CX byte count (amount of data to send)
DS:DX Address of buffer containing data to send
Number of bytes actually sent (if no error)
Info: Some character device drivers support this method of
communicating with a device driver to select special features
(such as setting the baud rate of a COM port). The layout and
contents of the control data varies from device to device.
(the rDevAttr field of the driver's DevHdrRec structure) is
set. Use Fn 4400H to check this bit.
Fn 4402H (Receive Control Data from Character Device)
Fn 4405H (Send Control Data to Block Device)