Read (10)
The Read(10) (also called 'read extended' in 1987) command was introduced to cope with 32-bit LBAs (2048 GiB) and uses a 16-bit Transfer Length field. It also included three performance-related enhancements. Disable Page Out (DPO) allows the initiator to warn the target that the data being read is unlikely to be requested again soon and so is not worth keeping in the target's data cache. Force Unit Access (FUA) tells the target to fetch the data from the media surface and to not use a cached copy. The RelAdr bit is used to indicate that the LBA value is relative (only used with linked commands). The '2' in the operation code indicates a "class 2" command, and the '8' is the real 'read' opcode. This command was present in a 1982 working document and as such it is likely to have been present from the first SCSI command definitions, showing remarkable foresight. The CDB structure is:
| bit→ ↓byte |
7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|
| 0 | Operation code = 28h | |||||||
| 1 | LUN | DPO | FUA | Reserved | RelAdr | |||
| 2–5 | LBA | |||||||
| 6 | Reserved | |||||||
| 7–8 | Transfer length | |||||||
| 9 | Control | |||||||
Read more about this topic: SCSI Read Commands
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