UDF
| Filesystems |
|---|
| Virtual Filesystems |
| Disk Filesystems |
| CD/DVD Filesystems |
| Network Filesystems |
| Flash Filesystems |
The Universal Disc Format or UDF is the filesystem used on DVDs and Blu-Ray discs. It is similar to the CDFS (ISO 9660) file system, but it includes many extended features, such as incremental updates, which allows files to be deleted or updated over time.
Volume Structure
Like the CDFS file system, the UDF file system uses a 2048 byte sector size, and it designates that the first 16 sectors can be used by the OS to store proprietary data or boot logic. It also, like CDFS, uses 2048 byte aligned descriptors, starting at sector 16, to record the UDF specific information to locate additional volumes, partitions and files located on the disc. It also supports the inclusion of descriptors from other file systems for compatibility with other media formats, operating systems and devices.
The descriptors located at sector 16 (0x10) all use the same general layout as the CDFS volume descriptor:
| Offset | Length (bytes) | Field name | Datatype | Description |
|---|---|---|---|---|
| 0 | 1 | Type | int8 | Volume Descriptor type code (see below). |
| 1 | 5 | Identifier | strA | Designates the type of descriptor. |
| 6 | 1 | Version | int8 | Volume Descriptor Version (0x01). |
| 7 | 2041 | Data | - | Depends on the volume descriptor type. |
The identifier field is used to determine the type of the descriptor:
| Identifier | Description |
|---|---|
| "CD001" | Indicates that this volume contains an ISO 9660 file system. |
| "BEA01" | Denotes the beginning of the extended descriptor section. |
| "NSR02" | Indicates that this volume contains a UDF file system. |
| "NSR03" | Indicates that this volume contains a UDF file system. |
| "BOOT2" | Includes information concerning boot loader location and entry point address. |
| "TEA01" | Denotes the end of the extended descriptor section. |
The presence of an NSR02 or NSR03 descriptor denotes that there is a valid UDF file system located on this volume, and that there is an Anchor Volume Descriptor Pointer located in sector 256 (0x100), with backup copies located in the last sector of the volume and/or the sector located 256 sectors from the end of the volume.
Anchor Volume Descriptor Pointer
The Anchor Volume Descriptor Pointer is a descriptor that contains the location of the Main Volume Descriptor and the backup Reserve Volume Descriptor.
| Offset | Length (bytes) | Field name | Datatype | Description |
|---|---|---|---|---|
| 0 | 16 | DescriptorTag | struct | Contains the descriptor type, version and checksum information for this descriptor. |
| 16 | 8 | MainVolumeDescriptorSequenceExtent | struct | Contains the sector and length of the main volume descriptor sequence. |
| 24 | 8 | ReserveVolumeDescriptorSequenceExtent | struct | Contains the sector and length of the backup volume descriptor sequence. |
| 32 | 480 | Reserved | - | Reserved for future use. |
Descriptor Tag
The descriptor tag structure is used by all UDF descriptors to designate the descriptor type and to verify data integrity.
| Offset | Length (bytes) | Field name | Datatype | Description |
|---|---|---|---|---|
| 0 | 2 | TagIdentifier | uint16 | Contains the descriptor type. |
| 2 | 2 | DescriptorVersion | uint16 | Contains the version of the descriptor tag. |
| 4 | 1 | TagChecksum | uint8 | Used to verify the data within the Descriptor Tag structure. |
| 5 | 1 | Reserved | uint8 | Reserved for future use. |
| 6 | 2 | TagSerialNumber | uint16 | Unique serial number for this volume |
| 8 | 2 | DescriptorCRC | uint16 | Used to verify the data with the Descriptor Tag structure. |
| 10 | 2 | DescriptorCRCLength | uint16 | Used to verify the data with the Descriptor Tag structure. |
| 12 | 4 | TagLocation | uint32 | Contains the sequence number of this descriptor. |
The TagIdentifier field will have one of the following values:
| Identifier (Hex) | Description |
|---|---|
| 0x0001 | Primary Volume Descriptor |
| 0x0002 | Anchor Volume Descriptor Pointer |
| 0x0003 | Volume Descriptor Pointer |
| 0x0004 | Implementation Use Volume Descriptor |
| 0x0005 | Partition Descriptor |
| 0x0006 | Logical Volume Descriptor |
| 0x0007 | Unallocated Space Descriptor |
| 0x0008 | Terminating Descriptor |
| 0x0009 | Logical Volume Integrity Descriptor |
| 0x0100 | File Set Descriptor |
| 0x0101 | File Identifier Descriptor |
| 0x0102 | Allocation Extent Descriptor |
| 0x0103 | Indirect Entry |
| 0x0104 | Terminal Entry |
| 0x0105 | File Entry |
| 0x0106 | Extended Attribute Header Descriptor |
| 0x0107 | Unallocated Space Entry |
| 0x0108 | Space Bitmap Descriptor |
| 0x0109 | Partition Integrity Entry |
| 0x010a | Extended File Entry |
Extent Structure
The extent structure is used to locate data on the current volume. It contains the sector index (relative to the beginning of the volume), and the length of the data.
| Offset | Length (bytes) | Field name | Datatype | Description |
|---|---|---|---|---|
| 0 | 4 | Length | uint32 | Contains the length, in bytes, of the data pointed to by this extent. |
| 4 | 4 | Location | uint32 | Contains the sector index of the data, relative to the beginning of the volume. |
Volume Descriptor Sequence
The Main (and Reserve) Volume Sequence can be found by reading the extent information located in the Anchor Volume Descriptor Pointer record above. Starting at the sector specified in the Location field of the extent structure, several Volume Descriptors will be found. Each of these descriptors will provide additional information about this volume. The Terminating Descriptor (0x0008) marks the end of the Volume Descriptor Sequence.
Note: Only Volume Descriptors (0x0001 to 0x0009) are allowed in the Volume Descriptor Sequence.
Note: The UDF Specifications state that the minimum extent size for the Volume Descriptor Sequence is 16 sectors, so there may be empty descriptor blocks after the Terminating Descriptor block, which can be ignored.
Logical Volume Descriptor
The Logical Volume Descriptor (0x0006) contains information concerning the Logical Volume, including a pointer to the File Set Descriptor.
| Offset | Length (bytes) | Field name | Datatype | Description |
|---|---|---|---|---|
| 0 | 16 | Tag | struct | The volume tag for this descriptor. |
| 16 | 4 | Volume Sequence Number | uint32 | The sequence number of this Logical Volume. |
| 20 | 64 | Descriptor Character Set | struct | The character set used in this descriptor. |
| 84 | 128 | Logical Volume Identifier | Compressed Unicode | The name of the logical volume. |
| 212 | 4 | Logical Block Size | uint32 | The block size of the logical volume. |
| 216 | 32 | Domain Identifier | struct | The domain identifier for this volume. |
| 248 | 16 | Logical Volume Contents Use | byte[] | Area reserved for content use. |
| 264 | 4 | Map Table Length | uint32 | The length of the map table in bytes. |
| 268 | 4 | Number Of Partition Maps | uint32 | The number of partition map records in this descriptor. |
| 272 | 32 | Implementation Identifier | struct | The implementation identifier for this volume. |
| 304 | 128 | Implementation Use | byte[] | Area reserved for implementation use. |
| 432 | 8 | Integrity Sequence Extent | struct | The extent of the integrity sequence, if any. |
| 440 | * | Partition Maps | struct[] | The map of partitions on this logical volume. |