Access method
Encyclopedia
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 API for programmers to transfer data to or from device, and could be compared to device driver
s in non-mainframe operating systems.
) that directs the operating system to issue the STARTIO on the application's behalf.
Access methods provide:
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.
, naturally enforce sequential access, but it can be used as well on direct access storage device
s (DASD), such as disk drives. In the latter case, a data set written with sequential access can be later processed in a direct manner,
operating system, two elements provide access methods:
Mainframe computer
Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...
operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer 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 API for programmers to transfer data to or from device, and could be compared to device driver
Device driver
In computing, a device driver or software driver is a computer program allowing higher-level computer programs to interact with a hardware device....
s in non-mainframe 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 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 instructionSupervisor call instruction
In the System/360 line of IBM mainframe computers up to contemporary zSeries, a Supervisor Call instruction is an instruction used to cause an interrupt in order to perform a Supervisor Call, which is distinct and separate from a system call, usually termed a Program Call .IBM mainframes in the...
) that directs the operating system to issue the STARTIO on the application's behalf.
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 and system security.
Storage access methods
Storage-oriented access methods in approximate chronological order:- BDAM - Basic direct access methodBasic direct access methodIn IBM mainframe operating systems, basic direct access method is an access method to read and write data sets directly. In BDAM, the programmer has complete control of the organization of the file...
- BSAMBasic sequential access methodIn IBM mainframe operating systems, Basic sequential access method is an access method to read and write datasets sequentially. BSAM is available on OS/360, OS/VS2, MVS, z/OS, and related high-end operating systems....
- Basic sequential access methodBasic sequential access methodIn IBM mainframe operating systems, Basic sequential access method is an access method to read and write datasets sequentially. BSAM is available on OS/360, OS/VS2, MVS, z/OS, and related high-end operating systems.... - QSAM - Queued sequential access methodQueued Sequential Access MethodIn IBM mainframe operating systems, Queued sequential access method is an access method to read and write datasets sequentially. QSAM is available on OS/360, OS/VS2, MVS, z/OS, and related high-end operating systems....
- BPAM - Basic partitioned access methodBasic partitioned access methodIn IBM mainframe operating systems, basic partitioned access method is an access method for libraries with a specific structure, called partitioned datasets . BPAM is used in OS/360, OS/VS2, MVS, z/OS, and others...
- ISAMISAMISAM stands for Indexed Sequential Access Method, a method for indexing data for fast retrieval. ISAM was originally developed by IBM for mainframe computers...
- Indexed sequential access method - VSAM - Virtual storage access methodVirtual storage access methodVirtual storage access method an IBM disk file storage access method, first used in the OS/VS1, OS/VS2 Release 1 and Release 2 operating systems, later used throughout the Multiple Virtual Storage architecture and now in z/OS...
- OAMObject access methodObject access method is an access method under z/OS which is designed for the storage of large numbers of large files, such as images. It has a number of distinguishing features, e.g. compared to VSAM:...
- Object access methodObject access methodObject access method is an access method under z/OS which is designed for the storage of large numbers of large files, such as images. It has a number of distinguishing features, e.g. compared to VSAM:...
Basic versus queued
Both types of access deal with recordsStorage record
In computer science, a storage record is:* A group of related data, words, or fields treated as a meaningful unit; for instance, a Name, Address, and Telephone Number can be a "Personal Record"....
of a data set
Data set (IBM mainframe)
data set , dataset , is a computer file having a record organization. The term pertains to the IBM mainframe operating system line, starting with OS/360, and is still used by its successors, including the current z/OS. Those systems historically preferred this term over a file...
. Queued methods are improvement over basic ones, as they support internal blocking
Block (data storage)
In computing , a block is a sequence of bytes or bits, having a nominal length . Data thus structured are said to be blocked. The process of putting data into blocks is called blocking. Blocking is used to facilitate the handling of the data-stream by the computer program receiving the data...
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 tapeMagnetic tape data storage
Magnetic tape data storage uses digital recording on to magnetic tape to store digital information. Modern magnetic tape is most commonly packaged in cartridges and cassettes. The device that performs actual writing or reading of data is a tape drive...
, naturally enforce sequential access, but it can be used as well on direct access storage device
Direct access storage device
In mainframe computers and some minicomputers, a direct access storage device, or DASD , is any secondary storage device which has relatively low access time relative to its capacity....
s (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:- BTAM - Basic telecommunications access methodBasic telecommunications access methodBasic Telecommunications Access Method was a low-level programming interface specified by IBM for use on the IBM System/360 for sending and receiving data through telecommunication lines...
- QTAMQTAMQueued Telecommunications Access Method is an alternative to the simpler Basic Telecommunications Access Method communications access method, introducing built-in queuing. It was developed by IBM as part of DOS/360 and OS/360 and used mainly to transmit batches of data...
- Queued teleprocessing access method - TCAM - Telecommunications access methodTelecommunications Access MethodTelecommunications Access Method is an access method, in IBM's OS/360 and successors computer operating systems on IBM System/360 and later, that provides access to terminals units within a teleprocessing network....
- VTAMVTAMVirtual Telecommunications Access Method is IBM's software package that provides communications via telecommunication devices for mainframe environments. It is the implementation of Systems Network Architecture for mainframes...
- Virtual telecommunications access method
Modern implementations
In the z/OSZ/OS
z/OS is a 64-bit operating system for mainframe computers, produced by IBM. It derives from and is the successor to OS/390, which in turn followed a string of MVS versions.Starting with earliest:*OS/VS2 Release 2 through Release 3.8...
operating system, two elements provide access methods:
- Data Facility Product
- Communications Server