Information Technology Reference
In-Depth Information
7.7.4 Transfer length
The transfer length field specifies the amount of data to be transferred (normally the number
of blocks). For several commands the transfer length indicates the requested number of bytes
to be sent as defined in the command description. A command that uses 1 byte for the trans-
fer length will thus allow up to 256 blocks of data for one command (a value of 0 identifies a
transfer bock of 256 blocks).
7.7.5 Parameter list length
The parameter list length specifies the number of bytes to be sent during the data-out phase.
It is typically used in command descriptor blocks for parameters that are sent to a target
(such as, mode parameters, diagnostic parameters, log parameters, and so on).
7.7.6 Allocation length
The allocation length field specifies the maximum number of bytes that an initiator has allo-
cated for returned data. The target terminates the data in phase when allocation length bytes
have been transferred or when all available data have been transferred to the initiator, which-
ever is less. The allocation length is used to limit the maximum amount of data (for example,
sense data, mode data, log data, diagnostic data, and so on) returned to an initiator.
7.7.7 Control field
The control field is the last byte of every command descriptor block. Its format is shown in
Figure 7.3. The flag bit specifies which message the target returns to the initiator if the link
bit is a 1 and the command completes without error. If the link bit is 0 then the flag bit
should be a 0, else the target returns check condition status.
b
7
b
6
b
5
b
4
b
3
b
2
b
1
b
0
Flag
Link
Vendor-
specific
Reserved
Figure 7.3
Control field
7.7.8 Command code
Commands for all device types are (bold type identifies the mandatory commands and the
operation code is given in brackets):
•
Change definition (40h). This command modifies the operating definition of the selected
logical unit or target with respect to commands from the selecting initiator or for all ini-
tiators.
•
Compare (39h). This command allows for a compare operation of data on one logical unit
with another or the same logical unit in a manner similar to the copy command.
•
Copy (18h). This command allows the copying of data from one logical unit to another or
the same logical unit. The logical unit that receives and performs the copy command is
the copy manager. It is responsible for copying data from the source device to the desti-
nation device.
•
Copy and compare (3Ah). This command performs the same function as the COPY
