Fix value interactions.

Adapt all Commodore machines to 6502Mk2.

Analyser/Static/Acorn/Target.hpp

@@ -44,6 +44,7 @@ struct BBCMicroTarget: public ::Analyser::Static::Target, public Reflection::Str
 	bool has_1770dfs = false;
 	bool has_adfs = false;
 	bool has_sideways_ram = true;
+	bool has_beebsid = false;
 
 	ReflectableEnum(TubeProcessor, None, WDC65C02, Z80);
 	TubeProcessor tube_processor = TubeProcessor::None;
@@ -56,6 +57,7 @@ private:
 		DeclareField(has_1770dfs);
 		DeclareField(has_adfs);
 		DeclareField(has_sideways_ram);
+		DeclareField(has_beebsid);
 		AnnounceEnum(TubeProcessor);
 		DeclareField(tube_processor);
 	}

ClockReceiver/JustInTime.hpp

@@ -325,7 +325,7 @@ public:
 	/// Flushes all accumulated time.
 	inline void flush() {
 		if(!is_flushed_) {
-			task_queue_.flush();
+			task_queue_.lock_flush();
 			object_.run_for(time_since_update_.template flush<TargetTimeScale>());
 			is_flushed_ = true;
 		}

Components/6560/6560.cpp

@@ -12,10 +12,9 @@
 
 using namespace MOS::MOS6560;
 
-AudioGenerator::AudioGenerator(Concurrency::AsyncTaskQueue<false> &audio_queue) :
+AudioGenerator::AudioGenerator(Outputs::Speaker::TaskQueue &audio_queue) :
 	audio_queue_(audio_queue) {}
 
-
 void AudioGenerator::set_volume(const uint8_t volume) {
 	audio_queue_.enqueue([this, volume]() {
 		volume_ = int16_t(volume) * range_multiplier_;
@@ -148,7 +147,7 @@ template void AudioGenerator::apply_samples<Outputs::Speaker::Action::Store>(
 template void AudioGenerator::apply_samples<Outputs::Speaker::Action::Ignore>(
 	std::size_t, Outputs::Speaker::MonoSample *);
 
-void AudioGenerator::set_sample_volume_range(std::int16_t range) {
+void AudioGenerator::set_sample_volume_range(const std::int16_t range) {
 	range_multiplier_ = int16_t(range / 64);
 }
 

Components/6560/6560.hpp

@@ -19,7 +19,7 @@ namespace MOS::MOS6560 {
 // audio state
 class AudioGenerator: public Outputs::Speaker::BufferSource<AudioGenerator, false> {
 public:
-	AudioGenerator(Concurrency::AsyncTaskQueue<false> &audio_queue);
+	AudioGenerator(Outputs::Speaker::TaskQueue &audio_queue);
 
 	void set_volume(uint8_t);
 	void set_control(int channel, uint8_t value);
@@ -30,7 +30,7 @@ public:
 	void set_sample_volume_range(std::int16_t);
 
 private:
-	Concurrency::AsyncTaskQueue<false> &audio_queue_;
+	Outputs::Speaker::TaskQueue &audio_queue_;
 
 	unsigned int counters_[4] = {2, 1, 0, 0};	// create a slight phase offset for the three channels
 	unsigned int shift_registers_[4] = {0, 0, 0, 0};
@@ -64,8 +64,7 @@ public:
 	MOS6560(BusHandler &bus_handler) :
 			bus_handler_(bus_handler),
 			crt_(65*4, 1, Outputs::Display::Type::NTSC60, Outputs::Display::InputDataType::Luminance8Phase8),
-			audio_generator_(audio_queue_),
-			speaker_(audio_generator_)
+			audio_(Cycles(4))
 	{
 		// default to s-video output
 		crt_.set_display_type(Outputs::Display::DisplayType::SVideo);
@@ -75,11 +74,11 @@ public:
 	}
 
 	~MOS6560() {
-		audio_queue_.flush();
+		audio_.stop();
 	}
 
 	void set_clock_rate(const double clock_rate) {
-		speaker_.set_input_rate(float(clock_rate / 4.0));
+		audio_.speaker().set_input_rate(float(clock_rate / 4.0));
 	}
 
 	void set_scan_target(Outputs::Display::ScanTarget *const scan_target) {
@@ -95,11 +94,11 @@ public:
 		return crt_.get_display_type();
 	}
 	Outputs::Speaker::Speaker *get_speaker() {
-		return &speaker_;
+		return &audio_.speaker();
 	}
 
 	void set_high_frequency_cutoff(const float cutoff) {
-		speaker_.set_high_frequency_cutoff(cutoff);
+		audio_.speaker().set_high_frequency_cutoff(cutoff);
 	}
 
 	/*!
@@ -180,7 +179,7 @@ public:
 	*/
 	inline void run_for(const Cycles cycles) {
 		// keep track of the amount of time since the speaker was updated; lazy updates are applied
-		cycles_since_speaker_update_ += cycles;
+		audio_ += cycles;
 
 		auto number_of_cycles = cycles.as_integral();
 		while(number_of_cycles--) {
@@ -377,8 +376,7 @@ public:
 		Causes the 6560 to flush as much pending CRT and speaker communications as possible.
 	*/
 	inline void flush() {
-		update_audio();
-		audio_queue_.perform();
+		audio_.perform();
 	}
 
 	/*!
@@ -420,14 +418,12 @@ public:
 			case 0xb:
 			case 0xc:
 			case 0xd:
-				update_audio();
-				audio_generator_.set_control(address - 0xa, value);
+				audio_->set_control(address - 0xa, value);
 			break;
 
 			case 0xe:
-				update_audio();
 				registers_.auxiliary_colour = colours_[value >> 4];
-				audio_generator_.set_volume(value & 0xf);
+				audio_->set_volume(value & 0xf);
 			break;
 
 			case 0xf: {
@@ -467,14 +463,7 @@ private:
 	BusHandler &bus_handler_;
 	Outputs::CRT::CRT crt_;
 
-	Concurrency::AsyncTaskQueue<false> audio_queue_;
-	AudioGenerator audio_generator_;
-	Outputs::Speaker::PullLowpass<AudioGenerator> speaker_;
-
-	Cycles cycles_since_speaker_update_;
-	void update_audio() {
-		speaker_.run_for(audio_queue_, Cycles(cycles_since_speaker_update_.divide(Cycles(4))));
-	}
+	Outputs::Speaker::PullLowpassSpeakerQueue<Cycles, AudioGenerator> audio_;
 
 	// register state
 	struct {

Components/SID/SID.cpp

@@ -0,0 +1,433 @@
+//
+//  SID.cpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 07/11/2025.
+//  Copyright © 2025 Thomas Harte. All rights reserved.
+//
+
+#include "SID.hpp"
+
+// Sources used:
+//
+//	(1) SID Article v0.2 at https://github.com/ImreOlajos/SID-Article
+//	(2) Technical SID Information/Software stuff at http://www.sidmusic.org/sid/sidtech2.html
+//	(3) SID 6581/8580 (Sound Interface Device) reference at https://oxyron.de/html/registers_sid.html
+
+using namespace MOS::SID;
+
+SID::SID(Concurrency::AsyncTaskQueue<false> &audio_queue) :
+	audio_queue_(audio_queue),
+	output_filter_(
+		SignalProcessing::BiquadFilter::Type::LowPass,
+		1000000.0f,
+		15000.0f
+	) {}
+
+// MARK: - Programmer interface.
+
+void SID::write(const Numeric::SizedInt<5> address, const uint8_t value) {
+	last_write_ = value;
+	audio_queue_.enqueue([=, this] {
+		const auto voice = [&]() -> Voice & {
+			return voices_[address.get() / 7];
+		};
+		const auto oscillator = [&]() -> Voice::Oscillator & {
+			return voice().oscillator;
+		};
+		const auto adsr = [&]() -> Voice::ADSR & {
+			return voice().adsr;
+		};
+
+		switch(address.get()) {
+			case 0x00:	case 0x07:	case 0x0e:
+				oscillator().pitch = (oscillator().pitch & 0xff'00'00) | uint32_t(value << 8);
+			break;
+			case 0x01:	case 0x08:	case 0x0f:
+				oscillator().pitch = (oscillator().pitch & 0x00'ff'00) | uint32_t(value << 16);
+			break;
+			case 0x02:	case 0x09:	case 0x10:
+				oscillator().pulse_width = (oscillator().pitch & 0xf0'00'00'00) | uint32_t(value << 20);
+			break;
+			case 0x03:	case 0x0a:	case 0x11:
+				// The top bit of the phase counter is inverted; since it'll be compared directly with the
+				// pulse width, invert that bit too.
+				oscillator().pulse_width =
+					(
+						(oscillator().pitch & 0x0f'f0'00'00) |
+						uint32_t(value << 28)
+					);
+			break;
+			case 0x04:	case 0x0b:	case 0x12:
+				voice().set_control(value);
+			break;
+			case 0x05:	case 0x0c:	case 0x13:
+				adsr().attack = value >> 4;
+				adsr().decay = value;
+				adsr().set_phase(adsr().phase);
+			break;
+			case 0x06:	case 0x0d:	case 0x14:
+				adsr().sustain = (value >> 4) | (value & 0xf0);
+				adsr().release = value;
+				adsr().set_phase(adsr().phase);
+			break;
+
+			case 0x15:
+				filter_cutoff_.load<0, 3>(value);
+				update_filter();
+			break;
+			case 0x16:
+				filter_cutoff_.load<3>(value);
+				update_filter();
+			break;
+			case 0x17:
+				filter_channels_ = value;
+				filter_resonance_ = value >> 4;
+				update_filter();
+			break;
+			case 0x18:
+				volume_ = value & 0x0f;
+				filter_mode_ = value >> 4;
+				voice3_disable_ = value & 0x80;
+				update_filter();
+			break;
+		}
+	});
+}
+
+void SID::set_potentometer_input(const int index, const uint8_t value) {
+	potentometers_[index] = value;
+}
+
+void SID::update_filter() {
+	using Type = SignalProcessing::BiquadFilter::Type;
+	Type type = Type::AllPass;
+
+	switch(filter_mode_.get()) {
+		case 0:
+			filter_ = SignalProcessing::BiquadFilter();
+		return;
+
+		case 1:
+		case 3: type = Type::LowPass;	break;
+
+		case 2: type = Type::BandPass;	break;
+		case 5: type = Type::Notch;		break;
+
+		case 4:
+		case 6: type = Type::HighPass;	break;
+
+		case 7: type = Type::AllPass;	break;
+	}
+
+	filter_.configure(
+		type,
+		1'000'000.0f,
+		30.0f + float(filter_cutoff_.get()) * 5.8f,
+		0.707f + float(filter_resonance_.get()) * 0.2862f,
+		6.0f,
+		true
+	);
+
+	// Filter cutoff: the data sheet provides that it is linear, and "approximate Cutoff Frequency
+	// ranges between 30Hz and 12KHz [with recommended externally-supplied capacitors]."
+	//
+	// It's an 11-bit number, so the above is "approximate"ly right.
+
+	// Resonance: a complete from-thin-air guess. The data sheet says merely:
+	//
+	//	"There are 16 Resonance settings ranging from about 0.707 (Critical Damping) for a count of 0
+	//	to a maximum for a count of 15"
+	//
+	// i.e. no information is given on the maximum. I've taken it to be 5-ish per commentary on more general sites
+	// that 5 is a typical ceiling for the resonance factor.
+}
+
+uint8_t SID::read(const Numeric::SizedInt<5> address) {
+	switch(address.get()) {
+		default:	return last_write_;
+
+		case 0x19:	return potentometers_[0];
+		case 0x1a:	return potentometers_[1];
+
+		case 0x1b:
+		case 0x1c:
+			// Ensure all channels are entirely up to date.
+			audio_queue_.spin_flush();
+			return (address == 0x1c) ? voices_[2].adsr.envelope : uint8_t(voices_[2].output(voices_[1]) >> 4);
+	}
+}
+
+// MARK: - Oscillators.
+
+void Voice::Oscillator::reset_phase() {
+	phase = PhaseReload;
+}
+
+bool Voice::Oscillator::did_raise_b23() const {
+	return previous_phase > phase;
+}
+
+bool Voice::Oscillator::did_raise_b19() const {
+	static constexpr int NoiseBit = 1 << (19 + 8);
+	return (previous_phase ^ phase) & phase & NoiseBit;
+}
+
+uint16_t Voice::Oscillator::sawtooth_output() const {
+	return (phase >> 20) ^ 0x800;
+}
+
+// MARK: - Noise generator.
+
+uint16_t Voice::NoiseGenerator::output() const {
+	// Uses bits: 20, 18, 14, 11, 9, 5, 2 and 0, plus four more zero bits.
+	const uint16_t output =
+		((noise >> 9) & 0b1000'0000'0000) |		// b20 -> b11
+		((noise >> 8) & 0b0100'0000'0000) |		// b18 -> b10
+		((noise >> 5) & 0b0010'0000'0000) |		// b14 -> b9
+		((noise >> 3) & 0b0001'0000'0000) |		// b11 -> b8
+		((noise >> 2) & 0b0000'1000'0000) |		// b9 -> b7
+		((noise << 1) & 0b0000'0100'0000) |		// b5 -> b6
+		((noise << 3) & 0b0000'0010'0000) |		// b2 -> b5
+		((noise << 4) & 0b0000'0001'0000);		// b0 -> b4
+
+	assert(output <= Voice::MaxWaveformValue);
+	return output;
+}
+
+void Voice::NoiseGenerator::update(const bool test) {
+	noise =
+		(noise << 1) |
+		(((noise >> 17) ^ ((noise >> 22) | test)) & 1);
+}
+
+// MARK: - ADSR.
+
+void Voice::ADSR::set_phase(const Phase new_phase) {
+	static constexpr uint16_t rate_prescaler[] = {
+		9, 32, 63, 95, 149, 220, 267, 313, 392, 977, 1954, 3126, 3907, 11720, 19532, 31251
+	};
+	static_assert(sizeof(rate_prescaler) / sizeof(*rate_prescaler) == 16);
+
+	phase = new_phase;
+	switch(phase) {
+		case Phase::Attack:			rate_counter_target = rate_prescaler[attack.get()]; break;
+		case Phase::DecayAndHold:	rate_counter_target = rate_prescaler[decay.get()]; break;
+		case Phase::Release:		rate_counter_target = rate_prescaler[release.get()]; break;
+	}
+}
+
+// MARK: - Voices.
+
+void Voice::set_control(const uint8_t new_control) {
+	const bool old_gate = gate();
+	control = new_control;
+	if(gate() && !old_gate) {
+		adsr.set_phase(ADSR::Phase::Attack);
+	} else if(!gate() && old_gate) {
+		adsr.set_phase(ADSR::Phase::Release);
+	}
+}
+
+bool Voice::noise() const		{ return control.bit<7>(); }
+bool Voice::pulse() const		{ return control.bit<6>(); }
+bool Voice::sawtooth() const	{ return control.bit<5>(); }
+bool Voice::triangle() const	{ return control.bit<4>(); }
+bool Voice::test() const		{ return control.bit<3>(); }
+bool Voice::ring_mod() const	{ return control.bit<2>(); }
+bool Voice::sync() const		{ return control.bit<1>(); }
+bool Voice::gate() const		{ return control.bit<0>(); }
+
+void Voice::update() {
+	// Oscillator.
+	oscillator.previous_phase = oscillator.phase;
+	if(test()) {
+		oscillator.phase = 0;
+	} else {
+		oscillator.phase += oscillator.pitch;
+
+		if(oscillator.did_raise_b19()) {
+			noise_generator.update(test());
+		}
+	}
+
+	// ADSR.
+
+	// First prescalar, which is a function of the programmer-set rate.
+	++ adsr.rate_counter;
+	if(adsr.rate_counter == adsr.rate_counter_target) {
+		adsr.rate_counter = 0;
+
+		// Second prescalar, which approximates an exponential.
+		static constexpr uint8_t exponential_prescaler[] = {
+			1,														// 0
+			30, 30, 30, 30, 30, 30,									// 1–6
+			16, 16, 16, 16, 16, 16, 16, 16,							// 7–14
+			8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,						// 15–26
+			4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
+			4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,				// 27–54
+			2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
+			2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
+			2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,					// 55–94
+			1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+			1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+			1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+			1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+			1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+			1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+			1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+			1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+			1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+			1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+			1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+			1, 1, 1, 1, 1, 1, 1,
+		};
+		static_assert(sizeof(exponential_prescaler) == 256);
+		static_assert(exponential_prescaler[0] == 1);
+		static_assert(exponential_prescaler[1] == 30);
+		static_assert(exponential_prescaler[6] == 30);
+		static_assert(exponential_prescaler[7] == 16);
+		static_assert(exponential_prescaler[14] == 16);
+		static_assert(exponential_prescaler[15] == 8);
+		static_assert(exponential_prescaler[26] == 8);
+		static_assert(exponential_prescaler[27] == 4);
+		static_assert(exponential_prescaler[54] == 4);
+		static_assert(exponential_prescaler[55] == 2);
+		static_assert(exponential_prescaler[94] == 2);
+		static_assert(exponential_prescaler[95] == 1);
+		static_assert(exponential_prescaler[255] == 1);
+
+		if(adsr.phase == ADSR::Phase::Attack) {
+			++adsr.envelope;
+			// TODO: what really resets the exponential counter? If anything?
+			adsr.exponential_counter = 0;
+
+			if(adsr.envelope == 0xff) {
+				adsr.set_phase(ADSR::Phase::DecayAndHold);
+			}
+		} else {
+			++adsr.exponential_counter;
+			if(adsr.exponential_counter == exponential_prescaler[adsr.envelope]) {
+				adsr.exponential_counter = 0;
+
+				if(adsr.envelope && (adsr.envelope != adsr.sustain || adsr.phase != ADSR::Phase::DecayAndHold)) {
+					--adsr.envelope;
+				}
+			}
+		}
+	}
+}
+
+void Voice::synchronise(const Voice &prior) {
+	// Only oscillator work to do here.
+	if(
+		sync() &&
+		prior.oscillator.did_raise_b23()
+	) {
+		oscillator.phase = Oscillator::PhaseReload;
+	}
+}
+
+uint16_t Voice::pulse_output() const {
+	return (
+		(oscillator.phase ^ 0x8000'0000) < oscillator.pulse_width
+	) ? 0 : MaxWaveformValue;
+}
+
+uint16_t Voice::triangle_output(const Voice &prior) const {
+	const uint16_t sawtooth = oscillator.sawtooth_output();
+	const uint16_t xor_mask1 = sawtooth;
+	const uint16_t xor_mask2 = ring_mod() ? prior.sawtooth() : 0;
+	const uint16_t xor_mask = ((xor_mask1 ^ xor_mask2) & 0x800) ? 0xfff : 0x000;
+	return ((sawtooth << 1) ^ xor_mask) & 0xfff;
+}
+
+uint16_t Voice::output(const Voice &prior) const {
+	// TODO: true composite waves.
+	//
+	// My current understanding on this: if multiple waveforms are enabled, the pull to zero beats the
+	// pull to one on any line where the two compete. But the twist is that the lines are not necessarily
+	// one per bit since they lead to a common ground. Ummm, I think.
+	//
+	// Anyway, first pass: logical AND. It's not right. It will temporarily do.
+
+	uint16_t output = MaxWaveformValue;
+
+	if(pulse())		output &= pulse_output();
+	if(sawtooth())	output &= oscillator.sawtooth_output();
+	if(triangle())	output &= triangle_output(prior);
+	if(noise())		output &= noise_generator.output();
+
+	return (output * adsr.envelope) / 255;
+}
+
+// MARK: - Wave generation
+
+void SID::set_sample_volume_range(const std::int16_t range) {
+	range_ = range;
+}
+
+bool SID::is_zero_level() const {
+	return false;
+}
+
+template <Outputs::Speaker::Action action>
+void SID::apply_samples(const std::size_t number_of_samples, Outputs::Speaker::MonoSample *const target) {
+	for(std::size_t c = 0; c < number_of_samples; c++) {
+		// Advance phase.
+		voices_[0].update();
+		voices_[1].update();
+		voices_[2].update();
+
+		// Apply hard synchronisations.
+		voices_[0].synchronise(voices_[2]);
+		voices_[1].synchronise(voices_[0]);
+		voices_[2].synchronise(voices_[1]);
+
+		// Construct filtered and unfiltered output.
+		const uint16_t outputs[3] = {
+			voices_[0].output(voices_[2]),
+			voices_[1].output(voices_[0]),
+			voices_[2].output(voices_[1]),
+		};
+
+		const uint16_t direct_sample =
+			(filter_channels_.bit<0>() ? 0 : outputs[0]) +
+			(filter_channels_.bit<1>() ? 0 : outputs[1]) +
+			(filter_channels_.bit<2>() || voice3_disable_ ? 0 : outputs[2]);
+
+		const int16_t filtered_sample =
+			filter_.apply(
+				(filter_channels_.bit<0>() ? outputs[0] : 0) +
+				(filter_channels_.bit<1>() ? outputs[1] : 0) +
+				(filter_channels_.bit<2>() ? outputs[2] : 0)
+			);
+
+		// Sum, apply volume and output.
+		const auto sample = output_filter_.apply(int16_t(
+			(
+				volume_ * (
+					direct_sample +
+					filtered_sample
+						- 227	// DC offset.
+				)
+				- 88732
+			) / 3
+		));
+		// Maximum range of above: 15 * (4095 * 3 - 227) = [-3405, 180870]
+		// So subtracting 88732 will move to the centre of the range, and 3 is the smallest
+		// integer that avoids clipping.
+
+		Outputs::Speaker::apply<action>(
+			target[c],
+			Outputs::Speaker::MonoSample((sample * range_) >> 16)
+		);
+	}
+}
+
+template void SID::apply_samples<Outputs::Speaker::Action::Mix>(
+	std::size_t, Outputs::Speaker::MonoSample *);
+template void SID::apply_samples<Outputs::Speaker::Action::Store>(
+	std::size_t, Outputs::Speaker::MonoSample *);
+template void SID::apply_samples<Outputs::Speaker::Action::Ignore>(
+	std::size_t, Outputs::Speaker::MonoSample *);

