Sound: UpdateVibratoSeeds

       Name: UpdateVibratoSeeds                                      [Show more]
       Type: Subroutine
   Category: Sound
    Summary: Update the sound seeds that are used to randomise the vibrato
             effect
  Deep dive: Sound effects in NES Elite
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * MakeSound calls UpdateVibratoSeeds


.UpdateVibratoSeeds

 LDA soundVibrato       ; Set A to soundVibrato with all bits cleared except for
 AND #%01001000         ; bits 3 and 6

 ADC #%00111000         ; Add %00111000, so if bit 3 of A is clear, we leave
                        ; bit 6 alone, otherwise bit 6 gets flipped
                        ;
                        ; The C flag doesn't affect this calculation, as it
                        ; will only affect bit 0, which we don't care about

 ASL A                  ; Set the C flag to bit 6 of A
 ASL A                  ;
                        ; So the C flag is:
                        ;
                        ;   * Bit 6 of soundVibrato if bit 3 of soundVibrato is
                        ;     clear
                        ;
                        ;   * Bit 6 of soundVibrato flipped if bit 3 of
                        ;     soundVibrato is set
                        ;
                        ; Or, to put it another way:
                        ;
                        ;   C = bit 6 of soundVibrato EOR bit 3 of soundVibrato

 ROL soundVibrato+3     ; Rotate soundVibrato(0 1 2 3) left, inserting the C
 ROL soundVibrato+2     ; flag into bit 0 of soundVibrato+3
 ROL soundVibrato+1
 ROL soundVibrato

 RTS                    ; Return from the subroutine