.MakeSoundOnSQ2 LDA effectOnSQ2 ; If effectOnSQ2 is non-zero then a sound effect is BNE msco1 ; being made on channel SQ2, so jump to msco1 to keep ; making it RTS ; Otherwise return from the subroutine .msco1 LDA soundByteSQ2+0 ; If the remaining number of iterations for this sound BNE msco3 ; effect in sound byte #0 is non-zero, jump to msco3 to ; keep making the sound LDX soundByteSQ2+12 ; If byte #12 of the sound effect data is non-zero, then BNE msco3 ; this sound effect keeps looping, so jump to msco3 to ; keep making the sound LDA enableSound ; If enableSound = 0 then sound is disabled, so jump to BEQ msco2 ; msco2 to silence the SQ2 channel and return from the ; subroutine ; If we get here then we have finished making the sound ; effect, so we now send the volume and pitch values for ; the music to the APU, so if there is any music playing ; it will pick up again, and we mark this sound channel ; as clear of sound effects LDA sq2Volume ; Send sq2Volume to the APU via SQ2_VOL, which is the STA SQ2_VOL ; volume byte of any music that was playing when the ; sound effect took precedence LDA sq2Lo ; Send (sq2Hi sq2Lo) to the APU via (SQ2_HI SQ2_LO), STA SQ2_LO ; which is the pitch of any music that was playing when LDA sq2Hi ; the sound effect took precedence STA SQ2_HI STX effectOnSQ2 ; Set effectOnSQ2 = 0 to mark the SQL channel as clear ; of sound effects, so the channel can be used for music ; and is ready for the next sound effect RTS ; Return from the subroutine .msco2 ; If we get here then sound is disabled, so we need to ; silence the SQ2 channel LDA #%00110000 ; Set the volume of the SQ2 channel to zero as follows: STA SQ2_VOL ; ; * Bits 6-7 = duty pulse length is 3 ; * Bit 5 set = infinite play ; * Bit 4 set = constant volume ; * Bits 0-3 = volume is 0 STX effectOnSQ2 ; Set effectOnSQ2 = 0 to mark the SQL channel as clear ; of sound effects, so the channel can be used for music ; and is ready for the next sound effect RTS ; Return from the subroutine .msco3 ; If we get here then we need to keep making the sound ; effect on channel SQ2 DEC soundByteSQ2+0 ; Decrement the remaining length of the sound in byte #0 ; as we are about to make the sound for another ; iteration DEC soundVolCountSQ2 ; Decrement the volume envelope counter so we count down ; towards the point where we apply the volume envelope BNE msco5 ; If the volume envelope counter has not reached zero ; then jump to msco5, as we don't apply the next entry ; from the volume envelope yet ; If we get here then the counter in soundVolCountSQ2 ; just reached zero, so we apply the next entry from the ; volume envelope LDA soundByteSQ2+11 ; Reset the volume envelope counter to byte #11 from the STA soundVolCountSQ2 ; sound effect's data, which controls how often we apply ; the volume envelope to the sound effect LDY soundVolIndexSQ2 ; Set Y to the index of the current byte in the volume ; envelope LDA soundVolumeSQ2 ; Set soundAddr(1 0) = soundVolumeSQ2(1 0) STA soundAddr ; LDA soundVolumeSQ2+1 ; So soundAddr(1 0) contains the address of the volume STA soundAddr+1 ; envelope for this sound effect LDA (soundAddr),Y ; Set A to the data byte at the current index in the ; volume envelope BPL msco4 ; If bit 7 is clear then we just fetched a volume value ; from the envelope, so jump to msco4 to apply it ; If we get here then A must be $80 or $FF, as those are ; the only two valid entries in the volume envelope that ; have bit 7 set ; ; $80 means we loop back to the start of the envelope, ; while $FF means the envelope ends here CMP #$80 ; If A is not $80 then we must have just fetched $FF BNE msco5 ; from the envelope, so jump to msco5 to exit the ; envelope ; If we get here then we just fetched a $80 from the ; envelope data, so we now loop around to the start of ; the envelope, so it keeps repeating LDY #0 ; Set Y to zero so we fetch data from the start of the ; envelope again LDA (soundAddr),Y ; Set A to the byte of envelope data at index 0, so we ; can fall through into msco4 to process it .msco4 ; If we get here then A contains an entry from the ; volume envelope for this sound effect, so now we send ; it to the APU to change the volume ORA soundByteSQ2+6 ; OR the envelope byte with the sound effect's byte #6, STA SQ2_VOL ; and send the result to the APU via SQ2_VOL ; ; Data bytes in the volume envelope data only use the ; low nibble (the high nibble is only used to mark the ; end of the data), and the sound effect's byte #6 only ; uses the high nibble, so this sets the low nibble of ; the APU byte to