Sound: ApplyEnvelopeTRI

       Name: ApplyEnvelopeTRI                                        [Show more]
       Type: Subroutine
   Category: Sound
    Summary: Apply volume and pitch changes to the TRI channel
  Deep dive: Music in NES Elite
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * MakeMusic calls ApplyEnvelopeTRI


.ApplyEnvelopeTRI

 LDA volumeCounterTRI   ; If volumeCounterTRI = 0 then we are not counting down
 BEQ muse1              ; to a volume change, so jump to muse1 to move on to the
                        ; pitch envelope

 DEC volumeCounterTRI   ; Decrement the counter for the volume change

 BNE muse1              ; If the counter is still non-zero, then we haven't yet
                        ; done counted down to the volume change, so jump to
                        ; muse1 to skip the following

 LDA #%00000000         ; Configure the TRI channel as follows:
 STA TRI_LINEAR         ;
                        ;   * Bit 7 clear = do not reload the linear counter
                        ;
                        ;   * Bits 0-6    = counter reload value of 0
                        ;
                        ; So this silences the TRI channel

.muse1

                        ; We now move on to the pitch envelope

 LDX pitchEnvelopeTRI   ; Set X to the number of the pitch envelope to apply

 LDA pitchEnvelopeLo,X  ; Set soundAddr(1 0) to the address of the data for
 STA soundAddr          ; pitch envelope X from the (i.e. pitchEnvelope0 for
 LDA pitchEnvelopeHi,X  ; envelope 0, pitchEnvelope1 for envelope 1, and so on)
 STA soundAddr+1

 LDY pitchIndexTRI      ; Set A to the byte of envelope data at the index in
 LDA (soundAddr),Y      ; pitchIndexTRI, which we increment to move through the
                        ; data one byte at a time

 CMP #$80               ; If A is not $80 then we just fetched a valid byte of
 BNE muse2              ; envelope data, so jump to muse2 to process it

                        ; If we get here then we just fetched a $80 from the
                        ; pitch envelope, which indicates the end of the list of
                        ; envelope values, so we now loop around to the start of
                        ; the list, so it keeps repeating

 LDY #0                 ; Set pitchIndexTRI = 0 to point to the start of the
 STY pitchIndexTRI      ; data for pitch envelope X

 LDA (soundAddr),Y      ; Set A to the byte of envelope data at index 0, so we
                        ; can fall through into muse2 to process it

.muse2

 INC pitchIndexTRI      ; Increment the index into the pitch envelope so we
                        ; move on to the next byte of data in the next iteration

 CLC                    ; Set triLo = triLoCopy + A
 ADC triLoCopy          ;
 STA triLo              ; So this alters the low byte of the pitch that we send
                        ; to the APU via TRI_LO, altering it by the amount in
                        ; the byte of data we just fetched from the pitch
                        ; envelope

 RTS                    ; Return from the subroutine