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:

bit→
↓byte
7 6 5 4 3 2 1 0
0 Operation code = 25h
1 LUN Reserved RelAdr
2 LBA (MSB)



LBA (LSB)
3
4
5
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:

bit→
↓byte
7 6 5 4 3 2 1 0
0–3 Returned LBA
4–7 Block length in bytes

Famous quotes containing the words read, capacity and/or command:

    I’ve never read a political poem that’s accomplished anything. Poetry makes things happen, but rarely what the poet wants.
    Howard Nemerov (1920–1991)

    People between twenty and forty are not sympathetic. The child has the capacity to do but it can’t know. It only knows when it is no longer able to do—after forty. Between twenty and forty the will of the child to do gets stronger, more dangerous, but it has not begun to learn to know yet. Since his capacity to do is forced into channels of evil through environment and pressures, man is strong before he is moral. The world’s anguish is caused by people between twenty and forty.
    William Faulkner (1897–1962)

    ‘Tis not in mortals to command success,
    But we’ll do more, Sempronius, we’ll deserve it.
    Joseph Addison (1672–1719)