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Anaccess method is a function of amainframeoperating system that enables access to data on disk, tape or other external devices. Access methods were present in several mainframe operating systems since the late 1950s, under a variety of names; the nameaccess method was introduced in 1963 in the IBMOS/360 operating system.[1] Access methods provide anapplication programming interface (API) for programmers to transfer data to or from device, and could be compared todevice drivers in non-mainframe operating systems, but typically provide a greater level of functionality.
System/360 and successor systems perform input/output using a special program for anI/O channel, a processor dedicated to control peripheral storage device access and data transfer to and from main memory.Channel programs are composed ofchannel command words (CCWs). Programming those is a complex task requiring detailed knowledge of the hardware characteristics. Channel programs are initiated by aSTART IO instruction issued by the operating system. This is usually front ended by theExecute 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.[2]
Access methods provide:
Unlike systems derived fromUnix, where all files and devices are considered to be an unformatted stream ofbytes, mainframes offer a variety of data options and formats, such as varying types and sizes ofrecords, 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 withrecords of adata set.Basic access methods read or write one physical record – block – at a time.Queued methods support internalblocking of data and also often read-ahead scheme.[1] Queued access methods generally provide better performance, while basic methods provide more flexibility.
Sequential access assumes that records can be processed only sequentially, as opposed todirect (or random) access. Some devices, such asmagnetic tape, naturally enforce sequential access, but it can be used as well ondirect 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.
Network-oriented access methods in approximate chronological order:
TheIBM Information Management System (IMS) uses the term "access method" to refer to its methods for manipulating "segments in a database record". These are:
This is a different use of the term from the other access methods mentioned in this article.
In thez/OS operating system, two elements provide access methods:
