SCS I Description
M axtor Atlas 10K III 5-3
An initiator communicates with the dr ive by sending a 6-, 10-, or 12-byte Comman d
Descriptor Bl oc k (CDB) t ha t cont a ins the para meter s f or the specific command. The
SCSI command's operation code is always the first byte in the CDB and a control field
is the last by te. For some commands, the CDB is accompanied by a list of para meters
sent during the data -out buffer transfer. Table 5-2 shows the format of a typical six-
byte CDB.
Typical (6-Byte) C omm and Descr iptor Block-D a ta Forma t
Reserved fields in CDBs and Parameters have a value of 0; Re-
served fields in states and other parameters sent t o an initiator are
se t to 0.
Table 5-3 conta ins 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) Logi cal Block Address (LBA)
2 - 3 Logical Block Addr ess
(LBA)
4 Transfer Length, Param eter List Length, or Alloc ation Length
5Control
Operati on Code The fi rst byte of a SCSI CDB contains an oper ation code . The oper-
ation code of t he CDB has a fiel d (bi ts 7-5) and a
field ( bits 4-0) . The 3-bit Gro up Code field provides for
eight groups of command codes. The 5-bit Command Code field
provides for 32 command codes in each group. A tot al, there fore,
of 25 6 possible ope ration codes exist. Oper ati on codes are de fined
in the SCSI command standards. The group code for CDBs specified
ther ein cor res pond to t he lengt h of the comm and descr iptor as be -
low:
Group Code Meaning
0 6-by te 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.