Unit attention check, Command descriptor block, Unit attention check 38 – HP C737990900 User Manual
Page 38: Command descriptor block 38
38 Commands—Introduction
Unit Attention Check
This checks if a
UNIT ATTENTION
condition exists for the host which sent the
command. If it does, the drive reports
CHECK CONDITION
status with a sense
key of
UNIT ATTENTION
. The remaining sense data will be set according to the
unit attention condition which exists. See Unit Attention Sense in the
description of the
REQUEST SENSE
.
Command Descriptor Block
A SCSI command descriptor block (CDB) is a sequence of 6, 10, 12 or 16
bytes sent by a host to a SCSI target with the bus in command phase. The CDB
tells the drive what action should be performed. The final byte is known as the
Control byte.
There are a number of fields in a CDB which are common to all commands.
These are shown in the following table.
7
6
5
4
3
2
1
0
0
Group Code
Operation Code
1
Reserved (0)
2
(MSB)
Multi-Byte Parameter
n
-1
(LSB)
n
Vendor Unique (0)
Reserved (0)
NACA(0) Flag (0) Link (0)
Group Code
and
Operation Code
The operation code uniquely identifies the command. The top three bits of the
operation code are known as the group code and these define the length of the
command descriptor block:
Group 0
Group 1
Group 2
Group 3
Group 4
Group 5
Group 6
Group 7
Six-byte commands
Ten-byte commands
Ten-byte commands
Six-byte commands
Sixteen-byte commands
Twelve-byte commands
not supported
not supported