The Theme: UpdateAliens (Part 5 of 5)

       Name: UpdateAliens (Part 5 of 5)                              [Show more]
       Type: Subroutine
   Category: The Theme
    Summary: Check whether the whole wave has been destroyed, and award points
             accordingly
  Deep dive: Alien feeding and growth patterns
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


 LDX #7                 \ We now work through all the aliens, from alien 7 down
                        \ to alien 0, to check whether they have all been
                        \ destroyed (i.e. whether they all have a negative
                        \ object ID), so set a loop counter in X

.upal18

 LDA alienObjectId,X    \ If the X-th alien object ID is positive, then that
 BPL upal19             \ alien is still at large, so jump to upal19 to return
                        \ from the subroutine, as we haven't destroyed all eight
                        \ aliens in this wave

 DEX                    \ Decrement the loop counter to point to the next alien

 BPL upal18             \ Loop back until we have checked all eight aliens

                        \ If we get here then all eight aliens have negative
                        \ object IDs, so they have all been destroyed

 LDA #8                 \ Set themeStatus = 8 to initiate a brand new wave of
 STA themeStatus        \ aliens

 LDX #&00               \ Add 500 points to the score and make a beep by calling
 LDA #&50               \ ScorePoints with (X A) = &0050
 JSR ScorePoints        \
                        \ This is the score for completing a wave in the Theme

 LDA alienSpeed         \ Set A = alienSpeed + 4
 CLC
 ADC #4

 CMP #19                \ If A >= 19, skip the following instruction, so
 BCS upal19             \ alienSpeed has a maximum value of 22 (as alienSpeed
                        \ starts with a value of 10, and increases by 4 with
                        \ each wave until it is >= 19, which happens when it
                        \ reaches 22)

 STA alienSpeed         \ Update alienSpeed with the faster speed so the next
                        \ wave has faster-moving aliens

.upal19

 RTS                    \ Return from the subroutine