User Manual

SCSI Commands Reference Manual, Rev. J 42
www.seagate.com General Concepts
2.2 Common CDB fields
2.2.1 OPERATION CODE
The first byte of a SCSI CDB shall contain an operation code identifying the operation being requested by the CDB. Some
operation codes provide for modification of their operation based on a service action (see 2.2.2). In such cases, the operation
code and service action code combine to identify the operation being requested. The location of the SERVICE ACTION field in the
CDB varies depending on the operation code value.
The OPERATION CODE (see table 9) of the CDB has a GROUP CODE field and a COMMAND CODE field. The three-bit GROUP CODE
field provides for eight groups of command codes. The five-bit COMMAND CODE field provides for thirty-two command codes in
each group. A total of 256 possible operation codes exist. Operation codes are defined in this manual and other command
standards. The group code value shall determine the length of the CDB (see table 10).
The value in the GROUP CODE field specifies one of the groups shown in Table 9.
Table 9 Group Code values
2.2.2 SERVICE ACTION
All CDB formats except the 6-byte format provide for a SERVICE ACTION field containing a coded value identifying a function to
be performed under the more general command function specified in the OPERATION CODE field. While the SERVICE ACTION
field is defined for CDB formats, it is used as described in this subclause only in those CDB formats that contain a SERVICE
ACTION field. When the specific field SERVICE ACTION is not defined in a CDB format, the bits identified as the SERVICE ACTION
field in a CDB shall be used or reserved as specified by the particular CDB format.
Table 8 OPERATION CODE byte
Bit 7 6 5 4 3 2 1 0
GROUP CODE COMMAND CODE
Group Code Meaning Typical CDB Format
000b 6 byte commands see Table 2
001b 10 byte commands see Table 3
010b 10 byte commands see Table 3
011b Reserved [a]
100b 16 byte commands see Table 5 and Table 6
101b 12 byte commands see Table 4
110b Vendor Specific
111b Vendor Specific
[a] The format of the commands using operation code 7Fh is described in 2.1.3. With
the exception of operation code 7Fh, all group code 011b operation codes are
reserved.