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History of IBM mainframe operating systems

Multiple Virtual Storage, more commonly called MVS, was the most commonly used operating system on the System/370 and System/390 IBM mainframe computers. It was developed by IBM, but is unrelated to IBM's other mainframe operating systems, e.g., VSE, VM, TPF.

First released in 1974, MVS was extended by program products with new names multiple times, first to MVS/SE (System Extension), next to MVS/SP (System Product) Version 1, next to MVS/XA (eXtended Architecture), next to MVS/ESA (Enterprise Systems Architecture), next to OS/390 and finally to z/OS (when 64-bit support was added with the zSeries models). IBM added Unix support (originally called OPEN EDITION) in MVS/SP V4.3 and has obtained POSIX and Unix certifications at several different levels. The MVS core remains fundamentally the same operating system. By design, programs written for MVS run on z/OS without modification.

At first IBM described MVS as simply a new release of OS/VS2, but it was, in fact a major rewrite. OS/VS2 release 1 was an upgrade of OS/360 MVT that retained most of the original code and, like MVT, was mainly written in assembly language. The MVS core was almost entirely written in Assembler XF, although a few modules were written in PL/S, but not the performance-sensitive ones, in particular not the Input/Output Supervisor (IOS). IBM's use of "OS/VS2" emphasized upwards compatibility: application programs that ran under MVT did not even need recompiling to run under MVS. The same Job Control Language files could be used unchanged; utilities and other non-core facilities like TSO ran unchanged. IBM and users almost unanimously called the new system MVS from the start, and IBM continued to use the term MVS in the naming of later major versions such as MVS/XA.

Evolution of MVS

OS/360 MFT (Multitasking with a Fixed number of Tasks) provided multitasking: several memory partitions, each of a fixed size, were set up when the operating system was installed and when the operator redefined them. For example, there could be a small partition, two medium partitions, and a large partition. If there were two large programs ready to run, one would have to wait until the other finished and vacated the large partition.

OS/360 MVT (Multitasking with a Variable number of Tasks) was an enhancement that further refined memory use. Instead of using fixed-size memory partitions, MVT allocated memory to regions for job steps as needed, provided enough contiguous physical memory was available. This was a significant advance over MFT's memory management, but had some weaknesses: if a job allocated memory dynamically (as most sort programs and database management systems do), the programmers had to estimate the job's maximum memory requirement and pre-define it for MVT. A job step that contained a mix of small and large programs wasted memory while the small programs ran. Most seriously, memory could become fragmented, i.e., the memory not used by current jobs could be divided into uselessly small chunks between the areas used by current jobs, and the only remedy was to wait some current jobs finished before starting any new ones.

In the early 1970s IBM sought to mitigate these difficulties by introducing virtual memory (which IBM called "virtual storage"), which allowed programs to request address spaces larger than physical memory. The original implementations had a single virtual address space, shared by all jobs. OS/VS1 was OS/360 MFT within a single virtual address space; OS/VS2 SVS was OS/360 MVT within a single virtual address space. So OS/VS1 and SVS in principle had the same disadvantages as MFT and MVT, but the impacts were less severe because jobs could request much larger address spaces and the requests came out of a 16 MiB pool even if physical storage was smaller.

MVS address spaces - global view
MVS (shared part of all address spaces)
App 1 App 2 App 3
Shared virtual area (controlled by MVS)
One application's view
App 1
Shared virtual area

In the mid-1970s IBM introduced MVS, which not only supported virtual storage that was larger than the available real storage,[NB 1] as did SVS, but also allowed an indefinite number of applications to run in different address spaces. Two concurrent programs might try to access the same virtual memory address, but the virtual memory system redirected these requests to different areas of physical memory. Each of these address spaces consisted of three areas: an operating system (one instance shared by all jobs), an application area unique for each application, and a shared virtual area used for various purposes, including inter-job communication. IBM promised that application areas would always be at least 8MB. This made MVS the perfect solution for business problems that resulted from the need to run more applications.

