SCSI Read Capacity Command

The SCSI Read Capacity command is used to obtain data capacity information from a target device.

The command descriptor block (CDB) structure is:

7 6 5 4 3 2 1 0
0 Operation code = 25h
1 LUN Reserved RelAdr

6 Reserved
7 Reserved
8 Reserved PMI
9 Control

The special control fields in the CDB have the following meaning:

  • RelAdr - indicates that the logical block address (LBA) value is relative (only used with linked commands).
  • PMI - partial medium indicator:
    • 0 - return value for the last LBA
    • 1 - return value for the last LBA after which a substantial delay in data transfer will be encountered (e.g., the current track or cylinder)

The target will return capacity data structured as follows:

7 6 5 4 3 2 1 0
0–3 Returned LBA
4–7 Block length in bytes

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