Access method: Difference between revisions

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[[Category:{{Short description|APIs in IBM mainframe computer operating systems]]}}

{{See also|Record-oriented filesystem|Record (computer science)|Input/Output Control System|Record Management Services}}

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{{refimprove|date=August 2012}}

{{POV check|talk=NPOV|date=July 2020}}

An '''access method''' is a function of a [[mainframe computer|mainframe]] [[operating system]] that enables access to data on disk, tape or other external devices. They were introduced in 1963 in IBM [[OS/360]] operating system.<ref name=aus>[http://researchweb.watson.ibm.com/journal/rd/255/auslander.pdf M. A. Auslander, D. C. Larkin, A. L. Scherr: "The Evolution of the MVS Operating System", pages 478-479]</ref> Access methods provide an [[API|application programming interface (API)]] for programmers to transfer data to or from device, and could be compared to [[device driver]]s in non-mainframe operating systems.

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An '''access method''' is a function of a [[mainframe computer|mainframe]] [[operating system]] that enables access to data on disk, tape or other external devices. TheyAccess methods were present in several mainframe operating systems since the late 1950s, under a variety of names; the name ''access method'' was introduced in 1963 in the IBM [[OS/360]] operating system.<ref name=aus>[http://researchweb.watson.ibm.com/journal/rd/255/auslander.pdf M. A. Auslander, D. C. Larkin, A. L. Scherr: "The Evolution of the MVS Operating System", pages 478-479] {{webarchive |url=https://web.archive.org/web/20081119054653/http://researchweb.watson.ibm.com/journal/rd/255/auslander.pdf |date=November 19, 2008 }}</ref> Access methods provide an [[API|application programming interface (API)]] for programmers to transfer data to or from device, and could be compared to [[device driver]]s in non-mainframe operating systems, but typically provide a greater level of functionality.

== Purpose of access methods ==

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* Ease of hardware replacement - programmer would no longer alter a program when data should be migrated to newer model of storage device, provided it supports the same access methods.<ref name=aus/>

* Ease shared data set access - an access method is a trusted program, that allows multiple programs to access the same file, while ensuring the basic data integrity and system security.<ref name=aus/>

* Read-ahead - Queued access methods may start as many I/O operations as there are [[data buffer|buffers]] available, anticipating application program requirements.

Unlike systems derived from [[Unix]], where all files and devices are considered to be an unformatted stream of [[byte]]s, mainframes offer a variety of data options and formats, such as varying types and sizes of [[record (computer science)|records]], and different ways of accessing data, such as via record keys. Access methods provide programs a way of dealing with this complexity.

* Programs can read or write a record or block of data and wait until the input/output operation is complete (''queued'' access methods) or allow the operation to be started and the program to continue to run, waiting for the completion at a later time (''basic'' access methods).

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* [[Hierarchical Indexed Direct Access Method]] ([[HIDAM]]),

* [[Hierarchical Indexed Sequential Access Method]] ([[HISAM]]),

* [[Hierarchical Sequential Access Method]] ([[HSAM (computing)|HSAM]]),

* [[Overflow sequential access method]] (OSAM),

* [[Partitioned Hierarchical Direct Access Method]] ([[PHDAM]]),

* [[Partitioned Hierarchical Indexed Direct Access Method]] ([[PHIDAM]]),

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{{Mainframe I/O access methods}}

[[Category:IBM mainframe operating systems]]

[[Category:Computer file systems]]

[[Category:Computer-related introductions in 1963]]

[[Category:IBM mainframe operating systems]]