FAT File Systems - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 182

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 1 of 2).
Byte Description Size

(bytes)

Comments

0 Jump instruction to boot code. 3 For bootable media, byte zero contains

EBh or E9h for an x86 unconditional
jump. Set to 00h for non-bootable media.

3 String that identifies the operating

system that formatted the media

8 Use “MSWIN4.1” for maximum

compatibility.

11 Number of bytes per sector 2 Use 512 for maximum compatibility.

Other allowed values are 1K, 2K, 4K, 8K,
16K, 32K, and 64K.

13 Number of sectors per cluster 1 Allowed values are 1, 2, 4, 8, 16, 32, 64,

and 128. For maximum compatibility,
cluster size must be 32K or less.

14 Number of reserved sectors 2 Includes all sectors that precede the FATs.

Typically 32.

16 Number of FATs (identical copies) 1 Use 2 for maximum compatibility.

17 Unused 2 0000h.

19 Unused 2 0000h.

21 Media descriptor 1 Use F8h for non-removable media and

F0h for removable media. This value is
also stored in the first byte of cluster 0
and is generally unused in both locations.

22 Unused 2 0000h.

24 Number of sectors per track 2 Not used in LBA.

26 Number of heads 2 Not used in LBA.

28 Number of hidden sectors 4 The number of hidden sectors that

precede the partition that contains this
FAT volume. Operating-system specific.

32 Total number of sectors 4 The total number of sectors in the

volume. The count begins with the boot
sector and includes all regions.

36 Number of sectors per FAT 4 The number of sectors in one FAT.

40 Flags 2 Bits 3..0: zero-based number of the active

FAT. Valid only if mirroring is disabled.
Bits 6..4: reserved
Bit 7: 0 if the FAT is mirrored at runtime
into all FATs; 1 if mirroring is disabled.