MVS (Multiple Virtual Storage)

MVS (Multiple Virtual Storage) is IBM's best-known operating system for mainframe and large server computers.

MVS (Multiple Virtual Storage) is an operating system from IBM that continues to run on many of IBM's mainframe and large server computers. MVS has been said to be the operating system that keeps the world going and the same could be said of its successor systems, OS/390 and z/OS. The payroll, accounts receivable, transaction processing, database management, and other programs critical to the world's largest businesses are usually run on an MVS or successor system. Although MVS has often been seen as a monolithic, centrally-controlled information system, IBM has in recent years repositioned it (and successor systems) as a "large server" in a network-oriented distributed environment, using a 3-tier application model.

The follow-on version of MVS, OS/390, no longer included the "MVS" in its name. Since MVS represents a certain epoch and culture in the history of computing and since many older MVS systems still operate, the term "MVS" will probably continue to be used for some time. Since OS/390 also comes with UNIX user and programming interfaces built in, it can be used as both an MVS system and a UNIX system at the same time. A more recent evolution of MVS is z/OS, an operating system for IBM's zSeries mainframes. MVS systems run older applications developed using COBOL and, for transaction programs, CICS. Older application programs written in PL/I and FORTRAN are still running. Older applications use the Virtual Storage Access Method access method for file management and Virtual Telecommunications Access Method for telecommunication with users. The most common program environment today uses the C and C++ languages. DB2 is IBM's primary relational database management system (RDBMS). Java applications can be developed and run under OS/390's UNIX environment.

MVS is a generic name for specific products that included MVS/SP (MVS/System Product), MVS/XA (MVS/Extended Architecture), and MVS/ESA (MVS/Enterprise Systems Architecture). Historically, MVS evolved from OS/360, the operating system for the System/360, which was released in 1964. It later became the OS/370 and the System/370. OS/370 evolved into the OS/VS, OS/MFT, OS/MVT, OS/MVS, MVS/SP, MVS/XA, MVS/ESA, and finally OS/390 and then z/OS. Throughout this evolution, application programs written for any operating system have always been able to run in any of the later operating systems. (This is called forward compatibility.)

An MVS system is a set of basic products and a set of optional products. This allows a customer to choose the set of functions they need and exclude the rest. In practice, most customers probably use almost all of the functions. The main user interface in MVS systems is TSO (Time Sharing Option). The Interactive System Productivity Facility (ISPF) is a set of menus for compiling and managing programs and for configuring the system. The main work management system is either Job Entry Subsystem 2 or 3 (JES2 or JES3). Storage (DASD) management is performed by DFSMS (Distributed File Storage Management Subsystem). MVS is considerably more complex and requires much more education and experience to operate than smaller server and personal computer operating systems.

The Virtual Storage in MVS refers to the use of virtual memory in the operating system. Virtual storage or memory allows a program to have access to the maximum amount of memory in a system even though this memory is actually being shared among more than one application program. The operating system translates the program's virtual address into the real physical memory address where the data is actually located. The Multiple in MVS indicates that a separate virtual memory is maintained for each of multiple task partitions.

Other IBM operating systems for their larger computers include or have included: the Transaction Processing Facility (TPF), used in some major airline reservation systems, and VM, an operating system designed to serve many interactive users at the same time.

This was first published in November 2005

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