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 read less and less. I have not forgiven books for their failure to tell me the truth and make me happy.”
—Mason Cooley (b. 1927)
“Popularity—The capacity for listening sympathetically when men boast of their wives and women complain of their husbands.”
—H.L. (Henry Lewis)
“Ordinary time is “quality time” too. Everyday activities are not just necessities that keep you from serious child rearing: they are the best opportunities for learning you can give your child...because her chief task in her first three years is precisely to gain command of the day-to-day life you take for granted.”
—Amy Laura Dombro (20th century)