exFAT Defined Directory Entries
Main exFAT Diectory entries defined in table below:
Defined Directory Entries list
| EntryType |
Primary |
Critical |
Code |
Directory Entry Name |
| 0x81 |
|
|
1 |
Allocation Bitmap |
| 0x82 |
|
|
2 |
Up-case Table |
| 0x83 |
|
|
3 |
Volume Label |
| 0x85 |
|
|
5 |
File |
| 0xA0 |
|
|
0 |
Volume GUID |
| 0xA1 |
|
|
1 |
TexFAT Padding |
| 0xA2 |
|
|
2 |
Windows CE Access Control Table |
| 0xC0 |
|
|
0 |
Stream Extension |
| 0xC1 |
|
|
1 |
File Name |
Read about Directory entries below:
Allocation Bitmap Directory Entry
| Offset |
Size |
Description |
Comments |
| 0 (0x00) |
1 |
Entry type |
0x81 |
| 1 (0x01) |
1 |
BitmapFlags (see below) |
Indicates which Allocation Bitmap the given entry describes |
| 2 (0x02) |
18 |
Reserved |
|
| 20 (0x14) |
4 |
First Cluster |
|
| 24 (0x18) |
8 |
Data Length |
|
Bitmap Flags
| Bits |
Size |
Description |
Comments |
| 0 |
1 |
BitmapIdentifier |
0 – 1st bitmap, 1 - 2nd bitmap |
| 1 |
7 |
Reserved |
|
The number of bitmaps and therefore a number of Bitmap Allocation entries is equal to the number of FATs. In case of TexFAT two FATs are used and bit 0 of Flags indicates which bitmap and FAT are referred.
The First Allocation Bitmap shall be used in conjunction with the First FAT and the Second Allocation Bitmap shall be used with the Second FAT. ActiveFat field in Boot Sector defines which FAT and Allocation Bitmap are active.
Bitmap size in bytes must be a number of clusters in the volume divided by 8 and rounded up.
Up-Case Table Directory Entry
| Offset |
Size |
Description |
Comments |
| 0 (0x00) |
1 |
Entry type |
0x82 |
| 1 (0x01) |
3 |
Reserved1 |
|
| 4 (0x04) |
4 |
TableChecksum |
Up-case Table checksum |
| 8 (0x08) |
12 |
Reserved2 |
|
| 20 (0x14) |
4 |
FirstCluster |
|
| 24 (0x18) |
8 |
DataLength |
|
The checksum is calculated against DataLength bytes of Up-case Table according to the following code:
Code:
UINT32 UpCaseTableChecksum(const unsigned char data[], int bytes)
{
UINT32 checksum = 0;
for (int i = 0; i < bytes; i++)
checksum = (checksum << 31) | (checksum >> 1) + data[i];
return checksum;
}
Volume Label Directory Entry
| Offset |
Size |
Description |
Comments |
| 0 (0x00) |
1 |
Entry type |
0x83 |
| 1 (0x01) |
1 |
CharacterCount |
Length in Unicode characters (max 11) |
| 2 (0x02) |
22 |
VolumeLabel |
Unicode string |
| 24 (0x18) |
8 |
Reserved |
|
If volume is formatted without a label, the Volume Label Entry will be present but Entry Type will be set to 0x03 (not in use).
File Directory Entry
File directory entry describes files and directories. It is a primary critical directory entry and must be immediately followed by 1 Stream Extension directory entry and from 1 to 17 File Name directory entries. Those 3-19 directory entries comprise a directory entry set describing a single file or a directory.