Components/SID/SID.hpp

@@ -0,0 +1,129 @@
+//
+//  SID.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 07/11/2025.
+//  Copyright © 2025 Thomas Harte. All rights reserved.
+//
+
+#pragma once
+
+#include "Numeric/SizedInt.hpp"
+
+#include "Concurrency/AsyncTaskQueue.hpp"
+#include "Outputs/Speaker/Implementation/BufferSource.hpp"
+#include "SignalProcessing/BiquadFilter.hpp"
+
+namespace MOS::SID {
+
+struct Voice {
+	static constexpr uint16_t MaxWaveformValue = (1 << 12) - 1;
+
+	struct Oscillator {
+		// Programmer inputs.
+		uint32_t pitch = 0;
+		uint32_t pulse_width = 0;
+
+		// State.
+		//
+		// A real SID has a 24-bit phase counter and does various things when the top bit transitions from 0 to 1.
+		// This implementation maintains a 32-bit phase counter in which the low byte is unused and the top bit
+		// is inverted. That saves the cost of any masking and makes the 0 -> 1 transition test actually a 1 -> 0
+		// transition test, which can be phrased simply as after < before. Sadly overflow of signed integers is
+		// still undefined behaviour in C++ at the time of writing.
+		static constexpr uint32_t PhaseReload = 0x8000'0000;
+		uint32_t phase = PhaseReload;
+		uint32_t previous_phase = PhaseReload;
+
+		void reset_phase();
+		bool did_raise_b23() const;
+		bool did_raise_b19() const;
+		uint16_t sawtooth_output() const;
+	} oscillator;
+	struct ADSR {
+		// Programmer inputs.
+		Numeric::SizedInt<4> attack;
+		Numeric::SizedInt<4> decay;
+		Numeric::SizedInt<4> release;
+
+		Numeric::SizedInt<8> sustain;
+
+		// State.
+		enum class Phase {
+			Attack,
+			DecayAndHold,
+			Release,
+		} phase = Phase::Release;
+		Numeric::SizedInt<15> rate_counter;
+		Numeric::SizedInt<15> rate_counter_target;
+
+		uint8_t exponential_counter;
+		uint8_t envelope;
+
+		void set_phase(const Phase);
+	} adsr;
+	struct NoiseGenerator {
+		static constexpr uint32_t NoiseReload = 0x7'ffff;
+		uint32_t noise = NoiseReload;
+
+		uint16_t output() const;
+		void update(const bool test);
+	} noise_generator;
+
+	void set_control(const uint8_t);
+	void update();
+	void synchronise(const Voice &prior);
+	uint16_t output(const Voice &prior) const;
+
+private:
+	Numeric::SizedInt<8> control;
+	bool noise() const;
+	bool pulse() const;
+	bool sawtooth() const;
+	bool triangle() const;
+	bool test() const;
+	bool ring_mod() const;
+	bool sync() const;
+	bool gate() const;
+
+	uint16_t pulse_output() const;
+	uint16_t triangle_output(const Voice &prior) const;
+};
+
+class SID: public Outputs::Speaker::BufferSource<SID, false> {
+public:
+	SID(Concurrency::AsyncTaskQueue<false> &audio_queue);
+
+	void write(Numeric::SizedInt<5> address, uint8_t value);
+	uint8_t read(Numeric::SizedInt<5> address);
+	void set_potentometer_input(int index, uint8_t value);
+
+	// Outputs::Speaker::BufferSource.
+	template <Outputs::Speaker::Action action>
+		void apply_samples(std::size_t, Outputs::Speaker::MonoSample *);
+	bool is_zero_level() const;
+	void set_sample_volume_range(std::int16_t);
+
+private:
+	Concurrency::AsyncTaskQueue<false> &audio_queue_;
+	Voice voices_[3];
+
+	uint8_t last_write_;
+
+	int16_t range_ = 0;
+	uint8_t volume_ = 0;
+
+	SignalProcessing::BiquadFilter filter_;
+	Numeric::SizedInt<11> filter_cutoff_;
+	Numeric::SizedInt<4> filter_resonance_;
+	Numeric::SizedInt<4> filter_channels_;
+	Numeric::SizedInt<3> filter_mode_;
+	bool voice3_disable_;
+	void update_filter();
+
+	SignalProcessing::BiquadFilter output_filter_;
+
+	uint8_t potentometers_[2]{};
+};
+
+}

Concurrency/AsyncTaskQueue.hpp

@@ -40,9 +40,12 @@ private:
 /// An implementation detail; provides a no-op implementation of time advances for TaskQueues without a Performer.
 template <> struct TaskQueueStorage<void> {
 	TaskQueueStorage() {}
+protected:
+	void update() {}
+};
 
-	protected:
-		void update() {}
+struct EnqueueDelegate {
+	virtual std::function<void(void)> prepare_enqueue() = 0;
 };
 
 /*!
@@ -67,10 +70,15 @@ template <> struct TaskQueueStorage<void> {
 template <
 	bool perform_automatically,
 	bool start_immediately = true,
+	bool use_enqueue_delegate = false,
 	typename Performer = void
 >
 class AsyncTaskQueue: public TaskQueueStorage<Performer> {
 public:
+	void set_enqueue_delegate(EnqueueDelegate *const delegate) {
+		enqueue_delegate_ = delegate;
+	}
+
 	template <typename... Args> AsyncTaskQueue(Args&&... args) :
 		TaskQueueStorage<Performer>(std::forward<Args>(args)...) {
 		if constexpr (start_immediately) {
@@ -90,6 +98,9 @@ public:
 	/// to 'now'.
 	void enqueue(const std::function<void(void)> &post_action) {
 		const std::lock_guard guard(condition_mutex_);
+		if constexpr (use_enqueue_delegate) {
+			actions_.push_back(enqueue_delegate_->prepare_enqueue());
+		}
 		actions_.push_back(post_action);
 
 		if constexpr (perform_automatically) {
@@ -97,6 +108,12 @@ public:
 		}
 	}
 
+	/// @returns The number of items currently enqueued.
+	size_t size() {
+		const std::lock_guard guard(condition_mutex_);
+		return actions_.size();
+	}
+
 	/// Causes any enqueued actions that are not yet scheduled to be scheduled.
 	void perform() {
 		static_assert(!perform_automatically);
@@ -131,7 +148,7 @@ public:
 
 	/// Schedules any remaining unscheduled work, then blocks synchronously
 	/// until all scheduled work has been performed.
-	void flush() {
+	void lock_flush() {
 		std::mutex flush_mutex;
 		std::condition_variable flush_condition;
 		bool has_run = false;
@@ -150,6 +167,23 @@ public:
 		flush_condition.wait(lock, [&has_run] { return has_run; });
 	}
 
+	/// Schedules any remaining unscheduled work, then spins
+	/// until all scheduled work has been performed, placing a memory barrier
+	/// in between.
+	void spin_flush() {
+		std::atomic<bool> has_run = false;
+
+		enqueue([&has_run] () {
+			has_run.store(true, std::memory_order::release);
+		});
+
+		if constexpr (!perform_automatically) {
+			perform();
+		}
+
+		while(!has_run.load(std::memory_order::acquire));
+	}
+
 	~AsyncTaskQueue() {
 		stop();
 	}
@@ -183,6 +217,8 @@ private:
 		};
 	}
 
+	EnqueueDelegate *enqueue_delegate_ = nullptr;
+
 	// The list of actions waiting be performed. These will be elided,
 	// increasing their latency, if the emulation thread falls behind.
 	using ActionVector = std::vector<std::function<void(void)>>;

Machines/Acorn/BBCMicro/BBCMicro.cpp

@@ -24,6 +24,7 @@
 #include "Components/6522/6522.hpp"
 #include "Components/6845/CRTC6845.hpp"
 #include "Components/6850/6850.hpp"
+#include "Components/SID/SID.hpp"
 #include "Components/SAA5050/SAA5050.hpp"
 #include "Components/SN76489/SN76489.hpp"
 #include "Components/uPD7002/uPD7002.hpp"
@@ -35,6 +36,7 @@
 #include "Outputs/Log.hpp"
 
 #include "Outputs/CRT/CRT.hpp"
+#include "Outputs/Speaker/Implementation/CompoundSource.hpp"
 #include "Outputs/Speaker/Implementation/LowpassSpeaker.hpp"
 #include "Concurrency/AsyncTaskQueue.hpp"
 
@@ -107,22 +109,48 @@ private:
 /*!
 	Combines an SN76489 with an appropriate asynchronous queue and filtering speaker.
 */
+
+// TODO: generalise the below and clean up across the project.
+template <bool has_beebsid>
 struct Audio {
+private:
+	using CompoundSource = Outputs::Speaker::CompoundSource<TI::SN76489, MOS::SID::SID>;
+	using Source = std::conditional_t<has_beebsid, CompoundSource, TI::SN76489>;
+	using Speaker = Outputs::Speaker::PullLowpass<Source>;
+
+	Source &speaker_source() {
+		if constexpr (has_beebsid) {
+			return compound_;
+		} else {
+			return sn76489_;
+		}
+	}
+
+public:
 	Audio() :
-		sn76489_(TI::SN76489::Personality::SN76489, audio_queue_, 2),
-		speaker_(sn76489_)
+		sn76489_(TI::SN76489::Personality::SN76489, audio_queue_, 4),
+		sid_(audio_queue_),
+		compound_(sn76489_, sid_),
+		speaker_(speaker_source())
 	{
-		// Combined with the additional divider specified above, implies this chip is clocked at 4Mhz.
-		speaker_.set_input_rate(2'000'000.0f);
+		// Combined with the additional divider specified above, implies the SN76489 is clocked at 4Mhz.
+		speaker_.set_input_rate(1'000'000.0f);
 	}
 
 	~Audio() {
-		audio_queue_.flush();
+		audio_queue_.lock_flush();
 	}
 
-	TI::SN76489 *operator ->() {
-		speaker_.run_for(audio_queue_, time_since_update_.flush<Cycles>());
-		return &sn76489_;
+	template <typename TargetT>
+	TargetT &get() {
+		post_time();
+
+		if constexpr (std::is_same_v<TargetT, TI::SN76489>) {
+			return sn76489_;
+		}
+		if constexpr (std::is_same_v<TargetT, MOS::SID::SID>) {
+			return sid_;
+		}
 	}
 
 	void operator +=(const Cycles duration) {
@@ -130,7 +158,7 @@ struct Audio {
 	}
 
 	void flush() {
-		speaker_.run_for(audio_queue_, time_since_update_.flush<Cycles>());
+		post_time();
 		audio_queue_.perform();
 	}
 
@@ -138,10 +166,20 @@ struct Audio {
 		return &speaker_;
 	}
 
+	size_t queue_size() {
+		return audio_queue_.size();
+	}
+
 private:
+	void post_time() {
+		speaker_.run_for(audio_queue_, time_since_update_.divide(Cycles(2)));
+	}
+
 	Concurrency::AsyncTaskQueue<false> audio_queue_;
 	TI::SN76489 sn76489_;
-	Outputs::Speaker::PullLowpass<TI::SN76489> speaker_;
+	MOS::SID::SID sid_;
+	CompoundSource compound_;
+	Outputs::Speaker::PullLowpass<Source> speaker_;
 	Cycles time_since_update_;
 };
 
@@ -174,19 +212,20 @@ protected:
 /*!
 	Models the system VIA, which connects to the SN76489 and the keyboard.
 */
-struct SystemVIAPortHandler;
-using SystemVIA = MOS::MOS6522::MOS6522<SystemVIAPortHandler>;
-
-struct SystemVIAPortHandler: public MOS::MOS6522::IRQDelegatePortHandler {
-	struct Delegate {
-		virtual void strobe_lightpen() = 0;
-	};
+struct VSyncReceiver {
+	virtual void set_vsync(bool) = 0;
+};
+struct SystemVIADelegate {
+	virtual void strobe_lightpen() = 0;
+};
 
+template <typename AudioT>
+struct SystemVIAPortHandler: public MOS::MOS6522::IRQDelegatePortHandler, public VSyncReceiver {
 	SystemVIAPortHandler(
-		Audio &audio,
+		AudioT &audio,
 		VideoBaseAddress &video_base,
-		SystemVIA &via,
-		Delegate &delegate,
+		MOS::MOS6522::MOS6522<SystemVIAPortHandler<AudioT>> &via,
+		SystemVIADelegate &delegate,
 		const std::vector<std::unique_ptr<Inputs::Joystick>> &joysticks,
 		const bool run_disk
 	) :
@@ -221,7 +260,7 @@ struct SystemVIAPortHandler: public MOS::MOS6522::IRQDelegatePortHandler {
 
 		// Check for a strobe on the audio output.
 		if((old_latch^latch_) & old_latch & LatchFlags::WriteToSN76489) {
-			audio_->write(port_a_output_);
+			audio_.template get<TI::SN76489>().write(port_a_output_);
 		}
 
 		// Pass on the video wraparound/base.
@@ -335,10 +374,10 @@ private:
 	uint8_t port_a_output_ = 0;
 	bool previous_cb2_ = false;
 
-	Audio &audio_;
+	AudioT &audio_;
 	VideoBaseAddress &video_base_;
 
-	SystemVIA &via_;
+	MOS::MOS6522::MOS6522<SystemVIAPortHandler<AudioT>> &via_;
 
 	// MARK: - Keyboard state and helpers.
 
@@ -369,7 +408,7 @@ private:
 				}
 		} ()).to_ulong() & 0xfe;	// Discard the first row.
 
-		via_.set_control_line_input<MOS::MOS6522::Port::A, MOS::MOS6522::Line::Two>(state);
+		via_.template set_control_line_input<MOS::MOS6522::Port::A, MOS::MOS6522::Line::Two>(state);
 	}
 
 	static inline const std::string caps_led = "CAPS";
@@ -379,7 +418,11 @@ private:
 	Activity::Observer *activity_observer_ = nullptr;
 
 	const std::vector<std::unique_ptr<Inputs::Joystick>> &joysticks_;
-	Delegate &delegate_;
+	SystemVIADelegate &delegate_;
+
+	void set_vsync(const bool vsync) override {
+		via_.template set_control_line_input<MOS::MOS6522::Port::A, MOS::MOS6522::Line::One>(vsync);
+	}
 };
 
 // MARK: - CRTC output.
@@ -389,10 +432,10 @@ private:
 */
 class CRTCBusHandler: public VideoBaseAddress {
 public:
-	CRTCBusHandler(const uint8_t *const ram, SystemVIA &system_via) :
+	CRTCBusHandler(const uint8_t *const ram, VSyncReceiver &vsync_receiver) :
 		crt_(1024, 1, Outputs::Display::Type::PAL50, Outputs::Display::InputDataType::Red1Green1Blue1),
 		ram_(ram),
-		system_via_(system_via)
+		vsync_receiver_(vsync_receiver)
 	{}
 
 	void set_dynamic_framing(const bool enable) {
@@ -440,7 +483,10 @@ public:
 		static_assert(!(PixelAllocationUnit % 16));
 		static_assert(!(PixelAllocationUnit % 12));
 
-		system_via_.set_control_line_input<MOS::MOS6522::Port::A, MOS::MOS6522::Line::One>(state.vsync);
+		if(state.vsync != vsync_) {
+			vsync_receiver_.set_vsync(state.vsync);
+			vsync_ = state.vsync;
+		}
 
 		// Count cycles since horizontal sync to insert a colour burst.
 		// TODO: this is copy/pasted from the CPC. How does the BBC do it?
@@ -675,7 +721,8 @@ private:
 	}
 
 	const uint8_t *const ram_ = nullptr;
-	SystemVIA &system_via_;
+	VSyncReceiver &vsync_receiver_;
+	bool vsync_ = false;
 
 	Mullard::SAA5050Serialiser saa5050_serialiser_;
 };
@@ -705,7 +752,7 @@ struct Tube<HostT, TubeProcessor::None> {
 
 // MARK: - ConcreteMachine.
 
