Basic partitioned access method

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In IBM mainframe operating systems, basic partitioned access method (BPAM) [1] 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.

IBM mainframes are large computer systems produced by IBM since 1952. During the 1960s and 1970s, IBM dominated the large computer market. Current mainframe computer in IBM's line of business computers are developments of the basic design of the IBM System/360.

Operating system collection of software that manages computer hardware resources

An operating system (OS) is system software that manages computer hardware and software resources and provides common services for computer programs.

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, but typically provide a greater level of functionality.

Contents

A PDS consists of members (internally identical to sequential data sets), registered in a list called the directory. The combination of members and directory is a single dataset on disk. The directory contains a list of member's names (8 characters, padded on the right with blanks, as required) and member's addresses. Addresses are relative to the start of the dataset in order to allow the PDS to be moved to a different disk location.

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

Partitioned datasets can store any type of data, but they are often used to store executable programs, or load modules, sometimes called binaries in other systems. Other uses include system assembler macro definitions, job control procedures, and program source code.

Executable file that can be run by a computer

In computing, executable code or an executable file or executable program, sometimes simply referred to as an executable, causes a computer "to perform indicated tasks according to encoded instructions", as opposed to a data file that must be parsed by a program to be meaningful.

Macro (computer science) rule or pattern that specifies how a certain input sequence should be mapped to a replacement output sequence according to a defined procedure

A macro in computer science is a rule or pattern that specifies how a certain input sequence should be mapped to a replacement output sequence according to a defined procedure. The mapping process that instantiates (transforms) a macro use into a specific sequence is known as macro expansion. A facility for writing macros may be provided as part of a software application or as a part of a programming language. In the former case, macros are used to make tasks using the application less repetitive. In the latter case, they are a tool that allows a programmer to enable code reuse or even to design domain-specific languages.

Job Control Language (JCL) is a name for scripting languages used on IBM mainframe operating systems to instruct the system on how to run a batch job or start a subsystem.

Application program interface

BPAM provides an application program interface (API) to allow programmers to access libraries directly. The BPAM API is similar to basic sequential access method (BSAM), but it adds functionality to process directories. Individual members of a PDS can also be processed using sequential access methods by specifying the member name on the job control DD statement.

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.

The programmer specifies DSORG=PO in their Data Control Block (DCB) to indicate use of BPAM. As a basic access method BPAM reads and writes member data in blocks and the I/O operation proceeds asynchronously and must be tested for completion using the CHECK macro. [2] BPAM uses the standard system macros OPEN, CLOSE, READ, WRITE,and CHECK. The NOTE macro instruction returns position of the last block read or written, and the POINT macro will reposition to the location identified by a previous NOTE. [2] The BLDL macro can be used to build a list of the addresses of members specified by the programmer for later use, if desired. FIND positions to a single member, specified by name, which requires a directory lookup on disk, or by address previously retrieved by BLDL. The STOW macro is used to update the directory when a member is added, deleted, changed (including renamed), or replaced. [3]

In IBM mainframe operating systems, such as OS/360, MVS, z/OS, a Data Control Block (DCB) is a description of a dataset in a program. A DCB is coded in Assembler programs using the DCB macro instruction. High level language programmers use library routines containing DCBs.

In computing, a block, sometimes called a physical record, is a sequence of bytes or bits, usually containing some whole number of records, having a maximum length, a block size. Data thus structured are said to be blocked. The process of putting data into blocks is called blocking, while deblocking is the process of extracting data from blocks. Blocked data is normally stored in a data buffer and read or written a whole block at a time. Blocking reduces the overhead and speeds up the handling of the data-stream. For some devices, such as magnetic tape and CKD disk devices, blocking reduces the amount of external storage required for the data. Blocking is almost universally employed when storing data to 9-track magnetic tape, NAND flash memory, and rotating media such as floppy disks, hard disks, and optical discs.

In computer science, asynchronous I/O is a form of input/output processing that permits other processing to continue before the transmission has finished.

