Keyboard: DK4

       Name: DK4                                                     [Show more]
       Type: Subroutine
   Category: Keyboard
    Summary: Scan for pause, configuration and secondary flight keys
  Deep dive: The key logger
    Context: See this subroutine in context in the source code
 Variations: See code variations for this subroutine in the different versions
 References: This subroutine is called as follows:
             * DOKEY calls DK4


 Scan for pause and configuration keys, and if this is a space view, also scan
 for secondary flight controls.

 Specifically:

   * Scan for the pause button (COPY) and if it's pressed, pause the game and
     process any configuration key presses until the game is unpaused (DELETE)

   * If this is a space view, scan for secondary flight keys and update the
     relevant bytes in the key logger


.DK4

 LDX KL                 \ Fetch the key pressed from byte #0 of the key logger

 CPX #&8B               \ If COPY is not being pressed, jump to DK2 below,
 BNE DK2                \ otherwise let's process the configuration keys

.FREEZE

                        \ COPY is being pressed, so we enter a loop that
                        \ listens for configuration keys, and we keep looping
                        \ until we detect a DELETE key press. This effectively
                        \ pauses the game when COPY is pressed, and unpauses
                        \ it when DELETE is pressed

 JSR WSCAN              \ Call WSCAN to wait for the vertical sync, so the whole
                        \ screen gets drawn

 JSR RDKEY              \ Scan the keyboard for a key press and return the
                        \ ASCII code of the key pressed in X (or 0 for no key
                        \ press)

 CPX #'Q'               \ If "Q" is not being pressed, skip to DK6
 BNE DK6

 LDX #&FF               \ "Q" is being pressed, so set DNOIZ to &FF to turn the
 STX DNOIZ              \ sound off

 LDX #'Q'               \ Set X to the ASCII for "Q" once again, so it doesn't
                        \ get changed by the above

.DK6

 LDY #0                 \ We now want to loop through the keys that toggle
                        \ various settings, so set a counter in Y to work our
                        \ way through them

.DKL4

 JSR DKS3               \ Call DKS3 to scan for the key given in Y, and toggle
                        \ the relevant setting if it is pressed

 INY                    \ Increment Y to point to the next toggle key

 CPY #9                 \ Check to see whether we have reached the last toggle
                        \ key

 BNE DKL4               \ If not, loop back to check for the next toggle key

 LDA VOL                \ Fetch the current volume setting into A

 CPX #'.'               \ If "." is being pressed (i.e. the ">" key) then jump
 BEQ DOVOL1             \ to DOVOL1 to increase the volume

 CPX #','               \ If "," is not being pressed (i.e. the "<" key) then
 BNE DOVOL4             \ jump to DOVOL4 to skip the following

 DEC A                  \ The volume down key is being pressed, so decrement the
                        \ volume level in A

 EQUB &24               \ Skip the next instruction by turning it into &24 &1A,
                        \ or BIT &001A, which does nothing apart from affect the
                        \ flags

.DOVOL1

 INC A                  \ The volume up key is being pressed, so increment the
                        \ volume level in A

 TAY                    \ Copy the new volume level to Y

 AND #%11111000         \ If any of bits 3-7 are set, skip to DOVOL3 as we have
 BNE DOVOL3             \ either increased the volume past the maximum volume of
                        \ 7, or we have decreased it below 0 to -1, and in
                        \ neither case do we want to change the volume as we are
                        \ already at the maximum or minimum level

 STY VOL                \ Store the new volume level in VOL

.DOVOL3

 PHX                    \ Store X on the stack so we can retrieve it below after
                        \ making a beep

 JSR BEEP               \ Call the BEEP subroutine to make a short, high beep at
                        \ the new volume level

 LDY #10                \ Wait for 10/50 of a second (0.2 seconds)
 JSR DELAY

 PLX                    \ Restore the value of X we stored above

.DOVOL4

 CPX #'B'               \ If "B" is not being pressed, skip to DOVOL2
 BNE DOVOL2

 LDA BSTK               \ Toggle the value of BSTK between 0 and &FF
 EOR #&FF
 STA BSTK

 STA JSTK               \ Configure JSTK to the same value, so when the Bitstik
                        \ is enabled, so is the joystick

 STA JSTE               \ Configure JSTE to the same value, so when the Bitstik
                        \ is enabled, the joystick is configured with reversed
                        \ channels

 BPL P%+5               \ If we just toggled the Bitstik off (i.e. to 0, which
                        \ is positive), then skip the first of these two
                        \ instructions, so we get two beeps for on and one beep
                        \ for off

 JSR BELL               \ We just enabled the Bitstik, so give two standard
                        \ system beeps (this being the first)

 JSR BELL               \ Make another system beep

.DOVOL2

 CPX #'S'               \ If "S" is not being pressed, jump to DK7
 BNE DK7

 LDA #0                 \ "S" is being pressed, so set DNOIZ to 0 to turn the
 STA DNOIZ              \ sound on

.DK7

 CPX #&1B               \ If ESCAPE is not being pressed, skip over the next
 BNE P%+5               \ instruction

 JMP DEATH2             \ ESCAPE is being pressed, so jump to DEATH2 to end
                        \ the game

 CPX #&7F               \ If DELETE is not being pressed, we are still paused,
 BNE FREEZE             \ so loop back up to keep listening for configuration
                        \ keys, otherwise fall through into the rest of the
                        \ key detection code, which unpauses the game

.DK2

 RTS                    \ Return from the subroutine