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→‎Structures: Added Main Boot Sector Structure (Start of information additions from Microsoft's newly publicly-released exFAT spec)
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{{Filesystems}}
{{Stub}}
{{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.
ExFAT, contrary to its name, is not a version of FAT, but it's own filesystem.
 
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 ==
 
{| {{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.
 
After a long period of keeping the official exFAT spec under their collective hats, Microsoft has finally published the official specification at https://docs.microsoft.com/en-us/windows/win32/fileio/exfat-specification. The major structures of the file system are described below.
 
== Structures ==
 
=== Main Boot Sector Structure ===
 
{| {{wikitable}}
Line 25 ⟶ 69:
| 0x048 || 8 || VolumeLength || The total size of this exFAT partition in sectors. Should not be smaller than 1 MiB.
|-
| 0x050 || 4 || FatOffsetFATOffset || Offset in sectors from the start of the partition to the first FAT
|-
| 0x054 || 4 || FatLengthFATLength || 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", better known as the "Data Area" on former FAT implementationsversions.
|-
| 0x05C || 4 || ClusterCount || The total number of clusters in the Cluster Heap.
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| 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 claculationcalculation; 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.
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|-
|}
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.
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| 0x06D || 1 || SectorsPerClusterShift || Similar to the above, except used to calculate sectors per cluster.
|-
| 0x06E || 1 || NumberOfFatsNumberOfFATs || 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
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| 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
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| 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, assimilar into 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.
 
|-
| ... || ... || ... || ...
|-
|}
 
 
 
 
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=== Filesystem Header ===
 
<sourcesyntaxhighlight lang="C">
struct bootsector {
char jump[3];
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char bootsign[2];
};
</syntaxhighlight>
</source>
 
Block bitmap is cluster 2, upcase tbl is cluster 3.
 
 
=== Boot Checksum Calculation ===
<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 ===
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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.
 
<sourcesyntaxhighlight lang="C">
struct {
uint8_t entrytype = 0x85;
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char pad[12];
} fileEntry;
</syntaxhighlight>
</source>
 
<sourcesyntaxhighlight lang="C">
struct {
uint8_t entryType = 0xC0;
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uint64_t filesize2;
} fileInfoEntry;
</syntaxhighlight>
</source>
 
<sourcesyntaxhighlight lang="C">
struct {
uint8_t entrytype = 0xC1, entrycount = 0x00;
uint16_t name[15];
} filenameEntry;
</syntaxhighlight>
</source>
 
 
Retrieved from "https://osdev.wiki/wiki/ExFAT"