Field

Description

 

 

Logical Block Address

The 6-byte READ, SEEK, and WRITE Command Descriptor Blocks contain a 21-bit Logical

 

Block Address. The 10-, 12-, and 16- Command Descriptor Blocks contain a 32-bit Logical

 

Block Address.

 

Commands that require additional parameter data specify the length of the Logical Block Ad-

 

dress that is needed. See the specific command descriptions for more detailed information.

 

Relative Addressing indicates a technique used to determine the next Logical Block Address to

 

be operated on. The drive does not support Relative Addressing, it defaults to a value of 0,

 

which specifies that the Logical Block Address specifies the first logical block of a range of logical

 

blocks to be operated on by the command.

 

 

Transfer Length

The transfer length field normally specifies the number of sectors to be transferred between the

 

initiator and the drive. For several commands, the transfer length indicates the number of bytes

 

(not sectors) to be sent. For these commands, this field may be identified by a different name

 

Commands that use one byte for the transfer length value allow up to 256 sectors of data to be

 

transferred by one command. A transfer length value of 0 indicates that 256 sectors are to be

 

sent. Transfer length values of 1 through 255 indicate the number of sectors to be transferred.

 

Commands that use multiple bytes for the transfer length value function differently. A transfer

 

length value of 0 indicates that no data transfer is to occur. Transfer length values of 1 or greater

 

indicate the number of sectors to be transferred.

 

 

Parameter List Length

The Parameter List Length is used to specify the number of bytes sent during the data-out buffer

 

transfer. This field is typically used for parameters that are sent to a drive (for example, mode,

 

diagnostic, and log parameters). A parameter list length of 0 indicates that no data is to be trans-

 

ferred.

 

 

Allocation Length

The Allocation Length field specifies the maximum number of bytes that the initiator has allo-

 

cated for returned data. The Allocation Length is used to limit the amount of data returned to

 

the initiator.

 

An Allocation Length of 0 indicates that no data is to be transferred from the drive to the initi-

 

ator. The drive terminates the data-in buffer transfer when the specified number of bytes have

 

been transferred to the initiator or when all available data has been transferred, whichever is less.

 

 

Control Field

The Control Field is the last byte of every Command Descriptor Block; its format is shown in

 

Figure 5-2 and described in Table 5-3.

 

 

Table 5-4Typical (6-Byte) Command Descriptor Block - Data Format

Bit

7

6

5

4

3

2

1

0

Byte

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5

Vendor Specific

 

Reserved

 

NACA

Flag

Link

 

 

 

 

 

 

 

 

 

Maxtor Atlas 10K V

5-5