About the SCSI CDB (Command Descriptor Block) allocation block

The "Allocation length" is the byte 4 in the typical CDB structure.  But "Allocation length" is the byte 3 and byte 4 in the Inquiry command structure.  And the byte 4 is the "LoEj" and "Start" in the Start/Stop Unit command.


It's poor that the allocation length seems not well definite in the CDB spec.


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From http://en.wikipedia.org/wiki/SCSI_CDB

Each CDB can be a total of 6, 10, 12, or 16 bytes, but later versions of the SCSI standard also allow for variable-length CDBs. The CDB consists of a one byte operation code followed by some command-specific parameters.

A typical CDB structure is:

bit→ ↓byte	7	6	5	4	3	2	1	0
0	Operation code = 03h
1	LUN			Reserved
2	Reserved
3	Reserved
4	Allocation length
5	Control

From http://en.wikipedia.org/wiki/SCSI_Inquiry_Command

The SCSI Inquiry command is used to obtain basic information from a target device.^[1] The CDB structure is:

bit→ ↓byte	7	6	5	4	3	2	1	0
0	Operation code = 12h
1	LUN			Reserved				EVPD
2	Page code
3	Allocation length (MSB)
4	Allocation length (LSB)
5	Control

From http://en.wikipedia.org/wiki/SCSI_Start_Stop_Unit_Command

The SCSI Start/Stop Unit command is used to control the motor in a rotary device such as a SCSI disk-drive. It is also used to load or eject removable media, such as a tape or compact disc. It is a frequently-used command and is often the first one issued after initial communication has been established between the initiator and the target.

The CDB structure is:

bit→ ↓byte	7	6	5	4	3	2	1	0
0	Operation code = 1Bh
1	LUN			Reserved				Immed
2	Reserved
3	Reserved
4	Reserved						LoEj	Start
5	Control