BL drive number (0=default, 1=A, 2=B, etc.)
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 block device drivers support this method of communicating
with a device driver to select special features. 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 4403H (Send Control Data to Character Device)
Fn 4404H (Receive Control Data from Block Device)