-template <TubeProcessor tube_processor, bool has_1770>
+template <TubeProcessor tube_processor, bool has_1770, bool has_beebsid>
 class ConcreteMachine:
 	public Activity::Source,
 	public Configurable::Device,
@@ -718,7 +765,7 @@ class ConcreteMachine:
 	public MachineTypes::TimedMachine,
 	public MOS::MOS6522::IRQDelegatePortHandler::Delegate,
 	public NEC::uPD7002::Delegate,
-	public SystemVIAPortHandler::Delegate,
+	public SystemVIADelegate,
 	public Utility::TypeRecipient<CharacterMapper>,
 	public WD::WD1770::Delegate
 {
@@ -731,7 +778,7 @@ public:
 		system_via_port_handler_(audio_, crtc_bus_handler_, system_via_, *this, joysticks_, target.should_shift_restart),
 		user_via_(user_via_port_handler_),
 		system_via_(system_via_port_handler_),
-		crtc_bus_handler_(ram_.data(), system_via_),
+		crtc_bus_handler_(ram_.data(), system_via_port_handler_),
 		crtc_(crtc_bus_handler_),
 		acia_(HalfCycles(2'000'000)), // TODO: look up real ACIA clock rate.
 		adc_(HalfCycles(2'000'000)),
@@ -990,6 +1037,12 @@ public:
 						}
 					break;
 				}
+			} else if(has_beebsid && address >= 0xfc20 && address < 0xfc40) {
+				if constexpr (is_read(operation)) {
+					value = audio_.template get<MOS::SID::SID>().read(+address);
+				} else {
+					audio_.template get<MOS::SID::SID>().write(+address, value);
+				}
 			} else {
 				Logger::error()
 					.append("Unhandled IO %s at %04x", is_read(operation) ? "read" : "write", address)
@@ -1045,8 +1098,7 @@ private:
 		return crtc_bus_handler_.get_scaled_scan_status();
 	}
 
-
-	// MARK: - SystemVIAPortHandler::Delegate.
+	// MARK: - SystemVIADelegate.
 	void strobe_lightpen() override {
 		crtc_.trigger_light_pen();
 	}
@@ -1129,7 +1181,10 @@ private:
 	}
 
 	void flush_output(const int outputs) final {
-		if(outputs & Output::Audio) {
+		// TODO: I think there's an infrastructural bug here on macOS; if the audio output has stalled out,
+		// the outer wrapper won't ask for an audio flush, which means the queue will never try to start,
+		// and the audio queue will just fill indefinitely. Could this be the mythical 'leak'?
+		if(outputs & Output::Audio || audio_.queue_size() > 200) {
 			audio_.flush();
 		}
 	}
@@ -1141,7 +1196,7 @@ private:
 
 	// MARK: - uPD7002::Delegate.
 	void did_change_interrupt_status(NEC::uPD7002 &) override {
-		system_via_.set_control_line_input<MOS::MOS6522::Port::B, MOS::MOS6522::Line::One>(adc_.interrupt());
+		system_via_.template set_control_line_input<MOS::MOS6522::Port::B, MOS::MOS6522::Line::One>(adc_.interrupt());
 	}
 
 	// MARK: - MediaTarget.
@@ -1200,9 +1255,9 @@ private:
 	CPU::MOS6502Mk2::Processor<CPU::MOS6502Mk2::Model::M6502, M6502Traits> m6502_;
 
 	UserVIAPortHandler user_via_port_handler_;
-	SystemVIAPortHandler system_via_port_handler_;
+	SystemVIAPortHandler<Audio<has_beebsid>> system_via_port_handler_;
 	UserVIA user_via_;
-	SystemVIA system_via_;
+	MOS::MOS6522::MOS6522<SystemVIAPortHandler<Audio<has_beebsid>>> system_via_;
 
 	void update_irq_line() {
 		const bool tube_irq =
@@ -1221,7 +1276,7 @@ private:
 		);
 	}
 
-	Audio audio_;
+	Audio<has_beebsid> audio_;
 
 	CRTCBusHandler crtc_bus_handler_;
 	CRTC crtc_;
@@ -1275,12 +1330,21 @@ using namespace BBCMicro;
 namespace {
 using Target = Analyser::Static::Acorn::BBCMicroTarget;
 
+template <Target::TubeProcessor processor, bool has_1770>
+std::unique_ptr<Machine> machine(const Target &target, const ROMMachine::ROMFetcher &rom_fetcher) {
+	if(target.has_beebsid) {
+		return std::make_unique<BBCMicro::ConcreteMachine<processor, has_1770, true>>(target, rom_fetcher);
+	} else {
+		return std::make_unique<BBCMicro::ConcreteMachine<processor, has_1770, false>>(target, rom_fetcher);
+	}
+}
+
 template <Target::TubeProcessor processor>
 std::unique_ptr<Machine> machine(const Target &target, const ROMMachine::ROMFetcher &rom_fetcher) {
 	if(target.has_1770dfs || target.has_adfs) {
-		return std::make_unique<BBCMicro::ConcreteMachine<processor, true>>(target, rom_fetcher);
+		return machine<processor, true>(target, rom_fetcher);
 	} else {
-		return std::make_unique<BBCMicro::ConcreteMachine<processor, false>>(target, rom_fetcher);
+		return machine<processor, false>(target, rom_fetcher);
 	}
 }
 }

Machines/Acorn/Electron/Electron.cpp

@@ -15,7 +15,6 @@
 #include "ClockReceiver/ClockReceiver.hpp"
 #include "ClockReceiver/ForceInline.hpp"
 #include "Configurable/StandardOptions.hpp"
-#include "Outputs/Speaker/Implementation/LowpassSpeaker.hpp"
 #include "Processors/6502/6502.hpp"
 
 #include "Storage/MassStorage/SCSI/SCSI.hpp"
@@ -27,6 +26,8 @@
 
 #include "ClockReceiver/JustInTime.hpp"
 
+#include "Outputs/Speaker/Implementation/LowpassSpeaker.hpp"
+
 #include "Interrupts.hpp"
 #include "Keyboard.hpp"
 #include "Plus3.hpp"
@@ -58,8 +59,11 @@ public:
 			hard_drive_(scsi_bus_, 0),
 			scsi_device_(scsi_bus_.add_device()),
 			video_(ram_),
-			sound_generator_(audio_queue_),
-			speaker_(sound_generator_) {
+			audio_(
+				2000000.0 / SoundGenerator::clock_rate_divider,
+				SoundGenerator::clock_rate_divider,
+				6000.0f
+			) {
 		memset(key_states_, 0, sizeof(key_states_));
 		for(int c = 0; c < 16; c++)
 			memset(roms_[c], 0xff, 16384);
@@ -67,9 +71,6 @@ public:
 		tape_.set_delegate(this);
 		set_clock_rate(2000000);
 
-		speaker_.set_input_rate(2000000 / SoundGenerator::clock_rate_divider);
-		speaker_.set_high_frequency_cutoff(6000);
-
 		::ROM::Request request = ::ROM::Request(::ROM::Name::AcornBASICII) && ::ROM::Request(::ROM::Name::AcornElectronMOS100);
 		if(target.has_pres_adfs) {
 			request = request && ::ROM::Request(::ROM::Name::PRESADFSSlot1) && ::ROM::Request(::ROM::Name::PRESADFSSlot2);
@@ -143,7 +144,7 @@ public:
 	}
 
 	~ConcreteMachine() {
-		audio_queue_.flush();
+		audio_.stop();
 	}
 
 	void set_key_state(uint16_t key, bool isPressed) final {
@@ -234,8 +235,7 @@ public:
 		const auto [cycles, video_interrupts] = run_for_access(address);
 		signal_interrupt(video_interrupts);
 
-		cycles_since_audio_update_ += cycles;
-		if(cycles_since_audio_update_ > Cycles(16384)) update_audio();
+		audio_ += cycles;
 		tape_.run_for(cycles);
 
 		if(typer_) typer_->run_for(cycles);
@@ -278,8 +278,7 @@ public:
 						// update speaker mode
 						bool new_speaker_is_enabled = (*value & 6) == 2;
 						if(new_speaker_is_enabled != speaker_is_enabled_) {
-							update_audio();
-							sound_generator_.set_is_enabled(new_speaker_is_enabled);
+							audio_->set_is_enabled(new_speaker_is_enabled);
 							speaker_is_enabled_ = new_speaker_is_enabled;
 						}
 
@@ -340,8 +339,7 @@ public:
 				break;
 				case 0xfe06:
 					if(!is_read(operation)) {
-						update_audio();
-						sound_generator_.set_divider(*value);
+						audio_->set_divider(*value);
 						tape_.set_counter(*value);
 					}
 				break;
@@ -510,8 +508,7 @@ public:
 
 	void flush_output(int outputs) final {
 		if(outputs & Output::Audio) {
-			update_audio();
-			audio_queue_.perform();
+			audio_.perform();
 		}
 	}
 
@@ -532,7 +529,7 @@ public:
 	}
 
 	Outputs::Speaker::Speaker *get_speaker() final {
-		return &speaker_;
+		return &audio_.speaker();
 	}
 
 	void run_for(const Cycles cycles) final {
@@ -682,10 +679,6 @@ private:
 	}
 
 	// MARK: - Work deferral updates.
-	inline void update_audio() {
-		speaker_.run_for(audio_queue_, cycles_since_audio_update_.divide(Cycles(SoundGenerator::clock_rate_divider)));
-	}
-
 	inline void signal_interrupt(uint8_t interrupt) {
 		if(!interrupt) {
 			return;
@@ -732,9 +725,6 @@ private:
 	uint8_t key_states_[14];
 	Electron::KeyboardMapper keyboard_mapper_;
 
-	// Counters related to simultaneous subsystems
-	Cycles cycles_since_audio_update_ = 0;
-
 	// Tape
 	Tape tape_;
 	bool use_fast_tape_hack_ = false;
@@ -770,10 +760,7 @@ private:
 
 	// Outputs
 	VideoOutput video_;
-
-	Concurrency::AsyncTaskQueue<false> audio_queue_;
-	SoundGenerator sound_generator_;
-	Outputs::Speaker::PullLowpass<SoundGenerator> speaker_;
+	Outputs::Speaker::PullLowpassSpeakerQueue<Cycles, SoundGenerator> audio_;
 
 	bool speaker_is_enabled_ = false;
 

Machines/Acorn/Electron/SoundGenerator.cpp

@@ -12,7 +12,7 @@
 
 using namespace Electron;
 
-SoundGenerator::SoundGenerator(Concurrency::AsyncTaskQueue<false> &audio_queue) :
+SoundGenerator::SoundGenerator(Outputs::Speaker::TaskQueue &audio_queue) :
 	audio_queue_(audio_queue) {}
 
 void SoundGenerator::set_sample_volume_range(std::int16_t range) {
@@ -40,13 +40,13 @@ template void SoundGenerator::apply_samples<Outputs::Speaker::Action::Mix>(std::
 template void SoundGenerator::apply_samples<Outputs::Speaker::Action::Store>(std::size_t, Outputs::Speaker::MonoSample *);
 template void SoundGenerator::apply_samples<Outputs::Speaker::Action::Ignore>(std::size_t, Outputs::Speaker::MonoSample *);
 
-void SoundGenerator::set_divider(uint8_t divider) {
+void SoundGenerator::set_divider(const uint8_t divider) {
 	audio_queue_.enqueue([this, divider]() {
 		divider_ = divider * 32 / clock_rate_divider;
 	});
 }
 
-void SoundGenerator::set_is_enabled(bool is_enabled) {
+void SoundGenerator::set_is_enabled(const bool is_enabled) {
 	audio_queue_.enqueue([this, is_enabled]() {
 		is_enabled_ = is_enabled;
 		counter_ = 0;

Machines/Acorn/Electron/SoundGenerator.hpp

@@ -9,13 +9,13 @@
 #pragma once
 
 #include "Outputs/Speaker/Implementation/BufferSource.hpp"
-#include "Concurrency/AsyncTaskQueue.hpp"
+#include "Outputs/Speaker/SpeakerQueue.hpp"
 
 namespace Electron {
 
 class SoundGenerator: public ::Outputs::Speaker::BufferSource<SoundGenerator, false> {
 public:
-	SoundGenerator(Concurrency::AsyncTaskQueue<false> &);
+	SoundGenerator(Outputs::Speaker::TaskQueue &);
 
 	void set_divider(uint8_t);
 	void set_is_enabled(bool);
@@ -28,7 +28,7 @@ public:
 	void set_sample_volume_range(std::int16_t range);
 
 private:
-	Concurrency::AsyncTaskQueue<false> &audio_queue_;
+	Outputs::Speaker::TaskQueue &audio_queue_;
 	unsigned int counter_ = 0;
 	unsigned int divider_ = 0;
 	bool is_enabled_ = false;

Machines/AmstradCPC/AmstradCPC.cpp

@@ -127,7 +127,7 @@ public:
 	}
 
 	~AYDeferrer() {
-		audio_queue_.flush();
+		audio_queue_.lock_flush();
 	}
 
 	/// Adds @c half_cycles half cycles to the amount of time that has passed.

Machines/Apple/AppleII/AppleII.cpp

@@ -678,7 +678,7 @@ public:
 	}
 
 	~ConcreteMachine() {
-		audio_queue_.flush();
+		audio_queue_.lock_flush();
 	}
 
 	void set_scan_target(Outputs::Display::ScanTarget *scan_target) final {

Machines/Apple/AppleIIgs/AppleIIgs.cpp

@@ -210,7 +210,7 @@ public:
 	}
 
 	~ConcreteMachine() {
-		audio_queue_.flush();
+		audio_queue_.lock_flush();
 	}
 
 	void run_for(const Cycles cycles) override {

Machines/Apple/Macintosh/Macintosh.cpp

@@ -172,7 +172,7 @@ public:
 	}
 
 	~ConcreteMachine() {
-		audio_.queue.flush();
+		audio_.queue.lock_flush();
 	}
 
 	void set_scan_target(Outputs::Display::ScanTarget *scan_target) final {

Machines/Atari/2600/Atari2600.cpp

@@ -159,7 +159,7 @@ public:
 
 	// to satisfy CRTMachine::Machine
 	void set_scan_target(Outputs::Display::ScanTarget *scan_target) final {
-		bus_->speaker_.set_input_rate(float(get_clock_rate() / double(CPUTicksPerAudioTick)));
+		bus_->audio_.speaker().set_input_rate(float(get_clock_rate() / double(CPUTicksPerAudioTick)));
 		bus_->tia_.set_crt_delegate(&frequency_mismatch_warner_);
 		bus_->tia_.set_scan_target(scan_target);
 	}
@@ -169,7 +169,7 @@ public:
 	}
 
 	Outputs::Speaker::Speaker *get_speaker() final {
-		return &bus_->speaker_;
+		return &bus_->audio_.speaker();
 	}
 
 	void run_for(const Cycles cycles) final {
@@ -206,11 +206,11 @@ private:
 	// a confidence counter
 	Analyser::Dynamic::ConfidenceCounter confidence_counter_;
 
-	void set_is_ntsc(bool is_ntsc) {
+	void set_is_ntsc(const bool is_ntsc) {
 		bus_->tia_.set_output_mode(is_ntsc ? TIA::OutputMode::NTSC : TIA::OutputMode::PAL);
 		const double clock_rate = is_ntsc ? NTSC_clock_rate : PAL_clock_rate;
-		bus_->speaker_.set_input_rate(float(clock_rate) / float(CPUTicksPerAudioTick));
-		bus_->speaker_.set_high_frequency_cutoff(float(clock_rate) / float(CPUTicksPerAudioTick * 2));
+		bus_->audio_.speaker().set_input_rate(float(clock_rate) / float(CPUTicksPerAudioTick));
+		bus_->audio_.speaker().set_high_frequency_cutoff(float(clock_rate) / float(CPUTicksPerAudioTick * 2));
 		set_clock_rate(clock_rate);
 	}
 };

Machines/Atari/2600/Bus.hpp

@@ -21,12 +21,10 @@ namespace Atari2600 {
 
 class Bus {
 public:
-	Bus() :
-		tia_sound_(audio_queue_),
-		speaker_(tia_sound_) {}
+	Bus() : audio_(Cycles(CPUTicksPerAudioTick * 3)) {}
 
 	virtual ~Bus() {
-		audio_queue_.flush();
+		audio_.stop();
 	}
 
 	virtual void run_for(const Cycles cycles) = 0;
@@ -34,31 +32,22 @@ public:
 	virtual void set_reset_line(bool state) = 0;
 	virtual void flush() = 0;
 
-	// the RIOT, TIA and speaker
+	// The RIOT, TIA and speaker.
 	PIA mos6532_;
 	TIA tia_;
+	Outputs::Speaker::PullLowpassSpeakerQueue<Cycles, TIASound> audio_;
 
-	Concurrency::AsyncTaskQueue<false> audio_queue_;
-	TIASound tia_sound_;
-	Outputs::Speaker::PullLowpass<TIASound> speaker_;
-
-	// joystick state
+	// Joystick state.
 	uint8_t tia_input_value_[2] = {0xff, 0xff};
 
 protected:
-	// speaker backlog accumlation counter
-	Cycles cycles_since_speaker_update_;
-	inline void update_audio() {
-		speaker_.run_for(audio_queue_, cycles_since_speaker_update_.divide(Cycles(CPUTicksPerAudioTick * 3)));
-	}
-
-	// video backlog accumulation counter
+	// Video backlog accumulation counter.
 	Cycles cycles_since_video_update_;
 	inline void update_video() {
 		tia_.run_for(cycles_since_video_update_.flush<Cycles>());
 	}
 
-	// RIOT backlog accumulation counter
+	// RIOT backlog accumulation counter.
 	Cycles cycles_since_6532_update_;
 	inline void update_6532() {
 		mos6532_.run_for(cycles_since_6532_update_.flush<Cycles>());

Machines/Atari/2600/Cartridges/Cartridge.hpp

@@ -70,10 +70,11 @@ public:
 		// leap to the end of ready only once ready is signalled because on a 6502 ready doesn't take
 		// effect until the next read; therefore it isn't safe to assume that signalling ready immediately
 		// skips to the end of the line.
-		if(operation == CPU::MOS6502::BusOperation::Ready)
+		if(operation == CPU::MOS6502::BusOperation::Ready) {
 			cycles_run_for = tia_.get_cycles_until_horizontal_blank(cycles_since_video_update_);
+		}
 
-		cycles_since_speaker_update_ += Cycles(cycles_run_for);
+		audio_ += Cycles(cycles_run_for);
 		cycles_since_video_update_ += Cycles(cycles_run_for);
 		cycles_since_6532_update_ += Cycles(cycles_run_for / 3);
 		bus_extender_.advance_cycles(cycles_run_for / 3);
@@ -171,11 +172,11 @@ public:
 						case 0x2c:	update_video(); tia_.clear_collision_flags();										break;
 
 						case 0x15:
-						case 0x16:	update_audio(); tia_sound_.set_control(decodedAddress - 0x15, *value);				break;
+						case 0x16:	audio_->set_control(decodedAddress - 0x15, *value);			break;
 						case 0x17:
-						case 0x18:	update_audio(); tia_sound_.set_divider(decodedAddress - 0x17, *value);				break;
+						case 0x18:	audio_->set_divider(decodedAddress - 0x17, *value);			break;
 						case 0x19:
-						case 0x1a:	update_audio(); tia_sound_.set_volume(decodedAddress - 0x19, *value);				break;
+						case 0x1a:	audio_->set_volume(decodedAddress - 0x19, *value);				break;
 					}
 				}
 			}
@@ -201,9 +202,8 @@ public:
 	}
 
 	void flush() override {
-		update_audio();
 		update_video();
-		audio_queue_.perform();
+		audio_.perform();
 	}
 
 protected:

Machines/Atari/2600/TIASound.cpp

@@ -10,7 +10,7 @@
 
 using namespace Atari2600;
 
-Atari2600::TIASound::TIASound(Concurrency::AsyncTaskQueue<false> &audio_queue) :
+Atari2600::TIASound::TIASound(Outputs::Speaker::TaskQueue &audio_queue) :
 	audio_queue_(audio_queue)
 {}
 

Machines/Atari/2600/TIASound.hpp

@@ -9,7 +9,7 @@
 #pragma once
 
 #include "Outputs/Speaker/Implementation/BufferSource.hpp"
-#include "Concurrency/AsyncTaskQueue.hpp"
+#include "Outputs/Speaker/SpeakerQueue.hpp"
 
 namespace Atari2600 {
 
@@ -19,7 +19,7 @@ constexpr int CPUTicksPerAudioTick = 2;
 
 class TIASound: public Outputs::Speaker::BufferSource<TIASound, false> {
 public:
-	TIASound(Concurrency::AsyncTaskQueue<false> &);
+	TIASound(Outputs::Speaker::TaskQueue &);
 
 	void set_volume(int channel, uint8_t volume);
 	void set_divider(int channel, uint8_t divider);
@@ -30,7 +30,7 @@ public:
 	void set_sample_volume_range(std::int16_t);
 
 private:
-	Concurrency::AsyncTaskQueue<false> &audio_queue_;
+	Outputs::Speaker::TaskQueue &audio_queue_;
 
 	uint8_t volume_[2];
 	uint8_t divider_[2];

Machines/Atari/ST/AtariST.cpp

@@ -144,7 +144,7 @@ public:
 	}
 
 	~ConcreteMachine() {
-		audio_queue_.flush();
+		audio_queue_.lock_flush();
 	}
 
 	// MARK: CRTMachine::Machine

Machines/ColecoVision/ColecoVision.cpp

@@ -168,7 +168,7 @@ public:
 	}
 
 	~ConcreteMachine() {
-		audio_queue_.flush();
+		audio_queue_.lock_flush();
 	}
 
 	const std::vector<std::unique_ptr<Inputs::Joystick>> &get_joysticks() final {

Machines/Commodore/1540/Implementation/C1540.cpp

@@ -17,7 +17,10 @@
 using namespace Commodore::C1540;
 
 namespace {
-ROM::Name rom_name(Personality personality) {
+
+// MARK: - Construction, including ROM requests.
+
+ROM::Name rom_name(const Personality personality) {
 	switch(personality) {
 		default:
 		case Personality::C1540:	return ROM::Name::Commodore1540;
@@ -26,11 +29,11 @@ ROM::Name rom_name(Personality personality) {
 }
 }
 
-ROM::Request Machine::rom_request(Personality personality) {
+ROM::Request Machine::rom_request(const Personality personality) {
 	return ROM::Request(rom_name(personality));
 }
 
-MachineBase::MachineBase(Personality personality, const ROM::Map &roms) :
+MachineBase::MachineBase(const Personality personality, const ROM::Map &roms) :
 		Storage::Disk::Controller(1000000),
 		m6502_(*this),
 		drive_VIA_(drive_VIA_port_handler_),
@@ -58,14 +61,13 @@ MachineBase::MachineBase(Personality personality, const ROM::Map &roms) :
 	std::memcpy(rom_, rom->second.data(), std::min(sizeof(rom_), rom->second.size()));
 }
 
-Machine::Machine(Personality personality, const ROM::Map &roms) :
+Machine::Machine(const Personality personality, const ROM::Map &roms) :
 	MachineBase(personality, roms) {}
 
-void Machine::set_serial_bus(Commodore::Serial::Bus &serial_bus) {
-	Commodore::Serial::attach(serial_port_, serial_bus);
-}
+// MARK: - 6502 bus.
 
