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 | |||||||
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—Gail Haley (20th century)
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—Mark Twain [Samuel Langhorne Clemens] (1835–1910)
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—James Boswell (1740–1795)