The deduplication table is a multi-level table of pointers to LBAs contained in the image.
It starts with the following header.
[source,c]
/* Undefined */
==== Field Descriptions
[cols="2,2,2,6",options="header"]
|===
|Type
|Size
|Name
|Description
|uint32_t
|4 bytes
|identifier
|The deduplication table identifier, always `DDT2` or `DDTS`. The first level of a table is always `DDT2` and its presence is mandatory. Subtables will have `DDTS`
|uint16_t
|2 bytes
|type
|The data type pointed by this table. See Annex B.
|uint16_t
|2 bytes
|compression
|The compression algorithm used in the table. See Annex C.
|uint8_t
|1 byte
|levels
|How many levels of subtables are present. 1 means this is the only level.
|uint8_t
|1 byte
|tableLevel
|What level does this table correspond to
|uint64_t
|8 bytes
|previousLevel
|Pointer to absolute byte offset in the image file where the previous table level resides
|uint16_t
|2 bytes
|negative
|The negative displacement of LBA numbers. For media that can have negative LBAs, this establishes the number to substract to the table entry number
|The number of blocks in the media. This includes all blocks in the media, including the ones represented by the negative displacement as well as overflow displacement.
|uint16_t
|2 bytes
|overflow
|The positive overflow displacement of LBA numbers. For media that can have blocks beyond the end of the user area, this is the number of how many such blocks there are in the image.
The *size type* parameter defines the memory-efficient data type used to encode deduplication table entries. Its primary purpose is to minimize memory consumption by selecting the smallest viable representation for the medium in question.
Implementations should select the minimal size type capable of accommodating the full extent of the image’s sector count. This decision must be based on the expected final size of the image file.
It is important to account for *live* images—those still in the process of being dumped. These images may expand as additional sectors are captured, potentially exceeding initial bounds. Consequently, deduplication tables associated with such live captures must allow for dynamic growth beyond the originally anticipated media size.
The following table enumerates the supported size types.
|Allocates 2 bytes per entry. The high nibble (mask `0xF000`) encodes status information, while the low byte serves as a pointer to either a target sector or a subordinate indexing level.
|Allocates 3 bytes per entry. The highest nibble (mask `0xF00000`) stores status data; the remaining two bytes represent a pointer to the corresponding sector or next-level structure.
|Allocates 4 bytes per entry. The upper nibble (mask `0xF0000000`) holds status bits, and the lower 3 bytes function as a pointer to the relevant sector or hierarchical level.
Each deduplication table entry includes a status nibble that conveys the current condition or interpretation of the corresponding sector(s). The following flags are defined:
|Sector contains valid MODE 1 data with regenerable suffix or prefix. This status is applicable only to deduplication tables of type `CdSectorPrefixCorrected` or `CdSectorSuffixCorrected`.
When status values are set in a deduplication table entry that references a subordinate level, the status applies collectively to all sectors represented by that sublevel—unless the specified status explicitly overrides or alters this behavior for individual sectors.
In most storage media, the accessible range of blocks or sectors—referred to as the *user area*—represents the logical region intended for data read and write operations.
However, certain media types contain additional blocks situated outside this user area that are accessible through alternate means. These blocks often hold metadata or structural information with significant preservation value. To ensure such data is retained, these sectors must be representable within the deduplication table.
Blocks located before the start of the user area are classified as *negative sectors*. Common examples include the first track pregap or Lead-In areas found on Compact Discs.
Conversely, sectors found beyond the end of the user area are categorized as *overflow sectors*. Examples include replication metadata on floppy disks, typically recorded in track 81, and the Lead-Out area of Compact Discs.
To calculate the precise number of user area sectors represented in the deduplication table, the total number of blocks on the medium is adjusted by subtracting both negative and overflow sectors. This ensures the deduplication map reflects only the standard user-accessible region while retaining awareness of displaced block data.
