Main loop: MainLoop (Part 6 of 15)

       Name: MainLoop (Part 6 of 15)                                 [Show more]
       Type: Subroutine
   Category: Main loop
    Summary: Check whether any aliens have been hit
  Deep dive: Program flow of the main game loop
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


.main7

 LDA themeStatus        \ If themeStatus is non-zero then either the Theme is
 BNE main12             \ not enabled, or it is enabled but we haven't yet added
                        \ all eight aliens to the current wave, so jump to
                        \ main12 as we only move aliens when the whole wave has
                        \ arrived

 LDA firingStatus       \ If firingStatus is zero then there are no bullets in
 BEQ main11             \ the air, so jump to main11, as we only need to check
                        \ whether an alien is hit when there are bullets around

 LDA #33                \ We now loop through objects 33 down to 30, which are
 STA objectId           \ all the alien objects, so we can check whether any of
                        \ them have been hit, so set a loop counter in objectId

.main8

 LDY objectId           \ Set Y to the object ID of the alien to check

 LDA alienStatus-30,Y   \ We copied the object status bytes for all four alien
 BPL main9              \ objects into alienStatus in part 1, so this checks
                        \ whether bit 7 of the alien's object status byte is
                        \ clear
                        \
                        \ If it is clear, then this object is not visible, so
                        \ we skip the following three instructions and move on
                        \ to the next alien, as we can't hit an alien that we
                        \ can't see

 JSR CheckIfAlienIsHit  \ This alien is visible, so check to see whether it has
                        \ been hit, and if so, initiate the explosion

 LDA hitTimer           \ If hitTimer is non-zero, then we just hit an alien, so
 BNE main10             \ jump to main10 to skip checking the rest of the aliens

.main9

 DEC objectId           \ Decrement the loop counter to the next alien

 LDA objectId           \ Loop back to check the next alien, until we have
 CMP #30                \ done all of them (from object 33 down to object 30)
 BCS main8

 BCC main11             \ Jump to main11 (this BCC is effectively a JMP as we
                        \ just passed through a BCS)

.main10

 STA distanceFromHit    \ Store the value of hitTimer (which will be 27 as we
                        \ just hit an alien) in distanceFromHit, so we are far
                        \ enough away to avoid any turbulence, for now (as
                        \ turbulence only kicks in when distanceFromHit < 16)

 LDA #0                 \ Set firingStatus = 0 to indicate that there are no
 STA firingStatus       \ longer any bullets are in the air, as they are now
                        \ embedded in an unfortunate alien

.main11

 JSR UpdateAliens       \ Update the aliens' statuses