Why Study FAT?

• Offers unique structure compared to Unix
• Still prevalent in various systems like camera file systems

File Allocation Table (FAT) Filesystem

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• Introduced in 1977, still relevant in 2018
• Uses linked lists for block allocation
• Utilizes FAT for metadata instead of inodes
• Despite limitations, remains widely used
  • Lacks support for hard links and permissions, making it less secure
  • But, it does make it suitable for single-user computers like digital cameras or USB sticks

Block Structures

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• Utilizes trees of “indirect” blocks
• Includes directory entries with details like…
  • names
  • dates
  • sizes
  • starting clusters
• Resembles a mix of inodes and directory entries

Linked Lists of Blocks:

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• Simple concept, simple implementation
• Enables arbitrary-length files but lacks random access

Some problems:

• Assume list of blocks:
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  • Accessing third block of file (cluster 6) requires iteration
• Related question:
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  • Where to store the pointers?

File Allocation Table

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• Neat solution to both problems:
  • FAT stores “next” pointer for each cluster
  • 12b, 16b, or 32b for each 2-32 kiB cluster

FAT Metadata

• Utilizes FAT instead of inodes
• Implications
  • No hard links
  • No permissions in FAT filesystem
• Design: (Again) suitable for single-user computers without protection mechanisms

FAT Name Lookup & Directories

• FAT Name Lookup: Root directory entry in cluster 2
  • Hard-coded in FAT12 and FAT16
  • Can be elsewhere in FAT32
• Directory Entries: Serialised into data blocks in the FAT filesystem
  • 32 bytes
  • map names to details like…
    • dates
    • times
    • sizes
    • starting cluster
  • Sort of like an inode/dirent crossover