libaaruformat/docs/spec/blocks/data.adoc

=== 📦 Data Block (`DBLK`)

A data block encapsulates media-derived content and is composed of a header followed by either compressed or uncompressed data.

The contents of a data block may represent user data—such as media sectors—or auxiliary data elements, including media or sector-specific tags.

When a data block includes multiple items (e.g., sectors or sector tags), the `sectorSize` field specifies the size, in bytes, of each individual item.
Conversely, if the block contains a single item (e.g., media tags), `sectorSize` must be set to 0.

==== Structure Definition

[source,c]
#define DATABLOCK_MAGIC 0x4B4C4244
/**Block header, precedes block data */
typedef struct BlockHeader
{
    uint32_t identifier;   ///< Block identifier, must be BlockType::DataBlock.
    uint16_t type;         ///< Logical data classification (value from \ref DataType).
    uint16_t compression;  ///< Compression algorithm used (value from \ref CompressionType).
    uint32_t sectorSize;   ///< Size in bytes of each logical sector represented in this block.
    uint32_t cmpLength;    ///< Size in bytes of the compressed payload immediately following this header.
    uint32_t length;       ///< Size in bytes of the uncompressed payload resulting after decompression.
    uint64_t cmpCrc64;     ///< CRC64-ECMA of the compressed payload (cmpLength bytes).
    uint64_t crc64;        ///< CRC64-ECMA of the uncompressed payload (length bytes).
} BlockHeader;

==== Field Descriptions

[cols="2,2,2,6",options="header"]
|===
|Type
|Size
|Name
|Description

|uint32_t
|4 bytes
|identifier
|The data block identifier, always `DBLK`

|uint16_t
|2 bytes
|type
|The data type contained in this block. See Annex B.

|uint16_t
|2 bytes
|compression
|The compression algorithm used in the data. See Annex C.

|uint32_t
|4 bytes
|sectorSize
|The size in bytes of the sectors contained in this data block if applicable.

|uint32_t
|4 bytes
|cmpLength
|The total size in bytes of the compressed payload that follows this header. For LZMA (IDs 1, 3) this includes the 5-byte properties prefix; see Appendix C for details.

|uint32_t
|4 bytes
|length
|The size in bytes of the data block when decompressed.

|uint64_t
|8 bytes
|cmpCrc64
|CRC64-ECMA of the compressed payload. For LZMA (IDs 1, 3) this covers the compressed stream only, excluding the 5-byte properties prefix; see Appendix C for details.

|uint64_t
|8 bytes
|crc64
|The CRC64-ECMA checksum of the decompressed data.
|===