-Cycles MachineBase::perform_bus_operation(CPU::MOS6502::BusOperation operation, uint16_t address, uint8_t *value) {
+template <CPU::MOS6502Mk2::BusOperation operation, typename AddressT>
+Cycles MachineBase::perform(const AddressT address, CPU::MOS6502Mk2::data_t<operation> value) {
 	/*
 		Memory map (given that I'm unsure yet on any potential mirroring):
 
@@ -75,24 +77,28 @@ Cycles MachineBase::perform_bus_operation(CPU::MOS6502::BusOperation operation,
 			0xc000-0xffff	ROM
 	*/
 	if(address < 0x800) {
-		if(is_read(operation))
-			*value = ram_[address];
+		if constexpr (is_read(operation))
+			value = ram_[address];
 		else
-			ram_[address] = *value;
+			ram_[address] = value;
 	} else if(address >= 0xc000) {
-		if(is_read(operation)) {
-			*value = rom_[address & 0x3fff];
+		if constexpr (is_read(operation)) {
+			value = rom_[address & 0x3fff];
 		}
 	} else if(address >= 0x1800 && address <= 0x180f) {
-		if(is_read(operation))
-			*value = serial_port_VIA_.read(address);
+		if constexpr (is_read(operation))
+			value = serial_port_VIA_.read(address);
 		else
-			serial_port_VIA_.write(address, *value);
+			serial_port_VIA_.write(address, value);
 	} else if(address >= 0x1c00 && address <= 0x1c0f) {
-		if(is_read(operation))
-			*value = drive_VIA_.read(address);
+		if constexpr (is_read(operation))
+			value = drive_VIA_.read(address);
 		else
-			drive_VIA_.write(address, *value);
+			drive_VIA_.write(address, value);
+	} else {
+		if constexpr (is_read(operation)) {
+			value = 0xff;
+		}
 	}
 
 	serial_port_VIA_.run_for(Cycles(1));
@@ -101,34 +107,42 @@ Cycles MachineBase::perform_bus_operation(CPU::MOS6502::BusOperation operation,
 	return Cycles(1);
 }
 
-void Machine::set_disk(std::shared_ptr<Storage::Disk::Disk> disk) {
-	get_drive().set_disk(disk);
-}
-
 void Machine::run_for(const Cycles cycles) {
 	m6502_.run_for(cycles);
 
-	const bool drive_motor = drive_VIA_port_handler_.get_motor_enabled();
+	const bool drive_motor = drive_VIA_port_handler_.motor_enabled();
 	get_drive().set_motor_on(drive_motor);
-	if(drive_motor)
+	if(drive_motor) {
 		Storage::Disk::Controller::run_for(cycles);
+	}
 }
 
-void MachineBase::set_activity_observer(Activity::Observer *observer) {
+// MARK: - External attachments.
+
+void Machine::set_serial_bus(Commodore::Serial::Bus &serial_bus) {
+	Commodore::Serial::attach(serial_port_, serial_bus);
+}
+
+void Machine::set_disk(std::shared_ptr<Storage::Disk::Disk> disk) {
+	get_drive().set_disk(disk);
+	drive_VIA_port_handler_.set_is_read_only(disk->is_read_only());
+}
+
+void MachineBase::set_activity_observer(Activity::Observer *const observer) {
 	drive_VIA_.bus_handler().set_activity_observer(observer);
 	get_drive().set_activity_observer(observer, "Drive", false);
 }
 
-// MARK: - 6522 delegate
+// MARK: - 6522 delegate.
 
 void MachineBase::mos6522_did_change_interrupt_status(void *) {
 	// both VIAs are connected to the IRQ line
-	m6502_.set_irq_line(serial_port_VIA_.get_interrupt_line() || drive_VIA_.get_interrupt_line());
+	m6502_.set<CPU::MOS6502Mk2::Line::IRQ>(serial_port_VIA_.get_interrupt_line() || drive_VIA_.get_interrupt_line());
 }
 
-// MARK: - Disk drive
+// MARK: - Disk drive.
 
-void MachineBase::process_input_bit(int value) {
+void MachineBase::process_input_bit(const int value) {
 	shift_register_ = (shift_register_ << 1) | value;
 	if((shift_register_ & 0x3ff) == 0x3ff) {
 		drive_VIA_port_handler_.set_sync_detected(true);
@@ -140,11 +154,11 @@ void MachineBase::process_input_bit(int value) {
 	if(bit_window_offset_ == 8) {
 		drive_VIA_port_handler_.set_data_input(uint8_t(shift_register_));
 		bit_window_offset_ = 0;
-		if(drive_VIA_port_handler_.get_should_set_overflow()) {
-			m6502_.set_overflow_line(true);
+		if(drive_VIA_port_handler_.should_set_overflow()) {
+			m6502_.set<CPU::MOS6502Mk2::Line::Overflow>(true);
 		}
 	}
-	else m6502_.set_overflow_line(false);
+	else m6502_.set<CPU::MOS6502Mk2::Line::Overflow>(false);
 }
 
 // the 1540 does not recognise index holes
@@ -164,7 +178,9 @@ void MachineBase::drive_via_did_set_data_density(DriveVIA &, const int density)
 
 template <MOS::MOS6522::Port port>
 uint8_t SerialPortVIA::get_port_input() const {
-	if(port) return port_b_;
+	if(port) {
+		return port_b_;
+	}
 	return 0xff;
 }
 
@@ -203,13 +219,15 @@ void SerialPortVIA::set_serial_port(Commodore::Serial::Port &port) {
 
 void SerialPortVIA::update_data_line() {
 	// "ATN (Attention) is an input on pin 3 of P2 and P3 that is sensed at PB7 and CA1 of UC3 after being inverted by UA1"
-	serial_port_->set_output(::Commodore::Serial::Line::Data,
-		Serial::LineLevel(!data_level_output_ && (attention_level_input_ != attention_acknowledge_level_)));
+	serial_port_->set_output(
+		::Commodore::Serial::Line::Data,
+		Serial::LineLevel(!data_level_output_ && (attention_level_input_ != attention_acknowledge_level_))
+	);
 }
 
 // MARK: - DriveVIA
 
-void DriveVIA::set_delegate(Delegate *delegate) {
+void DriveVIA::set_delegate(Delegate *const delegate) {
 	delegate_ = delegate;
 }
 
@@ -220,18 +238,22 @@ uint8_t DriveVIA::get_port_input() const {
 }
 
 void DriveVIA::set_sync_detected(const bool sync_detected) {
-	port_b_ = (port_b_ & 0x7f) | (sync_detected ? 0x00 : 0x80);
+	port_b_ = (port_b_ & ~0x80) | (sync_detected ? 0x00 : 0x80);
 }
 
-void DriveVIA::set_data_input(uint8_t value) {
+void DriveVIA::set_is_read_only(const bool is_read_only) {
+	port_b_ = (port_b_ & ~0x10) | (is_read_only ? 0x00 : 0x10);
+}
+
+void DriveVIA::set_data_input(const uint8_t value) {
 	port_a_ = value;
 }
 
-bool DriveVIA::get_should_set_overflow() {
+bool DriveVIA::should_set_overflow() {
 	return should_set_overflow_;
 }
 
-bool DriveVIA::get_motor_enabled() {
+bool DriveVIA::motor_enabled() {
 	return drive_motor_;
 }
 
@@ -240,36 +262,48 @@ void DriveVIA::set_control_line_output(const bool value) {
 	if(port == MOS::MOS6522::Port::A && line == MOS::MOS6522::Line::Two) {
 		should_set_overflow_ = value;
 	}
-}
 
-template <MOS::MOS6522::Port port>
-void DriveVIA::set_port_output(const uint8_t value, uint8_t) {
-	if(port) {
-		if(previous_port_b_output_ != value) {
-			// Record drive motor state.
-			drive_motor_ = value&4;
-
-			// Check for a head step.
-			const int step_difference = ((value&3) - (previous_port_b_output_&3))&3;
-			if(step_difference) {
-				if(delegate_) delegate_->drive_via_did_step_head(*this, (step_difference == 1) ? 1 : -1);
-			}
-
-			// Check for a change in density.
-			const int density_difference = (previous_port_b_output_^value) & (3 << 5);
-			if(density_difference && delegate_) {
-				delegate_->drive_via_did_set_data_density(*this, (value >> 5)&3);
-			}
-
-			// Post the LED status.
-			if(observer_) observer_->set_led_status("Drive", value&8);
-
-			previous_port_b_output_ = value;
+	if(port == MOS::MOS6522::Port::B && line == MOS::MOS6522::Line::Two) {
+		// TODO: 0 = write, 1 = read.
+		if(!value) {
+			printf("NOT IMPLEMENTED: write mode\n");
 		}
 	}
 }
 
-void DriveVIA::set_activity_observer(Activity::Observer *observer) {
+template <>
+void DriveVIA::set_port_output<MOS::MOS6522::Port::B>(const uint8_t value, uint8_t) {
+	if(previous_port_b_output_ != value) {
+		// Record drive motor state.
+		drive_motor_ = value&4;
+
+		// Check for a head step.
+		const int step_difference = ((value&3) - (previous_port_b_output_&3))&3;
+		if(step_difference && delegate_) {
+			delegate_->drive_via_did_step_head(*this, (step_difference == 1) ? 1 : -1);
+		}
+
+		// Check for a change in density.
+		const int density_difference = (previous_port_b_output_^value) & (3 << 5);
+		if(density_difference && delegate_) {
+			delegate_->drive_via_did_set_data_density(*this, (value >> 5)&3);
+		}
+
+		// Post the LED status.
+		if(observer_) {
+			observer_->set_led_status("Drive", value&8);
+		}
+
+		previous_port_b_output_ = value;
+	}
+}
+
+template <>
+void DriveVIA::set_port_output<MOS::MOS6522::Port::A>(const uint8_t value, uint8_t) {
+	printf("TODO: output is %02x\n", value);
+}
+
+void DriveVIA::set_activity_observer(Activity::Observer *const observer) {
 	observer_ = observer;
 	if(observer) {
 		observer->register_led("Drive");
@@ -277,9 +311,9 @@ void DriveVIA::set_activity_observer(Activity::Observer *observer) {
 	}
 }
 
-// MARK: - SerialPort
+// MARK: - SerialPort.
 
-void SerialPort::set_input(Serial::Line line, Serial::LineLevel level) {
+void SerialPort::set_input(const Serial::Line line, const Serial::LineLevel level) {
 	serial_port_via_->set_serial_line_state(line, bool(level), *via_);
 }
 

Machines/Commodore/1540/Implementation/C1540Base.hpp

@@ -8,7 +8,7 @@
 
 #pragma once
 
-#include "Processors/6502/6502.hpp"
+#include "Processors/6502Mk2/6502Mk2.hpp"
 #include "Components/6522/6522.hpp"
 
 #include "Machines/Commodore/SerialBus.hpp"
@@ -65,15 +65,17 @@ private:
 	It is wired up such that Port B contains:
 		Bits 0/1:	head step direction
 		Bit 2:		motor control
-		Bit 3:		LED control (TODO)
+		Bit 3:		LED control
 		Bit 4:		write protect photocell status (TODO)
 		Bits 5/6:	read/write density
 		Bit 7:		0 if sync marks are currently being detected, 1 otherwise.
 
 	... and Port A contains the byte most recently read from the disk or the byte next to write to the disk, depending on data direction.
 
-	It is implied that CA2 might be used to set processor overflow, CA1 a strobe for data input, and one of the CBs being definitive on
-	whether the disk head is being told to read or write, but it's unclear and I've yet to investigate. So, TODO.
+	Elsewhere:
+	* CA2 might is used to set processor overflow;
+	* CA1 a strobe for data input; and
+	* CB2 indicates read/write mode; 1 = read, 0 = write.
 */
 class DriveVIA: public MOS::MOS6522::IRQDelegatePortHandler {
 public:
@@ -88,8 +90,9 @@ public:
 
 	void set_sync_detected(bool);
 	void set_data_input(uint8_t);
-	bool get_should_set_overflow();
-	bool get_motor_enabled();
+	void set_is_read_only(bool);
+	bool should_set_overflow();
+	bool motor_enabled();
 
 	template <MOS::MOS6522::Port, MOS::MOS6522::Line>
 	void set_control_line_output(bool value);
@@ -122,7 +125,6 @@ private:
 };
 
 class MachineBase:
-	public CPU::MOS6502::BusHandler,
 	public MOS::MOS6522::IRQDelegatePortHandler::Delegate,
 	public DriveVIA::Delegate,
 	public Storage::Disk::Controller {
@@ -134,7 +136,8 @@ public:
 	void set_activity_observer(Activity::Observer *);
 
 	// to satisfy CPU::MOS6502::Processor
-	Cycles perform_bus_operation(CPU::MOS6502::BusOperation, uint16_t address, uint8_t *value);
+	template <CPU::MOS6502Mk2::BusOperation operation, typename AddressT>
+	Cycles perform(const AddressT, CPU::MOS6502Mk2::data_t<operation>);
 
 protected:
 	// to satisfy MOS::MOS6522::Delegate
@@ -144,7 +147,12 @@ protected:
 	void drive_via_did_step_head(DriveVIA &, int direction) override;
 	void drive_via_did_set_data_density(DriveVIA &, int density) override;
 
-	CPU::MOS6502::Processor<CPU::MOS6502::Personality::P6502, MachineBase, false> m6502_;
+	struct M6502Traits {
+		static constexpr auto uses_ready_line = false;
+		static constexpr auto pause_precision = CPU::MOS6502Mk2::PausePrecision::AnyCycle;
+		using BusHandlerT = MachineBase;
+	};
+	CPU::MOS6502Mk2::Processor<CPU::MOS6502Mk2::Model::M6502, M6502Traits> m6502_;
 
 	uint8_t ram_[0x800];
 	uint8_t rom_[0x4000];

Machines/Commodore/Plus4/Audio.hpp

@@ -9,13 +9,13 @@
 #pragma once
 
 #include "Outputs/Speaker/Implementation/BufferSource.hpp"
-#include "Concurrency/AsyncTaskQueue.hpp"
+#include "Outputs/Speaker/SpeakerQueue.hpp"
 
 namespace Commodore::Plus4 {
 
 class Audio: public Outputs::Speaker::BufferSource<Audio, false> {
 public:
-	Audio(Concurrency::AsyncTaskQueue<false> &audio_queue) :
+	Audio(Outputs::Speaker::TaskQueue &audio_queue) :
 		audio_queue_(audio_queue) {}
 
 	template <Outputs::Speaker::Action action>
@@ -122,7 +122,7 @@ public:
 
 private:
 	// Calling-thread state.
-	Concurrency::AsyncTaskQueue<false> &audio_queue_;
+	Outputs::Speaker::TaskQueue &audio_queue_;
 
 	// Audio-thread state.
 	int16_t external_volume_ = 0;

Machines/Commodore/Plus4/Plus4.cpp

@@ -16,7 +16,7 @@
 
 #include "Machines/MachineTypes.hpp"
 #include "Machines/Utility/MemoryFuzzer.hpp"
-#include "Processors/6502/6502.hpp"
+#include "Processors/6502Mk2/6502Mk2.hpp"
 #include "Analyser/Static/Commodore/Target.hpp"
 #include "Outputs/Log.hpp"
 #include "Outputs/Speaker/Implementation/LowpassSpeaker.hpp"
@@ -186,13 +186,11 @@ public:
 		interrupts_(*this),
 		timers_(interrupts_),
 		video_(video_map_, interrupts_),
-		audio_(audio_queue_),
-		speaker_(audio_)
+		audio_(clock_rate(false), Cycles(1))
 	{
 		const auto clock = clock_rate(false);
 		media_divider_ = Cycles(clock);
 		set_clock_rate(clock);
-		speaker_.set_input_rate(float(clock));
 
 		const auto kernel = ROM::Name::Plus4KernelPALv5;
 		const auto basic = ROM::Name::Plus4BASIC;
@@ -236,20 +234,17 @@ public:
 	}
 
 	~ConcreteMachine() {
-		audio_queue_.flush();
+		audio_.stop();
 	}
 
 	// HACK. NOCOMMIT.
 //	int pulse_num_ = 0;
 
-	Cycles perform_bus_operation(
-		const CPU::MOS6502::BusOperation operation,
-		const uint16_t address,
-		uint8_t *const value
-	) {
+	template <CPU::MOS6502Mk2::BusOperation operation, typename AddressT>
+	Cycles perform(const AddressT address, CPU::MOS6502Mk2::data_t<operation> value) {
 		// Determine from the TED video subsystem the length of this clock cycle as perceived by the 6502,
 		// relative to the master clock.
-		const auto length = video_.cycle_length(operation == CPU::MOS6502::BusOperation::Ready);
+		const auto length = video_.cycle_length(operation == CPU::MOS6502Mk2::BusOperation::Ready);
 
 		// Update other subsystems.
 		advance_timers_and_tape(length);
@@ -261,11 +256,10 @@ public:
 				c1541_->run_for(c1541_cycles_.divide(media_divider_));
 			}
 
-
-			time_since_audio_update_ += length;
+			audio_ += length;
 		}
 
-		if(operation == CPU::MOS6502::BusOperation::Ready) {
+		if(operation == CPU::MOS6502Mk2::BusOperation::Ready) {
 			return length;
 		}
 
@@ -282,17 +276,17 @@ public:
 			//	b1 = serial clock out and cassette write;
 			//	b0 = serial data out.
 