| Offset |
Size |
Description |
Comments |
| 0 (0x00) |
1 |
Entry type |
0x85 |
| 1 (0x01) |
1 |
SecondaryCount |
Must be from 2 to 18 |
| 2 (0x02) |
2 |
SetChecksum |
|
| 4 (0x04) |
2 |
FileAttributes (see below) |
|
| 6 (0x06) |
2 |
Reserved1 |
|
| 8 (0x08) |
4 |
CreateTimestamp |
|
| 12 (0x0C) |
4 |
LastModifiedTimestamp |
|
| 16 (0x10) |
4 |
LastAccessedTimestamp |
|
| 20 (0x14) |
1 |
Create10msIncrement |
0..199 |
| 21 (0x15) |
1 |
LastModified10msIncrement |
0..199 |
| 22 (0x16) |
1 |
CreateTimezoneOffset |
Offset from UTC in 15 min increments |
| 23 (0x17) |
1 |
LastModifiedTimezoneOffset |
Offset from UTC in 15 min increments |
| 24 (0x18) |
1 |
LastAccessedTimezoneOffset |
Offset from UTC in 15 min increments |
| 25 (0x19) |
7 |
Reserved2 |
|
File Attributes
| Bits |
Size |
Attribute |
Comments |
| 0 |
1 |
ReadOnly |
|
| 1 |
1 |
Hidden |
|
| 2 |
1 |
System |
|
| 3 |
1 |
Reserved1 |
|
| 4 |
1 |
Directory |
|
| 5 |
1 |
Archive |
|
| 6 |
10 |
Reserved2 |
|
Timestamp Format
| Bits |
Size |
Description |
Comments |
| 0-4 |
5 |
Seconds (as number of 2-second intervals) |
0..29 29 represents 58 seconds |
| 5-10 |
6 |
Minutes |
0..59 |
| 11-15 |
5 |
Hour |
0..23 |
| 16-20 |
5 |
Day |
1..31 |
| 21-24 |
4 |
Month |
1..12 |
| 25-31 |
7 |
Year (as offset from 1980) |
0 represents 1980 |
Timestamp format records seconds as 2 seconds intervals, so 10ms increments are used to increase precision from 2 seconds to 10 milliseconds. The valid values are from 0 to 199 in 10ms intervals which are added to correspondent timestamp. Timestamp is recorded in local time.
Time zone offset is expressed in 15 minutes increments.
Time Zone Offset Tablet
| TimezoneOffset field |
TZ Offset |
Time Zone |
Comments |
| 128 (0x80) |
UTC |
Greenwich Standard Time |
|
| 132 (0x84) |
UTC+01:00 |
Central Europe Time |
|
| 136 (0x88) |
UTC+02:00 |
Eastern Europe Standard Time |
|
| 140 (0x8C) |
UTC+03:00 |
Moscow Standard Time |
|
| 144 (0x90) |
UTC+04:00 |
Arabian Standard Time |
|
| 148 (0x94) |
UTC+05:00 |
West Asia Standard Time |
|
| 152 (0x98) |
UTC+06:00 |
Central Asia Standard Time |
|
| 156 (0x9C) |
UTC+07:00 |
North Asia Standard Time |
|
| 160 (0xA0) |
UTC+08:00 |
North Asia East Standard Time |
|
| 164 (0xA4) |
UTC+09:00 |
Tokyo Standard Time |
|
| 168 (0xA8) |
UTC+10:00 |
West Pacific Standard Time |
|
| 172 (0xAC) |
UTC+11:00 |
Central Pacific Standard Time |
|
| 176 (0xB0) |
UTC+12:00 |
New Zealand Standard Time |
|
| 180 (0xB4) |
UTC+13:00 |
Tonga Standard Time |
|
| 208 (0xD0) |
UTC-12:00 |
Dateline Standard Time |
|
| 212 (0xD4) |
UTC-11:00 |
Samoa Standard Time |
|
| 216 (0xD8) |
UTC-10:00 |
Hawaii Standard Time |
|
| 220 (0xDC) |
UTC-09:00 |
Alaska Standard Time |
|
| 224 (0xE0) |
UTC-08:00 |
Pacific Standard Time |
|
| 228 (0xE4) |
UTC-07:00 |
Mountain Standard Time |
|
| 232 (0xE8) |
UTC-06:00 |
Central Standard Time |
|
| 236 (0xEC) |
UTC-05:00 |
Eastern Standard Time |
|
| 240 (0xF0) |
UTC-04:00 |
Atlantic Standard time |
|
| 242 (0xF2) |
UTC-03:30 |
Newfoundland Standard Time |
|
| 244 (0xF4) |
UTC-03:00 |
Greenland Standard Time |
|
| 248 (0xF8) |
UTC-02:00 |
Mid-Atlantic Standard Time |
|
| 252 (0xFC) |
UTC-01:00 |
Azores Standard Time |
|
Volume GUID Directory Entry
In following table presented a benign primary directory entry and may not present in a file system.
