ExFAT

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This page or section is a work in progress and may thus be incomplete. Its content may be changed in the near future.

ExFAT, as its name suggests, is an extension of the traditional FAT filesystem. As opposed to older versions, such as FAT16, exFAT has been greatly modernized and has more conveniences from a programming standpoint.

After a long period of keeping the official exFAT spec under their collective hats, Microsoft has finally published it at https://github.com/MicrosoftDocs/win32/blob/docs/desktop-src/FileIO/exfat-specification.md on August 27th, 2019. The major structures of the file system are described below.

Partition Layout

Overview of the order of structures in an exFAT partition
Section	Contents	Offset (sector number)	Size (in sectors)
Main Boot Region	Boot Sector	0	1
	Extended Boot Sectors	1	8
	OEM Parameters	9	1
	Reserved sectors	10	1
	Boot checksum	11	1
Backup Boot Region	Boot Sector	12	1
	Extended Boot Sectors	13	8
	OEM Parameters	21	1
	Reserved sectors	22	1
	Boot checksum	23	1
FAT Region	FAT Alignment	24	FATOffset - 24
	First FAT	FATOffset	FATLength
	Second FAT	FATOffset + FATLength	FATLength * (NumberOfFATs - 1)
Data Region	Cluster Heap Alignment	FATOffset + FATLength * NumberOfFATs	ClusterHeapOffset - (FATOffset + FATLength * NumberOfFATs)
	Cluster Heap	ClusterheapOffset	ClusterCount * 2^{SectorsPerClusterShift}
	Excess Space (to end of partition or disk)	ClusterheapOffset + ClusterCount * 2^{SectorsPerClusterShift}	VolumeLength - (ClusterHeapOffset + ClusterCount * 2^{SectorsPerClusterShift})

The Backup Boot Region is a duplicate of the Main Boot Region.

The FATAlignment and Cluster Heap Alignment sections appear to be filler to ensure that the FAT and Cluster Heap both start on a good boundary. This has more meaning for removeable media such as flash drives than for hard drives.

Structures

Boot Sector Structure

Offset

Size

Name

Description

0x000

3

JumpBoot

Jump instruction to start of boot code. For x86 CPUs, the byte sequence is usually EB 76 90.

0x003

8

FileSystemName

The "EXFAT " signature.

0x00B

53

MustBeZero

This range of bytes is set null to help existing FAT drivers from mistakenly parsing an exFAT partition under the assumption that it contains valid FAT16 or FAT32 data.

0x040

8

PartitionOffset

The sector offset of the start of this partition relative to the start of the volume. Can be null.

0x048

8

VolumeLength

The total size of this exFAT partition in sectors. Should not be smaller than 1 MiB.

0x050

4

FATOffset

Offset in sectors from the start of the partition to the first FAT

0x054

4

FATLength

Size of each FAT in sectors

0x058

4

ClusterHeapOffset

Offset in sectors from the start of the partition to the start of the "Cluster Heap", known as the "Data Area" on former FAT versions.

0x05C

4

ClusterCount

The total number of clusters in the Cluster Heap.

0x060

4

FirstClusterOfRootDirectory

The starting cluster of the root directory's cluster chain.

0x064

4

VolumeSerialNumber

A serial number for this partition, usually comprised by calculating the date and time of format into a single 64-bit value. There is no standard for this calculation; each FAT driver is free to use its own technique.

0x068

2

FileSystemRevision

Specifies the version of exFAT specification to which the structures in this partition adhere. For now, there is only a single exFAT specification; version 1.00. However in the future there may be more than this, and a driver should not attempt to parse exFAT structures of an exFAT specification version with which it is not familiar.

0x06A

2

VolumeFlags

Bit	Name	Description
0	ActiveFAT	Used to implement transaction-safe mechanisms 0 - First FAT and first allocation bitmap are valid 1 - Second FAT and second allocation bitmap are valid
1	VolumeDirty	0 - Partition is likely in a good state 1 - Partition is likely not in a good state
2	MediaFailure	0 - No bad clusters aside from those already marked "bad" 1 - An I/O operation failed at some point indicating that the entire partition should be scanned for bad clusters at next mount.
3	ClearToZero	No real meaning... yet!
4 - 15	n/a	Reserved

This word is not included in checksumming.

0x06C

1

BytesPerSectorShift

The number of places one would need to shift to translate bytes to sectors or sectors to bytes. Serves as an assist to allow shifting instead of multiplication or division.

0x06D

1

SectorsPerClusterShift

Similar to the above, except used to calculate sectors per cluster.

0x06E

1

NumberOfFATs

Useful when implementing transaction-safe mechanisms

1 - Partition uses only a single FAT and Allocation Bitmaps
2 - Partition uses two FATs and two Allocation Bitmaps

0x06F

1

DriveSelect

Similar to the Drive Number field found at offset 0x24 in previous FAT versions. Usually 0x80 for hard disks.

0x070

1

PercentInUse

Percent of clusters in the Cluster Heap which are allocated, rounded down to the nearest whole number. This byte is not included in checksumming.

