OS/390 is the IBM operating system most commonly installed on its S/390 line of mainframe server. It is an evolved and newly renamed version of MVS (Multiple Virtual Storage), IBM's long-time, robust mainframe operating system. By whatever name, MVS has been said to be the operating system that keeps the world going. The payroll, accounts receivable, transaction processing, database management, and other programs critical to the world's largest businesses are usually run on an MVS system. Although MVS tends to be associated with a monolithic, centrally-controlled information system, IBM has in recent years repositioned it as a "large server" in a network-oriented distributed environment that would tend to use a 3-tier application model.

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. OS/390 (and earlier MVS) systems run older applications developed using Common Business Oriented Language and, for transaction programs, Customer Information Control System. Older application programs written in PL/I and Formula Translation 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 . Java applications can be developed and run under OS/390's UNIX environment.

For additional information about major components of OS/390, see MVS. 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 virtual machine, an operating system designed to serve many interactive users at the same time.

This was last updated in October 2008

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