High Quality Content by WIKIPEDIA articles! In IBM mainframe operating systems, basic direct access method (BDAM) is an access method to read and write data sets directly. In BDAM, the programmer has complete control of the organization of the file. Placement and retrieval is done by directly accessing the block on the device (physical) or within the file extent (relative) BDAM is available on OS/360, OS/VS2, MVS, z/OS, and related high-end operating systems. BDAM as with BSAM accesses the entire physical data record.