MVS maximized processing potential by providing multiprogramming and multiprocessing capabilities. Like its MVT and OS/VS2 SVS predecessors, MVS supported multiprogramming; program instructions and associated data are scheduled by a control program and given processing cycles. Unlike a single-programming operating system, these systems maximize the use of the processing potential by dividing processing cycles among the instructions associated with several different concurrently running programs. This way, the control program does not have to wait for the I/O operation to complete before proceeding. By executing the instructions for multiple programs, the computer is able to switch back and forth between active and inactive programs.

Early editions of MVS (mid-1970s) were among the first of the IBM OS series to support multiprocessor configurations, though the M65MP variant of OS/360 running on 360 Models 65 and 67 had provided limited multiprocessor support. The 360 Model 67 had also hosted the multiprocessor capable TSS/360, MTS and CP-67 operating systems. Because multiprocessing systems can execute instructions simultaneously, they offer greater processing power than single-processing system. As a result, MVS was able to address the business problems brought on by the need to process large amounts of data.

Multiprocessing systems are either loosely coupled, which means that each computer has access to a common workload, or tightly coupled, which means that the computers share the same real storage and are controlled by a single copy of the operating system. MVS retained both the loosely coupled multiprocessing of Attached Support Processor (ASP)[NB 2] and the tightly coupled multiprocessing of OS/360 Model 65 Multiprocessing. In tightly-coupled systems, two CPUs shared concurrent access to the same memory (and copy of the operating system) and peripherals, providing greater processing power and a degree of graceful degradation if one CPU failed. In loosely-coupled configurations each of a group of processors (single and / or tightly-coupled) had its own memory and operating system but shared peripherals and the operating system component JES3 allowed managing the whole group from one console. This provided greater resilience and let operators decide which processor should run which jobs from a central job queue. MVS JES3 gave users the opportunity to network together two or more data processing systems via shared disks and Channel-to-Channedl Adapters (CTCA's). This capability eventually became available to JES2 users as Multi-Access SPOOL (MAS).

MVS originally supported 24-bit addressing (i.e., up to 16MB). As the underlying hardware progressed, it supported 31-bit (XA and ESA; up to 2048MB) and now (as z/OS) 64-bit addressing. The most significant motives for the rapid upgrade to 31-bit addressing were the growth of large transaction-processing networks, mostly controlled by CICS, which ran in a single address space—and the DB2 relational database management system needed more than 8 MB of application address space to run efficiently. (Early versions were configured into two address spaces that communicated via the shared virtual area, but this imposed a significant overhead since all such communications had transmit via the operating system.)

The main user interfaces to MVS are: Job Control Language (JCL), which was originally designed for batch processing but from the 1970s onwards was also used to start and allocate resources to long-running interactive jobs such CICS; and TSO (Time Sharing Option), the interactive time-sharing interface, which was mainly used to run development tools and a few end-user information systems. ISPF is a TSO application for users on 3270-family terminals (and later, on VM as well), which allows the user to accomplish the same tasks as TSO's command line but in a menu and form oriented manner, and with a full screen editor and file browser. TSO's basic interface is command line, although facilities were added later for form-driven interfaces).

MVS took a major step forward in fault-tolerance, built on the earlier STAE facility, that IBM called software recovery. IBM decided to do this after years of practical real-world experience with MVT in the business world. System failures were now having major impacts on customer businesses, and IBM decided to take a major design jump, to assume that despite the very best software development and testing techniques, that 'problems WILL occur.' This profound assumption was pivotal in adding great percentages of fault-tolerance code to the system, but likely contributed to the system's success in tolerating software and hardware failures. Statistical information is hard to come by to prove the value of these design features (how can you measure 'prevented' or 'recovered' problems?), but IBM has, in many dimensions, enhanced these fault-tolerant software recovery and rapid problem resolution features, over time.

This design specified a hierarchy of error-handling programs, in system (kernel/'privileged') mode, called Functional Recovery Routines, and in user ('task' or 'problem program') mode, called "ESTAE" (Extended Specified Task Abnormal Exit routines) that were invoked in case the system detected an error (actually, hardware processor or storage error, or software error). Each recovery routine made the 'mainline' function reinvokable, captured error diagnostic data sufficient to debug the causing problem, and either 'retried' (reinvoke the mainline) or 'percolated' (escalated error processing to the next recovery routine in the hierarchy).

