Access method

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An access method is a function of a mainframe operating 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 name access method was introduced in 1963 in the IBM OS/360 operating system. [1] 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, but typically provide a greater level of functionality.

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Purpose of access methods

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. [2]

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 access methods

Storage-oriented access methods in approximate chronological order:

Basic versus queued

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. [1] Queued access methods generally provide better performance, while basic methods provide more flexibility.

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.

Networking access methods

Network-oriented access methods in approximate chronological order:

IMS

The IBM 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.

Modern implementations

In the z/OS operating system, two elements provide access methods:

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<span class="mw-page-title-main">MVS</span> Operating system for IBM mainframes

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In the context of IBM mainframe computers in the S/360 line, a data set or dataset is a computer file having a record organization. Use of this term began with, e.g., DOS/360, OS/360, and is still used by their successors, including the current z/OS. Documentation for these systems historically preferred this term rather than file.

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In IBM mainframe operating systems, basic partitioned access method (BPAM) is an access method for libraries, called partitioned datasets (PDSes) in IBM terminology. BPAM is used in OS/360, OS/VS2, MVS, z/OS, and others.

In IBM mainframe operating systems, Basic sequential access method (BSAM) is an access method to read and write datasets sequentially. BSAM is available on OS/360, OS/VS2, MVS, z/OS, and related operating systems.

In IBM mainframe operating systems, Execute Channel Program (EXCP) is a macro generating a system call, implemented as a Supervisor Call instruction, for low-level device access, where the programmer is responsible for providing a channel program—a list of device-specific commands (CCWs)—to be executed by I/O channels, control units and devices. EXCP for OS/360 and successors is more specifically described in the OS System Programmer's Guide.; EXCP for DOS/360 and successors is more specifically described in DOS Supervisor and I/O Macros. This article mostly reflects OS/360 through z/OS; some details are different for TOS/360 and DOS/360 through z/VSE.

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<span class="mw-page-title-main">OS/360 and successors</span> Operating system for IBM S/360 and later mainframes

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Basic Direct Access Method, or BDAM is an access method for IBM's OS/360 and successors computer operating systems on System/360 and later mainframes. BDAM "consists of routines used in retrieving data from, and storing data onto, direct access devices." BDAM is available on OS/360, OS/VS2, MVS, z/OS, and related high-end operating systems.

<span class="mw-page-title-main">OS/VS2 (SVS)</span>

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In MVS/370 and successor versions of IBM mainframe operating systems, Start Input/Output (STARTIO) is a macro instruction and a "branch entry" for low-level device access, where the programmer is responsible for providing a list of device-specific CCWs, that is, a channel program, to be executed by I/O channels, control units and devices and a number of "exits", several of which may be immediate returns to the Input/Output Supervisor (IOS). Invokers of STARTIO must be in supervisor mode and key 0. STARTIO interfaces directly with the IOS component of MVS.

In IBM mainframe operating systems, Execute Channel Program in Real Storage (EXCPVR) is a macro generating a system call, implemented as a Supervisor Call instruction, for low-level device access, where the programmer is responsible for providing a list of device-specific CCWs, that is, a channel program, to be executed by I/O channels, control units and devices. Additionally, the programmer is expected and required to ensure that the input/output data areas are in real storage and are "fixed", either prior to invoking EXCPVR or via the page fix appendage. EXCPVR can in some situations provide improved performance compared to EXCP.

The Input/Output Supervisor (IOS) is that portion of the control program in the IBM mainframe OS/360 operating system and successors which issues the privileged I/O instructions and supervises the resulting I/O interruptions for any program which requests I/O device operations until the normal or abnormal conclusion of those operations.

In IBM mainframe operating systems, Queued Sequential Access Method (QSAM) is an access method to read and write datasets sequentially. QSAM is available on OS/360, OS/VS2, MVS, z/OS, and related operating systems.

<span class="mw-page-title-main">Distributed Data Management Architecture</span> Open, published architecture for creating, managing and accessing data on a remote computer

Distributed Data Management Architecture (DDM) is IBM's open, published software architecture for creating, managing and accessing data on a remote computer. DDM was initially designed to support record-oriented files; it was extended to support hierarchical directories, stream-oriented files, queues, and system command processing; it was further extended to be the base of IBM's Distributed Relational Database Architecture (DRDA); and finally, it was extended to support data description and conversion. Defined in the period from 1980 to 1993, DDM specifies necessary components, messages, and protocols, all based on the principles of object-orientation. DDM is not, in itself, a piece of software; the implementation of DDM takes the form of client and server products. As an open architecture, products can implement subsets of DDM architecture and products can extend DDM to meet additional requirements. Taken together, DDM products implement a distributed file system.

References

  1. 1 2 3 4 5 M. A. Auslander, D. C. Larkin, A. L. Scherr: "The Evolution of the MVS Operating System", pages 478-479 Archived November 19, 2008, at the Wayback Machine
  2. ↑ ABCs of z/OS System Programming Volume 3
  3. ↑ IBM Corporation. "Types of IMS databases" . Retrieved September 4, 2011.
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