-			if(is_read(operation)) {
+			if constexpr (is_read(operation)) {
 				if(!address) {
-					*value = io_direction_;
+					value = io_direction_;
 				} else {
-					*value = io_input();
+					value = io_input();
 				}
 			} else {
 				if(!address) {
-					io_direction_ = *value;
+					io_direction_ = value;
 				} else {
-					io_output_ = *value;
+					io_output_ = value;
 				}
 
 				const auto output = io_output_ | ~io_direction_;
@@ -323,36 +317,36 @@ public:
 //				);
 //			}
 
-			if(
-				use_fast_tape_hack_ &&
-				operation == CPU::MOS6502Esque::BusOperation::ReadOpcode &&
-				(
-					(use_hle && address == 0xe5fd) ||
-					address == 0xe68b ||
-					address == 0xe68d
-				)
-			) {
-//				++pulse_num_;
-				if(use_hle) {
-					read_dipole();
-				}
+			if constexpr (is_read(operation)) {
+				if(
+					use_fast_tape_hack_ &&
+					operation == CPU::MOS6502Mk2::BusOperation::ReadOpcode &&
+					(
+						(use_hle && address == 0xe5fd) ||
+						address == 0xe68b ||
+						address == 0xe68d
+					)
+				) {
+	//				++pulse_num_;
+					if(use_hle) {
+						read_dipole();
+					}
 
-//				using Flag = CPU::MOS6502::Flag;
-//				using Register = CPU::MOS6502::Register;
-//				const auto flags = m6502_.value_of(Register::Flags);
-//				printf("to %lld: %c%c%c\n",
-//					tape_player_->serialiser()->offset(),
-//					flags & Flag::Sign ? 'n' : '-',
-//					flags & Flag::Overflow ? 'v' : '-',
-//					flags & Flag::Carry ? 'c' : '-'
-//				);
-				*value = 0x60;
-			} else {
-				if(is_read(operation)) {
-					*value = map_.read(address);
+	//				using Flag = CPU::MOS6502::Flag;
+	//				using Register = CPU::MOS6502::Register;
+	//				const auto flags = m6502_.value_of(Register::Flags);
+	//				printf("to %lld: %c%c%c\n",
+	//					tape_player_->serialiser()->offset(),
+	//					flags & Flag::Sign ? 'n' : '-',
+	//					flags & Flag::Overflow ? 'v' : '-',
+	//					flags & Flag::Carry ? 'c' : '-'
+	//				);
+					value = 0x60;
 				} else {
-					map_.write(address) = *value;
+					value = map_.read(address);
 				}
+			} else {
+				map_.write(address) = value;
 			}
 
 
@@ -393,12 +387,12 @@ public:
 //					ram_[0x90] = 0;
 //					ram_[0x93] = 0;
 //
-//					*value = 0x0c;	// NOP abs.
+//					value = 0x0c;	// NOP abs.
 //				}
 //			}
 		} else if(address < 0xff00) {
 			// Miscellaneous hardware. All TODO.
-			if(is_read(operation)) {
+			if constexpr (is_read(operation)) {
 				switch(address & 0xfff0) {
 					case 0xfd10:
 						// 6529 parallel port, about which I know only what I've found in kernel ROM disassemblies.
@@ -406,7 +400,7 @@ public:
 						// If play button is not currently pressed and this read is immediately followed by
 						// an AND 4, press it. The kernel will deal with motor control subsequently.
 						if(!play_button_) {
-							const uint16_t pc = m6502_.value_of(CPU::MOS6502::Register::ProgramCounter);
+							const uint16_t pc = m6502_.registers().pc.full;
 							const uint8_t next[] = {
 								map_.read(pc+0),
 								map_.read(pc+1),
@@ -422,22 +416,23 @@ public:
 							}
 						}
 
-						*value = 0xff ^ (play_button_ ? 0x4 :0x0);
+						value = 0xff ^ (play_button_ ? 0x4 :0x0);
 					break;
 
 					case 0xfdd0:
 					case 0xfdf0:
-						*value = uint8_t(address >> 8);
+						value = uint8_t(address >> 8);
 					break;
 
 					default:
+						value = 0xff;
 						Logger::info().append("TODO: read @ %04x", address);
 					break;
 				}
 			} else {
 				switch(address & 0xfff0) {
 					case 0xfd30:
-						keyboard_mask_ = *value;
+						keyboard_mask_ = value;
 					break;
 
 					case 0xfdd0: {
@@ -447,28 +442,28 @@ public:
 					} break;
 
 					default:
-						Logger::info().append("TODO: write of %02x @ %04x", *value, address);
+						Logger::info().append("TODO: write of %02x @ %04x", value, address);
 					break;
 				}
 			}
 		} else {
-			const auto pc = m6502_.value_of(CPU::MOS6502::Register::ProgramCounter);
+			const auto pc = m6502_.registers().pc.full;
 			const bool is_from_rom =
 				(rom_is_paged_ && pc >= 0x8000) ||
 				(pc >= 0x400 && pc < 0x500) ||
 				(pc >= 0x700 && pc < 0x800);
 			bool is_hit = true;
 
-			if(is_read(operation)) {
+			if constexpr (is_read(operation)) {
 				switch(address) {
-					case 0xff00:	*value = timers_.read<0>();		break;
-					case 0xff01:	*value = timers_.read<1>();		break;
-					case 0xff02:	*value = timers_.read<2>();		break;
-					case 0xff03:	*value = timers_.read<3>();		break;
-					case 0xff04:	*value = timers_.read<4>();		break;
-					case 0xff05:	*value = timers_.read<5>();		break;
-					case 0xff06:	*value = video_.read<0xff06>();	break;
-					case 0xff07:	*value = video_.read<0xff07>();	break;
+					case 0xff00:	value = timers_.read<0>();		break;
+					case 0xff01:	value = timers_.read<1>();		break;
+					case 0xff02:	value = timers_.read<2>();		break;
+					case 0xff03:	value = timers_.read<3>();		break;
+					case 0xff04:	value = timers_.read<4>();		break;
+					case 0xff05:	value = timers_.read<5>();		break;
+					case 0xff06:	value = video_.read<0xff06>();	break;
+					case 0xff07:	value = video_.read<0xff07>();	break;
 					case 0xff08: {
 						const uint8_t keyboard_input =
 							~(
@@ -487,127 +482,122 @@ public:
 							((joystick_mask_ & 0x02) ? 0xff : (joystick(0).mask() | 0x40)) &
 							((joystick_mask_ & 0x04) ? 0xff : (joystick(1).mask() | 0x80));
 
-						*value = keyboard_input & joystick_mask;
+						value = keyboard_input & joystick_mask;
 					} break;
-					case 0xff09:	*value = interrupts_.status();	break;
+					case 0xff09:	value = interrupts_.status();	break;
 					case 0xff0a:
-						*value = interrupts_.mask() | video_.read<0xff0a>() | 0xa0;
+						value = interrupts_.mask() | video_.read<0xff0a>() | 0xa0;
 					break;
-					case 0xff0b:	*value = video_.read<0xff0b>();	break;
-					case 0xff0c:	*value = video_.read<0xff0c>();	break;
-					case 0xff0d:	*value = video_.read<0xff0d>();	break;
-					case 0xff0e:	*value = ff0e_;					break;
-					case 0xff0f:	*value = ff0f_;					break;
-					case 0xff10:	*value = ff10_ | 0xfc;			break;
-					case 0xff11:	*value = ff11_;					break;
-					case 0xff12:	*value = ff12_ | 0xc0;			break;
-					case 0xff13:	*value = ff13_ | (rom_is_paged_ ? 1 : 0);	break;
-					case 0xff14:	*value = video_.read<0xff14>();	break;
-					case 0xff15:	*value = video_.read<0xff15>();	break;
-					case 0xff16:	*value = video_.read<0xff16>();	break;
-					case 0xff17:	*value = video_.read<0xff17>();	break;
-					case 0xff18:	*value = video_.read<0xff18>();	break;
-					case 0xff19:	*value = video_.read<0xff19>();	break;
-					case 0xff1a:	*value = video_.read<0xff1a>();	break;
-					case 0xff1b:	*value = video_.read<0xff1b>();	break;
-					case 0xff1c:	*value = video_.read<0xff1c>();	break;
-					case 0xff1d:	*value = video_.read<0xff1d>();	break;
-					case 0xff1e:	*value = video_.read<0xff1e>();	break;
-					case 0xff1f:	*value = video_.read<0xff1f>();	break;
+					case 0xff0b:	value = video_.read<0xff0b>();	break;
+					case 0xff0c:	value = video_.read<0xff0c>();	break;
+					case 0xff0d:	value = video_.read<0xff0d>();	break;
+					case 0xff0e:	value = ff0e_;					break;
+					case 0xff0f:	value = ff0f_;					break;
+					case 0xff10:	value = ff10_ | 0xfc;			break;
+					case 0xff11:	value = ff11_;					break;
+					case 0xff12:	value = ff12_ | 0xc0;			break;
+					case 0xff13:	value = ff13_ | (rom_is_paged_ ? 1 : 0);	break;
+					case 0xff14:	value = video_.read<0xff14>();	break;
+					case 0xff15:	value = video_.read<0xff15>();	break;
+					case 0xff16:	value = video_.read<0xff16>();	break;
+					case 0xff17:	value = video_.read<0xff17>();	break;
+					case 0xff18:	value = video_.read<0xff18>();	break;
+					case 0xff19:	value = video_.read<0xff19>();	break;
+					case 0xff1a:	value = video_.read<0xff1a>();	break;
+					case 0xff1b:	value = video_.read<0xff1b>();	break;
+					case 0xff1c:	value = video_.read<0xff1c>();	break;
+					case 0xff1d:	value = video_.read<0xff1d>();	break;
+					case 0xff1e:	value = video_.read<0xff1e>();	break;
+					case 0xff1f:	value = video_.read<0xff1f>();	break;
 
-					case 0xff3e:	*value = 0;						break;
-					case 0xff3f:	*value = 0;						break;
+					case 0xff3e:	value = 0;						break;
+					case 0xff3f:	value = 0;						break;
 
 					default:
 						Logger::info().append("TODO: TED read at %04x", address);
+						value = 0xff;
 						is_hit = false;
 				}
 			} else {
 				switch(address) {
-					case 0xff00:	timers_.write<0>(*value);		break;
-					case 0xff01:	timers_.write<1>(*value);		break;
-					case 0xff02:	timers_.write<2>(*value);		break;
-					case 0xff03:	timers_.write<3>(*value);		break;
-					case 0xff04:	timers_.write<4>(*value);		break;
-					case 0xff05:	timers_.write<5>(*value);		break;
-					case 0xff06:	video_.write<0xff06>(*value);	break;
+					case 0xff00:	timers_.write<0>(value);		break;
+					case 0xff01:	timers_.write<1>(value);		break;
+					case 0xff02:	timers_.write<2>(value);		break;
+					case 0xff03:	timers_.write<3>(value);		break;
+					case 0xff04:	timers_.write<4>(value);		break;
+					case 0xff05:	timers_.write<5>(value);		break;
+					case 0xff06:	video_.write<0xff06>(value);	break;
 					case 0xff07:
-						video_.write<0xff07>(*value);
-						update_audio();
-						audio_.set_divider(*value);
+						video_.write<0xff07>(value);
+						audio_->set_divider(value);
 					break;
 					case 0xff08:
 						// Observation here: the kernel posts a 0 to this
 						// address upon completing each keyboard scan cycle,
 						// once per frame.
-						if(typer_ && !*value) {
+						if(typer_ && !value) {
 							if(!typer_->type_next_character()) {
 								clear_all_keys();
 								typer_.reset();
 							}
 						}
 
-						joystick_mask_ = *value;
+						joystick_mask_ = value;
 					break;
 					case 0xff09:
-						interrupts_.set_status(*value);
+						interrupts_.set_status(value);
 					break;
 					case 0xff0a:
-						interrupts_.set_mask(*value);
-						video_.write<0xff0a>(*value);
+						interrupts_.set_mask(value);
+						video_.write<0xff0a>(value);
 					break;
-					case 0xff0b:	video_.write<0xff0b>(*value);	break;
-					case 0xff0c:	video_.write<0xff0c>(*value);	break;
-					case 0xff0d:	video_.write<0xff0d>(*value);	break;
+					case 0xff0b:	video_.write<0xff0b>(value);	break;
+					case 0xff0c:	video_.write<0xff0c>(value);	break;
+					case 0xff0d:	video_.write<0xff0d>(value);	break;
 					case 0xff0e:
-						ff0e_ = *value;
-						update_audio();
-						audio_.set_frequency_low<0>(*value);
+						ff0e_ = value;
+						audio_->set_frequency_low<0>(value);
 					break;
 					case 0xff0f:
-						ff0f_ = *value;
-						update_audio();
-						audio_.set_frequency_low<1>(*value);
+						ff0f_ = value;
+						audio_->set_frequency_low<1>(value);
 					break;
 					case 0xff10:
-						ff10_ = *value;
-						update_audio();
-						audio_.set_frequency_high<1>(*value);
+						ff10_ = value;
+						audio_->set_frequency_high<1>(value);
 					break;
 					case 0xff11:
-						ff11_ = *value;
-						update_audio();
-						audio_.set_control(*value);
+						ff11_ = value;
+						audio_->set_control(value);
 					break;
 					case 0xff12:
-						ff12_ = *value & 0x3f;
-						video_.write<0xff12>(*value);
+						ff12_ = value & 0x3f;
+						video_.write<0xff12>(value);
 
-						if((*value & 4)) {
+						if((value & 4)) {
 							page_video_rom();
 						} else {
 							page_video_ram();
 						}
 
-						update_audio();
-						audio_.set_frequency_high<0>(*value);
+						audio_->set_frequency_high<0>(value);
 					break;
 					case 0xff13:
-						ff13_ = *value & 0xfe;
-						video_.write<0xff13>(*value);
+						ff13_ = value & 0xfe;
+						video_.write<0xff13>(value);
 					break;
-					case 0xff14:	video_.write<0xff14>(*value);	break;
-					case 0xff15:	video_.write<0xff15>(*value);	break;
-					case 0xff16:	video_.write<0xff16>(*value);	break;
-					case 0xff17:	video_.write<0xff17>(*value);	break;
-					case 0xff18:	video_.write<0xff18>(*value);	break;
-					case 0xff19:	video_.write<0xff19>(*value);	break;
-					case 0xff1a:	video_.write<0xff1a>(*value);	break;
-					case 0xff1b:	video_.write<0xff1b>(*value);	break;
-					case 0xff1c:	video_.write<0xff1c>(*value);	break;
-					case 0xff1d:	video_.write<0xff1d>(*value);	break;
-					case 0xff1e:	video_.write<0xff1e>(*value);	break;
-					case 0xff1f:	video_.write<0xff1f>(*value);	break;
+					case 0xff14:	video_.write<0xff14>(value);	break;
+					case 0xff15:	video_.write<0xff15>(value);	break;
+					case 0xff16:	video_.write<0xff16>(value);	break;
+					case 0xff17:	video_.write<0xff17>(value);	break;
+					case 0xff18:	video_.write<0xff18>(value);	break;
+					case 0xff19:	video_.write<0xff19>(value);	break;
+					case 0xff1a:	video_.write<0xff1a>(value);	break;
+					case 0xff1b:	video_.write<0xff1b>(value);	break;
+					case 0xff1c:	video_.write<0xff1c>(value);	break;
+					case 0xff1d:	video_.write<0xff1d>(value);	break;
+					case 0xff1e:	video_.write<0xff1e>(value);	break;
+					case 0xff1f:	video_.write<0xff1f>(value);	break;
 
 					case 0xff3e:	page_cpu_rom();					break;
 					case 0xff3f:	page_cpu_ram();					break;
@@ -626,22 +616,26 @@ public:
 	}
 
 private:
-	using Processor = CPU::MOS6502::Processor<CPU::MOS6502::Personality::P6502, ConcreteMachine, true>;
-	Processor m6502_;
+	struct M6502Traits {
+		static constexpr auto uses_ready_line = true;
+		static constexpr auto pause_precision = CPU::MOS6502Mk2::PausePrecision::BetweenInstructions;
+		using BusHandlerT = ConcreteMachine;
+	};
+	CPU::MOS6502Mk2::Processor<CPU::MOS6502Mk2::Model::M6502, M6502Traits> m6502_;
 
 	Outputs::Speaker::Speaker *get_speaker() override {
-		return &speaker_;
+		return &audio_.speaker();
 	}
 
 	void set_activity_observer(Activity::Observer *const observer) final {
 		if(c1541_) c1541_->set_activity_observer(observer);
 	}
 
-	void set_irq_line(bool active) override {
-		m6502_.set_irq_line(active);
+	void set_irq_line(const bool active) override {
+		m6502_.template set<CPU::MOS6502Mk2::Line::IRQ>(active);
 	}
-	void set_ready_line(bool active) override {
-		m6502_.set_ready_line(active);
+	void set_ready_line(const bool active) override {
+		m6502_.template set<CPU::MOS6502Mk2::Line::Ready>(active);
 	}
 
 	void page_video_rom() {
@@ -684,16 +678,12 @@ private:
 
 	void run_for(const Cycles cycles) final {
 		m6502_.run_for(cycles);
-
-		// I don't know why.
-		update_audio();
-		audio_queue_.perform();
+		audio_.perform();
 	}
 
 	void flush_output(int outputs) override {
 		if(outputs & Output::Audio) {
-			update_audio();
-			audio_queue_.perform();
+			audio_.perform();
 		}
 	}
 
@@ -720,14 +710,7 @@ private:
 	Cycles timers_subcycles_;
 	Timers timers_;
 	Video video_;
-
-	Concurrency::AsyncTaskQueue<false> audio_queue_;
-	Audio audio_;
-	Cycles time_since_audio_update_;
-	Outputs::Speaker::PullLowpass<Audio> speaker_;
-	void update_audio() {
-		speaker_.run_for(audio_queue_, time_since_audio_update_.flush<Cycles>());
-	}
+	Outputs::Speaker::PullLowpassSpeakerQueue<Cycles, Audio> audio_;
 
 	// MARK: - MappedKeyboardMachine.
 	MappedKeyboardMachine::KeyboardMapper *get_keyboard_mapper() override {
@@ -788,26 +771,19 @@ private:
 	// TODO: substantially simplify the below; at the minute it's a
 	// literal transcription of the original as a simple first step.
 	void read_dipole() {
-		using Register = CPU::MOS6502::Register;
-		using Flag = CPU::MOS6502::Flag;
+		using Flag = CPU::MOS6502Mk2::Flag;
 
 		//
 		// Get registers now and ensure they'll be written back at function exit.
 		//
-		CPU::MOS6502Esque::LazyFlags flags(uint8_t(m6502_.value_of(Register::Flags)));
-		uint8_t x, y, a;
-		uint8_t s = uint8_t(m6502_.value_of(Register::StackPointer));
+		auto registers = m6502_.registers();
 		struct ScopeGuard {
 			ScopeGuard(std::function<void(void)> at_exit) : at_exit_(at_exit) {}
 			~ScopeGuard() {	at_exit_();	}
 		private:
 			std::function<void(void)> at_exit_;
-		} registers([&] {
-			m6502_.set_value_of(Register::Flags, flags.get());
-			m6502_.set_value_of(Register::A, a);
-			m6502_.set_value_of(Register::X, x);
-			m6502_.set_value_of(Register::Y, y);
-			m6502_.set_value_of(Register::StackPointer, s);
+		} store_registers([&] {
+			m6502_.set_registers(registers);
 		});
 
 		//
@@ -822,38 +798,38 @@ private:
 		// 6502 pseudo-ops.
 		//
 		const auto ldabs = [&] (uint8_t &target, const uint16_t address) {
-			flags.set_nz(target = map_.read(address));
+			registers.flags.set_per<Flag::NegativeZero>(target = map_.read(address));
 		};
 		const auto ldimm = [&] (uint8_t &target, const uint8_t value) {
-			flags.set_nz(target = value);
+			registers.flags.set_per<Flag::NegativeZero>(target = value);
 		};
 		const auto pha = [&] () {
-			map_.write(0x100 + s) = a;
-			--s;
+			map_.write(0x100 + registers.s) = registers.a;
+			--registers.s;
 		};
 		const auto pla = [&] () {
-			++s;
-			a = map_.read(0x100 + s);
+			++registers.s;
+			registers.a = map_.read(0x100 + registers.s);
 		};
 		const auto bit = [&] (const uint8_t value) {
-			flags.zero_result = a & value;
-			flags.negative_result = value;
-			flags.overflow = value & CPU::MOS6502Esque::Flag::Overflow;
+			registers.flags.set_per<Flag::Zero>(registers.a & value);
+			registers.flags.set_per<Flag::Negative>(value);
+			registers.flags.set_per<Flag::Overflow>(value);
 		};
 		const auto cmp = [&] (const uint8_t value) {
-			const uint16_t temp16 = a - value;
-			flags.set_nz(uint8_t(temp16));
-			flags.carry = ((~temp16) >> 8)&1;
+			const uint16_t temp16 = registers.a - value;
+			registers.flags.set_per<Flag::NegativeZero>(uint8_t(temp16));
+			registers.flags.set_per<Flag::Carry>(((~temp16) >> 8)&1);
 		};
 		const auto andimm = [&] (const uint8_t value) {
-			a &= value;
-			flags.set_nz(a);
+			registers.a &= value;
+			registers.flags.set_per<Flag::NegativeZero>(registers.a);
 		};
 		const auto ne = [&]() -> bool {
-			return flags.zero_result;
+			return !registers.flags.get<Flag::Zero>();
 		};
 		const auto eq = [&]() -> bool {
-			return !flags.zero_result;
+			return registers.flags.get<Flag::Zero>();
 		};
 
