Access method

Access method

An access method is a function of a 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. [ M. A. Auslander, D. C. Larkin, A. L. Scherr: "The Evolution of the MVS Operating System", pages 478-479] ] Access methods provide an API for programmers to transfer data to or from device, and could be compared to device drivers in modern operating systems.

Reasons for introducing access methods

Without access methods, a programmer must write a special program for an I/O channel, a processor dedicated to control peripheral storage device access and data transfer to and from main memory. This processor requires programs written with special instructions, called Channel Command Words (CCWs). Programming those is a complex and arduous task. Channel programs are kicked off from mainframe programs by a "startIO" instruction 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 asks the operating system to issue the startIO on the application's behalf. [ [] IBM Redbook for system programmers]

Access methods provide:
* Ease of programming - programmer would no longer deal with a specific device procedures, including error detection and recovery tactics in each and every program. A program designed to process a sequence of 80-character records would work no matter where the data are stored.
* 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.
* Easy 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.

Storage access methods

Storage-oriented access methods in approximate chronological order:
* BDAM - Basic direct access method
* BSAM - Basic sequential access method
* QSAM - Queued sequential access method
* BPAM - Basic partitioned access method
* ISAM - Indexed sequential access method
* VSAM - Virtual storage access method
* OAM - Object access method

Basic versus queued

Both types of access deal with records of a data set. "Queued" methods are improvement over "basic" ones, as they support internal blocking of data and also often read-ahead scheme. This meant multiple records could be combined into one block for increased performance.

Sequential versus direct

"Sequential" access assumes that records can be processed only sequentially, as opposed to "direct" (or random) access. Some devices, such as magnetic tape, naturally enforce sequential access, but it can be used as well on direct access storage devices (DASD), such as disk drives. In the latter case, a data set written with sequential access can be later processed in a direct manner, and vice versa.

Indexed access is a later improvement of direct access.

Networking access methods

Network-oriented access methods in approximate chronological order:
* BTAM - Basic telecommunications access method
* QTAM - Queued teleprocessing access method
* TCAM - Telecommunications access method
* VTAM - Virtual telecommunications access method

Modern implementations

In the z/OS operating system, two elements provide access methods:
* Data Facility Product
* Communications Server


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