FAT Folder Structure

There is a 32-byte Folder Entry for each file and subfolder. A Folder Entry includes the following information:

  • Name (eight-plus-three characters)
  • Attribute byte (8 bits)
  • Create time (24 bits)
  • Create date (16 bits)
  • Last access date (16 bits)
  • Last modified time (16 bits)
  • Last modified date (16 bits)
  • Starting cluster number in FAT table (16 bits)
  • File size (32 bits)

Because all entries in a folder are the same size, the attribute byte for each entry in a folder describes what kind of entry it is. One bit indicates that the entry is for a subfolder, while another bit marks the entry as a volume label. The operating system controls the settings of these bits.

A FAT file also has four attribute bits that can be set by the user: archive, system, hidden and read-only.

  • The archive attribute is used to indicate that a file has been modified since the last backup and should be backed up again the net time a backup is made. It is normally set automatically by the software that modified the file and cleared by the backup software.
  • The system attribute is set automatically for system files, such as OS components, which should not be modified by users.
  • The hidden attribute is used to prevent a file from being displayed in DOS oe Windows directory listings. This is usually applied to files which would not normally be modified by users.
  • The read-only attribute is set to indicate that a file can be read, but cannot be written to, eg: word processed documents that you want to let other users read, but not modify.

FAT32 is a derivative of the FAT file system that supports drives with over 2GB of storage. Because FAT32 drives can contain more than 65,526 clusters, smaller clusters are used than on large FAT16 drives. This method results in more efficient space allocation on the FAT32 drive. The largest possible file for a FAT32 drive is 4GB minus 2 bytes.

Next: NTFS File System