Fujitsu MAX3073NC - SCSI,80, Dell DC961,Hard Drive Scsi Logical Interface Specifications page 22

Command Processing
(4) 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 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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