 		//
@@ -862,7 +838,7 @@ private:
 		const auto dipok = [&] {
 			//       clc             	; everything's fine
 			//       rts
-			flags.carry = 0;
+			registers.flags.set_per<Flag::Carry>(0);
 		};
 		const auto rshort = [&] {
 			//       bit  tshrtd     	; got a short
@@ -878,7 +854,7 @@ private:
 		const auto rderr1 = [&] {
 			//       sec             	; i'm confused
 			//       rts
-			flags.carry = Flag::Carry;
+			registers.flags.set_per<Flag::Carry>(Flag::Carry);
 		};
 
 		//
@@ -891,8 +867,8 @@ private:
 		//rddipl
 		//       ldx  dsamp1     	; setup x,y with 1st sample point
 		//       ldy  dsamp1+1
-		ldabs(x, dsamp1);
-		ldabs(y, dsamp1 + 1);
+		ldabs(registers.x, dsamp1);
+		ldabs(registers.y, dsamp1 + 1);
 		advance_cycles(8);
 
 		//badeg1
@@ -901,9 +877,9 @@ private:
 			//       pha
 			//       lda  dsamp2
 			//       pha
-			ldabs(a, dsamp2 + 1);
+			ldabs(registers.a, dsamp2 + 1);
 			pha();
-			ldabs(a, dsamp2);
+			ldabs(registers.a, dsamp2);
 			pha();
 			advance_cycles(14);
 
@@ -911,7 +887,7 @@ private:
 			//rwtl   			; wait till rd line is high
 			//       bit  port	[= $0001]
 			//       beq  rwtl       	; !ls!
-			ldimm(a, 0x10);
+			ldimm(registers.a, 0x10);
 			advance_cycles(2);
 			do {
 				bit(io_input());
@@ -933,8 +909,8 @@ private:
 
 			//       stx  timr2l
 			//       sty  timr2h
-			timers_.write<2>(x);
-			timers_.write<3>(y);
+			timers_.write<2>(registers.x);
+			timers_.write<3>(registers.y);
 			advance_cycles(8);
 
 
@@ -945,9 +921,9 @@ private:
 			//       pla
 			//       sta  timr3h     	;go! ...tb
 			pla();
-			timers_.write<4>(a);
+			timers_.write<4>(registers.a);
 			pla();
-			timers_.write<5>(a);
+			timers_.write<5>(registers.a);
 			advance_cycles(14);
 
 
@@ -955,8 +931,8 @@ private:
 			//
 			//       lda  #$50       	; clr ta,tb
 			//       sta  tedirq
-			ldimm(a, 0x50);
-			interrupts_.set_status(a);
+			ldimm(registers.a, 0x50);
+			interrupts_.set_status(registers.a);
 			advance_cycles(6);
 
 
@@ -969,7 +945,7 @@ private:
 			//       and  #$10       	; a look at that edge again
 			//       bne  badeg1     	; woa! got a bad edge trigger  !ls!
 			do {
-				ldimm(a, io_input());
+				ldimm(registers.a, io_input());
 				cmp(io_input());
 				if(advance_cycles(9)) {
 					return;
@@ -992,7 +968,7 @@ private:
 
 		//       lda  #$10
 		//wata   			; wait for ta to timeout
-		ldimm(a, 0x10);
+		ldimm(registers.a, 0x10);
 		advance_cycles(3);
 		do {
 			//       bit  port       	; kuldge, kludge, kludge !!! <<><>>
@@ -1020,7 +996,7 @@ private:
 		do {
 			//       lda  port
 			//       cmp  port
-			ldimm(a, io_input());
+			ldimm(registers.a, io_input());
 			cmp(io_input());
 
 			if(advance_cycles(9)) {
@@ -1048,7 +1024,7 @@ private:
 		//
 		//; wait for tb to timeout
 		//; now do the dipole sample #2
-		ldimm(a, 0x40);
+		ldimm(registers.a, 0x40);
 		advance_cycles(3);
 		do {
 			bit(interrupts_.status());
@@ -1063,7 +1039,7 @@ private:
 		//       cmp  port
 		//       bne  casdb3
 		do {
-			ldimm(a, io_input());
+			ldimm(registers.a, io_input());
 			cmp(io_input());
 			if(advance_cycles(9)) {
 				return;
@@ -1084,10 +1060,10 @@ private:
 		//       sta  timr2l
 		//       lda  zcell+1
 		//       sta  timr2h
-		ldabs(a, zcell);
-		timers_.write<2>(a);
-		ldabs(a, zcell + 1);
-		timers_.write<3>(y);
+		ldabs(registers.a, zcell);
+		timers_.write<2>(registers.a);
+		ldabs(registers.a, zcell + 1);
+		timers_.write<3>(registers.y);
 		advance_cycles(16);
 
 
@@ -1096,9 +1072,9 @@ private:
 		//       lda  #$10
 		//       sta  tedirq	; verify +180 half of word dipole
 		//       lda  #$10
-		ldimm(a, 0x10);
-		interrupts_.set_status(a);
-		ldimm(a, 0x10);
+		ldimm(registers.a, 0x10);
+		interrupts_.set_status(registers.a);
+		ldimm(registers.a, 0x10);
 		advance_cycles(8);
 
 		//wata2
@@ -1116,7 +1092,7 @@ private:
 		//       cmp  port
 		//       bne  casdb4
 		do {
-			ldimm(a, io_input());
+			ldimm(registers.a, io_input());
 			cmp(io_input());
 			if(advance_cycles(9)) {
 				return;

Machines/Commodore/Vic-20/Vic20.cpp

@@ -13,7 +13,7 @@
 #include "Activity/Source.hpp"
 #include "Machines/MachineTypes.hpp"
 
-#include "Processors/6502/6502.hpp"
+#include "Processors/6502Mk2/6502Mk2.hpp"
 #include "Components/6560/6560.hpp"
 #include "Components/6522/6522.hpp"
 
@@ -72,26 +72,30 @@ public:
 		// Port A provides information about the presence or absence of a tape, and parts of
 		// the joystick and serial port state, both of which have been statefully collected
 		// into port_a_.
-		if(!port) {
+		if constexpr (port == MOS::MOS6522::Port::A) {
 			return port_a_ | (tape_->has_tape() ? 0x00 : 0x40);
 		}
 		return 0xff;
 	}
 
 	/// Receives announcements of control line output change from the 6522.
-	template <MOS::MOS6522::Port port, MOS::MOS6522::Line line> void set_control_line_output(const bool value) {
-		// The CA2 output is used to control the tape motor.
-		if(port == MOS::MOS6522::Port::A && line == MOS::MOS6522::Line::Two) {
+	template <MOS::MOS6522::Port port, MOS::MOS6522::Line line>
+	void set_control_line_output(const bool value) {
+		// CA2: control the tape motor.
+		if constexpr (port == MOS::MOS6522::Port::A && line == MOS::MOS6522::Line::Two) {
 			tape_->set_motor_control(!value);
 		}
 	}
 
 	/// Receives announcements of changes in the serial bus connected to the serial port and propagates them into Port A.
-	void set_serial_line_state(Commodore::Serial::Line line, const bool value) {
+	void set_serial_line_state(const Commodore::Serial::Line line, const bool value) {
+		const auto set = [&](const uint8_t bit) {
+			port_a_ = (port_a_ & ~bit) | (value ? bit : 0x00);
+		};
 		switch(line) {
 			default: break;
-			case ::Commodore::Serial::Line::Data: port_a_ = (port_a_ & ~0x02) | (value ? 0x02 : 0x00);	break;
-			case ::Commodore::Serial::Line::Clock: port_a_ = (port_a_ & ~0x01) | (value ? 0x01 : 0x00);	break;
+			case ::Commodore::Serial::Line::Data: 	set(0x02);	break;
+			case ::Commodore::Serial::Line::Clock: 	set(0x01);	break;
 		}
 	}
 
@@ -146,7 +150,7 @@ public:
 
 	/// Sets all keys as unpressed.
 	void clear_all_keys() {
-		memset(columns_, 0xff, sizeof(columns_));
+		std::fill(std::begin(columns_), std::end(columns_), 0xff);
 	}
 
 	/// Called by the 6522 to get input. Reads the keyboard on Port A, returns a small amount of joystick state on Port B.
@@ -230,8 +234,6 @@ struct Vic6560BusHandler {
 	// It is assumed that these pointers have been filled in by the machine.
 	const uint8_t *video_memory_map[16]{};	// Segments video memory into 1kb portions.
 	const uint8_t *colour_memory{};			// Colour memory must be contiguous.
-
-	// TODO: make the above const.
 };
 
 /*!
@@ -280,7 +282,6 @@ class ConcreteMachine:
 	public MachineTypes::MappedKeyboardMachine,
 	public MachineTypes::JoystickMachine,
 	public Configurable::Device,
-	public CPU::MOS6502::BusHandler,
 	public MOS::MOS6522::IRQDelegatePortHandler::Delegate,
 	public Utility::TypeRecipient<CharacterMapper>,
 	public Storage::Tape::BinaryTapePlayer::Delegate,
@@ -455,7 +456,7 @@ public:
 	}
 
 	void set_key_state(const uint16_t key, const bool is_pressed) final {
-		if(key < 0xfff0) {
+		if(key < KeyUp) {
 			keyboard_via_port_handler_.set_key_state(key, is_pressed);
 		} else {
 			switch(key) {
@@ -481,28 +482,27 @@ public:
 
 	void clear_all_keys() final {
 		keyboard_via_port_handler_.clear_all_keys();
+		set_key_state(KeyRestore, false);
 	}
 
 	const std::vector<std::unique_ptr<Inputs::Joystick>> &get_joysticks() final {
 		return joysticks_;
 	}
 
-	// to satisfy CPU::MOS6502::Processor
-	forceinline Cycles perform_bus_operation(
-		const CPU::MOS6502::BusOperation operation,
-		const uint16_t address,
-		uint8_t *const value
-	) {
+	template <CPU::MOS6502Mk2::BusOperation operation, typename AddressT>
+	Cycles perform(const AddressT address, CPU::MOS6502Mk2::data_t<operation> value) {
 		// Tun the phase-1 part of this cycle, in which the VIC accesses memory.
 		cycles_since_mos6560_update_++;
 
 		// Run the phase-2 part of the cycle, which is whatever the 6502 said it should be.
-		const bool is_from_rom = m6502_.value_of(CPU::MOS6502::Register::ProgramCounter) > 0x8000;
-		if(is_read(operation)) {
+		const auto is_from_rom = [&]() {
+			return m6502_.registers().pc.full > 0x8000;
+		};
+		if constexpr (is_read(operation)) {
 			const auto page = processor_read_memory_map_[address >> 10];
 			uint8_t result;
 			if(!page) {
-				if(!is_from_rom) confidence_.add_miss();
+				if(!is_from_rom()) confidence_.add_miss();
 				result = 0xff;
 			} else {
 				result = processor_read_memory_map_[address >> 10][address & 0x3ff];
@@ -515,7 +515,7 @@ public:
 				if(address & 0x10) result &= user_port_via_.read(address);
 				if(address & 0x20) result &= keyboard_via_.read(address);
 
-				if(!is_from_rom) {
+				if(!is_from_rom()) {
 					if((address & 0x100) && !(address & 0x30)) {
 						confidence_.add_miss();
 					} else {
@@ -523,10 +523,10 @@ public:
 					}
 				}
 			}
-			*value = result;
+			value = result;
 
 			// Consider applying the fast tape hack.
-			if(use_fast_tape_hack_ && operation == CPU::MOS6502::BusOperation::ReadOpcode) {
+			if(use_fast_tape_hack_ && operation == CPU::MOS6502Mk2::BusOperation::ReadOpcode) {
 				if(address == 0xf7b2) {
 					// Address 0xf7b2 contains a JSR to 0xf8c0 ('RDTPBLKS') that will fill the tape buffer with the
 					// next header. Skip that via a three-byte NOP and fill in the next header programmatically.
@@ -551,10 +551,10 @@ public:
 					ram_[0x90] = 0;
 					ram_[0x93] = 0;
 
-					*value = 0x0c;	// i.e. NOP abs, to swallow the entire JSR
+					value = 0x0c;	// i.e. NOP abs, to swallow the entire JSR
 				} else if(address == 0xf90b) {
-					const auto x = uint8_t(m6502_.value_of(CPU::MOS6502::Register::X));
-					if(x == 0xe) {
+					auto registers = m6502_.registers();
+					if(registers.x == 0xe) {
 						Storage::Tape::Commodore::Parser parser(TargetPlatform::Vic20);
 						const auto tape_position = tape_->serialiser()->offset();
 						const std::unique_ptr<Storage::Tape::Commodore::Data> data = parser.get_next_data(*tape_->serialiser());
@@ -576,14 +576,13 @@ public:
 
 							// set tape status, carry and flag
 							ram_[0x90] |= 0x40;
-							uint8_t	flags = uint8_t(m6502_.value_of(CPU::MOS6502::Register::Flags));
-							flags &= ~uint8_t((CPU::MOS6502::Flag::Carry | CPU::MOS6502::Flag::Interrupt));
-							m6502_.set_value_of(CPU::MOS6502::Register::Flags, flags);
+							registers.flags.set_per<CPU::MOS6502Mk2::Flag::Carry>(0);
+							registers.flags.set_per<CPU::MOS6502Mk2::Flag::Interrupt>(0);
 
 							// to ensure that execution proceeds to 0xfccf, pretend a NOP was here and
 							// ensure that the PC leaps to 0xfccf
-							m6502_.set_value_of(CPU::MOS6502::Register::ProgramCounter, 0xfccf);
-							*value = 0xea;	// i.e. NOP implied
+							registers.pc.full = 0xfccf;
+							value = 0xea;	// i.e. NOP implied
 							hold_tape_ = true;
 							Logger::info().append("Found data");
 						} else {
@@ -592,27 +591,28 @@ public:
 							Logger::info().append("Didn't find data");
 						}
 					}
+					m6502_.set_registers(registers);
 				}
 			}
 		} else {
 			uint8_t *const ram = processor_write_memory_map_[address >> 10];
 			if(ram) {
 				update_video();
-				ram[address & 0x3ff] = *value;
+				ram[address & 0x3ff] = value;
 			}
 			// Anything between 0x9000 and 0x9400 is the IO area.
 			if((address&0xfc00) == 0x9000) {
 				// The VIC is selected by bit 8 = 0
 				if(!(address&0x100)) {
 					update_video();
-					mos6560_.write(address, *value);
+					mos6560_.write(address, value);
 				}
 				// The first VIA is selected by bit 4 = 1.
-				if(address & 0x10) user_port_via_.write(address, *value);
+				if(address & 0x10) user_port_via_.write(address, value);
 				// The second VIA is selected by bit 5 = 1.
-				if(address & 0x20) keyboard_via_.write(address, *value);
+				if(address & 0x20) keyboard_via_.write(address, value);
 
-				if(!is_from_rom) {
+				if(!is_from_rom()) {
 					if((address & 0x100) && !(address & 0x30)) {
 						confidence_.add_miss();
 					} else {
@@ -620,13 +620,13 @@ public:
 					}
 				}
 			} else if(!ram) {
-				if(!is_from_rom) confidence_.add_miss();
+				if(!is_from_rom()) confidence_.add_miss();
 			}
 		}
 
 		user_port_via_.run_for(Cycles(1));
 		keyboard_via_.run_for(Cycles(1));
-		if(typer_ && address == 0xeb1e && operation == CPU::MOS6502::BusOperation::ReadOpcode) {
+		if(typer_ && address == 0xeb1e && operation == CPU::MOS6502Mk2::BusOperation::ReadOpcode) {
 			if(!typer_->type_next_character()) {
 				clear_all_keys();
 				typer_.reset();
@@ -672,8 +672,8 @@ public:
 	}
 
 	void mos6522_did_change_interrupt_status(void *) final {
-		m6502_.set_nmi_line(user_port_via_.get_interrupt_line());
-		m6502_.set_irq_line(keyboard_via_.get_interrupt_line());
+		m6502_.template set<CPU::MOS6502Mk2::Line::NMI>(user_port_via_.get_interrupt_line());
+		m6502_.template set<CPU::MOS6502Mk2::Line::IRQ>(keyboard_via_.get_interrupt_line());
 	}
 
 	void type_string(const std::string &string) final {
@@ -718,10 +718,16 @@ public:
 	}
 
 private:
+	struct M6502Traits {
+		static constexpr auto uses_ready_line = false;
+		static constexpr auto pause_precision = CPU::MOS6502Mk2::PausePrecision::BetweenInstructions;
+		using BusHandlerT = ConcreteMachine;
+	};
+	CPU::MOS6502Mk2::Processor<CPU::MOS6502Mk2::Model::M6502, M6502Traits> m6502_;
+
 	void update_video() {
 		mos6560_.run_for(cycles_since_mos6560_update_.flush<Cycles>());
 	}
-	CPU::MOS6502::Processor<CPU::MOS6502::Personality::P6502, ConcreteMachine, false> m6502_;
 
 	std::vector<uint8_t> character_rom_;
 	std::vector<uint8_t> basic_rom_;
@@ -745,12 +751,22 @@ private:
 			++address;
 		}
 	}
-	void write_to_map(const uint8_t **const map, const uint8_t *area, uint16_t address, size_t length) {
+	void write_to_map(
+		const uint8_t **const map,
+		const uint8_t *const area,
+		const uint16_t address,
+		const size_t length
+	) {
 		write_to_map([&](const uint16_t address, const size_t offset) {
 			map[address] = &area[offset];
 		}, address, length);
 	}
-	void write_to_map(uint8_t **const map, uint8_t *area, uint16_t address, size_t length) {
+	void write_to_map(
+		uint8_t **const map,
+		uint8_t *const area,
+		const uint16_t address,
+		const size_t length
+	) {
 		write_to_map([&](const uint16_t address, const size_t offset) {
 			map[address] = &area[offset];
 		}, address, length);

Machines/Enterprise/Dave.cpp

@@ -12,7 +12,7 @@ using namespace Enterprise::Dave;
 
 // MARK: - Audio generator
 
-Audio::Audio(Concurrency::AsyncTaskQueue<false> &audio_queue) :
+Audio::Audio(Outputs::Speaker::TaskQueue &audio_queue) :
 	audio_queue_(audio_queue) {}
 
 void Audio::write(uint16_t address, const uint8_t value) {

Machines/Enterprise/Dave.hpp

@@ -11,7 +11,7 @@
 #include <cstdint>
 
 #include "ClockReceiver/ClockReceiver.hpp"
-#include "Concurrency/AsyncTaskQueue.hpp"
+#include "Outputs/Speaker/SpeakerQueue.hpp"
 #include "Numeric/LFSR.hpp"
 #include "Outputs/Speaker/Implementation/BufferSource.hpp"
 
@@ -28,7 +28,7 @@ enum class Interrupt: uint8_t {
 */
 class Audio: public Outputs::Speaker::BufferSource<Audio, true> {
 public:
-	Audio(Concurrency::AsyncTaskQueue<false> &audio_queue);
+	Audio(Outputs::Speaker::TaskQueue &);
 
 	/// Modifies an register in the audio range; only the low 4 bits are
 	/// used for register decoding so it's assumed that the caller has
@@ -41,7 +41,7 @@ public:
 	void apply_samples(std::size_t number_of_samples, Outputs::Speaker::StereoSample *target);
 
 private:
-	Concurrency::AsyncTaskQueue<false> &audio_queue_;
+	Outputs::Speaker::TaskQueue &audio_queue_;
 
 	// Global divider (i.e. 8MHz/12Mhz switch).
 	uint8_t global_divider_;

Machines/Enterprise/Enterprise.cpp

@@ -103,12 +103,10 @@ public:
 		min_ram_slot_(min_ram_slot(target)),
 		z80_(*this),
 		nick_(ram_.end() - 65536),
-		dave_audio_(audio_queue_),
-		speaker_(dave_audio_) {
+		audio_(float(clock_rate) / float(DaveDivider), DaveDivider) {
 
 		// Request a clock of 4Mhz; this'll be mapped upwards for Nick and downwards for Dave elsewhere.
 		set_clock_rate(clock_rate);
-		speaker_.set_input_rate(float(clock_rate) / float(dave_divider));
 
 		ROM::Request request;
 		using Target = Analyser::Static::Enterprise::Target;
@@ -257,7 +255,7 @@ public:
 	}
 
 	~ConcreteMachine() {
-		audio_queue_.flush();
+		audio_.stop();
 	}
 
 	// MARK: - Z80::BusHandler.
@@ -344,7 +342,7 @@ public:
 		}
 
 		const HalfCycles full_length = cycle.length + penalty;
-		time_since_audio_update_ += full_length;
+		audio_ += full_length;
 		advance_nick(full_length);
 		if(dave_timer_ += full_length) {
 			set_interrupts(dave_timer_.last_valid()->get_new_interrupts(), dave_timer_.last_sequence_point_overrun());
@@ -475,8 +473,7 @@ public:
 					case 0xa4:	case 0xa5:	case 0xa6:	case 0xa7:
 					case 0xa8:	case 0xa9:	case 0xaa:	case 0xab:
 					case 0xac:	case 0xad:	case 0xae:	case 0xaf:
-						update_audio();
-						dave_audio_.write(address, *cycle.value);
+						audio_->write(address, *cycle.value);
 						dave_timer_->write(address, *cycle.value);
 					break;
 
@@ -563,8 +560,7 @@ public:
 			nick_.flush();
 		}
 		if(outputs & Output::Audio) {
-			update_audio();
-			audio_queue_.perform();
+			audio_.perform();
 		}
 	}
 
@@ -650,7 +646,7 @@ private:
 	// MARK: - AudioProducer
 
 	Outputs::Speaker::Speaker *get_speaker() final {
-		return &speaker_;
+		return &audio_.speaker();
 	}
 
 	// MARK: - TimedMachine
@@ -726,20 +722,13 @@ private:
 	bool previous_nick_interrupt_line_ = false;
 	// Cf. timing guesses above.
 
