mirror of https://github.com/bsnes-emu/bsnes.git
94 lines
2.0 KiB
C++
94 lines
2.0 KiB
C++
unsigned APU::Noise::divider() const {
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if(divisor == 0) return 4;
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return divisor * 8;
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}
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void APU::Noise::run() {
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if(period && --period == 0) {
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period = divider() << frequency;
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if(frequency < 14) {
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bool bit = (lfsr ^ (lfsr >> 1)) & 1;
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lfsr = (lfsr >> 1) ^ (bit << (narrowlfsr ? 6 : 14));
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}
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}
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output = volume;
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if(enable == false || (lfsr & 1)) output = 0;
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}
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void APU::Noise::clocklength() {
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if(enable && counter) {
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if(++length == 0) enable = false;
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}
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}
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void APU::Noise::clockenvelope() {
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if(enable && envelope.frequency && --envelope.period == 0) {
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envelope.period = envelope.frequency;
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if(envelope.direction == 0 && volume > 0) volume--;
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if(envelope.direction == 1 && volume < 15) volume++;
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}
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}
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uint8 APU::Noise::read(unsigned addr) const {
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switch(addr) {
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case 1: return (length << 0);
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case 2: return (envelope.frequency << 0) | (envelope.direction << 3) | (envelope.volume << 4);
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case 3: return (divisor << 0) | (narrowlfsr << 3) | (frequency << 4);
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case 4: return (counter << 6) | (initialize << 7);
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}
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}
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void APU::Noise::write(unsigned addr, uint8 byte) {
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switch(addr) {
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case 1: //NR41
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length = byte >> 0;
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break;
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case 2: //NR42
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envelope.frequency = byte >> 0;
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envelope.direction = byte >> 3;
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envelope.volume = byte >> 4;
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if(envelope.dacenable() == false) enable = false;
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break;
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case 3: //NR43
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divisor = byte >> 0;
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narrowlfsr = byte >> 3;
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frequency = byte >> 4;
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period = divider() << frequency;
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break;
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case 4: //NR44
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counter = byte >> 6;
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initialize = byte >> 7;
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if(initialize) {
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enable = envelope.dacenable();
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lfsr = ~0u;
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envelope.period = envelope.frequency;
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volume = envelope.volume;
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}
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break;
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}
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}
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void APU::Noise::power() {
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envelope.frequency = 0;
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envelope.direction = 0;
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envelope.volume = 0;
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envelope.period = 0;
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length = 0;
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divisor = 0;
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narrowlfsr = 0;
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frequency = 0;
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counter = 0;
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initialize = 0;
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enable = 0;
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lfsr = 0;
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output = 0;
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period = 0;
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volume = 0;
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}
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