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:
“It was one of those evenings when men feel that truth, goodness and beauty are one. In the morning, when they commit their discovery to paper, when others read it written there, it looks wholly ridiculous.”
—Aldous Huxley (1894–1963)
“It is part of the educator’s responsibility to see equally to two things: First, that the problem grows out of the conditions of the experience being had in the present, and that it is within the range of the capacity of students; and, secondly, that it is such that it arouses in the learner an active quest for information and for production of new ideas. The new facts and new ideas thus obtained become the ground for further experiences in which new problems are presented.”
—John Dewey (1859–1952)
“It is manifest therefore that they who have sovereign power, are immediate rulers of the church under Christ, and all others but subordinate to them. If that were not, but kings should command one thing upon pain of death, and priests another upon pain of damnation, it would be impossible that peace and religion should stand together.”
—Thomas Hobbes (1579–1688)