-	Concurrency::AsyncTaskQueue<false> audio_queue_;
-	Dave::Audio dave_audio_;
-	Outputs::Speaker::PullLowpass<Dave::Audio> speaker_;
-	HalfCycles time_since_audio_update_;
-
+	Outputs::Speaker::PullLowpassSpeakerQueue<HalfCycles, Dave::Audio> audio_;
 	HalfCycles dave_delay_ = HalfCycles(2);
 
 	// The divider supplied to the JustInTimeActor and the manual divider used in
-	// update_audio() should match.
-	static constexpr int dave_divider = 8;
-	JustInTimeActor<Dave::TimedInterruptSource, HalfCycles, 1, dave_divider> dave_timer_;
-	inline void update_audio() {
-		speaker_.run_for(audio_queue_, time_since_audio_update_.divide_cycles(Cycles(dave_divider)));
-	}
+	// the spekaer queue should match.
+	static constexpr int DaveDivider = 8;
+	JustInTimeActor<Dave::TimedInterruptSource, HalfCycles, 1, DaveDivider> dave_timer_;
 
 	// MARK: - EXDos card.
 	EXDos exdos_;

Machines/MSX/MSX.cpp

@@ -360,7 +360,7 @@ public:
 	}
 
 	~ConcreteMachine() {
-		speaker_.audio_queue.flush();
+		speaker_.audio_queue.lock_flush();
 	}
 
 	void set_scan_target(Outputs::Display::ScanTarget *scan_target) final {

Machines/MasterSystem/MasterSystem.cpp

@@ -178,7 +178,7 @@ public:
 	}
 
 	~ConcreteMachine() {
-		audio_queue_.flush();
+		audio_queue_.lock_flush();
 	}
 
 	ChangeEffect effect_for_file_did_change(const std::string &) const final {

Machines/Oric/Oric.cpp

@@ -406,7 +406,7 @@ public:
 	}
 
 	~ConcreteMachine() {
-		audio_queue_.flush();
+		audio_queue_.lock_flush();
 	}
 
 	void set_key_state(uint16_t key, bool is_pressed) final {

Machines/PCCompatible/PCCompatible.cpp

@@ -817,7 +817,7 @@ public:
 	}
 
 	~ConcreteMachine() {
-		speaker_.queue.flush();
+		speaker_.queue.lock_flush();
 	}
 
 	// MARK: - TimedMachine.

Machines/Sinclair/ZX8081/ZX8081.cpp

@@ -114,7 +114,7 @@ public:
 	}
 
 	~ConcreteMachine() {
-		audio_queue_.flush();
+		audio_queue_.lock_flush();
 	}
 
 	forceinline HalfCycles perform_machine_cycle(const CPU::Z80::PartialMachineCycle &cycle) {

Machines/Sinclair/ZXSpectrum/ZXSpectrum.cpp

@@ -213,7 +213,7 @@ public:
 	}
 
 	~ConcreteMachine() {
-		audio_queue_.flush();
+		audio_queue_.lock_flush();
 	}
 
 	static constexpr unsigned int clock_rate() {

Numeric/SizedInt.hpp

@@ -24,8 +24,9 @@ struct SizedInt {
 	constexpr SizedInt(const IntT start_value) noexcept : counter_(start_value & Mask) {}
 	SizedInt() = default;
 
+	template <int begin = 0>
 	IntT get() const {
-		return counter_;
+		return counter_ >> begin;
 	}
 
 	SizedInt operator +(const SizedInt offset) const {	return SizedInt<bits>(counter_ + offset.counter_); }

OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj

@@ -753,6 +753,10 @@
 		4B92E26B234AE35100CD6D1B /* MFP68901.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B92E268234AE35000CD6D1B /* MFP68901.cpp */; };
 		4B92EACA1B7C112B00246143 /* 6502TimingTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4B92EAC91B7C112B00246143 /* 6502TimingTests.swift */; };
 		4B9378E422A199C600973513 /* Audio.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B9378E222A199C600973513 /* Audio.cpp */; };
+		4B96DECC2EBEE7D100505298 /* SID.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B96DECA2EBEE7D100505298 /* SID.cpp */; };
+		4B96DECD2EBEE7D100505298 /* SID.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B96DECA2EBEE7D100505298 /* SID.cpp */; };
+		4B96DECE2EBEE7D100505298 /* SID.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B96DECA2EBEE7D100505298 /* SID.cpp */; };
+		4B96DED32EC3ECDA00505298 /* SIDTests.mm in Sources */ = {isa = PBXBuildFile; fileRef = 4B96DED22EC3ECDA00505298 /* SIDTests.mm */; };
 		4B96F7CE263E33B10092AEE1 /* DSK.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B96F7CC263E33B10092AEE1 /* DSK.cpp */; };
 		4B96F7CF263E33B10092AEE1 /* DSK.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B96F7CC263E33B10092AEE1 /* DSK.cpp */; };
 		4B98A05E1FFAD3F600ADF63B /* CSROMFetcher.mm in Sources */ = {isa = PBXBuildFile; fileRef = 4B98A05D1FFAD3F600ADF63B /* CSROMFetcher.mm */; };
@@ -1952,6 +1956,10 @@
 		4B95FA9C1F11893B0008E395 /* ZX8081Controller.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = ZX8081Controller.swift; sourceTree = "<group>"; };
 		4B961408222760E0001A7BF2 /* Screenshot.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = Screenshot.hpp; sourceTree = "<group>"; };
 		4B96DEC32EBEA88C00505298 /* TubeProcessor.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = TubeProcessor.hpp; sourceTree = "<group>"; };
+		4B96DEC92EBEE7D100505298 /* SID.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = SID.hpp; sourceTree = "<group>"; };
+		4B96DECA2EBEE7D100505298 /* SID.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = SID.cpp; sourceTree = "<group>"; };
+		4B96DED22EC3ECDA00505298 /* SIDTests.mm */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.objcpp; path = SIDTests.mm; sourceTree = "<group>"; };
+		4B96DED42EC53BC300505298 /* BiquadFilter.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = BiquadFilter.hpp; sourceTree = "<group>"; };
 		4B96F7CB263E30B00092AEE1 /* RawSectorDump.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = RawSectorDump.hpp; sourceTree = "<group>"; };
 		4B96F7CC263E33B10092AEE1 /* DSK.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = DSK.cpp; sourceTree = "<group>"; };
 		4B96F7CD263E33B10092AEE1 /* DSK.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = DSK.hpp; sourceTree = "<group>"; };
@@ -2515,6 +2523,7 @@
 		4BEF6AA81D35CE9E00E73575 /* DigitalPhaseLockedLoopBridge.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = DigitalPhaseLockedLoopBridge.h; sourceTree = "<group>"; };
 		4BEF6AA91D35CE9E00E73575 /* DigitalPhaseLockedLoopBridge.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; path = DigitalPhaseLockedLoopBridge.mm; sourceTree = "<group>"; };
 		4BEF6AAB1D35D1C400E73575 /* DPLLTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = DPLLTests.swift; sourceTree = "<group>"; };
+		4BEF9CA92EC8294E00DDD0F6 /* SpeakerQueue.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = SpeakerQueue.hpp; sourceTree = "<group>"; };
 		4BF0BC67297108D100CCA2B5 /* MemorySlotHandler.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = MemorySlotHandler.cpp; sourceTree = "<group>"; };
 		4BF0BC6F2973318E00CCA2B5 /* RP5C01.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; path = RP5C01.cpp; sourceTree = "<group>"; };
 		4BF0BC702973318E00CCA2B5 /* RP5C01.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = RP5C01.hpp; sourceTree = "<group>"; };
@@ -3017,6 +3026,7 @@
 				4BC76E671C98E31700E6EF73 /* FIRFilter.cpp */,
 				4BC76E681C98E31700E6EF73 /* FIRFilter.hpp */,
 				4B24095A1C45DF85004DA684 /* Stepper.hpp */,
+				4B96DED42EC53BC300505298 /* BiquadFilter.hpp */,
 			);
 			name = SignalProcessing;
 			path = ../../SignalProcessing;
@@ -4354,6 +4364,15 @@
 			path = 68901;
 			sourceTree = "<group>";
 		};
+		4B96DECB2EBEE7D100505298 /* SID */ = {
+			isa = PBXGroup;
+			children = (
+				4B96DEC92EBEE7D100505298 /* SID.hpp */,
+				4B96DECA2EBEE7D100505298 /* SID.cpp */,
+			);
+			path = SID;
+			sourceTree = "<group>";
+		};
 		4B9F11C72272375400701480 /* QL Startup */ = {
 			isa = PBXGroup;
 			children = (
@@ -4872,6 +4891,7 @@
 				4BD4A8CF1E077FD20020D856 /* PCMTrackTests.mm */,
 				4B3F76B825A1635300178AEC /* PowerPCDecoderTests.mm */,
 				4BE76CF822641ED300ACD6FA /* QLTests.mm */,
+				4B96DED22EC3ECDA00505298 /* SIDTests.mm */,
 				4B8DD3672633B2D400B3C866 /* SpectrumVideoContentionTests.mm */,
 				4B2AF8681E513FC20027EE29 /* TIATests.mm */,
 				4B1D08051E0F7A1100763741 /* TimeTests.mm */,
@@ -5168,6 +5188,7 @@
 				4BF0BC6E2973318E00CCA2B5 /* RP5C01 */,
 				4B8855A42E84D51B00E251DD /* SAA5050 */,
 				4B0ACBFF237756EC008902D0 /* Serial */,
+				4B96DECB2EBEE7D100505298 /* SID */,
 				4BB0A6582044FD3000FB3688 /* SN76489 */,
 				4B47F3B42E7B9A14005D4DEC /* uPD7002 */,
 			);
@@ -5292,6 +5313,7 @@
 		4BD060A41FE49D3C006E14BE /* Speaker */ = {
 			isa = PBXGroup;
 			children = (
+				4BEF9CA92EC8294E00DDD0F6 /* SpeakerQueue.hpp */,
 				4BD060A51FE49D3C006E14BE /* Speaker.hpp */,
 				4B8EF6051FE5AF830076CCDD /* Implementation */,
 			);
@@ -6163,6 +6185,7 @@
 				4B894521201967B4007DE474 /* StaticAnalyser.cpp in Sources */,
 				4B47F3B62E7BAB94005D4DEC /* uPD7002.cpp in Sources */,
 				4B8318B522D3E548006DB630 /* Macintosh.cpp in Sources */,
+				4B96DECD2EBEE7D100505298 /* SID.cpp in Sources */,
 				4B8DF4FA254E36AE00F3433C /* Video.cpp in Sources */,
 				4B7BA03123C2B19C00B98D9E /* Jasmin.cpp in Sources */,
 				4B7F188F2154825E00388727 /* MasterSystem.cpp in Sources */,
@@ -6557,6 +6580,7 @@
 				4BC080D026A257A200D03FD8 /* StaticAnalyser.cpp in Sources */,
 				4B4DC8211D2C2425003C5BF8 /* Vic20.cpp in Sources */,
 				4B71368E1F788112008B8ED9 /* Parser.cpp in Sources */,
+				4B96DECC2EBEE7D100505298 /* SID.cpp in Sources */,
 				4B12C0ED1FCFA98D005BFD93 /* Keyboard.cpp in Sources */,
 				4BA0F68E1EEA0E8400E9489E /* ZX8081.cpp in Sources */,
 				429B13602B1F7BDA006BB4CB /* StaticAnalyser.cpp in Sources */,
@@ -6683,6 +6707,7 @@
 				4B06AAF82C6460760034D014 /* IntelligentKeyboard.cpp in Sources */,
 				4B7752B628217EE70073E2C5 /* DSK.cpp in Sources */,
 				4B06AAD12C645F130034D014 /* 1770.cpp in Sources */,
+				4B96DECE2EBEE7D100505298 /* SID.cpp in Sources */,
 				4B778F2523A5EDF40000D260 /* Encoder.cpp in Sources */,
 				4B778F4223A5F1A70000D260 /* MemoryFuzzer.cpp in Sources */,
 				4B778F0123A5EBA00000D260 /* MacintoshIMG.cpp in Sources */,
@@ -6778,6 +6803,7 @@
 				4B778F4623A5F1D80000D260 /* StaticAnalyser.cpp in Sources */,
 				4B778F1323A5EC890000D260 /* Z80Base.cpp in Sources */,
 				4B778F2923A5EF030000D260 /* CommodoreROM.cpp in Sources */,
+				4B96DED32EC3ECDA00505298 /* SIDTests.mm in Sources */,
 				4B06AADC2C645F720034D014 /* BD500.cpp in Sources */,
 				4B778F4823A5F1E70000D260 /* StaticAnalyser.cpp in Sources */,
 				4B92EACA1B7C112B00246143 /* 6502TimingTests.swift in Sources */,

OSBindings/Mac/Clock Signal/Machine/CSMachine.mm

@@ -39,7 +39,7 @@
 namespace {
 
 struct MachineUpdater {
-	void perform(Time::Nanos duration) {
+	void perform(const Time::Nanos duration) {
 		// Top out at 1/20th of a second; this is a safeguard against a negative
 		// feedback loop if emulation starts running slowly.
 		const auto seconds = std::min(Time::seconds(duration), 0.05);
@@ -51,7 +51,7 @@ struct MachineUpdater {
 	MachineTypes::TimedMachine *timed_machine = nullptr;
 };
 
-using Updater = Concurrency::AsyncTaskQueue<true, false, MachineUpdater>;
+using Updater = Concurrency::AsyncTaskQueue<true, false, false, MachineUpdater>;
 
 }
 

OSBindings/Mac/Clock Signal/MachinePicker/Base.lproj/MachinePicker.xib

@@ -1,8 +1,8 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<document type="com.apple.InterfaceBuilder3.Cocoa.XIB" version="3.0" toolsVersion="24128" targetRuntime="MacOSX.Cocoa" propertyAccessControl="none" useAutolayout="YES" customObjectInstantitationMethod="direct">
+<document type="com.apple.InterfaceBuilder3.Cocoa.XIB" version="3.0" toolsVersion="24412" targetRuntime="MacOSX.Cocoa" propertyAccessControl="none" useAutolayout="YES" customObjectInstantitationMethod="direct">
     <dependencies>
         <deployment identifier="macosx"/>
-        <plugIn identifier="com.apple.InterfaceBuilder.CocoaPlugin" version="24128"/>
+        <plugIn identifier="com.apple.InterfaceBuilder.CocoaPlugin" version="24412"/>
         <capability name="documents saved in the Xcode 8 format" minToolsVersion="8.0"/>
     </dependencies>
     <objects>

OSBindings/Mac/Clock SignalTests/SIDTests.mm

@@ -0,0 +1,83 @@
+//
+//  SIDTests.mm
+//  Clock SignalTests
+//
+//  Created by Thomas Harte on 11/11/2025.
+//  Copyright © 2025 Thomas Harte. All rights reserved.
+//
+
+#import <XCTest/XCTest.h>
+
+#include "Components/SID/SID.hpp"
+
+@interface SIDTests : XCTestCase
+@end
+
+@implementation SIDTests
+
+- (void)testOscillator {
+	MOS::SID::Voice prior;
+	MOS::SID::Voice voice;
+	const uint32_t pulse_width = 0x02'3;
+
+	voice.oscillator.pitch = 0x00'1000'00;
+	voice.oscillator.pulse_width = pulse_width << 20;
+	voice.oscillator.reset_phase();
+
+	int c = 0;
+
+	// Run for first half of a cycle.
+	while(!voice.oscillator.did_raise_b23()) {
+		// Force envelope.
+		voice.adsr.envelope = 255;
+
+		// Test sawtooth.
+		voice.set_control(0x20);
+		XCTAssertEqual(voice.output(prior), c);
+
+		// Test triangle.
+		voice.set_control(0x10);
+		XCTAssertEqual(voice.output(prior), c << 1);
+
+		// Test pulse.
+		voice.set_control(0x40);
+		XCTAssertEqual(voice.output(prior), (c < pulse_width) ? 0 : 4095);
+
+		// Advance.
+		voice.update();
+		++c;
+	}
+
+	// B23 should go up halfway through the 12-bit range.
+	XCTAssertEqual(c, 2048);
+
+	// Run for second half of a cycle.
+	while(c < 4096) {
+		// Force envelope.
+		voice.adsr.envelope = 255;
+
+		// Test sawtooth.
+		voice.set_control(0x20);
+		XCTAssertEqual(voice.output(prior), c);
+
+		// Test triangle.
+		voice.set_control(0x10);
+		XCTAssertEqual(voice.output(prior), 4095 - ((c << 1) & 4095));
+
+		// Test pulse.
+		voice.set_control(0x40);
+		XCTAssertEqual(voice.output(prior), (c <= pulse_width) ? 0 : 4095);
+
+		// Advance.
+		voice.update();
+		++c;
+
+		XCTAssert(!voice.oscillator.did_raise_b23());
+	}
+
+	// Check that B23 doesn't false rise again.
+	voice.update();
+	XCTAssert(!voice.oscillator.did_raise_b23());
+}
+
+@end