Thus, with each error the system captured diagnostic data, and attempted to perform a repair and keep the system up. The worst thing possible was to take down a user address space (a 'job') in the case of unrepaired errors. Though it was an initial design point, it was not until the most recent MVS version (z/OS), that recovery program was not only guaranteed its own recovery routine, but each recovery routine now has its own recovery routine. This recovery structure was embedded in the basic MVS control program, and programming facilities are available and used by application program developers and 3rd party developers.

Practically, the MVS software recovery made problem debugging both easier and more difficult. Software recovery requires that programs leave 'tracks' of where they are and what they are doing, thus facilitating debugging—but the fact that processing progresses despite an error can overwrite the tracks. Early date capture at the time of the error maximizes debugging, and facilities exist for the recovery routines (task and system mode, both) to do this.

IBM included additional criteria for a major software problem that required IBM service. If a mainline component failed to initiate software recovery, that was considered a valid reportable failure. Also, if a recovery routine failed to collect significant diagnostic data such that the original problem was solvable by data collected by that recovery routine, IBM standards dictated that this fault was reportable and required repair. Thus, IBM standards, when rigorously applied, encouraged continuous improvement.

IBM introduced an on-demand hypervisor, a major serviceability tool, called Dynamic Support System (DSS), in the first release of MVS. This facility could be invoked to initiate a session to create diagnostic procedures, or invoke already-stored procedures. The procedures 'trapped' special events, such as the loading of a program, device I/O, system procedure calls, and then triggered the activation of the previously-defined procedures. These procedures, which could be invoked recursively, allowed for reading and writing of data, and alteration of instruction flow. Program Event Recording hardware was used. Due to the overhead of this tool, it was removed from customer-available MVS systems. Program-Event Recording (PER) exploitation was performed by the enhancement of the diagnostic "SLIP" command with the introduction of the PER support (SLIP/Per) in SU 64/65 (1978).

Multiple copies of MVS (or other IBM operating systems) could share the same machine if that machine was controlled by VM/370. In this case VM/370 was the real operating system, and regarded the "guest" operating systems as applications with unusually high privileges. As a result of later hardware enhancements one instance of an operating system (either MVS, or VM with guests, or other) could also occupy a Logical Partition (LPAR) instead of an entire physical system.

Multiple MVS instances can be organized and collectively administered in a structure called a systems complex or sysplex, introduced in September, 1990. Instances interoperate through a software component called a Cross-system Coupling Facility (XCF) and a hardware component called a Hardware Coupling Facility (CF or Integrated Coupling Facility, ICF, if co-located on the same mainframe hardware). Multiple sysplexes can be joined via standard network protocols such as IBM's proprietary Systems Network Architecture (SNA) or, more recently, via TCP/IP. The z/OS operating system (MVS' most recent descendant) also has native support to execute POSIX applications.

Files are properly called VSAM files themselves.

The native encoding scheme of MVS and its peripherals is Big Endian EBCDIC, but over time IBM added hardware-accelerated services to perform translation and support of ASCII, Little Endian, and Unicode.

MVS filesystem

Data set names (DSNs, mainframe term for filenames) are organized in a hierarchy whose levels are separated with dots, e.g. "DEPT01.SYSTEM01.FILE01". Each level in the hierarchy can be up to eight characters long. The total filename length is a maximum of 44 characters including dots. By convention, the components separated by the dots are used to organize files similarly to directories in other operating systems. For example there were utility programs that performed similar functions to those of Windows Explorer (but without the GUI and usually in batch processing mode) - adding, renaming or deleting new elements and reporting all the contents of a specified element. However, unlike in many other systems, these levels are not usually[NB 3] actual directories but just a naming convention (like the original Macintosh File System, where folder hierarchy was an illusion maintained by the Finder). TSO supports a default prefix for files (similar to a "current directory" concept), and RACF supports setting up access controls based on filename patterns, analogous to access controls on directories on other platforms.

As with other members of the OS family, MVS' data sets were record-oriented. MVS inherited three main types from its predecessors:

  • Sequential data sets were normally read one record at a time from beginning to end.
  • In BDAM (direct access) data sets, the application program had to specify the physical location of the data it wanted to access (usually by specifying the offset from the start of the data set).
  • In ISAM data sets a specified section of each record was defined as a key that could be used as a key to look up specific records. The key quite often consisted of multiple fields but these had to be contiguous and in the right order; and key values had to be unique. Hence an IBM ISAM file could have only one key, equivalent to the primary key of a relational database table; ISAM could not support foreign keys.

