Keyboard: ScanKeyboard

       Name: ScanKeyboard                                            [Show more]
       Type: Subroutine
   Category: Keyboard
    Summary: Scan the keyboard for a specific key
  Deep dive: The key logger
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DrawGunSights calls ScanKeyboard
             * MainLoop (Part 7 of 15) calls ScanKeyboard
             * MainLoop (Part 11 of 15) calls ScanKeyboard
             * MainLoop (Part 14 of 15) calls ScanKeyboard
             * ProcessVolumeKeys calls ScanKeyboard
             * TerminateGame calls ScanKeyboard
             * ToggleJoystick calls ScanKeyboard
             * UpdateKeyLogger calls ScanKeyboard


 Arguments:

   X                    The internal key number of the key to scan for


 Returns:

   Z flag               If set (BEQ) then the key is being pressed, if clear
                        (BNE) then it is not being pressed


.ScanKeyboard

 LDA #129               \ Call OSBYTE with A = 129, X = key number and Y = &FF
 LDY #&FF               \ to scan the keyboard for the key in X, returning the
 JSR OSBYTE             \ following in both X and Y:
                        \
                        \   * 0 = the key is not being pressed
                        \
                        \   * &FF = the key is being pressed

 CPX #&FF               \ Set the Z flag depending on whether the key is being
                        \ pressed

 RTS                    \ Return from the subroutine