In computer file systems, a block availability map (BAM)[1][2][3][4][5] is a data structure used to track disk blocks that are considered free (available for new data).
The count was held in a single byte, as all formats had 256 or fewer blocks per track.
[8] In the bitmap of any format, a 1 bit indicated the block was available (free), while a 0 bit indicated the block was not available (used), and the bitmap data was stored low-byte first.
Storage devices by Creative Micro Designs, intended for use with CBM computers, also used a Block Availability Map which served the same purpose.
Although the bits were reversed (compared to CBM formats), the bytes were still stored in the same order (low-byte first).