.MakeSoundOnSQ1 LDA effectOnSQ1 ; If effectOnSQ1 is non-zero then a sound effect is BNE mscz1 ; being made on channel SQ1, so jump to mscz1 to keep ; making it RTS ; Otherwise return from the subroutine .mscz1 LDA soundByteSQ1+0 ; If the remaining number of iterations for this sound BNE mscz3 ; effect in sound byte #0 is non-zero, jump to mscz3 to ; keep making the sound LDX soundByteSQ1+12 ; If byte #12 of the sound effect data is non-zero, then BNE mscz3 ; this sound effect keeps looping, so jump to mscz3 to ; keep making the sound LDA enableSound ; If enableSound = 0 then sound is disabled, so jump to BEQ mscz2 ; mscz2 to silence the SQ1 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 sq1Volume ; Send sq1Volume to the APU via SQ1_VOL, which is the STA SQ1_VOL ; volume byte of any music that was playing when the ; sound effect took precedence LDA sq1Lo ; Send (sq1Hi sq1Lo) to the APU via (SQ1_HI SQ1_LO), STA SQ1_LO ; which is the pitch of any music that was playing when LDA sq1Hi ; the sound effect took precedence STA SQ1_HI STX effectOnSQ1 ; Set effectOnSQ1 = 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 .mscz2 ; If we get here then sound is disabled, so we need to ; silence the SQ1 channel LDA #%00110000 ; Set the volume of the SQ1 channel to zero as follows: STA SQ1_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 effectOnSQ1 ; Set effectOnSQ1 = 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 .mscz3 ; If we get here then we need to keep making the sound ; effect on channel SQ1 DEC soundByteSQ1+0 ; Decrement the remaining length of the sound in byte #0 ; as we are about to make the sound for another ; iteration DEC soundVolCountSQ1 ; Decrement the volume envelope counter so we count down ; towards the point where we apply the volume envelope BNE mscz5 ; If the volume envelope counter has not reached zero ; then jump to mscz5, as we don't apply the next entry ; from the volume envelope yet ; If we get here then the counter in soundVolCountSQ1 ; just reached zero, so we apply the next entry from the ; volume envelope LDA soundByteSQ1+11 ; Reset the volume envelope counter to byte #11 from the STA soundVolCountSQ1 ; sound effect's data, which controls how often we apply ; the volume envelope to the sound effect LDY soundVolIndexSQ1 ; Set Y to the index of the current byte in the volume ; envelope LDA soundVolumeSQ1 ; Set soundAddr(1 0) = soundVolumeSQ1(1 0) STA soundAddr ; LDA soundVolumeSQ1+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 mscz4 ; If bit 7 is clear then we just fetched a volume value ; from the envelope, so jump to mscz4 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 mscz5 ; from the envelope, so jump to mscz5 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 mscz4 to process it .mscz4 ; 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 soundByteSQ1+6 ; OR the envelope byte with the sound effect's byte #6, STA SQ1_VOL ; and send the result to the APU via SQ1_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 soundVolIndexSQ1 ; envelope so on the next iteration we move on to the ; next byte in the envelope .mscz5 ; Now that we are done with the volume envelope, it's ; time to move on to the pitch of the sound effect LDA soundPitCountSQ1 ; If the byte #1 counter has not yet run down to zero, BNE mscz8 ; jump to mscz8 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 soundPitCountSQ1 ; (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 soundByteSQ1+12 ; If byte #12 is non-zero then the sound effect loops BNE mscz6 ; infinitely, so jump to mscz6 to send pitch data to the ; APU LDA soundByteSQ1+9 ; Otherwise, if the counter in byte #9 has not run down BNE mscz6 ; then we haven't yet sent pitch data for enough ; iterations, so jump to mscz6 to send pitch data to the ; APU RTS ; Return from the subroutine .mscz6 ; If we get here then we are sending pitch data to the ; APU on this iteration, so now we do just that DEC soundByteSQ1+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 soundByteSQ1+1 ; Reset the soundPitCountSQ1 counter to the value of STA soundPitCountSQ1 ; byte #1 so it can start counting down again to trigger ; the next pitch change after this one LDA soundByteSQ1+2 ; Set A to the low byte of the sound effect's current ; pitch, which is in byte #2 of the sound data LDX soundByteSQ1+7 ; If byte #7 is zero then vibrato is disabled, so jump BEQ mscz7 ; to mscz7 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) .mscz7 STA soundLoSQ1 ; Store the value of A (i.e. the low byte of the sound ; effect's pitch, possibly with added vibrato) in ; soundLoSQ1 STA SQ1_LO ; Send the value of soundLoSQ1 to the APU via SQ1_LO LDA soundByteSQ1+3 ; Set A to the high byte of the sound effect's current ; pitch, which is in byte #3 of the sound data STA soundHiSQ1 ; Store the value of A (i.e. the high byte of the sound ; effect's pitch) in soundHiSQ1 STA SQ1_HI ; Send the value of soundHiSQ1 to the APU via SQ1_HI .mscz8 DEC soundPitCountSQ1 ; Decrement the byte #1 counter, as we have now done one ; more iteration of the sound effect LDA soundByteSQ1+13 ; If byte #13 of the sound data is zero then we apply BEQ mscz9 ; pitch variation in every iteration (if enabled), so ; jump to mscz9 to skip the following and move straight ; to the pitch variation checks DEC soundPitchEnvSQ1 ; Otherwise decrement the