Keyboard: ProcessOtherKeys

       Name: ProcessOtherKeys                                        [Show more]
       Type: Subroutine
   Category: Keyboard
    Summary: Apply the undercarriage, brakes, flaps and fire keys
  Deep dive: The key logger
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * UpdateFlightModel (Part 3 of 4) calls ProcessOtherKeys


 Arguments:

   X                    The offset within the key logger for the keys to check:

                          * 4 = "U" or "B" (undercarriage, brakes)

                          * 5 = "F" or SHIFT (flaps, fire)


 Returns:

   A                    Returns the status of the relevant key presses:

                          * 0 = neither key in the pair is being pressed, or a
                                key is still being held down from a previous
                                call to the routine, so it has already been
                                processed

                          * 1 = One of "U" and "F" (undercarriage, flaps) is
                                being pressed

                          * 128 = One of "B" and SHIFT (brakes, fire) is being
                                pressed


.ProcessOtherKeys

 LDA keyLoggerLo,X      \ Fetch the low byte for this key pair

 BNE poth2              \ If A is non-zero then a key from this key pair is
                        \ being pressed, so jump down to poth2 to process the
                        \ key press

 STA pressingUFBS-4,X   \ Zero the relevant value in pressingUFBS (for U) or
                        \ pressingUFBS+1 (for F) to indicate that the first key
                        \ from this pair is not being held down

 STA pressingUFBS-4+3,X \ Zero the relevant value in pressingUFBS+3 (for B) or
                        \ pressingUFBS+4 (for SHIFT) to indicate that the second
                        \ key from this pair is not being held down

.poth1

 LDA #0                 \ Set A = 0 as the return value

 RTS                    \ Return from the subroutine

.poth2

 TAY                    \ Copy the low value into Y, so we have:
                        \
                        \   * Y = 4 if "U" is being pressed (undercarriage)
                        \   * Y = 5 if "F" is being pressed (flaps)
                        \   * Y = 7 if "B" is being pressed (brakes)
                        \   * Y = 8 if SHIFT is being pressed (fire)

 LDA pressingUFBS-4,Y   \ Fetch the relevant value from pressingUFBS to see if
 BNE poth1              \ the key press has already been processed, and if it is
                        \ non-zero, this indicates that it is still being held
                        \ down from a previous visit to this routine, so jump to
                        \ poth1 to return a value of 0, so we can ignore the key
                        \ press

 LDA ucStatus-4,Y       \ Flip the value of the relevant status byte, as
 EOR #1                 \ follows:
 STA ucStatus-4,Y       \
                        \   * Flip ucStatus if "U" is being pressed
                        \   * Flip flapsStatus if "F" is being pressed
                        \   * Flip brakesStatus if "B" is being pressed
                        \   * Flip brakesStatus+1 if SHIFT is being pressed
                        \      (which has no effect)

 LDA #1                 \ Set the relevant value in pressingUFBS to denote that
 STA pressingUFBS-4,Y   \ this key is being pressed, so if we revisit this
                        \ routine before the key is released, we don't keep on
                        \ flipping the status byte

 CPY #7                 \ If Y < 7, i.e. U or F are being pressed, jump to poth3
 BCC poth3              \ to return a result of 1

 LDA #128               \ Y >= 7, i.e. B or SHIFT are being pressed, so set
                        \ A = 128 as the return value

 RTS                    \ Return from the subroutine

.poth3

 LDA #1                 \ Set A = 1 as the return value

 RTS                    \ Return from the subroutine