Skip to navigation

⌂ ∞ ▽ Elite on the 6502

Sound: NOISE2

[Commodore 64 version]

       Name: NOISE2                                                  [Show more]
       Type: Subroutine
   Category: Sound
    Summary: Make a sound effect with a specific volume and release length

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * EXNO calls NOISE2
             * EXNO2 calls NOISE2
             * HYPNOISE calls NOISE2
             * Main game loop (Part 5 of 6) calls NOISE2




 Arguments:

   A                    Determines the release length and sustain volume of the
                        sound effect

                          * Bits 0-3 contain the release length

                          * Bits 4-7 contain the sustain volume

   X                    The frequency of the sound effect



.NOISE2

 BIT SOUR1              ; SOUR1 contains an RTS instruction, which has opcode
                        ; $60 (or %01100000), and as the BIT instructions sets
                        ; the V flag to bit 6 of its operand, this instruction
                        ; sets the V flag
                        ;
                        ; There is no SEV instruction in the 6502, hence the
                        ; need for this workaround

 STA XX15               ; Store the sustain volume/release length in XX15

 STX XX15+1             ; Store the frequency in XX15+1

 EQUB $50               ; Skip the next instruction by turning it into
                        ; $50 $B8, or BVC $B8, which does nothing because we
                        ; set the V flag above

                        ; Fall through into NOISE with the V flag set