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SCSI Description
Maxtor Atlas 10K III 5-3
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.
Typical (6-Byte) Command Descriptor Block-Data Format
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.
Command Descriptor Block-Field Descriptions
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
5Control
Operation Code The first byte of a SCSI CDB contains an operation code. The oper-
ation code of the CDB has a
field (bits 7-5) and a
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. Operation codes are defined
in the SCSI command standards. The group code for CDBs specified
therein correspond to the length of the command descriptor as be-
low:
Group Code
Meaning
0 6-byte commands
1 10-byte commands
2 10-byte commands
3 Reserved
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 commands and their operation codes are con-
tained in Table 5-1.