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Unit Attention Check
Command Descriptor Block
0
1
2
n -1
n
Group Code and
The operation code uniquely identifies the command. The top three bits of the
Operation Code
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
38
Commands—Introduction
This checks if a
UNIT ATTENTION
command. If it does, the drive reports
key of
UNIT ATTENTION
unit attention condition which exists. See Unit Attention Sense in the
description of the
REQUEST SENSE
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.
7
6
Group Code
(MSB)
Vendor Unique (0)
There are a number of fields in a CDB which are common to all commands.
These are shown in the following table.
Six-byte commands
Ten-byte commands
Ten-byte commands
Six-byte commands
Sixteen-byte commands
Twelve-byte commands
not supported
not supported
condition exists for the host which sent the
CHECK CONDITION
. The remaining sense data will be set according to the
command on
5
4
3
Reserved (0)
Multi-Byte Parameter
Reserved (0)
status with a sense
page
132.
2
1
Operation Code
NACA(0) Flag (0) Link (0)
0
(LSB)
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