INT 15H 84H: Joystick Support

 Expects: AH    84H
          DX    subfn code: 0 = read buttons (switches) into AL
                            1 = read joystick positions into AX,BX,CX,DX
 Returns: AH    84H
          AL    (DX=0) bits 4-7 indicate state of joystick buttons
          AX    (DX=1) joystick A horizontal coordinate
          BX           joystick A vertical coordinate
          CX           joystick B horizontal coordinate
          DX           joystick B vertical coordinate
          CF    CY (1) if DX was invalid
                NC (0) no error
    Info: This reads the current buttons or resistive inputs of the
          joystick (or other general analog-to-digital device).

          See Game I/O Adapter Port (201H) for layout of the bits for the
          switches (buttons) and related info.

See Also: INT 15H (extended AT services)
          BIOS Data Area
          ROM-BIOS Functions