.DropARockFromTheSky STMFD R13!, {R0-R2, R8, R14} \ Store the particle coordinates on the \ stack so we can retrieve them below, and \ also store R8 and the return address so \ we can pass them to the particle adding \ routine below \ We start by setting up a three-channel \ colour, with the red channel in R0, the \ green channel in R1 and the blue channel \ in R2 BL GetRandomNumbers \ Set R0 and R1 to random numbers MOV R1, R1, LSR #29 \ Set R1 to a random number in the range 0 \ to 7 MOV R2, R0, LSR #30 \ Set R2 to a random number in the range 0 \ to 3 AND R0, R0, #7 \ Set R0 to a random number in the range 0 \ to 7 ADD R0, R0, #4 \ Set R0 to a random number in the range 4 \ to 11, to use in the red channel ADD R1, R1, #2 \ Set R1 to a random number in the range 2 \ to 2, to use for the green channel ADD R2, R2, #4 \ Set R2 to a random number in the range 4 \ to 7, to use in the blue channel \ This sets the channels to a randomly \ purple-brownish-green colour \ We now build a VIDC colour number in R7 \ by combining the three channels into one \ byte, which we then replicate four times \ to get a 32-bit colour number \ \ The byte is of the form: \ \ * Bit 7 = blue bit 3 \ * Bit 6 = green bit 3 \ * Bit 5 = green bit 2 \ * Bit 4 = red bit 3 \ * Bit 3 = blue bit 2 \ * Bit 2 = red bit 2 \ * Bit 1 = sum of red/green/blue bit 1 \ * Bit 0 = sum of red/green/blue bit 0 \ \ We now build this colour number in R7 from \ the red, green and blue values in R0, R1 \ and R2 ORR R7, R1, R2 \ Set R7 to the bottom three bits of: AND R7, R7, #%00000011 \ ORR R7, R7, R0 \ (the bottom two bits of R1 OR R2) OR R0 AND R7, R7, #7 \ \ So this sets bits 0, 1 and 2 of R7 as \ required TST R0, #%00001000 \ If bit 3 of the red channel in R0 is set, ORRNE R7, R7, #%00010000 \ set bit 4 of R7 AND R1, R1, #%00001100 \ Clear all bits of the green channel in R1 \ except bits 2-3 ORR R7, R7, R1, LSL #3 \ And stick them into bits 5-6 of R7 TST R2, #%00000100 \ If bit 2 of the blue channel in R2 is set, ORRNE R7, R7, #%00001000 \ set bit 3 of R7 TST R2, #%00001000 \ If bit 3 of the blue channel in R2 is set, ORRNE R7, R7, #%10000000 \ set bit 7 of R7 ORR R7, R7, #&00FE0000 \ Set bits 17 to 23 of the particle flags, \ so that's: \ \ * Bit 17 set = particle is a rock \ * Bit 18 set = splash on impact with sea \ * Bit 19 set = bounce on ground \ * Bit 20 set = apply gravity to particle \ * Bit 21 set = can destroy objects \ * Bit 23 set = splash size is big \ * Bit 24 set = explode on hitting ground LDMFD R13!, {R0-R2} \ Retrieve the particle coordinates that we \ stored above into (R0, R1, R2) MOV R9, #27 \ Set the random element of the particle's \ lifespan to the range 0 to 2^(32 - 27), \ i.e. 0 to 32 MOV R8, #10 \ Set the random element of the particle's \ velocity to the range +/- 2^(32 - 10), \ i.e. -&400000 to +&400000 MOV R6, #170 \ Set the particle's lifespan counter to \ 170 iterations of the main loop, so it \ won't disappear before it hits the ground B AddStaticParticleToBuffer \ Add a rock to the particle data buffer \ that starts off static, and with a random \ element being added to its velocity and \ lifespan counter, returning from the \ subroutine using a tail callName: DropARockFromTheSky [Show more] Type: Subroutine Category: Particles Summary: Drop a rock from the specified coordinates by spawning it as a particle, albeit a very big particle with an associated 3D object Deep dive: Particles and particle clouds Screen memory in the ArchimedesContext: See this subroutine in context in the source code References: This subroutine is called as follows: * DropRocksFromTheSky calls DropARockFromTheSky * SpawnRock calls DropARockFromTheSky
Arguments: (R0, R1, R2) The 3D coordinate (x, y, z) where we spawn the rock
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Subroutine AddStaticParticleToBuffer (category: Particles)
Add a particle to the particle data buffer that starts off static, adding a random element to its velocity and lifespan counter
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Subroutine GetRandomNumbers (category: Maths (Arithmetic))
Generate pseudo-random numbers from the random number seeds