A block availability mask (BAM) block is a 4KB block that describes the availability of the corresponding blocks in a RACF® database. Each BAM block starts with a header followed by a table of two-byte "block masks".

The BAM high-water mark contained in the ICB is the address of the BAM block from which segments were last allocated or deallocated. This BAM block is used first when starting a search for space for a new allocation.