Read (6)
The READ(6) SCSI command uses a six byte command descriptor block and was first defined in the Shugart Associates System Interface specification from 1979. READ(6) is limited to a 21-bit logical block address or LBA, thus setting a 1 GiB limit, and an 8-bit transfer length. The CDB structure is:
| bit→ ↓byte |
7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|
| 0 | Operation code = 08h | |||||||
| 1 | LUN | LBA | ||||||
| 2 | LBA | |||||||
| 3 | LBA | |||||||
| 4 | Transfer length | |||||||
| 5 | Control | |||||||
When READ(6) is used on a stream device (e.g., a tape drive) LBA is zero and the transfer length is equal to the number of bytes to be transferred. Otherwise, LBA is the desired zero-based logical block address and transfer length is expressed in logical blocks—a transfer length of zero will be interpreted as 256 logical blocks. READ(6) has been declared obsolete by the T10 standardization committee but is still supported in many devices for backward compatibility reasons.
Read more about this topic: SCSI Read Commands
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—James Thurber (1894–1961)