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⌂ ∞ ▽ Lander on the Acorn Archimedes

Particles: BounceParticle

       Name: BounceParticle                                          [Show more]
       Type: Subroutine
   Category: Particles
    Summary: Bounce a particle off the ground
  Deep dive: Particles and particle clouds

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * MoveAndDrawParticles (Part 1 of 4) calls BounceParticle




 Arguments:

   (R0, R1, R2)                     Particle coordinates

   (R3, R4, R5)                     Particle velocity

   R7                               Particle flags

   R9                               The altitude of the landscape directly below
                                    the particle



.BounceParticle

 MOV     R1, R9                     \ Set R1 to R9, so (R0, R1, R2) now contains
                                    \ the coordinates of the point on the
                                    \ landscape directly below the particle

 CMP     R9, #SEA_LEVEL             \ If the particle is above the sea, jump to
 BEQ     SplashParticleIntoSea      \ SplashParticleIntoSea to splash the
                                    \ particle into the sea, returning from the
                                    \ subroutine using a tail call

 TST     R7, #&00080000             \ If bit 19 of the particle flags is clear,
 BEQ     DeleteParticleData         \ jump to DeleteParticleData to delete the
                                    \ particle without bouncing or exploding

 TST     R7, #&01000000             \ If bit 24 of the particle flags is set,
 BNE     AddSmallExplosionToBuffer  \ jump to AddSmallExplosionToBuffer to
                                    \ destroy the particle in a small explosion

 MOV     R3, R3, ASR #1             \ Otherwise we bounce the particle off the
 MOV     R4, R4, ASR #1             \ ground by setting the particle's velocity
 MOV     R5, R5, ASR #1             \ vector to half its previous speed, and in
 RSB     R4, R4, #0                 \ the opposite direction in the y-axis

 MOV     PC, R14                    \ Return from the subroutine