Sound: MakeSound

       Name: MakeSound                                               [Show more]
       Type: Subroutine
   Category: Sound
    Summary: Make a sound
  Deep dive: The engine sounds
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ApplyTyresAndSkids calls MakeSound
             * MakeDrivingSounds calls MakeSound
             * ApplyBounce calls via MakeSound-3
             * CheckForCrash calls via MakeSound-3
             * MakeDrivingSounds calls via MakeSound-3
             * SquealTyres calls via MakeSound-3


 Arguments:

   A                    The sound number from the soundData table (0 to 4)

   Y                    The volume level to use for the sound, or the envelope
                        number (the latter is used for sound #3 only, and is
                        always set to envelope 1, which is the only envelope)


 Other entry points:

   MakeSound-3          Make the sound at the current volume level


 LDY volumeLevel        \ Set Y to the current volumeLevel, to use as the sound
                        \ amplitude below

.MakeSound

 STX xStoreSound        \ Store the value of X in xStoreSound, so we can
                        \ preserve it through the call to the MakeSound routine

 ASL A                  \ Set A = A * 8
 ASL A                  \
 ASL A                  \ so we can use it as an index into the soundData table,
                        \ which has 8 bytes per entry

 CLC                    \ Set (Y X) = soundData + A
 ADC #LO(soundData)     \
 TAX                    \ starting with the low byte in X, which gets set to the
                        \ following, as LO(soundData) is 16:
                        \
                        \   * 16 for sound #0
                        \   * 24 for sound #1
                        \   * 32 for sound #2
                        \   * 40 for sound #3
                        \   * 48 for sound #4
                        \
                        \ This means that soundData - 16 + X points to the sound
                        \ data block for the sound we are making, which we now
                        \ use to set the volume or envelope for the sound to Y,
                        \ and flag the correct sound buffer as being in use

 TYA                    \ Set byte #2 of the sound data (low byte of amplitude
 STA soundData-16+2,X   \ or envelope number) to Y

 LDA soundData-16,X     \ Set Y to byte #0 of the sound data (channel/flush),
 AND #3                 \ and extract the channel number into Y
 TAY

 LDA #7                 \ Set A = 7 for the OSWORD command to make a sound

 STA soundBuffer,Y      \ Set the Y-th sound buffer status to 7, which is
                        \ non-zero and indicates that we are making a sound on
                        \ this channel

 BNE MakeSoundEnvelope  \ Jump to MakeSoundEnvelope to set up Y and apply the
                        \ OSWORD command to the (Y X) block, which makes the
                        \ relevant sound (this BNE is effectively a JMP as A is
                        \ never zero)