0x071

7

Reserved

Do not use

0x078

390

BootCode

Boot code pointed to by the initial jump instruction at the beginning of the structure

0x1FE

2

BootSignature

A series of bytes (0x55, 0xAA) which indicates this is a proper boot sector, similar to previous FAT versions. Should be verified prior to depending on any other fields in the Boot Sector.

Extended Boot Sectors Structure

Offset	Size	Name	Description
0x00	2^{BytesPerSectorShift} - 4	ExtendedBootCode	Additional space for boot code.
2^{BytesPerSectorShift} - 4	4	ExtendedBootSignature	A series of bytes (0x00, 0x00, 0x55, 0xAA) which indicates this is a proper boot sector, similar to previous FAT versions, but now a 32-bit value instead of 16-bit. Should be verified prior to depending on any other fields in the Extended Boot Sector. Note that this signature will appear on each of the Extended Boot Sectors.

OEM Parameters Structure

Offset

Size

Name

Description

0x000

48

Parameters[0]

A group of parameters in Generic Parameters format.

Each 48-byte entry may be one of:

NullParameters
Offset	Size	Name	Description
0x00	16	ParametersGUID	The "NullParameters" GUID: 00000000-0000-0000-0000-000000000000
0x10	32	CustomDefined	Reserved, do not use

or

FlashParameters
Offset	Size	Name	Description
0x00	16	ParametersGUID	The "FlashParameters" GUID: 0A0C7E46-3399-4021-90C8-FA6D389C4BA2
0x10	4	EraseBlockSize	The size of the flash media's erase block, in bytes
0x14	4	PageSize	The page size, in bytes, of the flash media
0x18	4	SpareSectors	The number of sectors the flash media has free for use in sparing operations
0x1C	4	RandomAccessTime	The average number of nanoseconds the flash media needs to randomly access its memory
0x20	4	ProgrammingTime	The average number of nanoseconds the flash media needs for a programming operation
0x24	4	ReadCycle	The average number of nanoseconds the flash media needs to perform a read cycle
0x28	4	WriteCycle	The average number of nanoseconds the flash media needs to perform a write cycle
0x2C	4	Reserved	Do not use

As of this writing, the exFAT specification only defines the two GUIDs above; additional ones may be defined in future versions.

0x030

48

Parameters[1]

0x060

48

Parameters[2]

0x090

48

Parameters[3]

0x0C0

48

Parameters[4]

0x0F0

48

Parameters[5]

0x120

48

Parameters[6]

0x150

48

Parameters[7]

0x180

48

Parameters[8]

0x1B0

48

Parameters[9]

0x1E0

2^{BytesPerSectorShift} - 480

Reserved

Do not use

FAT Region Structure

Offset

Size

Name

Description

0x00

4

FatEntry[0]

Comprised of a Media Descriptor byte (always 0xF8) followed by three bytes of 0xFF each

0x04

4

FatEntry[1]

Always 0xFFFFFFFF. Do not use, interpret, or change.

0x08

4

FatEntry[2]

One of the following:

Value	Description
0x00000000	Denotes a free cluster available for allocation
0x00000002 through ClusterCount - 1	A pointer to the next cluster in the chain
0xFFFFFFF7	Bad cluster
0xFFFFFFFF	End-of-chain markeer

These are identical to the values used in previous FAT versions, with the difference that 0x00000001, 0xFFFFFFF0 - 0xFFFFFFF6, and 0xFFFFFFF8 - 0xFFFFFFFE are no longer ascribed any significance.

...

C Code Examples

Filesystem Header

struct bootsector {
	char jump[3];
	char fsid[8];
	char pad[53];
	uint64_t unk; // sectors before first cluster too?
	uint64_t fsSizeInSectors;
        uint32_t sectorsToStartOfFat;
        uint32_t sectorsUsedForFat;
        uint32_t sectorStartOfFirstCluster;
        uint32_t clusterCountInFs;
        uint32_t clusterForRootDirectory;
        uint32_t unk[2];
        uint8_t logBytesPerSector, logSectorsPerCluster, fatCount, driveId; // 2-log for both. bytesPerCluster = 1 << (logBytesPerSector + logSectorsPerCluster).
        uint8_t percentInUse; // for display ala windows computer explorer
        uint8_t unk[7];
        char bootcode[390];
        char bootsign[2];
};

Block bitmap is cluster 2, upcase tbl is cluster 3.

