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)
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INT 15H 84H: Joystick Support