5.2Command 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 5-2shows the format of a typical six- byte CDB.

Table 5-2Typical (6-Byte) Command Descriptor Block-Data Format

Bit

7

6

5

4

 

3

2

 

1

0

Byte

 

 

 

 

 

 

 

 

 

 

 

 

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

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Note: Reserved fields in CDBs and Parameters have a value of 0; Reserved fields in states and other parameters sent to an initiator are set to 0.

Table 5-3contains a description of the CDB fields.

Table 5-4describes the data format for a typical CDB.

 

Table 5-3Command Descriptor Block-Field Descriptions

 

 

 

 

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

 

 

 

 

 

 

 

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 com-

 

 

mands and their operation codes are contained in Table 5-1.

 

 

 

 

 

5-4 Maxtor Atlas 10K V