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Transfer Data Length
In this field, the length of data to be transferred between INIT and TARG when the command
is executed is specified by the number of logical data blocks or the number of bytes. In
subsequent descriptions, the former is called the "transfer block count" and the latter is called
the "transfer byte length" or "parameter list length."
Furthermore, this field may be used with a different meaning, or it may not have any meaning
at all, depending on the type of command. There are also some commands which allocate 3 or
more bytes as the transfer data length field. Detailed specifications of these commands are
described in the individual command specifications in Chapter 3.
a. Transfer Block Count
When the "Transfer Data Length" is specified as the "Transfer Block Count," this field
specifies the number of logical data blocks to be transferred between INIT and the IDD.
In commands where this field is 1 byte in length, if the field's specified value is 0, it is
regarded as specifying 256 blocks, and it is possible to specify a block count ranging from
1 to 256 blocks. On the other hand, in commands where this field is 2 bytes in length, if
the field's specified value is 0, no data transfer is executed. It is possible to specify a
block count ranging from 0 to 65,535 blocks.
b. Transfer Byte Length or Parameter List Length
When this field is specified as the "Transfer Byte Length" or "Parameter List Length," that
command specifies data length to be transferred between the INIT and the IDD, expressed
as the number of bytes. When 0 is specified in this field, data transfer is not executed,
except in cases where it is expressly stated in the individual command specifications in
Chapter 3.
In commands which send parameters necessary for executing a command from the INIT to
the IDD, this field is called the "Parameter List Length," and it specifies the total number
of types in the parameter list which the INIT is sending.
On the other hand, in commands for receiving information from the IDD (REQUEST
SENSE, INQUIRY, etc.), this field is called the "Transfer Byte Length," and specifies the
maximum number of bytes (the which the INIT can receive (the number of bytes of area
secured within the INIT for receiving information). The IDD transfers either the number
of effective bytes of the type of information specified in the command, or the value
specified in the "Transfer Byte Length" field, whichever is the smallest number of bytes,
and only that number, to the INIT.
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