The Theme: UpdateAliens (Part 4 of 5)

       Name: UpdateAliens (Part 4 of 5)                              [Show more]
       Type: Subroutine
   Category: The Theme
    Summary: When an alien reaches the next feeding stage, double its size
  Deep dive: Scheduling tasks in the main loop
             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


.upal15

 LDA mainLoopCounter    \ If the mainLoopCounter is a multiple of 128, jump to
 AND #127               \ upal19 to skip the following (so we only do this part
 BNE upal19             \ on 2 out of every 256 iterations round the main loop,
                        \ just like part 1)

 LDY #31                \ We now work our way through alien slots 31 and 32,
                        \ so set a counter in Y for the slot number

.upal16

 LDX alienSlot-30,Y     \ If alien slot Y contains a negative number, then the
 BMI upal17             \ slot is empty, so jump to upal17 to move on to the
                        \ next slot

 LDA alienState,X       \ If the alien's state is < 5, jump to upal17 to move on
 CMP #5                 \ to the next slot, as the alien is still in feeding
 BCC upal17             \ stage 1 and doesn't need resizing

 CMP #20                \ If the alien's state is >= 20, jump to upal17 to move
 BCS upal17             \ on to the next slot, as the alien has already reached
                        \ the last feeding stage and can't get any bigger

 AND #%00000011         \ Check whether the feeding state is in the form
 BNE upal17             \ %xxxxxx00, and if not, jump to upal17 to move on to
                        \ the next slot

                        \ If we get here, then the alien's state is between 6
                        \ and 19, and matches %xxxxxx00, so it's one of these:
                        \
                        \   * 8  = %00001000
                        \   * 12 = %00001100
                        \   * 16 = %00010000
                        \
                        \ These are the points in the feeding stage when the
                        \ size of the alien doubles

 JSR ResizeFeedingAlien \ Double the size of the feeding alien to move it onto
                        \ the next feeding stage

.upal17

 INY                    \ Increment the counter to point to the next alien slot

 CPY #33                \ Loop back until we have done alien slots 31 and 32
 BNE upal16