Keyboard: DKS2

       Name: DKS2                                                    [Show more]
       Type: Subroutine
   Category: Keyboard
    Summary: Read the joystick position
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DOKEY calls DKS2


 Return the value of ADC channel in X (used to read the joystick). The value
 will be inverted if the game has been configured to reverse both joystick
 channels (which can be done by pausing the game and pressing J).


 Arguments:

   X                    The ADC channel to read:

                          * 1 = joystick X

                          * 2 = joystick Y


 Returns:

   (A X)                The 16-bit value read from channel X, with the value
                        inverted if the game has been configured to reverse the
                        joystick


 Other entry points:

   DKS2-1               Contains an RTS


.DKS2

 LDA #128               \ Call OSBYTE with A = 128 to fetch the 16-bit value
 JSR OSBYTE             \ from ADC channel X, returning (Y X), i.e. the high
                        \ byte in Y and the low byte in X
                        \
                        \   * Channel 1 is the x-axis: 0 = right, 65520 = left
                        \
                        \   * Channel 2 is the y-axis: 0 = down,  65520 = up

 TYA                    \ Copy Y to A, so the result is now in (A X)

 EOR JSTE               \ The high byte A is now EOR'd with the value in
                        \ location JSTE, which contains &FF if both joystick
                        \ channels are reversed and 0 otherwise (so A now
                        \ contains the high byte but inverted, if that's what
                        \ the current settings say)

 RTS                    \ Return from the subroutine