Basic partitioned access method

In IBM mainframe operating systems, basic partitioned access method (BPAM) is an access method for libraries with a specific structure, called partitioned datasets (PDSes). BPAM is used in OS/360, OS/VS2, MVS, z/OS, and others. A PDS consists of members (that are internally identical to 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 (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 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 the member's directory entry contains additional attribute information specific to load modules. When used for storing load modules, directories also contain, amongst 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. Other uses for libraries include system assembler "macro" definitions.

While executable programs are written to libraries by the linkage editor and loaded into user-acquired storage by the Loader (itself an application program) or into system-acquired storage by Program Fetch (itself a specialized component of the OS's supervisor), BPAM also provides an API to programmers to access libraries directly. The BPAM API is fairly similar to basic sequential access method (BSAM), but it adds functionality to process directories. The Linkage Editor is a utility program which organizes a load module in a very 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 EXCP (on pre-MVS instances of the OS) and with one STARTIO (on MVS/370 and later instances of the OS) by Program Fetch.

The closest parallel for PDSes in other operating systems such as Unix or Windows is the static library, such as produced by the ar utility. In fact, the nomenclature for libraries in make, lib(member), is directly derived from OS/360. 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. The need for libraries relates to the fact that mainframe operating systems (until very recently) did not have a hierarchical file system.