BL drive number (0=default, 1=A, 2=B, etc.)
CX buffer size (maximum number of bytes to receive)
DS:DX Address of buffer to receive data
Number of bytes actually received (if no error)
Info: Some block device drivers support this method of providing
device-specific information to programs; for instance, it is used
to tell DoubleSpace to flush its cache. The layout and contents
of the returned 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)