Boot Checksum Calculation

UInt32 BootChecksum (UCHAR * Sectors, USHORT BytesPerSector)
{
  /*
    UCHAR * Sectors, above, points to an in-memory copy of the 11 sectors
    That is, 0 - 10 for the Main Boot Region, or 12 - 22 for the Backup Boot Region
  */
 
  UInt32 NumberOfBytes = (UInt32)BytesPerSector * 11;
  UInt32 Checksum = 0;
  UInt32 Index;
 
  for (Index = 0; Index < NumberOfBytes; Index++)
  {
    if ((Index == 106) || (Index == 107) || (Index == 112))
    {
      continue;
    }
    Checksum = ((Checksum&1) ? 0x80000000 : 0) + (Checksum>>1) + (UInt32)Sectors[Index];
  }
  return Checksum;
}

Directory entries

32-byte entries. Entries contain one of a few possible entry types, first byte identifies which type it is. The second byte identifies how many subsequent entries are part of this entry. All bytes after are dependent on the type of entry. Any entry with the highest bit of entrytype set is not used.

struct {
        uint8_t entrytype = 0x85;
        uint8_t entrycount; // This is the number of subsequent entries that also belong to this "file" entry. Should be at least 2, one 0xC0 and one 0xC1 for info and filename.
        uint8_t pad[2];
        uint32_t flags; // 0x10 == directory, probably identical to fat32
        uint32_t creation, modification, access;
        char pad[12];
} fileEntry;

struct {
        uint8_t entryType = 0xC0;
        uint8_t flags;
        uint8_t unk; // = 0. May be part of next field, but then it'd be in big-endian order which is unlikely.
        uint8_t filenameLengthInBytes;
        uint64_t filesize;
        uint32_t unk;
        uint32_t startCluster; // minus 2! Same as fat12/16/32.
        uint64_t filesize2;
} fileInfoEntry;

struct {
        uint8_t entrytype = 0xC1, entrycount = 0x00;
        uint16_t name[15];
} filenameEntry;

Disk Hexdump Examples

Short filenames

   01040300  85 02 c8 f9 10 00 00 00  5b a9 47 45 6f a9 47 45  |........[.GEo.GE|
   01040310  6f a9 47 45 25 25 88 88  88 00 00 00 00 00 00 00  |o.GE%%..........|
   01040320  c0 03 00 05 ae 26 00 00  00 00 02 00 00 00 00 00  |.....&..........|
   01040330  00 00 00 00 17 00 00 00  00 00 02 00 00 00 00 00  |................|
   01040340  c1 00 69 00 6d 00 61 00  67 00 65 00 00 00 00 00  |..i.m.a.g.e.....|
   01040350  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|

Longer filenames

   01080060  85 03 34 aa 10 00 00 00  1c a5 47 45 92 0a 48 45  |..4.......GE..HE|
   01080070  92 0a 48 45 94 94 88 88  88 00 00 00 00 00 00 00  |..HE............|
   01080080  c0 03 00 18 23 20 00 00  00 00 02 00 00 00 00 00  |....# ..........|
   01080090  00 00 00 00 07 00 00 00  00 00 02 00 00 00 00 00  |................|
   010800a0  c1 00 63 00 6f 00 6d 00  2e 00 67 00 6f 00 6f 00  |..c.o.m...g.o.o.|
   010800b0  67 00 6c 00 65 00 2e 00  61 00 6e 00 64 00 72 00  |g.l.e...a.n.d.r.|
   010800c0  c1 00 6f 00 69 00 64 00  2e 00 6d 00 75 00 73 00  |..o.i.d...m.u.s.|
   010800d0  69 00 63 00 00 00 00 00  00 00 00 00 00 00 00 00  |i.c.............|

Really Long filenames

   01100640  85 05 a9 6f 20 00 00 00  25 38 48 45 26 38 48 45  |...o ...%8HE&8HE|
   01100650  25 38 48 45 64 64 00 00  00 00 00 00 00 00 00 00  |%8HEdd..........|
   01100660  c0 01 00 32 c6 a5 00 00  d8 52 76 00 00 00 00 00  |...2.....Rv.....|
   01100670  00 00 00 00 14 46 00 00  d8 52 76 00 00 00 00 00  |.....F...Rv.....|
   01100680  c1 00 30 00 30 00 33 00  20 00 2d 00 20 00 4c 00  |..0.0.3. .-. .L.|
   01100690  65 00 64 00 20 00 5a 00  65 00 70 00 70 00 65 00  |e.d. .Z.e.p.p.e.|
   011006a0  c1 00 6c 00 69 00 6e 00  20 00 2d 00 20 00 53 00  |..l.i.n. .-. .S.|
   011006b0  74 00 61 00 69 00 72 00  77 00 61 00 79 00 20 00  |t.a.i.r.w.a.y. .|
   011006c0  c1 00 74 00 6f 00 20 00  68 00 65 00 61 00 76 00  |..t.o. .h.e.a.v.|
   011006d0  65 00 6e 00 20 00 2d 00  20 00 31 00 39 00 37 00  |e.n. .-. .1.9.7.|
   011006e0  c1 00 32 00 2e 00 6d 00  70 00 33 00 00 00 00 00  |..2...m.p.3.....|    
   011006f0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|

ExFAT

Contents

Partition Layout

Structures

Boot Sector Structure

Extended Boot Sectors Structure

OEM Parameters Structure

FAT Region Structure

C Code Examples

Filesystem Header

Boot Checksum Calculation

Directory entries

Disk Hexdump Examples

Short filenames

Longer filenames

Really Long filenames

Personal tools

Namespaces

Variants

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