Block Bitmap

On small file systems, the "Block Bitmap" is normally located at the first block, or second block if a superblock backup is present, of each block group. Its official location can be determined by reading the "bg_block_bitmap" in its associated group descriptor.

Each bit represent the current state of a block within that block group, where 1 means "used" and 0 "free/available". The first block of this block group is represented by bit 0 of byte 0, the second by bit 1 of byte 0. The 8th block is represented by bit 7 (most significant bit) of byte 0 while the 9th block is represented by bit 0 (least significant bit) of byte 1.