The Bitmap keeps track of what blocks on disk are in use and which aren't. The
Bitmap is stored in special Bitmap blocks. For this purpose
the bitmap contains a single bit for each block on the disk. The status of this bit
determines whether a specific block on disk is in use or not.
The number of bitmap blocks depends on how many blocks a disk consists of and how many
bits can be fitted in a single bitmap block (minus space for the block header). In
the picture above a bitmap block holds 16 bits. For a disk of 22 blocks you'd need 2
bitmap blocks. As you can see the last bitmap block isn't used completely.
Note that this is a very downsized example. A bitmap block of 512 bytes has
enough room to store over 4000 bits. Furthermore, a partition of 1 GB can easily
have over 500 bitmap blocks.