mirror of
https://github.com/aaru-dps/libaaruformat.git
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86 lines
1.9 KiB
Plaintext
86 lines
1.9 KiB
Plaintext
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=== The twin sector table (`TWTB`)
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This table enumerates hardware sectors that share an identical sector number.
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Such sectors are referred to as “twin sectors,” although the grouping may consist of more than two instances.
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The associated pointer is resolved following the same logic applied in a last-level deduplication table.
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[source,c]
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/* Undefined */
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==== Field Descriptions
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[cols="2,2,2,6",options="header"]
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|===
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|Type
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|Size
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|Name
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|Description
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|uint32_t
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|4 bytes
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|identifier
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|The twin sector table identifier, always `TWTB`
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|uint8_t
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|1 byte
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|alignment
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|Shift of alignment of all blocks in the image. This must be the same in all deduplication tables and subtables.
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|uint8_t
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|1 byte
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|shift
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|The shift used to calculate the position of a sector in a data block pointed by this table, or how many sectors are pointed by the next level.
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|uint64_t
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|8 bytes
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|entries
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|How many pointers follow this header.
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|uint32_t
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|4 bytes
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|length
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|The size in bytes of the table block.
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|uint64_t
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|8 bytes
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|crc64
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|The CRC64-ECMA checksum of the decompressed table.
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|===
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==== Twin sector entries
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[cols="2,2,2,6",options="header"]
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|===
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|Type
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|Size
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|Name
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|Description
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|uint32_t
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|8 bytes
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|pointer
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|Pointer to the sector.
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|===
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==== Pointer-Based Data Block Resolution
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To determine the corresponding data block:
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. Right-shift the pointer value using the specified `shift` parameter.
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. Multiply the result by the `alignment` value.
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. The remainder from this operation indicates the sector's offset within the target data block.
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Each data block contains a fixed number of bytes per sector, which remains constant across blocks.
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This invariant size allows for more efficient storage of pointer values.
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===== Example
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Given the following parameters:
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* Pointer Value: `0x8003`
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* Shift Value: `5`
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* Alignment: `9`
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The data block is located at byte offset `524288`.
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The sector referenced by the pointer is the **third entry** within this block.
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