Keyboard: check_keys

       Name: check_keys                                              [Show more]
       Type: Subroutine
   Category: Keyboard
    Summary: Wait until a key is pressed, quitting the game if the game is
             paused and ESCAPE is pressed
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ships_ag calls check_keys
             * trading calls check_keys
             * ships_ag calls via l_release


 Returns:

   X                    The key that was pressed, or 0 if we paused the game
                        (COPY) and unpaused it again (DELETE)


 Other entry points:

   l_release            If a key is currently being pressed, wait until it is
                        released


.check_keys

 JSR WSCAN              \ Call WSCAN to wait for the vertical sync

 JSR RDKEY              \ Scan the keyboard for a key press and return the
                        \ internal key number in X (or 0 for no key press)

 CPX #&69               \ If COPY is not being pressed, jump to not_freeze to
 BNE not_freeze         \ return the key pressed in X

.freeze_loop

 JSR WSCAN              \ Call WSCAN to wait for the vertical sync

 JSR RDKEY              \ Scan the keyboard for a key press and return the
                        \ internal key number in X (or 0 for no key press)

 CPX #&70               \ If ESCAPE is not being pressed, jump to dont_quit to
 BNE dont_quit          \ skip the next

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

.dont_quit

\CPX #&37               \ These instructions are commented out in the original
\BNE dont_dump          \ source
\JSR printer
\.dont_dump

 CPX #&59               \ If DELETE is not being pressed, we are still paused,
 BNE freeze_loop        \ so loop back up to keep listening for configuration
                        \ keys, otherwise fall through into the rest of the
                        \ key detection code, which waits for the key to be
                        \ released before unpausing the game

.l_release

 JSR RDKEY              \ Scan the keyboard for a key press and return the
                        \ internal key number in X (or 0 for no key press)

 BNE l_release          \ If a key is being pressed, loop back to l_release
                        \ until it is released

 LDX #0                 \ Set X = 0 to indicate no key is being pressed

.not_freeze

 RTS                    \ Return from the subroutine