PC Hardware in a Nutshell, 3rd Edition

FAT16

MS-DOS and the initial release of Windows 95 support only the FAT16 filesystem. FAT16 uses 16-bit addressing, which limits it to 65,536 discrete addresses. If FAT16 addressed individual sectors, it could access only (65,536 sectors × 512 bytes/sector = 33,554,432 bytes), or 32 MB. To get around this small limit, FAT addresses clusters rather than individual sectors. A cluster is a group of sectors. The number of sectors/cluster is always a power of two, is constant within a volume, and is determined automatically based on the size of that volume. Table 14-1 lists the cluster sizes that FAT16 uses for various partition sizes.

Table 14-1. The relationship of FAT16 partition size to cluster size

Partition size (MB)	Sectors / Cluster	Cluster size
0 - 32	1	512 bytes
33 - 64	2	1 KB
65 - 128	4	2 KB
129 - 255	8	4 KB
256 - 511	16	8 KB
512 - 1023	32	16 KB
1024 - 2047	64	32 KB

The downside to using clusters is that a cluster is the smallest addressable unit on the volume, so each file must occupy at least one cluster, and every file on average wastes half a cluster. This means, for example, that storing a 1-byte file on a FAT16 volume larger than 1 GB requires 32,768 bytes of disk space, and that storing a 32,769-byte file requires 65,536 bytes of disk space.

This allocated but unoccupied and unusable disk space is referred to as slack space. Large FAT16 volumes with many files have a lot of it. For example, a 1+ GB FAT16 volume with 15,000 files, each of which ...