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5-4 Maxtor Atlas 10K V
5.2 Command Descriptor Block
An initiator communicates with the drive by sending a 6-, 10-, or 12-byte Command
Descriptor Block (CDB) that contains the parameters for the specific command. The
SCSI command's operation code is always the first byte in the CDB and a control field
is the last byte. For some commands, the CDB is accompanied by a list of parameters
sent during the data-out buffer transfer. Table 5-2 shows the format of a typical six-
byte CDB.
Table 5-2 Typical (6-Byte) Command Descriptor Block-Data Format
Note: Reserved fields in CDBs and Parameters have a value of 0; Re-
served fields in states and other parameters sent to an initiator are
set to 0.
Table 5-3 contains a description of the CDB fields.
Table 5-4 describes the data format for a typical CDB.
Table 5-3 Command Descriptor Block-Field Descriptions
Bit
Byte
7654321 0
0 Operation Code
1 Reserved
(MSB)
Logical Block Address
(LBA)
2 - 3
Logical Block Address
(LBA)
4 Transfer Length, Parameter List Length, or Allocation Length
5 Control
Field Description
Operation Code The first byte of a SCSI CDB contains an operation code. The operation code of the CDB has
a Group Code field (bits 7-5) and a Command Code field (bits 4-0). The 3-bit Group Code field
provides for eight groups of command codes. The 5-bit Command Code field provides for 32
command codes in each group. A total, therefore, of 256 possible operation codes exist. Oper-
ation codes are defined in the SCSI command standards. The group code for CDBs specified
therein correspond to the length of the command descriptor as below:
Group Code
Meaning
0 6-byte commands
1 10-byte commands
2 10-byte commands
3Reserved
4 16-byte commands
5 12-byte commands
6 Vendor specific
7 Vendor specific
The operation code specifies the command being requested. The list of supported SCSI com-
mands and their operation codes are contained in Table 5-1.