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12-507: A DCB is one of the many control blocks used in these operating systems. A control block is a data area with a predefined structure, very similar to a C struct , but typically only related to system's functions. A DCB may be compared to a FILE structure in C, but it is much more complex, offering many more options for various access methods . The control block acted as the Application programming interface between Logical IOCS and

24-566: A DCB may be left blank (i.e., these default to zero). The OPEN process results in a merge of the constants and variables specified in the DD JCL statement, and the dataset label for existing magnetic tape and direct-access datasets, into the DCB, replacing the zero values with actual, non-zero values. A control block called the JFCB (Job File Control Block) initially holds the information extracted from

36-557: A JCL-specified BLKSIZE (physical block size), with the merge of the two becoming the permanent definition of the dataset. This operating-system -related article is a stub . You can help Misplaced Pages by expanding it . This mainframe computer -related article is a stub . You can help Misplaced Pages by expanding it . 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. Access methods were present in several mainframe operating systems since

48-409: 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

60-494: 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. 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

72-430: 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

84-544: 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

96-421: The application program and usually was defined within (and resided within) the application program itself. The addresses of I/O subroutines would be resolved during a linkedit phase after compilation or else dynamically inserted at OPEN time. The equivalent control block for IBM DOS/360 , DOS/VSE and z/VSE operating systems is a "DTF" (Define the file) Many of the constants and variables contained within

108-583: The z/OS operating system, two elements provide access methods: Storage record In computer science , a storage record is: In record-oriented filesystems , a record is a basic unit of device-to-program data transfers. Files in record-oriented filesystems are structured collections of records. Records may have a fixed length or variable length. In Unix-like systems, a number of programs (for example, awk , join , and sort ) are designed to process data consisting of records (called lines ) each separated by newlines , where each record may contain

120-704: The DD statement for the dataset. The results of the merge are stored in the JFCB which may also be written into the DSCB during the CLOSE process, thereby making the dataset definition permanent. An example is the BLKSIZE= variable, which may be (and usually is) specified in the DCB as zero. In the DD statement, the BLKSIZE is specified as a non-zero value and this, then, results in a program-specified LRECL (logical record length) and

132-503: The late 1950s, under a variety of names; the name access method was introduced in 1963 in the 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, but typically provide a greater level of functionality. System/360 and successor systems perform input/output using

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144-434: 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: 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. In

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