Assembly Language Help (alang.hlp) (Table of Contents; Topic list)
Int 15h Function 84h
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     An error condition is returned if DX does not contain a valid
     subfunction number.
 
     If no game adapter is installed, AL is returned as 00h for
     Subfunction 00h (all switches open); AX, BX, CX, and DX are
     returned containing 00h for Subfunction 01h.
 
     Using a 250K Ohm joystick, the potentiometer values usually lie
     within the range 0-416 (0000-01A0h).
                                    -♦-