Following the BAM block header is a table of block masks. A block mask is a two-byte field (16 bits) that corresponds to one 4KB block in the RACF® database. The first block mask corresponds to the block whose RBA is defined in Bytes 12-17 of the BAM header. Subsequent block masks in the BAM correspond successively to the 4KB blocks following this RBA. Bytes 18-19 of the BAM header contains the number of block masks in this BAM.

Each bit in the block mask corresponds, successively, to a 256-byte slot within the mask's corresponding 4KB block. If a bit is set to 1, its corresponding slot in the block is free. Otherwise, this slot is in use. Figure 1 is an example of a block mask for a block whose first six slots are in use.

Figure 1. Block Masks of the RACF Database