Boot Sector

Offset Size Description Comments
0 (0x00) 3 JumpBoot 0xEB7690
3 (0x03) 8 FileSystemName "EXFAT>
11 (0x0B) 53 MustBeZero
64 (0x40) 8 PartitionOffset In sectors; if 0, shall be ignored
72 (0x48) 8 VolumeLength Size of exFAT volume in sectors
80 (0x50) 4 FatOffset In sectors
84 (0x54) 4 FatLength In sectors. May exceed the required space in order to align the second FAT
88 (0x58) 4 ClusterHeapOffset In sectors
92 (0x5C) 4 ClusterCount 232 - 11 is the maximum number of clusters could be described
96 (0x60) 4 RootDirectoryCluster
100 (0x64) 4 VolumeSerialNumber
104 (0x68) 2 FileSystemRevision as MAJOR.minor, major revision is high byte, minor is low byte; currently 01.00
106 (0x6A) 2 VolumeFlags *
108 (0x6C) 1 BytesPerSectorShift Power of 2. Minimum 9 (512 bytes per sector), maximum 12 (4096 bytes per sector)
109 (0x6D) 1 SectorsPerCluster Shift Power of 2. Minimum 0 (1 sector per cluster), maximum 25 - BytesPerSectorShift, so max cluster size is 32 MB
110 (0x6E) 1 NumberOfFats 2 is for TexFAT only
111 (0x6F) 1 DriveSelect Extended INT 13h drive number; typically 0x80
112 (0x70) 1 PercentInUse 0..100 - percentage of allocated clusters rounded down to the integer 0xFF – percentage is not available
113 (0x71) 7 Reserved
120 (0x78) 390 BootCode
510 (0x1FE) 2 BootSignature 0xAA55
512 (0x200) 2BytesPerSectorShift - 512 ExcessSpace Not used

*VolumeFlags

Offset Size Field
0 1 ActiveFat 0 - First FAT and Allocation Bitmap are active, 1- Second.
1 1 VolumeDirty (0-clean, 1-dirty)
2 1 MediaFailure (0 - no failures reported or they already marked as BAD clusters) 1- some read/write operations failed)
3 1 ClearToZero (no meaning)
4 12 Reserved (no meaning)