Windows File System - FAT

1
Windows File System - FAT

• originally invented as a method for storing data
  on floppy disks.
• later used by MS-DOS with supports for a
  hierarchical directory structure, and fixed disks
  as well as floppy disks
• still used by Windows 3.x and Win95 supporting
  long filenames and allowing drive size to 2047
  gigabytes

2
FAT File Structure

• Cluster - a fundamental unit of disk storage (a
  collection of sectors)
• File Allocation Table (FAT)
• used to track of the usage of a volumes clusters
• each entry for a cluster represents whether it is
  used, which cluster is next if used, or whether
  it contains a bad sector
• FAT12/16/32 means the size of FAT entry (12/16/32
  bits)
• two identical copies of FAT for recovery

...
pointed by directory entry
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FAT File Structure

• Directory Entry
• keeps track of information about a file or
  directory
• short name entry
• name and extension, attributes, size, and time
• a starting cluster number
• long name entry - used by Win95 to support long
  filename
• sequence, type(longname entry)
• name (wchar_t 13) 2-byte character for
  international characters
• A filename can have 256 characters.
• one short name entry can have upto 20 longname
  entries

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FAT File Structure

• Example - C\fooDir\barLongnameFile
• The root directory starts at 0 cluster

...
0 1 2 3 4 5
Clusters
...
0 1 2
3 4 5
5
FAT File Structure

• Disk Size vs Cluster Size and FAT Size

FAT12 FAT16 FAT32
DISK SIZE
(Cluster Size, FAT Size)
512M (229) bytes (128K, 6K) ( 8K,
256K) (4K, 2M)

1Giga (230) bytes (256K, 6K) (16K,
256K) (4K, 4M)
2Giga (231) bytes (512K, 6K) (32K,
256K) (4K, 8M)
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FAT File Structure

• Disk Size, Cluster Size vs FAT Size
• e.g. Disk Size 1 Giga (230) bytes ( 1 sector
  512(29) bytes )
• Cluster Size
• FAT12 (212) clusters (218) bytes/cluster
  (256K bytes/cluster)
• FAT16 (216) clusters (214) bytes/cluster
  (16K bytes/cluster)
• FAT32 (232) clusters (28) bytes/cluster -
  ??
• (218) clusters (212)
  bytes/cluster (4K bytes/cluster)
• FAT Size
• FAT12 (212) entry 12 bits/entry 3(211)
  bytes 6K bytes
• FAT16 (216) entry 16 bits/entry (218)
  bytes 256K bytes
• FAT32 (218) entry 32 bits/entry (222)
  bytes 4M bytes