| Offset |
Size |
Description |
Comments |
| 0 (0x00) |
1 |
EntryType |
0xA0 |
| 1 (0x01) |
1 |
SecondaryCount |
Must be 0x00 |
| 2 (0x02) |
2 |
SetChecksum |
|
| 4 (0x04) |
2 |
GeneralPrimaryFlags (See below) |
|
| 6 (0x06) |
16 |
VolumeGuid |
All values are valid except null GUID {00000000-0000-0000-0000-000000000000} |
| 22 (0x16) |
10 |
Reserved |
|
Primary Flags Definitions
| Bits |
Size |
Description |
Comments |
| 0 |
1 |
AllocationPossible |
Must be 0 |
| 1 |
1 |
NoFatChain |
Must be 0 |
| 2 |
14 |
CustomDefined |
|
TexFAT Padding Directory Entry
| Offset |
Size |
Description |
Comments |
| 0 (0x00) |
1 |
EntryType |
0xA1 |
| 1 (0x01) |
31 |
Reserved |
|
exFAT 1.00 does not define TexFAT Padding directory entry. TexFAT Padding directory entries are only valid in the first cluster of directory and occupy every directory entry of the cluster. The implementations should not move TexFAT Padding directory entries.
Windows CE Access Control Table Directory Entry
| Offset |
Size |
Description |
Comments |
| 0 (0x00) |
1 |
EntryType |
0xA2 |
| 1 (0x01) |
31 |
Reserved |
|
exFAT 1.00 does not define Windows CE Access Control Table Directory Entry.
Stream Extension Directory Entry
| Offset |
Size |
Description |
Comments |
| 0 (0x00) |
1 |
EntryType |
0xC0 |
| 1 (0x01) |
1 |
GeneralSecondaryFlags (see below) |
|
| 2 (0x02) |
1 |
Reserved1 |
|
| 3 (0x03) |
1 |
NameLength |
Length of Unicode name contained in subsequent File Name directory entries |
| 4 (0x04) |
2 |
NameHash |
Hash of up-cased file name |
| 6 (0x06) |
2 |
Reserved2 |
|
| 8 (0x08) |
8 |
ValidDataLength |
Must be between 0 and DataLength |
| 16 (0x10) |
4 |
Reserved3 |
|
| 20 (0x14) |
4 |
FirstCluster |
|
| 24 (0x18) |
8 |
DataLength |
For directories maximum 256 MB |
Secondary Flags Definitions
| Bits |
Size |
Description |
Comments |
| 0 |
1 |
AllocationPossible |
Must be 1 |
| 1 |
1 |
NoFatChain |
|
| 2 |
14 |
CustomDefined |
|
Stream Extension directory entry must immediately follow the File directory entry in the set. It could be only one Stream Extension entry in the set. If NoFatChain flag is set, all allocated clusters are contiguous.
The NameHash field facilitates the purpose of fast file name comparison and is performed on up-cased file name. NameHash verify against a mismatch, however matching hashes cannot guarantee the equality of file names. If name hashes match, a subsequent full name comparison must be performed.
Code:
// fileName points to up-cased file name
UINT16 NameHash(WCHAR *fileName, int nameLength)
{
UINT16 hash = 0;
unsigned char *data = (unsigned char *)fileName;
for (int i = 0; i < nameLength * 2; i++)
hash = (hash << 15) | (hash >> 1) + data[i];
return hash;
}
ValidDataLength determines how much actual data written to the file. Implementation shall update this field as data has been written. The data beyond the valid data length is undefined and implementation
shall return zeros.
File Name Directory Entry
| Offset |
Size |
Description |
Comments |
| 0 (0x00) |
1 |
EntryType |
0xC1 |
| 1 (0x01) |
1 |
GeneralSecondaryFlags (see below) |
|
| 2 (0x02) |
30 |
FileName |
|
Secondary Flags Definitions
| Bits |
Size |
Description |
Comments |
| 0 |
1 |
AllocationPossible |
Must be 0 |
| 1 |
1 |
NoFatChain |
Must be 0 |
| 2 |
14 |
CustomDefined |
|
File Name directory entries must immediately follow the Steam Extension directory entry in the number of NameLength/15 rounded up. The maximum number of File Name entries is 17, each can hold up to 15 Unicode characters and the maximum file name length is 255. Unused portion of FileName field must be set to 0x0000.
Invalid File Name Characters
| Character Code |
Character |
Description |
| 0x0000 – 0x001F |
|
Control codes |
| 0x0022 |
“ |
Quotation mark |
| 0x002A |
* |
Asterisk |
| 0x002F |
/ |
Forward slash |
| 0x003A |
: |
Colon |
| 0x003C |
< |
Less than |
| 0x003E |
> |
Greater than |
| 0x003F |
? |
Question mark |
| 0x005C |
\ |
Back slash |
| 0x007C |
| |
Vertical bar |
See Also:
exFAT File System |
Volume Layout |
Directory Structure |
Cluster Heap
