BDAM

BDAM, short for Basic Direct Access Method, is a low-level data access method historically used on IBM mainframe operating systems such as DOS/360 and OS/360. It provides direct, block-addressable access to data stored on direct-access storage devices without built-in indexing or key-based retrieval. Data sets managed by BDAM are organized into fixed-size blocks, with a specified logical record length (LRECL) and block size (BLKSIZE). The number of records per block is typically BLKSIZE divided by LRECL, and applications address data by relative block numbers (RBAs) and, within a block, by fixed offsets.

Operation and characteristics: BDAM does not maintain an index or keys for records. To read or write data, a program computes the target block number and offset, then issues block-level I/O to access the desired content. There is no automatic data organization beyond what the application provides, so free-space management and record layout are responsibility of the software using BDAM. This makes BDAM a straightforward, low-overhead mechanism suitable for simple, high-performance direct access, or as a primitive upon which other data access methods could be built.

Context and usage: BDAM was common in earlier mainframe environments and is often contrasted with more feature-rich methods like ISAM, KSDS, and ESDS, which offer indexing and structured data sets. While largely superseded in modern systems by these indexed formats, BDAM remains documented in historical IBM literature and may be encountered in legacy applications, emulators, or discussions of early data storage techniques.