
Main article: MVT
MVT was considerably larger and more complex than MFT and therefore was used on the most powerful System/360 CPUs. 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. This scheme was more flexible than MFT's and in principle used memory more efficiently, but was liable to fragmentation – after a while one could find that, although there was enough spare memory in total to run a program, it was divided into separate chunks none of which was large enough.: 372–373
In 1971 the Time Sharing Option (TSO) for use with MVT was added. TSO became widely used for program development because it provided: an editor, debuggers for some of the programming languages used on System/360, and the ability to submit batch jobs, be notified of their completion, and view the results without waiting for printed reports. TSO communicated with terminals by using TCAM (Telecommunications Access Method), which eventually replaced the earlier Queued Telecommunications Access Method (QTAM). TCAM's name suggests that IBM hoped it would become the standard access method for data communications, but in fact, TCAM was used almost entirely for TSO and was largely superseded by VTAM from the late 1970s onwards.
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. In 1968 IBM released IMS to handle transaction processing, and in 1969 it released CICS, a simpler transaction processing system which a group of IBM's staff had developed for a customer. IMS was only available for OS/360 and its successors, but CICS was also available for DOS/360 and its successors. For many years this type of product was known as a "TP (teleprocessing) monitor". Strictly speaking TP monitors were not operating system components but application programs which managed other application programs. In the 1970s and 1980s, several third-party TP monitors competed with CICS (notably COM-PLETE, DATACOM/DC, ENVIRON/1, INTERCOMM, SHADOW II, TASK/MASTER and WESTI), but IBM gradually improved CICS to the point where most customers abandoned the alternatives.
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 1957 IBM signed a development contract with American Airlines for the development of a computerized reservations system, which became known as SABRE. The first experimental system went live in 1960 and the system took over all booking functions in 1964 – in both cases using IBM 7090 mainframes. 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.
In SABRE and early versions of PARS there was no separation between the application and operating system components of the software, but in 1968 IBM divided it into PARS (application) and ACP (operating system). Later versions of ACP were named ACP / TPF and then TPF (Transaction Processing Facility) as non-airline businesses adopted this operating system for handling large volumes of online transactions. The latest version is z/TPF.
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.
The last "public domain" version of ACP, hence its last "free" version, was ACP 9.2, which was distributed on a single mini-reel with an accompanying manual set (about two dozen manuals, which occupied perhaps 48 lineal inches of shelf space) and which could be restored to IBM 3340 disk drives and which would, thereby, provide a fully functional ACP system.
ACP 9.2 was intended, primarily, for bank cards like MasterCard and other financial applications, but it could also be utilized for airline reservation systems, too, as by this time ACP had become a more general-purpose OS.
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.
In some instances, production work was run under VS2 under the hypervisor, including, possibly, IMS DB.
The Model 20 was labeled as part of the System/360 range because it could be connected to some of the same peripherals, but it was a 16-bit machine and not entirely program-compatible with other members of the System/360 range. Three operating systems were developed by IBM's labs in Germany, for different 360/20 configurations; DPS—with disks (minimum memory required: 12 KB); TPS—no disk but with tapes (minimum memory required: 8 KB); and CPS—punched-card-based (minimum memory required: 4 KB). These had no direct successors since IBM introduced the System/3 range of small business computers in 1969 and System/3 had a different internal design from the 360/20 and different peripherals from IBM's mainframes.
Main article: IBM System/360 Model 44
The 360/44 is another processor that uses the System/360 peripherals but has a modified instruction set. It was designed for scientific computation using floating point numbers, such as geological or meteorological analyses. Because of the internal differences and the specialized type of work for which it was designed, the 360/44 has its own operating system, PS/44. An optional feature allows a System/360 emulator to run in hidden storage and implement the missing instructions in order to run OS/360. The 360/44 and PS/44 have no direct successors.
System/370 was announced in 1970 with essentially the same facilities as System/360 but with about 4 times the processor speeds of similarly-priced System/360 CPUs. 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.
Most of the new operating systems are distinguished from their predecessors by the presence of "/VS" in their names. "VS" stands for "Virtual Storage". IBM avoided the term "virtual memory", allegedly because the word "memory" might be interpreted to imply that IBM computers could forget things.
All modern IBM mainframe operating systems except z/TPF are descendants of those included in the "System/370 Advanced Functions" announcement – z/TPF is a descendant of ACP, the system which IBM initially developed to support high-volume airline reservations applications.
DOS/VS is the successor to DOS/360, and offers similar facilities, with the addition of virtual memory. In addition to virtual memory DOS/VS provided other enhancements:
DOS/VS was followed by significant upgrades: DOS/VSE and VSE/SP (1980s), VSE/ESA (1991), and z/VSE (2005).
OS/VS1 succeeded MFT, with similar facilities, and adding virtual memory. IBM released fairly minor enhancements of OS/VS1 until 1983, and in 1984 announced that there would be no more. OS/VS1 and TSS/370 are the only IBM System/370 operating systems that do not have modern descendants.
The Special Real Time Operating System (SRTOS), Programming RPQ Z06751, is a variant of OS/VS1 extended to support real-time computing. It was targeted at such industries as electric utility energy management and oil refinery applications.
OS/VS2 Release 1 (SVS) is a replacement for MVT with virtual memory. There are many changes, but it retains the overall structure of MVT.
In 1974 IBM released what it described as OS/VS2 Release 2 but which is a major rewrite that was upwards-compatible with the earlier OS/VS2 SVS. The new system's most noticeable feature is support for multiple virtual address spaces. Different applications thought they were using the same range of virtual addresses, but the new system's virtual memory facilities mapped these to different ranges of real memory addresses. As a result, the new system rapidly became known as "MVS" (Multiple Virtual Storages), the original OS/VS2 became known as "SVS" (Single Virtual Storage). IBM itself accepted this terminology and labelled MVS's successors "MVS/...".
The other distinctive features of MVS are: its main catalog must be a VSAM catalog; it supports "tightly-coupled multiprocessing" (2 or more CPUs share the same memory and copy of the operating system); it includes a System Resources Manager (renamed Workload Manager in later versions) which allows users to load additional work on to the system without reducing the performance of high-priority jobs.
IBM has released several MVS upgrades: MVS/SE, MVS/SP Version 1, MVS/XA (1981), MVS/ESA (1985), OS/390 (1996) and currently z/OS (2001).
Further information on the early history of VM/370: History of CP/CMS and VM (operating system) § History
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. This combination was a direct descendant of CP/CMS. The virtual machine facility was often used for testing new software while normal production work continues on another virtual machine, and the CMS timesharing system was widely used for program development.
VM/370 was followed by a series of upgrades: VM/SEPP ("Systems Extensions Program Product"), VM/BSEPP ("Basic Systems Extensions Program Product"), VM/SP (System Product), VM/SP HPO ("High Performance Option"), VM/XA MA ("Extended Architecture Migration Aid"), VM/XA SF ("Extended Architecture System Facility"), VM/XA SP ("Extended Architecture System Product"), VM/ESA ("Enterprise Systems Architecture"), and z/VM. IBM also produced optional microcode assists for VM and successors, to speed up the hypervisor's emulation of privileged instructions (those which only operating systems can use) on behalf of "guest" operating systems. As part of 370/Extended Architecture, IBM added the Start Interpretive Execution (SIE) instruction to allow a further speedup of the CP hypervisor.
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