Sequential and ISAM datasets could store either fixed-length or variable length records, and all types could occupy more than one disk volume.

All of these are based on the VTOC disk structure.

Early IBM database management systems used various combinations of ISAM and BDAM datasets - usually BDAM for the actual data storage and ISAM for indexes.

In the early 1970s IBM's virtual memory operating systems introduced a new file management component, VSAM, which provided similar facilities:

  • Entry-Sequenced Datasets (ESDS) provided facilities similar to those of both sequential and BDAM datasets, since they could be read either from start to finish or directly by specifying an offset from the start.
  • Key-Sequenced Datasets (KSDS) were a major upgrade from ISAM: they allowed secondary keys with non-unique values and keys formed by concatenating non-contiguous fields in any order; they greatly reduced the performance problems caused by overflow records in ISAM; and they greatly reduced the risk that a software or hardware failure in the middle of an index update might corrupt the index.

These VSAM formats became the basis of IBM's database management systems, IMS/VS and DB2 - usually ESDS for the actual data storage and KSDS for indexes.

VSAM also included a catalog component used for MVS' master catalog.

Partitioned datasets (PDS) were sequential datasets subdivided into "members" that could be processed as sequential files in their own right. The most important use of PDS was for program libraries - system administrators used the main PDS as a way to allocate disk space to a project and the project team then created and edited the members.

Generation Data Groups (GDGs) were originally designed to support grandfather-father-son backup procedures - if a file was modified, the changed version became the new "son", the previous "son" became the "father", the previous "father" became the "grandfather" and the previous "grandfather" was deleted. But one could set up GDGs with a lot more than 3 generations and some applications used GDGs to collect data from several sources and feed the information to one program - each collecting program created a new generation of the file and the final program read the whole group as a single sequential file (by not specifying a generation in the JCL).

Modern versions of MVS (e.g., z/OS) also support POSIX-compatible "slash" filesystems along with facilities for integrating the two filesystems. That is, the OS can make an MVS dataset appear as a file to a POSIX program or subsystem. These newer filesystems include Hierarchical File System (HFS) (not to be confused with Apple's Hierarchical File System) and zFS (not to be confused with Sun's ZFS).

History and modernity

MVS is now a part of z/OS, older MVS releases are no longer supported by IBM and since 2007 only 64-bit z/OS releases are supported. z/OS supports running older 24-bit and 31-bit MVS applications alongside 64-bit applications.

MVS releases up to 3.8j (24-bit, released in 1981) were freely available and it is now possible to run the MVS 3.8j release in mainframe emulators for free.


MVS/370 is a generic term for all versions of the MVS operating system prior to MVS/XA.[NB 4] The System/370 architecture, at the time MVS was released, supported only 24-bit virtual addresses, so the MVS/370 operating system architecture is based on a 24-bit address. Because of this 24-bit address length, programs running under MVS/370 are each given 16 megabytes of contiguous virtual storage.


MVS/XA, or Multiple Virtual Storage/Extended Architecture, was a version of MVS that supported the 370-XA architecture, which expanded addresses from 24 bits to 31 bits, providing a 2 gigabyte addressable memory area.[1] It also supported a 24-bit legacy addressing mode for older 24-bit applications (i.e. those that stored a 24-bit address in the lower 24 bits of a 32-bit word and utilized the upper 8 bits of that word for other purposes).


MVS/ESA: MVS Enterprise System Architecture. Version of MVS, first introduced as MVS/SP Version 3 in February 1988. Replaced by/renamed as OS/390 late 1995 and subsequently as z/OS.

MVS/ESA OpenEdition: upgrade to Version 4 Release 3 of MVS/ESA announced February 1993 with support for vanilla MVS/ESA SP Version 5 Release 1. Under OS/390, it became UNIX System Services, and has kept that name under z/OS.