the volume level from the data, and ; the high nibble of the APU byte to the configuration ; in byte #6 (which sets the duty pulse, looping and ; constant flags for the volume) INY ; Increment the index of the current byte in the volume STY soundVolIndexSQ2 ; envelope so on the next iteration we move on to the ; next byte in the envelope .msco5 ; Now that we are done with the volume envelope, it's ; time to move on to the pitch of the sound effect LDA soundPitCountSQ2 ; If the byte #1 counter has not yet run down to zero, BNE msco8 ; jump to msco8 to skip the following, so we don't send ; pitch data to the APU on this iteration ; If we get here then the counter in soundPitCountSQ2 ; (which counts down from the value of byte #1) has run ; down to zero, so we now send pitch data to the ALU if ; if we haven't yet sent it all LDA soundByteSQ2+12 ; If byte #12 is non-zero then the sound effect loops BNE msco6 ; infinitely, so jump to msco6 to send pitch data to the ; APU LDA soundByteSQ2+9 ; Otherwise, if the counter in byte #9 has not run down BNE msco6 ; then we haven't yet sent pitch data for enough ; iterations, so jump to msco6 to send pitch data to the ; APU RTS ; Return from the subroutine .msco6 ; If we get here then we are sending pitch data to the ; APU on this iteration, so now we do just that DEC soundByteSQ2+9 ; Decrement the counter in byte #9, which contains the ; number of iterations for which we send pitch data to ; the APU (as that's what we are doing) LDA soundByteSQ2+1 ; Reset the soundPitCountSQ2 counter to the value of STA soundPitCountSQ2 ; byte #1 so it can start counting down again to trigger ; the next pitch change after this one LDA soundByteSQ2+2 ; Set A to the low byte of the sound effect's current ; pitch, which is in byte #2 of the sound data LDX soundByteSQ2+7 ; If byte #7 is zero then vibrato is disabled, so jump BEQ msco7 ; to msco7 to skip the following instruction ADC soundVibrato ; Byte #7 is non-zero, so add soundVibrato to the pitch ; of the sound in A to apply vibrato (this also adds the ; C flag, which is not in a fixed state, so this adds an ; extra level of randomness to the vibrato effect) .msco7 STA soundLoSQ2 ; Store the value of A (i.e. the low byte of the sound ; effect's pitch, possibly with added vibrato) in ; soundLoSQ2 STA SQ2_LO ; Send the value of soundLoSQ2 to the APU via SQ2_LO LDA soundByteSQ2+3 ; Set A to the high byte of the sound effect's current ; pitch, which is in byte #3 of the sound data STA soundHiSQ2 ; Store the value of A (i.e. the high byte of the sound ; effect's pitch) in soundHiSQ2 STA SQ2_HI ; Send the value of soundHiSQ2 to the APU via SQ2_HI .msco8 DEC soundPitCountSQ2 ; Decrement the byte #1 counter, as we have now done one ; more iteration of the sound effect LDA soundByteSQ2+13 ; If byte #13 of the sound data is zero then we apply BEQ msco9 ; pitch variation in every iteration (if enabled), so ; jump to msco9 to skip the following and move straight ; to the pitch variation checks DEC soundPitchEnvSQ2 ; Otherwise decrement the byte #13 counter to count down ; towards the point where we apply pitch variation BNE msco11 ; If the counter is not yet zero, jump to msco11 to ; return from the subroutine without applying pitch ; variation, as the counter has not yet reached that ; point ; If we get here then the byte #13 counter just ran down ; to zero, so we need to apply pitch variation (if ; enabled) STA soundPitchEnvSQ2 ; Reset the soundPitchEnvSQ2 counter to the value of ; byte #13 so it can start counting down again, for the ; next pitch variation after this one .msco9 LDA soundByteSQ2+8 ; Set A to byte #8 of the sound data, which determines ; whether pitch variation is enabled BEQ msco11 ; If A is zero then pitch variation is not enabled, so ; jump to msco11 to return from the subroutine without ; applying pitch variation ; If we get here then pitch variation is enabled, so now ; we need to apply it BMI msco10 ; If A is negative then we need to add the value to the ; pitch's period, so jump to msco10 ; If we get here then we need to subtract the 16-bit ; value in bytes #4 and #5 from the pitch's period in ; (soundHiSQ2 soundLoSQ2) ; ; Reducing the pitch's period increases its frequency, ; so this makes the note frequency higher LDA soundLoSQ2 ; Subtract the 16-bit value in bytes #4 and #5 of the SEC ; sound data from (soundHiSQ2 soundLoSQ2), updating SBC soundByteSQ2+4 ; (soundHiSQ2 soundLoSQ2) to the result, and sending STA soundLoSQ2 ; it to the APU via (SQ2_HI SQ2_LO) STA SQ2_LO ; LDA soundHiSQ2 ; Note that bits 2 to 7 of the high byte are cleared so SBC soundByteSQ2+5 ; the length counter does not reload AND #%00000011 STA soundHiSQ2 STA SQ2_HI RTS ; Return from the subroutine .msco10 ; If we get here then we need to add the 16-bit value ; in bytes #4 and #5 to the pitch's period in ; (soundHiSQ2 soundLoSQ2) ; ; Increasing the pitch's period reduces its frequency, ; so this makes the note frequency lower LDA soundLoSQ2 ; Add the 16-bit value in bytes #4 and #5 of the sound CLC ; data to (soundHiSQ2 soundLoSQ2), updating ADC soundByteSQ2+4 ; (soundHiSQ2 soundLoSQ2) to the result, and sending STA soundLoSQ2 ; it to the APU via (SQ2_HI SQ2_LO) STA SQ2_LO ; LDA soundHiSQ2 ; Note that bits 2 to 7 of the high byte are cleared so ADC soundByteSQ2+5 ; the length counter does not reload AND #%00000011 STA soundHiSQ2 STA SQ2_HI .msco11 RTS ; Return from the subroutineName: MakeSoundOnSQ2 [Show more] Type: Subroutine Category: Sound Summary: Make the current sound effect on the SQ2 channel Deep dive: Sound effects in NES EliteContext: See this subroutine in context in the source code References: This subroutine is called as follows: * MakeSound calls MakeSoundOnSQ2
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Configuration variable SQ2_HI = $4007
The APU period register (high byte) for square wave channel 2
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Configuration variable SQ2_LO = $4006
The APU period register (low byte) for square wave channel 2
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Configuration variable SQ2_VOL = $4004
The APU duty cycle and volume register for square wave channel 2
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Variable effectOnSQ2 in workspace WP
Records whether a sound effect is being made on the SQ2 channel
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Variable enableSound in workspace WP
Controls sound effects and music in David Whittaker's sound module
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Label msco1 is local to this routine
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Label msco10 is local to this routine
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Label msco11 is local to this routine
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Label msco2 is local to this routine
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Label msco3 is local to this routine
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Label msco4 is local to this routine
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Label msco5 is local to this routine
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Label msco6 is local to this routine
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Label msco7 is local to this routine
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Label msco8 is local to this routine
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Label msco9 is local to this routine
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Variable soundByteSQ2 in workspace WP
The 14 sound bytes for the sound effect being made on channel SQ2
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Variable soundHiSQ2 in workspace WP
The value that we are going to send to the APU via SQ2_HI for the current sound effect
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Variable soundLoSQ2 in workspace WP
The value that we are going to send to the APU via SQ2_LO for the current sound effect
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Variable soundPitCountSQ2 in workspace WP
Controls how often we send pitch data to the APU for the sound effect on channel SQ2
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Variable soundPitchEnvSQ2 in workspace WP
Controls how often we apply the pitch envelope to the sound effect on channel SQ2
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Variable soundVibrato in workspace WP
The four-byte seeds for adding randomised vibrato to the current sound effect
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Variable soundVolCountSQ2 in workspace WP
Controls how often we apply the volume envelope to the sound effect on channel SQ2
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Variable soundVolIndexSQ2 in workspace WP
The index into the volume envelope data of the next volume byte to apply to the sound effect on channel SQ2
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Variable soundVolumeSQ2 in workspace WP
The address of the volume envelope data for the sound effect currently being made on channel SQ2