In computer file systems, a block availability map is a data structure used to trackdisk blocks that are considered free. It is used along with a directory to manage files on a disk. In terms of Commodore DOS compatible disk drives, the BAM was a data structure stored in a reserved area of the disk. For each track, the BAM consisted of a bitmap of available blocks and a count of the available blocks. The count was held in a single byte, as all formats had 256 or fewer blocks per track. The count byte was simply the sum of all 1-bits in the bitmap of bytes for the current track. The following table illustrates the layout of Commodore 1541 BAM. The table would be larger for higher-capacity disks.
Total byte
Bitmap byte 1
Bitmap byte 2
Bitmap byte 3
Track 1
blocks available
Blocks 0-7
Blocks 8-15
Blocks 16-23
Track 2
blocks available
Blocks 0-7
Blocks 8-15
Blocks 16-23
...
...
...
...
...
Track 35
blocks available
Blocks 0-7
Blocks 8-15
Blocks 16-23
The bitmap was contained in 3 bytes for Commodore 1541 format disks because it had 17 to 20sectors per track. Similarly, the Commodore 1571 used 3 bytes for the bitmap of each track, but the BAM was twice the size because there were twice as many tracks when formatted as double-sided. In contrast, the Commodore 1581disk drive used 5 bytes for the bitmap because the disk format had 40 blocks per track. In the bitmap of any format, a 1 bit indicated the block was available, while a 0 bit indicated the block was not available, and the bitmap data was stored low-byte first. So the first byte held a map for blocks 0 to 7, the second byte held a map for blocks 8 to 15, and so on. Within a byte, the bitmap was orderedlow-bit first. For example, the first byte would represent block 0 with the least significant bit and block 7 with the most significant bit. Storage devices by Creative Micro Designs, intended for use with CBMcomputers, also used a Block Availability Map which served the same purpose. However, these devices did not include a count byte, and the bits in each byte were reversed. Although the bits were reversed, the bytes were still stored in the same order.
