The SpawnRock subroutine - Lander on the Acorn Archimedes

       Name: SpawnRock                                               [Show more]
       Type: Subroutine
   Category: Particles
    Summary: An unused routine that spawns a rock in the sky, at half the
             altitude of the rocks in the DropRocksFromTheSky routine
  Deep dive: Unused code in Lander
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


.SpawnRock

 STMFD   R13!, {R6-R9, R14}         \ Store the registers that we want to use on
                                    \ the stack so they can be preserved

                                    \ Drop a rock from the sky, spawning the
                                    \ rock at coordinates (R0, R1, R2), which
                                    \ are set to half the height of a normal
                                    \ rock and PLAYER_FRONT_Z tiles forwards
                                    \ from the camera coordinates, which is
                                    \ fairly high in the sky above the ship's
                                    \ current position and one tile in front of
                                    \ the ship

 LDR     R0, [R11, #xCamera]        \ Set R0 = xCamera

 MVN     R1, #ROCK_HEIGHT / 2       \ Set R1 = ~ROCK_HEIGHT / 2
                                    \        = -(ROCK_HEIGHT + 1) / 2

 LDR     R2, [R11, #zCamera]        \ Set R2 = zCamera - PLAYER_FRONT_Z
 SUB     R2, R2, #PLAYER_FRONT_Z

 BL      DropARockFromTheSky        \ Drop a rock from the coordinates in
                                    \ (R0, R1, R2)

 LDMFD   R13!, {R6-R9, PC}          \ Retrieve the registers that we stored on
                                    \ the stack and return from the subroutine

[X]

Subroutine DropARockFromTheSky (category: Particles)

Drop a rock from the specified coordinates by spawning it as a particle, albeit a very big particle with an associated 3D object

[X]

Configuration variable PLAYER_FRONT_Z = (TILES_Z - 5) * TILE_SIZE

The distance along the z-axis between the tile in front of the player and the camera, which is where we spawn explosions and falling rocks (set to six tiles forwards from the camera)

[X]

Configuration variable ROCK_HEIGHT = TILE_SIZE * 32

The height from which we drop rocks from the sky (32 tile sizes)

[X]

Configuration variable xCamera = &13C

The 3D x-coordinate of the camera position (though note that the camera position is actually at the back of the on-screen landscape, not the front)

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Configuration variable zCamera = &144

The 3D z-coordinate of the camera position (though note that the camera position is actually at the back of the on-screen landscape, not the front, so the camera's z-coordinate is larger than it would be for a more traditional camera position; it is more like the camera's focal point than position, in a sense)