Version analysis of DKS2 - Elite on the 6502

This code appears in the following versions (click to see it in the source code):

Cassette, Disc (flight), Disc (docked), 6502 Second Processor and Electron

Code variations between these versions are shown below.

       Name: DKS2
       Type: Subroutine
   Category: Keyboard
    Summary: Read the joystick position

Code variation 1 of 3A variation in the comments only

This variation is blank in the Cassette, Disc (flight), Disc (docked) and 6502 Second Processor versions.

Electron
 This routine is never called in the Electron version, as the Electron doesn't
 have ADC channels as standard and doesn't support joysticks (though a lot of
 the joystick code from the other versions is still present, it just isn't
 called).

 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

Code variation 2 of 3A variation in the comments only

This variation is blank in the 6502 Second Processor version.

Cassette, Flight, Docked, Electron
 Other entry points:

   DKS2-1               Contains an RTS


.DKS2

Code variation 3 of 3Related to Elite's use of the Tube

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Docked, Electron
6502SP
 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)
 LDA KTRAN+7,X          \ Fetch either the joystick X value or joystick Y value
                        \ from the key logger buffer, depending on the value of
                        \ X (i.e. fetch either KTRAN+8 or KTRAN+0)

 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