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MVS

From Academic Kids

See MVS (Ukraine) for Ukrainian police authority.

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, first to MVS/XA (eXtended Architecture), next to MVS/ESA (Enterprise Systems Architecture), then to OS/390 when UNIX services were added, and finally to z/OS when 64-bit support was added on the zSeries models; but it remains fundamentally the same operating system. Programs designed and built for MVS can still run on z/OS without modification.

MVS descends from SVS (Single Virtual Storage), which in turn descends from MVT, one of the original variants of OS/360. The first variant of OS/360, PCP (Primary Control Program), did not support multitasking; MVT (Multitasking with a Variable number of Tasks) was an enhancement that provided that functionality. SVS 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 (XA and ESA) 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. COBOL programs were traditionally used with transaction processing systems like IMS and CICS. For a program running in CICS special EXEC CICS statements are inserted in the COBOL source code. A preprocessor (translator) replaces those EXEC CICS statements with the appropriate COBOL code to call CICS (compare with SQL used to call DB2) before the program is compiled. Applications can also be written in other languages such as C, C++, Java, Assembler, Fortran, BASIC, RPG, and REXX. Language support is packaged as a component called "Language Environment" or "LE".

MVS systems are traditionally accessed by 3270 terminals, or by PCs running 3270 emulators; many mainframe applications these days have custom web or Windows interfaces. The z/OS operating system has built-in support for TCP/IP. System management, done in the past with a 3270 terminal, is now done through the Hardware Management Console (HMC). Operator consoles are provided through 2074 emulators. So you are unlikely to see any S/390 or zSeries processor with a real 3270 connected to it. The z/OS operating system also has native support to execute POSIX applications.

Files are called "data sets" in MVS; these files are organized in "catalogs". The native encoding scheme of MVS is Big Endian EBCDIC but MVS provides services to perform translation and support of ASCII, Little Endian, and Unicode.

One instance of an MVS operating system is called a "Logical Partition" or "LPAR"". Multiple LPARs can be organized and collectively administered in a structure called a "Systems Complex" or "Sysplex". The LPARs interoperate through a software component called a "Cross-system Coupling Facility" or "XCF" and a hardware component called a "Hardware Coupling Facility" or "CF". Multiple Sysplexes can be joined via standard network protocols such as TCP/IP or IBM's proprietary Systems Network Architecture (SNA). The Sysplex was introduced in September, 1990. An LPAR can also be initialized to run other operating systems, such as Linux, VSE, TPF, or VM.

MVS filesystem

MVS has a record-oriented filesystem.

Filenames are organised hierarchially with dots, up to a maximum of 44 characters. Generally, the components separated by the dots are used to organise files similarly to directories in other operating systems: the higher level components generally represent project and user name. However, unlike other systems, these are not actual directories, just a naming convention - but TSO supports a default prefix for files (similar to a current directory), and RACF supports setting up access controls based on filename patterns, which are similar to access controls on directories on other platforms.

Partitioned datasets are in some ways similar to single level directories.

MVS supports a wide array of file access methods, mainly due to legacy needs. These include VSAM, BSAM, QSAM, and others.

The MVS filesystem is based on the VTOC disk structure that IBM has used for many years.

Modern versions of MVS also support POSIX compatible hierarchial 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, and an MVS dataset can be made to appear as a file to the POSIX subsystem.

References

  • Bob DuCharme: "The Operating Systems Handbook, Part 6: MVS" (available online here (http://www.snee.com/bob/opsys/part6mvs.pdf))

External links

fr:Multiple Virtual Storage

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