OSBindings/Qt/clksignal.pro

@@ -76,6 +76,7 @@ SOURCES += \
 	$$SRC/Components/KonamiSCC/*.cpp \
 	$$SRC/Components/OPx/*.cpp \
 	$$SRC/Components/RP5C01/*.cpp \
+	$$SRC/Components/SID/*.cpp \
 	$$SRC/Components/SAA5050/*.cpp \
 	$$SRC/Components/Serial/*.cpp \
 	$$SRC/Components/SN76489/*.cpp \
@@ -213,6 +214,7 @@ HEADERS += \
 	$$SRC/Components/OPx/*.hpp \
 	$$SRC/Components/OPx/Implementation/*.hpp \
 	$$SRC/Components/RP5C01/*.hpp \
+	$$SRC/Components/SID/*.hpp \
 	$$SRC/Components/SAA5050/*.hpp \
 	$$SRC/Components/Serial/*.hpp \
 	$$SRC/Components/SN76489/*.hpp \

OSBindings/SDL/SConstruct

@@ -61,6 +61,7 @@ SOURCES += glob.glob('../../Components/KonamiSCC/*.cpp')
 SOURCES += glob.glob('../../Components/OPx/*.cpp')
 SOURCES += glob.glob('../../Components/RP5C01/*.cpp')
 SOURCES += glob.glob('../../Components/SAA5050/*.cpp')
+SOURCES += glob.glob('../../Components/SID/*.cpp')
 SOURCES += glob.glob('../../Components/SN76489/*.cpp')
 SOURCES += glob.glob('../../Components/Serial/*.cpp')
 SOURCES += glob.glob('../../Components/uPD7002/*.cpp')

Outputs/Speaker/Implementation/BufferSource.hpp

@@ -9,6 +9,7 @@
 #pragma once
 
 #include "Outputs/Speaker/Speaker.hpp"
+#include "Concurrency/AsyncTaskQueue.hpp"
 
 #include <algorithm>
 #include <array>
@@ -26,7 +27,7 @@ enum class Action {
 	Ignore,
 };
 
-template <Action action, typename SampleT> void apply(SampleT &lhs, SampleT rhs) {
+template <Action action, typename SampleT> void apply(SampleT &lhs, const SampleT rhs) {
 	switch(action) {
 		case Action::Mix:	lhs += rhs;	break;
 		case Action::Store:	lhs = rhs;	break;
@@ -34,7 +35,8 @@ template <Action action, typename SampleT> void apply(SampleT &lhs, SampleT rhs)
 	}
 }
 
-template <Action action, typename IteratorT, typename SampleT> void fill(IteratorT begin, IteratorT end, SampleT value) {
+template <Action action, typename IteratorT, typename SampleT>
+void fill(IteratorT begin, const IteratorT end, const SampleT value) {
 	switch(action) {
 		case Action::Mix:
 			while(begin != end) {
@@ -56,45 +58,45 @@ template <Action action, typename IteratorT, typename SampleT> void fill(Iterato
 */
 template <typename SourceT, bool stereo>
 class BufferSource {
-	public:
-		/*!
-			Indicates whether this component will write stereo samples.
-		*/
-		static constexpr bool is_stereo = stereo;
+public:
+	/*!
+		Indicates whether this component will write stereo samples.
+	*/
+	static constexpr bool is_stereo = stereo;
 
-		/*!
-			Should 'apply' the next @c number_of_samples to @c target ; application means applying @c action which can be achieved either via the
-			helper functions above — @c apply and @c fill — or by semantic inspection (primarily, if an obvious quick route for @c Action::Ignore is available).
+	/*!
+		Should 'apply' the next @c number_of_samples to @c target ; application means applying @c action which can be achieved either via the
+		helper functions above — @c apply and @c fill — or by semantic inspection (primarily, if an obvious quick route for @c Action::Ignore is available).
 
-			No default implementation is provided.
-		*/
-		template <Action action>
-		void apply_samples(std::size_t number_of_samples, typename SampleT<stereo>::type *target);
+		No default implementation is provided.
+	*/
+	template <Action action>
+	void apply_samples(std::size_t number_of_samples, typename SampleT<stereo>::type *);
 
-		/*!
-			@returns @c true if it is trivially true that a call to get_samples would just
-				fill the target with zeroes; @c false if a call might return all zeroes or
-				might not.
-		*/
-//		bool is_zero_level() const						{	return false;	}
+	/*!
+		@returns @c true if it is trivially true that a call to get_samples would just
+			fill the target with zeroes; @c false if a call might return all zeroes or
+			might not.
+	*/
+//	bool is_zero_level() const						{	return false;	}
 
-		/*!
-			Sets the proper output range for this sample source; it should write values
-			between 0 and volume.
-		*/
-//		void set_sample_volume_range(std::int16_t volume);
+	/*!
+		Sets the proper output range for this sample source; it should write values
+		between 0 and volume.
+	*/
+//	void set_sample_volume_range(std::int16_t volume);
 
-		/*!
-			Permits a sample source to declare that, averaged over time, it will use only
-			a certain proportion of the allocated volume range. This commonly happens
-			in sample sources that use a time-multiplexed sound output — for example, if
-			one were to output only every other sample then it would return 0.5.
+	/*!
+		Permits a sample source to declare that, averaged over time, it will use only
+		a certain proportion of the allocated volume range. This commonly happens
+		in sample sources that use a time-multiplexed sound output — for example, if
+		one were to output only every other sample then it would return 0.5.
 
-			This is permitted to vary over time but there is no contract as to when it will be
-			used by a speaker. If it varies, it should do so very infrequently and only to
-			represent changes in hardware configuration.
-		*/
-		double average_output_peak() const { return 1.0; }
+		This is permitted to vary over time but there is no contract as to when it will be
+		used by a speaker. If it varies, it should do so very infrequently and only to
+		represent changes in hardware configuration.
+	*/
+	double average_output_peak() const { return 1.0; }
 };
 
 ///
@@ -140,7 +142,7 @@ public:
 
 	// TODO: use a concept here, when C++20 filters through.
 	//
-	// Until then: sample sources should implement this.
+	// Until then: sample sources can implement this rather than apply_samples.
 //	typename SampleT<stereo>::type level() const;
 //	void advance();
 

Outputs/Speaker/Implementation/LowpassSpeaker.hpp

@@ -10,6 +10,7 @@
 
 #include "BufferSource.hpp"
 #include "Outputs/Speaker/Speaker.hpp"
+#include "Outputs/Speaker/SpeakerQueue.hpp"
 #include "SignalProcessing/FIRFilter.hpp"
 #include "ClockReceiver/ClockReceiver.hpp"
 #include "Concurrency/AsyncTaskQueue.hpp"
@@ -375,10 +376,13 @@ public:
 		if(cycles == Cycles(0)) {
 			return;
 		}
+		queue.enqueue(update_for(cycles));
+	}
 
-		queue.enqueue([this, cycles] {
+	std::function<void(void)> update_for(const Cycles cycles) {
+		return [this, cycles] {
 			run_for(cycles);
-		});
+		};
 	}
 
 private:
@@ -414,4 +418,7 @@ private:
 	}
 };
 
+template <typename CyclesT, typename GeneratorT>
+using PullLowpassSpeakerQueue = SpeakerQueue<CyclesT, Outputs::Speaker::PullLowpass<GeneratorT>, GeneratorT>;
+
 }

Outputs/Speaker/SpeakerQueue.hpp

@@ -0,0 +1,68 @@
+//
+//  SpeakerQueue.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 14/11/2025.
+//  Copyright © 2025 Thomas Harte. All rights reserved.
+//
+
+#pragma once
+
+#include "Concurrency/AsyncTaskQueue.hpp"
+#include "ClockReceiver/ClockReceiver.hpp"
+
+namespace Outputs::Speaker {
+using TaskQueue = Concurrency::AsyncTaskQueue<false, true, true>;
+
+template <typename CyclesT, typename SpeakerT, typename GeneratorT>
+struct SpeakerQueue: private Concurrency::EnqueueDelegate {
+	constexpr SpeakerQueue(const CyclesT divider) noexcept :
+		generator_(queue_), speaker_(generator_), divider_(divider)
+	{
+		queue_.set_enqueue_delegate(this);
+	}
+
+	constexpr SpeakerQueue(const float input_rate, const CyclesT divider, const float high_cutoff = -1.0f) noexcept :
+		SpeakerQueue(divider)
+	{
+		speaker_.set_input_rate(input_rate);
+		if(high_cutoff >= 0.0) {
+			speaker_.set_high_frequency_cutoff(high_cutoff);
+		}
+	}
+
+	void operator += (const CyclesT &duration) {
+		time_since_update_ += duration;
+	}
+
+	void stop() {
+		queue_.stop();
+	}
+
+	void perform() {
+		// TODO: is there a way to avoid the empty lambda?
+		queue_.enqueue([]() {});
+		queue_.perform();
+	}
+
+	SpeakerT &speaker() {
+		return speaker_;
+	}
+
+	GeneratorT *operator ->() {
+		return &generator_;
+	}
+
+private:
+	TaskQueue queue_;
+	GeneratorT generator_;
+	SpeakerT speaker_;
+	CyclesT divider_;
+	CyclesT time_since_update_;
+
+	std::function<void(void)> prepare_enqueue() final {
+		return speaker_.update_for(time_since_update_.template divide<Cycles>(divider_));
+	}
+};
+
+}

Processors/6502Mk2/6502Mk2.hpp

@@ -66,6 +66,7 @@ enum class Line {
 	PowerOn,
 	Overflow,
 	NMI,
+	Ready,
 };
 
 // MARK: - Address bus.
@@ -185,14 +186,24 @@ public:
 					(inputs_.interrupt_requests & Inputs::InterruptRequest::PowerOn);
 			break;
 
-			// Level triggered.
+			// Level triggered interrupts.
 			case Line::Reset:		level_sample(Inputs::InterruptRequest::Reset);						break;
 			case Line::IRQ:			level_sample(Inputs::InterruptRequest::IRQ);						break;
 
-			// Edge triggered.
-			case Line::Overflow:	edge_sample(Inputs::InterruptRequest::Reset, inputs_.overflow);		break;
+			// Edge triggered interrupts.
 			case Line::NMI:			edge_sample(Inputs::InterruptRequest::NMI, inputs_.nmi);			break;
 
+			// Leval-capturing state.
+			case Line::Ready:		inputs_.ready = value;												break;
+
+			// Edge-triggered state.
+			case Line::Overflow:
+				if(!inputs_.overflow && value) {
+					registers_.flags.set_per<Flag::Overflow>(Flag::Overflow);
+				}
+				inputs_.overflow = value;
+			break;
+
 			default:
 				__builtin_unreachable();
 		}

SignalProcessing/BiquadFilter.hpp

@@ -0,0 +1,214 @@
+//
+//  BiquadFilter.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 12/11/2025.
+//  Copyright © 2025 Thomas Harte. All rights reserved.
+//
+
+#pragma once
+
+#include <cassert>
+#include <cmath>
+#include <numbers>
+
+namespace SignalProcessing {
+
+/*!
+	A biquad[ratic] filter approximates the real analogue thing in taking a 1d PCM signal and applying a
+	filter to it as a function of the current input plus the two most-recent inputs plus the two most-recent outputs.
+
+	So both IIR and three-tap FIR filters are degenerate cases of the biquad.
+
+	It is used quite often in real designs, hence an implementation of this filter specifically.
+
+	... and the below is largely textbook; I can't claim any great knowledge. I am especially indebted to
+	the W3C Group's audio EQ cookbook at https://www.w3.org/TR/audio-eq-cookbook/ .
+*/
+class BiquadFilter {
+public:
+	enum class Type {
+		LowPass,
+		HighPass,
+		BandPass,
+		Notch,
+		AllPass,
+		Peaking,
+		LowShelf,
+		HighShelf
+	};
+
+	// Default construction: a filter that produces _nothing_.
+	BiquadFilter() {}
+
+	BiquadFilter(
+		const Type type,
+		const float sample_rate,
+		const float frequency,
+		const float resonance = 0.707f,
+		const float gain = 8,
+		const bool normalise = true
+	) {
+		configure(type, sample_rate, frequency, resonance, gain, normalise);
+	}
+
+	void configure(
+		const Type type,
+		const float sample_rate,
+		const float frequency,
+		const float resonance = 0.707f,
+		const float gain = 8,
+		const bool normalise = true
+	) {
+		const float w0 = 2.0f * std::numbers::pi_v<float> * frequency / sample_rate;
+		const float alpha = std::sin(w0) / (2.0f * resonance);
+		const float cos_w0 = std::cos(w0);
+
+		float coefficients[5];
+		float magnitude = 1.0f;
+		switch(type) {
+			case Type::LowPass:
+				coefficients[0] = (1.0f - cos_w0) / 2.0f;
+				coefficients[1] = 1.0f - cos_w0;
+				coefficients[2] = (1.0f - cos_w0) / 2.0f;
+				magnitude = 1.0f + alpha;
+				coefficients[3] = -2.0f * cos_w0;
+				coefficients[4] = 1.0f - alpha;
+			break;
+
+			case Type::HighPass:
+				coefficients[0] = (1.0f - cos_w0) / 2.0f;
+				coefficients[1] = -(1.0f + cos_w0);
+				coefficients[2] = (1.0f - cos_w0) / 2.0f;
+				magnitude = 1.0f + alpha;
+				coefficients[3] = -2.0f * cos_w0;
+				coefficients[4] = 1.0f - alpha;
+			break;
+
+			case Type::BandPass:
+				coefficients[0] = alpha;
+				coefficients[1] = 0.0f;
+				coefficients[2] = -alpha;
+				magnitude = 1.0f + alpha;
+				coefficients[3] = -2.0f * cos_w0;
+				coefficients[0] = 1.0f - alpha;
+			break;
+
+			case Type::Notch:
+				coefficients[0] = 1.0f;
+				coefficients[1] = -2.0f * cos_w0;
+				coefficients[2] = 1.0f;
+				magnitude = 1.0f + alpha;
+				coefficients[3] = -2.0f * cos_w0;
+				coefficients[4] = 1.0f - alpha;
+			break;
+
+			case Type::AllPass:
+				coefficients[0] = 1.0f - alpha;
+				coefficients[1] = -2.0f * cos_w0;
+				coefficients[2] = 1.0f + alpha;
+				magnitude = 1.0f + alpha;
+				coefficients[3] = -2.0f * cos_w0;
+				coefficients[4] = 1.0f - alpha;
+			break;
+
+			case Type::Peaking: {
+				const float a = std::pow(10.0f, gain / 40.0f);
+
+				coefficients[0] = 1.0f + (alpha * a);
+				coefficients[1] = -2.0f * cos_w0;
+				coefficients[2] = 1.0f - (alpha * a);
+				magnitude = 1.0f + (alpha / a);
+				coefficients[3] = -2.0f * cos_w0;
+				coefficients[4] = 1.0f - (alpha / a);
+			} break;
+
+			case Type::LowShelf: {
+				const float a_ls = std::pow(10.0f, gain / 40.0f);
+				const float sqrt_a = std::sqrt(a_ls);
+				const float alpha_ls =
+					std::sin(w0) / 2.0f * std::sqrt((a_ls + 1.0f / a_ls) * (1.0f / resonance - 1.0f) + 2.0f);
+
+				coefficients[0] = a_ls * ((a_ls + 1.0f) - (a_ls - 1.0f) * cos_w0 + 2.0f * sqrt_a * alpha_ls);
+				coefficients[1] = 2.0f * a_ls * ((a_ls - 1.0f) - (a_ls + 1.0f) * cos_w0);
+				coefficients[2] = a_ls * ((a_ls + 1.0f) - (a_ls - 1.0f) * cos_w0 - 2.0f * sqrt_a * alpha_ls);
+				magnitude = (a_ls + 1.0f) + (a_ls - 1.0f) * cos_w0 + 2.0f * sqrt_a * alpha_ls;
+				coefficients[3] = -2.0f * ((a_ls - 1) + (a_ls + 1) * cos_w0);
+				coefficients[4] = (a_ls + 1.0f) + (a_ls - 1.0f) * cos_w0 - 2.0f * sqrt_a * alpha_ls;
+			} break;
+
+			case Type::HighShelf: {
+				const float a_hs = std::pow(10.0f, gain / 40.0f);
+				const float sqrt_a_hs = std::sqrt(a_hs);
+				const float alpha_hs =
+					std::sin(w0) / 2.0f * std::sqrt((a_hs + 1.0f / a_hs) * (1.0f / resonance - 1.0f) + 2.0f);
+
+				coefficients[0] = a_hs * ((a_hs + 1.0f) + (a_hs - 1.0f) * cos_w0 + 2.0f * sqrt_a_hs * alpha_hs);
+				coefficients[1] = -2.0f * a_hs * ((a_hs - 1.0f) + (a_hs + 1.0f) * cos_w0);
+				coefficients[2] = a_hs * ((a_hs + 1.0f) + (a_hs - 1.0f) * cos_w0 - 2.0f * sqrt_a_hs * alpha_hs);
+				magnitude = (a_hs + 1.0f) - (a_hs - 1.0f) * cos_w0 + 2.0f * sqrt_a_hs * alpha_hs;
+				coefficients[3] = 2.0f * ((a_hs - 1.0f) - (a_hs + 1.0f) * cos_w0);
+				coefficients[4] = (a_hs + 1.0f) - (a_hs - 1.0f) * cos_w0 - 2.0f * sqrt_a_hs * alpha_hs;
+			} break;
+		}
+
+		if(normalise) {
+			for(int c = 0; c < 5; c++) {
+				coefficients[c] /= magnitude;
+			}
+		}
+		for(int c = 0; c < 5; c++) {
+#ifdef FIXED
+			coefficients_[c] = FixedType(coefficients[c] * FixedMultiplier);
+#else
+			coefficients_[c] = coefficients[c];
+#endif
+		}
+	}
+
+	int16_t apply(const int16_t input) {
+#ifdef FIXED
+		const auto applied =
+			coefficients_[0] * input +
+			coefficients_[1] * inputs_[0] +
+			coefficients_[2] * inputs_[1] -
+			coefficients_[3] * outputs_[0] -
+			coefficients_[4] * outputs_[1];
+
+		const auto output = int16_t(applied >> FixedShift);
+#else
+		const float output =
+			coefficients_[0] * float(input) +
+			coefficients_[1] * inputs_[0] +
+			coefficients_[2] * inputs_[1] -
+			coefficients_[3] * outputs_[0] -
+			coefficients_[4] * outputs_[1];
+#endif
+
+		inputs_[1] = inputs_[0];
+		inputs_[0] = input;
+		outputs_[1] = outputs_[0];
+		outputs_[0] = output;
+
+		return int16_t(output);
+	}
+
+private:
+#ifdef FIXED
+	int16_t inputs_[2]{};
+	int16_t outputs_[2]{};
+
+	using FixedType = int64_t;
+	static constexpr int FixedShift = 48;
+	static constexpr auto FixedMultiplier = static_cast<float>(int64_t(1) << FixedShift);
+	FixedType coefficients_[5]{};
+#else
+	float inputs_[2]{};
+	float outputs_[2]{};
+	float coefficients_[5]{};
+#endif
+		// Coefficients indices versus common textbook terms:
+		//	0 = b0; 1 = b1; 2 = b2; 3 = a1; 4 = a2
+};
+
+}

Storage/Disk/DiskImage/DiskImageImplementation.hpp

@@ -75,7 +75,7 @@ Track *DiskImageHolder<T>::track_at_position(Track::Address address) const {
 
 template <typename T>
 DiskImageHolder<T>::~DiskImageHolder() {
-	if(update_queue_) update_queue_->flush();
+	if(update_queue_) update_queue_->lock_flush();
 }
 
 template <typename T>

cmake/CLK_SOURCES.cmake

@@ -59,6 +59,7 @@ set(CLK_SOURCES
 	Components/OPx/OPLL.cpp
 	Components/RP5C01/RP5C01.cpp
 	Components/SAA5050/SAA5050.cpp
+	Components/SID/SID.cpp
 	Components/SN76489/SN76489.cpp
 	Components/Serial/Line.cpp
 	Components/uPD7002/uPD7002.cpp