Load modules

The operating system requires all executable programs to be stored in libraries because the member's directory entry contains additional attribute information specific to load modules. When used for storing load modules, directories also contain, among other data, the size of the load module and the address of the first "text record", which is different from the address of the first member data. Executable programs are written to libraries by the linkage editor and loaded into user-acquired storage by the Loader (an application program) or into system-acquired storage by Program Fetch (a component of the OS supervisor).

Kernel (operating system) main component of most computer operating systems

The kernel is a computer program that is the core of a computer's operating system, with complete control over everything in the system. On most systems, it is one of the first programs loaded on start-up. It handles the rest of start-up as well as input/output requests from software, translating them into data-processing instructions for the central processing unit. It handles memory and peripherals like keyboards, monitors, printers, and speakers.

The Linkage Editor organizes a load module in a specialized format consisting of alternating "text records" and "control/relocation dictionary records". This organization allows a load module to be completely loaded and relocated with one input/output operation by Program Fetch (EXCP on pre-MVS systems, or STARTIO on MVS/370 and later systems).

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A direct-access storage device (DASD) is a secondary storage device in which "each physical record has a discrete location and a unique address". IBM coined the term DASD as a shorthand describing hard disk drives, magnetic drums, and data cells. Later, optical disc drives and flash memory units are also classified as DASD. The term DASD contrasts with sequential storage media such as magnetic tape, and unit record equipment such as card devices like card readers and punches.

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Virtual Storage Access Method (VSAM) is an IBM DASD file storage access method, first used in the OS/VS1, OS/VS2 Release 1 (SVS) and Release 2 (MVS) operating systems, later used throughout the Multiple Virtual Storage (MVS) architecture and now in z/OS. Originally a record-oriented filesystem, VSAM comprises four data set organizations: Key Sequenced Data Set (KSDS), Relative Record Data Set (RRDS), Entry Sequenced Data Set (ESDS) and Linear Data Set (LDS). The KSDS, RRDS and ESDS organizations contain records, while the LDS organization simply contains a sequence of pages with no intrinsic record structure, for use as a memory-mapped file.

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In IBM mainframe operating systems, Execute Channel Program (EXCP) is a supervisor call 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 is more specifically described in the OS System Programmer's Guide.

OS/360 and successors operating system for IBM mainframes

OS/360, officially known as IBM System/360 Operating System, is a discontinued batch processing operating system developed by IBM for their then-new System/360 mainframe computer, announced in 1964; it was heavily influenced by the earlier IBSYS/IBJOB and Input/Output Control System (IOCS) packages. It was one of the earliest operating systems to require the computer hardware to include at least one direct access storage device.

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.

Single Virtual Storage (SVS) refers to Release 1 of Operating System/Virtual Storage 2 (OS/VS2); it is the successor system to the MVT option of Operating System/360. OS/VS2 (SVS) was a stopgap measure pending the availability of MVS, although IBM provided support and enhancements to SVS long after shipping MVS.

In IBM mainframe operating systems, Execute Direct Access Program (XDAP) is a pseudo access method for accessing direct access datasets on a block by block basis. XDAP is more specifically described in the OS System Programmer's Guide.

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.

The OS/360 Object File Format is the standard object module file format for the IBM DOS/360, OS/360 and VM/370, Univac VS/9, and Fujitsu BS2000 mainframe operating systems. In the 1990s, the format was given an extension with the XSD-type record for the MVS Operating System to support longer module names in the C Programming Language. This format is still in use by the z/VSE operating system. In contrast, it has been superseded by the GOFF file format on the MVS Operating System and on the z/VM Operating System. Since the MVS and z/VM loaders will still handle this older format, some compilers have chosen to continue to produce this format instead of the newer GOFF format.

The GOFF specification was developed for the IBM zSystem Mainframe computer to supersede the IBM OS/360 Object File Format to compensate for weaknesses in the older format.

References

  1. ↑ IBM System/360 Operating System Sequential Access Methods Program Logic Manual (PDF). IBM. January 1967. Y28-6604-1.
  2. 1 2 IBM Corporation (June 1973). OS Data Management Macro Instructions (PDF). p. 157. Retrieved August 19, 2016.
  3. ↑ IBM Corporation (July 1973). OS Data Management Services Guide (PDF). pp. 75–85. Retrieved August 19, 2016.
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