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.