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PC Hardware in a Nutshell, 3rd Edition
book

PC Hardware in a Nutshell, 3rd Edition

by Robert Bruce Thompson, Barbara Fritchman Thompson
July 2003
Beginner to intermediate content levelBeginner to intermediate
874 pages
38h 13m
English
O'Reilly Media, Inc.
Content preview from 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 ...

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Publisher Resources

ISBN: 059600513XErrata Page