Basic partitioned access method: Difference between revisions

Revision as of 11:51, 7 October 2007

Latest revision as of 01:23, 20 June 2025

(35 intermediate revisions by 13 users not shown)

Line 1:

In [[IBM mainframe]] [[operating system]]s, a '''basic partitioned access method''' ('''BPAM''')<ref>{{cite book|publisher=IBM|title=IBM System/360 Operating System Sequential Access Methods Program Logic Manual|id=Y28-6604-1|date=January 1967|url=http://www.bitsavers.org/pdf/ibm/360/os/plm_1966-67/Y28-6604-1_Sequential_Access_Methods_PLM_Jan67.pdf}}</ref> is an [[access method]] for libraries[[Library with a specific structure(computing)|libraries]], called [[partitioned dataset]]s (PDSPDSes) in IBM terminology. BPAM is used in [[OS/360]], [[OS/VS2]], [[MVS]], [[z/OS]], and others. A PDS consists of members (that are internally identical to [[data set (IBM mainframe)|sequential data sets]]; a member cannot be a PDS itself), registered in a list called ''directory'', and the combination of members and directory is a single dataset on disk. The directory contains a list of member's names (max. 8 characters) 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.

A PDS consists of members (internally identical to [[data set (IBM mainframe)|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.

While in theory such libraries can store any type of data, they are typically are used to store executable programs, or ''load modules'', or what is called binaries in modern terms. The operating system requires all executable programs to be stored in libraries, because directory entries contain additional attribute information for load modules.

Partitioned datasets can store any type of data, but they are often used to store [[executable|executable programs]], or ''load modules'', sometimes called binaries in other systems. Other uses include system assembler [[Macro (computer science)|macro]] definitions, [[Job Control Language|job control]] procedures, and program source code.

While executable programs typically are written to libraries by utility software such as a [[linkage editor]] and loaded by operating functions, BPAM also provides an [[API]] to programmers to access such libraries directly. The BPAM [[API]] is fairly similar to [[basic sequential access method]] (BSAM), but it adds functionality to process directories.

==Application program interface==

There is no real parallel for PDSes in more modern operating systems such as Unix or Windows. It may be compared to a directory, that can contain only files, no subdirectories, and at the same time that is physically stored in a single file, like a [[tar]] archive. The need for libraries relates to the fact that mainframe operating systems (until very recently) did not have a hierarchical [[file system]]. The number of files (called datasets) on mainframes must therefore be limited, for management and overhead reasons.{{huh|date=October 2007}} This calls for a subdivision of (some) datasets into members.

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.

The programmer specifies <code>DSORG=PO</code> in their [[Data Control Block]] (DCB) to indicate use of BPAM.

[[Category:IBM Mainframe computer operating systems]]

As a ''basic'' access method BPAM reads and writes member data in [[Block (data storage)|blocks]] and the I/O operation proceeds [[Asynchronous I/O|asynchronously]] and must be tested for completion using the <code>CHECK</code> macro.<ref name=Macro>{{cite book|last1=IBM Corporation|title=OS Data Management Macro Instructions|date=June 1973|page=157|url=http://bitsavers.informatik.uni-stuttgart.de/pdf/ibm/360/os/R21.7_Apr73/GC26-3794-1_OS_Data_Management_Macro_Instructions_Rel_21.7_Jun73.pdf|accessdate=August 19, 2016}}</ref> BPAM uses the standard system macros <code>OPEN</code>, <code>CLOSE</code>, <code>READ</code>, <code>WRITE</code>, and <code>CHECK</code>. The <code>NOTE</code> macro instruction returns position of the last block read or written, and the <code>POINT</code> macro will reposition to the ___location identified by a previous <code>NOTE</code>.<ref name=Macro /> The <code>BLDL</code> macro can be used to build a list of the addresses of members specified by the programmer for later use, if desired. <code>FIND</code> positions to a single member, specified by name, which requires a directory lookup on disk, or by address previously retrieved by <code>BLDL</code>. The <code>STOW</code> macro is used to update the directory when a member is added, deleted, changed (including renamed), or replaced.<ref>{{cite book|last1=IBM Corporation|title=OS Data Management Services Guide|date=July 1973|pages=75–85|url=http://bitsavers.informatik.uni-stuttgart.de/pdf/ibm/360/os/R21.7_Apr73/GC26-3746-2_OS_Data_Management_Services_Guide_Rel_21.7_Jul73.pdf|accessdate=August 19, 2016}}</ref>

[[Category:Computer file systems]]

==Load modules==

{{compu-storage-stub}}

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 [[Kernel (operating system)|supervisor]]).

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 ([[Execute Channel Program|EXCP]] on pre-MVS systems, or [[Start Input/Output|STARTIO]] on MVS/370 and later systems).

==References==

<references />

{{Mainframe I/O access methods}}

[[Category:IBM Mainframe computermainframe operating systems]]

[[Category:Computer file systems]]