Basic sequential access method

In IBM mainframe operating systems, basic sequential access method (BSAM) is an access method to read and write data sets sequentially. BSAM is available on OS/360, OS/VS2, MVS, z/OS, and related high-end operating systems.

BSAM is used both for devices that are naturally sequential, such as punch card readers and printers, and for data on devices that could also be addressed directly, such as magnetic disks. BSAM offers device independence: to the extent possible, the same API calls are used for different devices.

BSAM is - as its name says - basic. It allows programs to read and write physical blocks of data, as opposed to the more advancced queued sequential access method (QSAM) that allows programs to access records within physical blocks of data.

The BSAM application program interface can be compared with the interface offered by open, read, write and close calls (using file handles) in more modern operating systems such as Unix and Windows.