High Quality Content by WIKIPEDIA articles! 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.