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{{Filesystems}}
Filesystem Header:
{{In Progress}}
 
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.
struct bootsector {
 
char jump[3];
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.
char fsid[8];
 
char pad[53];
== Partition Layout ==
uint64_t unk; // sectors before first cluster too?
 
uint64_t fsSizeInSectors;
{| {{wikitable}}
|+ Overview of the order of structures in an exFAT partition
! Section !! Contents !! Offset (sector number) !! Size (in sectors)
|-
| rowspan = 5 | Main Boot Region || [[#Boot_Sector_Structure|Boot Sector]] || 0 || 1
|-
| [[#Extended_Boot_Sectors_Structure|Extended Boot Sectors]] || 1 || 8
|-
| [[#OEM_Parameters_Structure|OEM Parameters]] || 9 || 1
|-
| Reserved sectors || 10 || 1
|-
| [[#Boot_Checksum_Calculation|Boot checksum]] || 11 || 1
|-
| rowspan = 5 | Backup Boot Region || Boot Sector || 12 || 1
|-
| Extended Boot Sectors || 13 || 8
|-
| OEM Parameters || 21 || 1
|-
| Reserved sectors || 22 || 1
|-
| Boot checksum || 23 || 1
|-
| rowspan = 3 | FAT Region || FAT Alignment || 24 || FATOffset - 24
|-
| [[#FAT_Region_Structure|First FAT]] || FATOffset || FATLength
|-
| Second FAT || FATOffset + FATLength || FATLength * (NumberOfFATs - 1)
|-
| rowspan = 3 | Data Region || Cluster Heap Alignment || FATOffset + FATLength * NumberOfFATs || ClusterHeapOffset - (FATOffset + FATLength * NumberOfFATs)
|-
| Cluster Heap || ClusterheapOffset || ClusterCount * 2<sup>SectorsPerClusterShift</sup>
|-
| Excess Space (to end of partition or disk) || ClusterheapOffset + ClusterCount * 2<sup>SectorsPerClusterShift</sup> || VolumeLength - (ClusterHeapOffset + ClusterCount * 2<sup>SectorsPerClusterShift</sup>)
|-
|}
 
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 ===
 
{| {{wikitable}}
|-
! 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&nbsp;&nbsp;&nbsp;" 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 ||
{| {{wikitable}}
|-
! Bit !! Name !! Description
|-
| 0 || ActiveFAT || Used to implement transaction-safe mechanisms<br>
0 - First FAT and first allocation bitmap are valid<br>
1 - Second FAT and second allocation bitmap are valid
|-
| 1 || VolumeDirty || 0 - Partition is likely in a good state<br>
1 - Partition is likely not in a good state
|-
| 2 || MediaFailure || 0 - No bad clusters aside from those already marked "bad"<br>
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<br>
1 - Partition uses only a single FAT and Allocation Bitmaps<br>
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 ===
 
{| {{wikitable}}
|-
! Offset !! Size !! Name !! Description
|-
| 0x00 || 2<sup>BytesPerSectorShift</sup> - 4 || ExtendedBootCode || Additional space for boot code.
|-
| 2<sup>BytesPerSectorShift</sup> - 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 <i>each</i> of the Extended Boot Sectors.
|-
|}
 
 
 
=== OEM Parameters Structure ===
 
{| {{wikitable}}
|-
! Offset !! Size !! Name !! Description
|-
| 0x000 || 48 || Parameters[0] || rowspan = 10 | A group of parameters in Generic Parameters format.
 
Each 48-byte entry may be one of:
{| {{wikitable}}
|+ NullParameters
|-
! Offset !! Size !! Name !! Description
|-
| 0x00 || 16 || ParametersGUID || The "NullParameters" GUID:<br>
00000000-0000-0000-0000-000000000000
|-
| 0x10 || 32 || CustomDefined || Reserved, do not use
|-
|}
 
or
 
{| {{wikitable}}
|+ FlashParameters
|-
! Offset !! Size !! Name !! Description
|-
| 0x00 || 16 || ParametersGUID || The "FlashParameters" GUID:<br>
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<sup>BytesPerSectorShift</sup> - 480 || Reserved || Do not use
|-
|}
 
 
 
=== FAT Region Structure ===
{| {{wikitable}}
|-
! 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:
 
{| {{wikitable}}
|-
! 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 ===
 
<syntaxhighlight lang="C">
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;
Line 18 ⟶ 270:
char bootcode[390];
char bootsign[2];
};
</syntaxhighlight>
Block bitmap is cluster 2, upcase tbl is cluster 3.
 
block bitmap is cluster 2, upcase tbl is cluster 3.
 
=== Boot Checksum Calculation ===
Directory entries:
<syntaxhighlight lang="C">
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;
}
</syntaxhighlight>
 
 
 
=== 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.
 
<syntaxhighlight lang="C">
struct {
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.
Line 33 ⟶ 314:
uint32_t creation, modification, access;
char pad[12];
} fileEntry;
</syntaxhighlight>
 
<syntaxhighlight lang="C">
struct {
struct {
uint8_t entryType = 0xC0;
uint8_t flags;
Line 44 ⟶ 327:
uint32_t startCluster; // minus 2! Same as fat12/16/32.
uint64_t filesize2;
} fileInfoEntry;
</syntaxhighlight>
 
<syntaxhighlight lang="C">
struct {
struct {
uint8_t entrytype = 0xC1, entrycount = 0x00;
uint16_t name[15];
} filenameEntry;
</syntaxhighlight>
 
 
Short filename:
 
== 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%%..........|
Line 59 ⟶ 349:
01040350 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
 
=== Longer filename: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............|
Line 69 ⟶ 359:
010800d0 69 00 63 00 00 00 00 00 00 00 00 00 00 00 00 00 |i.c.............|
 
=== Really longLong filenames filename:===
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..........|
Line 82 ⟶ 372:
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 |................|
 
[[Category:Filesystems]]
Retrieved from "https://osdev.wiki/wiki/ExFAT"