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. Access methods provide an application programming interface (API) for programmers to transfer data to or from device, and could be compared to device drivers in non-mainframe operating systems.

System/360 and successor systems perform input/output using 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. Channel programs are composed of channel command words (CCWs). Programming those is a complex task requiring detailed knowledge of the hardware characteristics. Channel programs are initiated by a START IO instruction issued by the operating system. This is usually front ended by the Execute Channel Program (EXCP) macro for application programmer convenience. EXCP issues an SVC (supervisor call instruction) that directs the operating system to issue the START IO on the application's behalf.

Access methods provide:

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

Storage-oriented access methods in approximate chronological order:

Both types of access deal with records of a data set. Basic access methods read or write one physical record – block – at a time. Queued methods support internal blocking of data and also often read-ahead scheme. Queued access methods generally provide better performance, while basic methods provide more flexibility.

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