Closely related operating systems

Japanese mainframe manufacturers Fujitsu and Hitachi both repeatedly and illegally obtained IBM's MVS source code and internal documentation in one of the 20th century's most famous cases of industrial espionage. Fujitsu relied heavily on IBM's code in its MSP mainframe operating system, and likewise Hitachi did the same for its VOS3 operating system. MSP and VOS3 were heavily marketed in Japan, where they still hold a substantial share of the mainframe installed base, but also to some degree in other countries, notably Australia. Even IBM's bugs and documentation misspellings were faithfully copied. IBM cooperated with the U.S. Federal Bureau of Investigation in a sting operation, reluctantly supplying Fujitsu and Hitachi with proprietary MVS and mainframe hardware technologies during the course of multi-year investigations culminating in the early 1980s—investigations which implicated senior company managers and even some Japanese government officials. Amdahl, however, was not involved in Fujitsu's theft of IBM's intellectual property. Any communications from Amdahl to Fujitsu were through "Amdahl Only Specifications" which were scrupulously cleansed of any IBM IP or any references to IBM's IP.

Subsequent to the investigations, IBM reached multi-million dollar settlements with both Fujitsu and Hitachi, collecting substantial fractions of both companies' profits for many years. Reliable reports indicate that the settlements exceeded US$500,000,000. The three companies have long since amicably agreed to many joint business ventures. For example, in 2002 IBM and Hitachi collaborated on developing the IBM z800 mainframe model.

Because of this historical copying, MSP and VOS3 are properly classified as "forks" of MVS, and many third party software vendors with MVS-compatible products were able to produce MSP- and VOS3-compatible versions with little or no modification.[5][6][7]

When IBM introduced its 64-bit z/Architecture mainframes in the year 2000, IBM also introduced the 64-bit z/OS operating system, the direct successor to OS/390 and MVS. Fujitsu and Hitachi opted not to license IBM's z/Architecture for their quasi-MVS operating systems and hardware systems, and so MSP and VOS3, while still nominally supported by their vendors, maintain most of MVS's 1980s architectural limitations to the present day. Since z/OS still supports MVS-era applications and technologies—indeed, z/OS still contains most of MVS's code, albeit greatly enhanced and improved over decades of evolution—applications (and operational procedures) running on MSP and VOS3 can move to z/OS much more easily than to other operating systems.

See also


  1. ^ Some processors could take more physical storage than the size of a single address space, but still much smaller than the aggregate size of a typical workload's virtual storage.
  2. ^ Via Job Entry Subsystem 3 (JES3)
  3. ^ The exceptions are mostly CVOL and user catalog alias names at the beginning of a dataset name.
  4. ^ OS/VS2 Release 2 through 3.8, MVS/SE and MVS/SP Version 1


  • Bob DuCharme: "The Operating Systems Handbook, Part 06: MVS" (available online here)
  • IBM (June 1978), OS/VS2 MVS Overview, First Edition, GC28-0984-0. 
  1. ^ Hoskins, Jim; Frank, Bob. Exploring IBM eServer zSeries and S/390 Servers: See Why IBM's Redesigned Mainframe Computer Family Has Become More Popular than Ever!. Maximum Press (FL). pp. 210–290.  
  2. ^ IBM (December 1993), Introducing OpenEdition MVS, First Edition, GC23-3010-00. 
  3. ^ IBM (February 1993), OpenEdition MVS POSIX.1 Conformance Document, First Edition, GC23-3011-00. 
  4. ^ IBM (December 1993), OpenEdition MVS POSIX.2 Conformance Document, First Edition, GC23-3012-00. 
  5. ^ Alexander, Charles; Buderi, Bob (5 July 1982). "Now, from the FBI: Japanscam". Time. 
  6. ^ Malone, Michael S. (16 May 1983). "Hitachi-F.B.I. Tapes Are Released". The New York Times. 
  7. ^ Marie Anchordoguy, "Reprogramming Japan: The High Tech Crisis Under Communitarian Capitalism," p. 159.

External links

  • IBM: z/OS V1R11.0 MVS Manuals
  • IBM: z/OS V1R8.0 MVS manuals
  • MVS: the operating system that keeps the world going
  • MVS... a long history
  • Functional structure of IBM virtual storage operating systems Part II: OS/VS2-2 concepts and philosophies by A. L. Scherr
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