Logical Unit Number (Lun); Logical Block Address; Relative Address Bit - Seagate SCSI Interface Product Manual

SCSI Interface Product Manual, Rev. J
Table 15. Typical Command Descriptor Block for ten byte commands
Bit 7
Byte
0
1 Logical Unit No.
0
2
3
4
5
6
7
8
9
[1] For twelve byte commands, there are two more bytes of transfer length, bytes 9 and 10, and the control
byte is byte 11.
4.2.2

Logical Unit Number (LUN)

The Logical Unit Number (LUN) addresses one of up to eight physical devices or virtual devices attached to a
target. The only valid LUN number for the drives supported by this manual (pre-J revision) has been zero. How-
ever, future drives may support a non-zero LUN value.
The LUN in the CDB is provided for systems that do not implement the Identify Message. If an Identify mes-
sage is sent to the drive, the drive will use the LUN number specified in this message. In this case, the drive
shall ignore the LUN specified within the command descriptor block.
The drive will reject commands that select an invalid LUN (except Request Sense and Inquiry) by requesting
and accepting the command bytes, then going to Status phase and sending Check Condition status. Note that
the LUN is sent in the LUN field of a CDB (if no Identify message has been received for this selection) or by the
LUN field of an Identify message.
Request Sense commands selecting an invalid LUN will receive a Sense Data block with the Illegal Request
Sense Key and an Invalid LUN Error Code. Inquiry commands will return Inquiry Data with the Peripheral
Device Type field set to Logical Unit Not Present (7Fh). Request Sense and Inquiry commands will not send
Check Condition status in response to an invalid LUN selection.
4.2.3

Logical block address

The logical block address in the command descriptor block shall begin with block zero and be continuous up to
the last logical block on drive.
Group 0 command descriptor block contains 21 bit logical block addresses. Groups 1 and 2 command descrip-
tor blocks contain 32 bit logical block addresses.
The logical block concept implies that the initiator and target shall have previously established the number of
data bytes per logical block. This may be established through the use of the Read Capacity command or the
Mode Sense command or by prior arrangement.
The maximum logical block address for the drive which is accessible by the Initiator is defined in Read Capacity
Command data in Section 5.21.
4.2.4

Relative address bit

Relative addressing is a technique useful in accessing structured data in a uniform manner. Relative address-
ing is only allowed when commands are linked. Details are given with those commands that use this feature.
6 5
0 0
Logical Block Address (if required) (MSB)
Logical Block Address (if required)
Logical Block Address (if required)
Logical Block Address (if required) (LSB)
Transfer Length (if required) (MSB) [1]
Transfer Length (if required) (LSB) [1]
4 3
Operation Code
Reserved
Reserved
Control Byte [1]
2 1
45
0
Rel
Adr