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Command Processing
106 C141-C013
Group 3 ("011"): Reserved Operation Code (Shown in Section (6))
Group 4 ("100"): 16-byte CDB
Group 5 ("101"): 12-byte CDB (Shown in Table 2.3)
Group 6 ("110"): Vendor unique CDB (shown in Section 2.8.3)
Group 7 ("111"): Vendor unique CDB (shown in Section 2.8.3)
b. Command code
The command code specifies the type of command in each group.
(2) Logical block address
This field shows the top logical data block address of the data block group on the
disk medium that is to be processed by the command. In a Group 0 CDB, 21-bit
block addressing can be used, and in a Group 1, Group 2 or Group 5 CDB, 32-bit
block addressing can be used. Standards for logical data block addressing in the
HDD are described in Section 2.9.
(3) Transfer data length
This field specifies the length of data to be transferred between the INIT and the
TARG by execution of the command, either as the number of logical data blocks
or the number of bytes. In subsequent descriptions, the former is called the
"Number of Transfer Blocks" and the latter is called the "Transfer Byte Length"
or the "Parameter List Length".
Furthermore, depending on the command, use of this field can have other
meanings, or no meaning at all. There are also some commands which allocate 3
or more bytes as the transfer data length field. Detailed standards concerning
these commands are described in the specifications for individual commands in
Chapter 4.
a. Transfer block count
When the "Transfer Data Length" field is specified as the "Number of
Transfer Blocks", this field specifies the number of logical data blocks
transferred between the INIT and HDD.
In a command with a 1-byte length, if the value specified in this field is zero,
it is regarded that 256 blocks have been specified, and specification in a range
of from 1 to 256 blocks is possible. On the other hand, in a command with a
2-byte length, when the value specified in this field is zero, data transfer is
not executed. Specification in a range of from 0 to 65,535 blocks is possible.