Keyboard: ReadADCChannel

       Name: ReadADCChannel                                          [Show more]
       Type: Subroutine
   Category: Keyboard
    Summary: Read the joystick position from one of the ADC channels
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ReadJoystick calls ReadADCChannel


 Arguments:

   X                    The ADC channel to read:

                          * 1 = joystick X

                          * 2 = joystick Y


 Returns:

   A                    The joystick position, inverted and clipped to the range
                        -118 to +116


.ReadADCChannel

 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)

 CMP #247               \ If A < 247, jump to radc1 to skip the next instruction
 BCC radc1

 LDA #246               \ Set A = 246, so A now has a maximum value of 246

.radc1

 CMP #12                \ If A >= 12, jump to radc2 to skip the next instruction
 BCS radc2

 LDA #12                \ Set A = 12, so A now has a minimum value of 12

.radc2

                        \ By the time we get here, A is in the range 12 to 246

 SEC                    \ Set A = A - 128, so A is now in the range -116 to +118
 SBC #128

 EOR #&FF               \ Negate A using two's complement, so A is now in the
 CLC                    \ range -118 to +116
 ADC #1

 RTS                    \ Return from the subroutine