Volume Layout

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Volume Layout

Offset, sectors Size, sectors Block Comments
Main Boot Region
0 1 Boot Sector  
1 8 Extended Boot Sectors  
9 1 OEM Parameters  
10 1 Reserved  
11 1 Boot Checksum  
Backup Boot Region
12 1 Boot Sector  
13 8 Extended Boot Sectors  
21 1 OEM Parameters  
22 1 Reserved  
23 1 Boot Checksum  
FAT Region
24 FatOffset - 24 FAT Alignment Boot Sectors contain FatOffset
FatOffset FatLength First FAT Boot Sectors contain FatOffset and FatLength
FatOffset + FatLength FatLength Second FAT For TexFAT only
Data Region
FatOffset + FatLength * NumberOfFats ClusterHeapOffset – (FatOffset + FatLength * NumberOfFats) Cluster Heap Alignment  
ClusterHeapOffset ClusterCount * 2^SectorsPerClusterShift Cluster Heap  
ClusterHeapOffset + ClusterCount * 2^SectorsPerClusterShift VolumeLength – (ClusterHeapOffset + ClusterCount * 2^SectorsPerClusterShift) Excess Space  

Navigate to detailed volume specification using following links:

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 2^32-11 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 (see below)  
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) 2^BytesPerSectorShift - 512 ExcessSpace Not used

Volume Flags

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

Extended Boot Sector

Size Description Comments
0 (0x00) 2^BytesPerSectorShift - 4 ExtendedBootCode  
2^BytesPerSectorShift - 4 4 ExtendedBootSignature 0xAA550000  

Whole sector is used for boot code except last 4 bytes used for signature in each sector. If Extended Boot Sector is not used, it should be filled with 0x00. Extended signature must be preserved.

OEM Parameters

Offset Size Description Comments
0 (0x00) 48 Parameters[0]  
432 (0x1B0) 48 Parameters[9]  
480 (0x01E0) 2^BytesPerSectorShift - 480 Reserved  

OEM parameters are ignored by Windows but can be used by OEM implementations. OEMs can define their own parameters with unique GUIDs. All unused Parameters fields must be described as unused by GUID_NULL in ParameterType.

This structure must be preserved during exFAT formatting, except in the case of secure wipe.

OEM Parameter Record

Offset Size Description Comments
0x00 16 ParameterType OEM defined GUID , GUID_NULL indicate that parameter value is not used
0x10 32 ParameterValue OEM specific


#define OEM_FLASH_PARAMETER_GUID 0A0C7E46-3399-4021-90C8-FA6D389C4BA2
  GUID OemParameterType;   //Value is OEM_FLASH_PARAMETER_GUID
  UINT32 EraseBlockSize;   //Erase block size in bytes
  UINT32 PageSize;
  UINT32 NumberOfSpareBlocks;
  UINT32 tRandomAccess;   //Random Access Time in nanoseconds
  UINT32 tProgram;    //Program time in nanoseconds
  UINT32 tReadCycle;  // Serial read cycle time in nanoseconds
  UINT32 tWriteCycle;   // Write Cycle time in nanoseconds
  UCHAR Reserved[4];

Boot Checksum

This sector contains a repeating 32-bit checksum of the previous 11 sectors. The checksum calculation excludes VolumeFlags and PercentInUse fields in Boot Sector (bytes 106, 107, 112). The checksum is repeated until the end of the sector. The number of repetitions depends on the size of the sector.


UINT32 BootChecksum(const unsigned char data[], int bytes)
  UINT32 checksum = 0;

  for (int i = 0; i < bytes; i++)
    if (i == 106 || i == 107 || i == 112)
    checksum = (checksum << 31) | (checksum >> 1) + data[i];
  return checksum;

File Allocation Table (FAT)

File Allocation Table (FAT) may contain 1 or 2 FATs, as defined in NumberOfFats field. ActiveFat field in VolumeFlags in the Main Boot Sector determines which FAT is active.

The first cluster is cluster 2, as in FAT32. Each FatEntry represents one cluster

In exFAT, FAT is not used for tracking an allocation; an Allocation Bitmap is used for this purpose. FAT is only used for keeping chains of clusters of fragmented files. If a file is not fragmented, FAT table does not need to be updated. A Stream Extensions Directory Entry should be consulted to determine if the FAT chain is valid or not. If FAT chain is not valid, it does not need to be zeroed.

Offset Size Description Comments
0 (0x00) 4 FatEntry[0] Media type (should be 0xFFFFFFF8)
4 (0x04) 4 FatEntry[1] Must be 0xFFFFFFFF
8 (0x08) 4 FatEntry[2] First cluster
(ClusterCount + 1) * 4 4 FatEntry[ClusterCount + 1] Last cluster
(ClusterCount + 2) * 4 Remainder of sector ExcessSpace  

Valid values of FAT entries:


ClusterCount +1 (max 0xFFFFFFF6) – next cluster in the chain
bad cluster
media descriptor
end of file (EOF mark)

Value 0x00000000 does not mean the cluster is free, it is an undefined value.

The second FAT table (presents only in TexFAT) is located immediately after the first one and has the same size.

See Also:

exFAT File System | Directory Structure | Defined Directory Entries | Cluster Heap

Data Recovery

Data Utility

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Data Backup

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