INT 15 - BIOS - JOYSTICK SUPPORT (XT after 11/8/82,AT,XT286,PS)
        AH = 84h
        DX = subfunction
            0000h read joystick switches
                Return: AL bits 7-4 = switch settings
            0001h read positions of joysticks
                Return: AX = X position of joystick A
                        BX = Y position of joystick A
                        CX = X position of joystick B
                        DX = Y position of joystick B
Return: CF set on error
            AH = status
                80h invalid command (PC,PCjr)
                86h function not supported (other)
        CF clear if successful
Notes:  if no game port is installed, subfunction 0000h returns AL=00h (all
          switches open) and subfunction 0001h returns AX=BX=CX=DX=0000h
        a 250kOhm joystick typically returns 0000h-01A0h