FAT File Systems - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 183

Table 8-3: A volume's boot sector contains low-level information about the
media's formatting. These values are for a FAT32 volume (Sheet 2 of 2).
Byte Description Size

(bytes)

Comments

42 Version number of the FAT32

volume

2 The high byte is the major revision num-

ber and the low byte is the minor revision
number. Use 0000h.

44 Cluster number of the first cluster

of the root directory.

4 Use 2 for maximum compatibility.

48 Sector number of the FSINFO

structure within the reserved
region

2 Usually 1.

50 The sector number of a backup

copy of the boot record within the
reserved area

2 Set to 6 for maximum compatibility. Set

to zero if no backup copy is available.

52 Reserved 12 Set all bytes to 00h.

64 Logical drive number of the parti-

tion

1 Operating-system specific.

65 Reserved 1 00h.

66 Extended boot signature 1 Set to 29h to indicate that the three fields

that follow are present.

67 Volume serial number 4 Typically created using the date and time

of formatting.

71 Volume label 11 Text that identifies the volume.

82 File system type 8 “FAT32”. Not used to determine the file

system type,

• The total number of sectors is always at offset 32 because a FAT32 vol-

ume is always at least 32 KB.

• The number of sectors per FAT is in bytes 24h-27h.

• The cluster number of the root directory is at offset 44 because FAT32

has no dedicated location for the root directory.

• Byte 48 holds the location of the FSInfo structure and byte 50 holds the

location of the backup boot sector. FAT16 doesn't have these compo-
nents.

• A Flags field at byte 40 indicates whether the file system maintains iden-

tical FATs or whether only one FAT should be considered valid. If only