byte #13 counter to count down ; towards the point where we apply pitch variation BNE mscz11 ; If the counter is not yet zero, jump to mscz11 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 soundPitchEnvSQ1 ; Reset the soundPitchEnvSQ1 counter to the value of ; byte #13 so it can start counting down again, for the ; next pitch variation after this one .mscz9 LDA soundByteSQ1+8 ; Set A to byte #8 of the sound data, which determines ; whether pitch variation is enabled BEQ mscz11 ; If A is zero then pitch variation is not enabled, so ; jump to mscz11 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 mscz10 ; If A is negative then we need to add the value to the ; pitch's period, so jump to mscz10 ; If we get here then we need to subtract the 16-bit ; value in bytes #4 and #5 from the pitch's period in ; (soundHiSQ1 soundLoSQ1) ; ; Reducing the pitch's period increases its frequency, ; so this makes the note frequency higher LDA soundLoSQ1 ; Subtract the 16-bit value in bytes #4 and #5 of the SEC ; sound data from (soundHiSQ1 soundLoSQ1), updating SBC soundByteSQ1+4 ; (soundHiSQ1 soundLoSQ1) to the result, and sending STA soundLoSQ1 ; it to the APU via (SQ1_HI SQ1_LO) STA SQ1_LO ; LDA soundHiSQ1 ; Note that bits 2 to 7 of the high byte are cleared so SBC soundByteSQ1+5 ; the length counter does not reload AND #%00000011 STA soundHiSQ1 STA SQ1_HI RTS ; Return from the subroutine .mscz10 ; If we get here then we need to add the 16-bit value ; in bytes #4 and #5 to the pitch's period in ; (soundHiSQ1 soundLoSQ1) ; ; Increasing the pitch's period reduces its frequency, ; so this makes the note frequency lower LDA soundLoSQ1 ; Add the 16-bit value in bytes #4 and #5 of the sound CLC ; data to (soundHiSQ1 soundLoSQ1), updating ADC soundByteSQ1+4 ; (soundHiSQ1 soundLoSQ1) to the result, and sending STA soundLoSQ1 ; it to the APU via (SQ1_HI SQ1_LO) STA SQ1_LO ; LDA soundHiSQ1 ; Note that bits 2 to 7 of the high byte are cleared so ADC soundByteSQ1+5 ; the length counter does not reload AND #%00000011 STA soundHiSQ1 STA SQ1_HI .mscz11 RTS ; Return from the subroutineName: MakeSoundOnSQ1 [Show more] Type: Subroutine Category: Sound Summary: Make the current sound effect on the SQ1 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 MakeSoundOnSQ1
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Configuration variable SQ1_HI = $4003
The APU period register (high byte) for square wave channel 1
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Configuration variable SQ1_LO = $4002
The APU period register (low byte) for square wave channel 1
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Configuration variable SQ1_VOL = $4000
The APU duty cycle and volume register for square wave channel 1
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Variable effectOnSQ1 in workspace WP
Records whether a sound effect is being made on the SQ1 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 mscz1 is local to this routine
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Label mscz10 is local to this routine
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Label mscz11 is local to this routine
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Label mscz2 is local to this routine
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Label mscz3 is local to this routine
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Label mscz4 is local to this routine
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Label mscz5 is local to this routine
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Label mscz6 is local to this routine
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Label mscz7 is local to this routine
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Label mscz8 is local to this routine
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Label mscz9 is local to this routine
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Variable soundByteSQ1 in workspace WP
The 14 sound bytes for the sound effect being made on channel SQ1
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Variable soundHiSQ1 in workspace WP
The value that we are going to send to the APU via SQ1_HI for the current sound effect
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Variable soundLoSQ1 in workspace WP
The value that we are going to send to the APU via SQ1_LO for the current sound effect
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Variable soundPitCountSQ1 in workspace WP
Controls how often we send pitch data to the APU for the sound effect on channel SQ1
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Variable soundPitchEnvSQ1 in workspace WP
Controls how often we apply the pitch envelope to the sound effect on channel SQ1
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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 soundVolCountSQ1 in workspace WP
Controls how often we apply the volume envelope to the sound effect on channel SQ1
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Variable soundVolIndexSQ1 in workspace WP
The index into the volume envelope data of the next volume byte to apply to the sound effect on channel SQ1
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Variable soundVolumeSQ1 in workspace WP
The address of the volume envelope data for the sound effect currently being made on channel SQ1