MVS

MVS (Multiple Virtual Storage) was the most commonly used operating system on the System/370 and System/390 IBM mainframe computers. It is unrelated to IBM's other mainframe operating system called VM/CMS. First released in 1974, MVS was later renamed by IBM to OS/390 when UNIX services were added, and then to z/OS when 64-bit support was added on the zSeries models; but it remains fundamentally the same operating system. It is descended from SVS, which is in turn descended from MVT, MFT and finally OS/360.

OS/360 originally did not support multitasking. MFT (Multitasking with a Fixed number of Tasks) added multitasking, but only allowed a fixed number of tasks to execute at once. MVT (Multitasking with a Variable number of Tasks) was an enhancement to MFT that supported a variable number of tasks. SVS (Single Virtual Storage) added virtual storage (more commonly known outside IBM as virtual memory), with the same address space being shared by all tasks. Finally, MVS allowed different tasks to have different address spaces.

MVS originally supported 24-bit addressing; as the underlying hardware was extended it was also to progressively support 31-bit and now (as z/OS) 64-bit addressing.

The main interfaces to MVS are JCL (Job Control Language), the batch processing interface, and TSO (Time Sharing Option), the interactive[?] time-sharing interface, which originally was optional but is now a standard component. ISPF is an interface which allows to accomplish the same tasks as TSO but in a menu and form oriented manner.

The system is typically used in business and banking, and applications are often written in COBOL. For some tasks which are difficult to accomplish in COBOL, such as form-based user interaction, a certain language called CICS is used; CICS statements are inserted in the COBOL source code and a preprocessor replaces them with the appropriate COBOL code before the program is compiled.

MVS systems are traditionally accessed by 3270 terminals[?], or by PCs running 3270 emulators; many mainframe applications these days have custom WWW or Windows interfaces, but 3270 access is still necessary for programming and system administration.

Files are called "data sets" in MVS; these files are organized in "catalogs". Many MVS programs expect their data files in EBCDIC rather than in the now more common ASCII.

External Links:

• Thierry Falissard: "MVS: the operating system that keeps the world going", http://os390-mvs.hypermart.net/mvs.htm
• Bob DuCharme: "The Operating Systems Handbook, Part 6: MVS", http://www.snee.com/bob/opsys