Access method: Difference between revisions

Revision as of 23:27, 17 September 2013

Revision as of 00:13, 18 September 2013

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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. 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.

== ReasonsPurpose for introducingof access methods ==

System/360 and successor systems perform input/output using a special program for an [[Channel I/O|I/O channel]], a processor dedicated to control peripheral storage device access and data transfer to and from main memory. These ''channel programs'' are composed of special instructions, called ''channel command words'' (CCWs). Programming those is a complex task requiring detailed knowledge of the hardware characteristics. Channel programs are initiated by a '''STARTIO''' macro issued by the operating system. This is usually front ended by the '''Execute Channel Program''' ([[EXCP]]) macro for application programmer convenience. This macro issues an SVC ([[supervisor call instruction]]) that directs the operating system to issue the STARTIO on the application's behalf.<ref>[http://www.redbooks.ibm.com/redbooks/pdfs/sg246983.pdf] ABCs of z/OS System Programming Volume 3</ref>

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* [[ISAM]] - [[Indexed sequential access method]]

* [[VSAM]] - [[Virtual storage access method]], introduced with [[OS/VS]]

* [[Object access method|OAM]] <!-- the acronym links to other topic --> - [[Object access method]], introduced in MVS/SP (1989)

=== Basic versus queued ===

Both types of access deal with [[storage record|records]] of a [[data set (IBM mainframe)|data set]]. ''QueuedBasic'' access methods areread improvementor write one physical record &ndash; block &ndash; at a time. over ''basicQueued'' ones, as theymethods support internal [[Block (data storage)|blocking]] of data and also often read-ahead scheme.<ref name=aus/> ThisQueued meantaccess multiplemethods recordsgenerally couldprovide bebetter combinedperformance, intowhile onebasic blockmethods forprovide increasedmore performanceflexibility.

=== Sequential versus direct ===