feat(audio): implement PSG noise channel 4

crates/gba-core/src/audio.rs

@@ -1,7 +1,9 @@
-//! Nominal Direct Sound and PSG pulse channels 1–2. No host audio output.
+//! Nominal Direct Sound, PSG pulse channels 1–2, and noise channel 4. No host audio output.
 
 mod modulation;
+mod noise;
 mod pulse;
+use noise::Noise;
 use pulse::Pulse;
 
 const SEQUENCER_PERIOD: u32 = 32768; // 16,777,216 Hz / 512 Hz.
@@ -80,6 +82,7 @@ pub(crate) struct Audio {
     enabled: bool,
     psg: [u16; 17],
     pulses: [Pulse; 2],
+    noise: Noise,
     sequencer_phase: u16,
     next_step: u8,
     control: u16,
@@ -100,7 +103,7 @@ impl Audio {
         {
             return Some("reserved PSG volume selection");
         }
-        if self.enabled && matches!(address, 0x0400_0075 | 0x0400_007d) && value & 0x80 != 0 {
+        if self.enabled && address == 0x0400_0075 && value & 0x80 != 0 {
             Some("PSG channel trigger (synthesis not implemented)")
         } else {
             None
@@ -123,6 +126,9 @@ impl Audio {
     }
 
     pub(crate) fn read8(&self, address: u32) -> Option<u8> {
+        if (0x0400_0078..=0x0400_007d).contains(&address) {
+            return Some(self.noise.read(address - 0x0400_0078));
+        }
         if let Some((index, offset)) = Self::pulse_register(address) {
             return Some(self.pulses[index].read(offset));
         }
@@ -135,8 +141,6 @@ impl Audio {
                     0x0400_0074 => 0x4000,
                     0x0400_0070 => 0x00e0,
                     0x0400_0072 => 0xe000,
-                    0x0400_0078 => 0xff00,
-                    0x0400_007c => 0x40ff,
                     0x0400_0080 => 0xff77,
                     _ => 0,
                 };
@@ -147,6 +151,7 @@ impl Audio {
                 (u16::from(self.enabled) << 7)
                     | u16::from(self.pulses[0].modulation.active)
                     | (u16::from(self.pulses[1].modulation.active) << 1)
+                    | (u16::from(self.noise.modulation.active) << 3)
             }
             SOUNDBIAS => self.bias,
             0x0400_0086 | 0x0400_008a => 0,
@@ -173,6 +178,13 @@ impl Audio {
                 }
                 continue;
             }
+            if (0x0400_0078..=0x0400_007d).contains(&address) {
+                if self.enabled {
+                    self.noise
+                        .write(address - 0x0400_0078, value, self.next_step);
+                }
+                continue;
+            }
             let shift = (address & 1) * 8;
             let merge = |old: u16| (old & !(255 << shift)) | (u16::from(value) << shift);
             match address & !1 {
@@ -194,6 +206,7 @@ impl Audio {
                     if self.enabled && value & 0x80 == 0 {
                         self.psg = [0; 17];
                         self.pulses = [Pulse::default(); 2];
+                        self.noise = Noise::default();
                         for fifo in &mut self.fifo {
                             fifo.reset_queue();
                         }
@@ -222,12 +235,14 @@ impl Audio {
             for pulse in &mut self.pulses {
                 pulse.advance(count);
             }
+            self.noise.advance(count);
             let phase = u32::from(self.sequencer_phase) + count;
             if phase == SEQUENCER_PERIOD {
                 self.sequencer_phase = 0;
                 for pulse in &mut self.pulses {
                     pulse.clock(self.next_step);
                 }
+                self.noise.clock(self.next_step);
                 self.next_step = (self.next_step + 1) & 7;
             } else {
                 self.sequencer_phase = phase as u16;
@@ -278,6 +293,9 @@ impl Audio {
                     psg_sample += pulse.sample();
                 }
             }
+            if psg_control & (1 << (11 + side * 4)) != 0 {
+                psg_sample += self.noise.sample();
+            }
             let volume = ((psg_control >> (side * 4)) & 7) as i16 + 1;
             // Sum PSG channels before scaling, so fractional bits are discarded once.
             *level += (psg_sample * volume * ratio) >> 2;

crates/gba-core/src/audio/noise.rs

@@ -0,0 +1,144 @@
+//! Nominal GBA channel 4 polynomial counter. No random numbers or host clocks.
+use super::modulation::Modulation;
+
+#[derive(Debug, Default, Clone, Copy, PartialEq, Eq)]
+pub(super) struct Noise {
+    pub(super) modulation: Modulation,
+    polynomial: u8,
+    lfsr: u16,
+    remaining: u32,
+    high: bool,
+}
+
+impl Noise {
+    pub(super) fn read(&self, offset: u32) -> u8 {
+        match offset {
+            1 => self.modulation.envelope,
+            4 => self.polynomial,
+            5 => u8::from(self.modulation.length_enabled) << 6,
+            _ => 0,
+        }
+    }
+
+    fn short(&self) -> bool {
+        self.polynomial & 8 != 0
+    }
+
+    fn period(&self) -> u32 {
+        let ratio = u32::from(self.polynomial & 7);
+        let base = if ratio == 0 { 32 } else { 64 * ratio };
+        base << (self.polynomial >> 4)
+    }
+
+    pub(super) fn write(&mut self, offset: u32, value: u8, next_step: u8) {
+        match offset {
+            0 => self.modulation.write_length(value),
+            1 => self.modulation.write_envelope(value),
+            4 => self.polynomial = value, // Preserve counter and remaining interval on live writes.
+            5 => {
+                if self.modulation.write_control(value, next_step) {
+                    self.lfsr = if self.short() { 0x40 } else { 0x4000 };
+                    self.remaining = self.period();
+                    self.high = false;
+                }
+            }
+            _ => {}
+        }
+    }
+
+    pub(super) fn advance(&mut self, cycles: u32) {
+        if !self.modulation.active || cycles == 0 {
+            return;
+        }
+        if cycles < self.remaining {
+            self.remaining -= cycles;
+            return;
+        }
+        let rest = cycles - self.remaining;
+        let period = self.period();
+        let edges = 1 + rest / period;
+        // The output is the carry of the final shift, not the final register's low bit.
+        let before_last = jump(self.lfsr, edges - 1, self.short());
+        self.high = before_last & 1 != 0;
+        self.lfsr = shift(before_last, self.short());
+        self.remaining = period - rest % period;
+    }
+
+    pub(super) fn clock(&mut self, step: u8) {
+        self.modulation.clock(step);
+    }
+
+    pub(super) fn sample(&self) -> i16 {
+        if !self.modulation.active {
+            return 0;
+        }
+        if self.high {
+            i16::from(self.modulation.volume)
+        } else {
+            -i16::from(self.modulation.volume)
+        }
+    }
+}
+
+const fn shift(state: u16, short: bool) -> u16 {
+    (state >> 1)
+        ^ if state & 1 != 0 {
+            if short {
+                0x60
+            } else {
+                0x6000
+            }
+        } else {
+            0
+        }
+}
+
+// Each column maps one input bit. Squaring the linear map doubles its step count.
+// This also handles transient upper bits after a live width change; reducing by
+// the 127-step short-mode period would incorrectly discard those transient bits.
+type Transform = [u16; 15];
+const JUMPS: [[Transform; 32]; 2] = [powers(false), powers(true)];
+
+const fn apply(map: &Transform, mut state: u16) -> u16 {
+    let mut result = 0;
+    while state != 0 {
+        result ^= map[state.trailing_zeros() as usize];
+        state &= state - 1;
+    }
+    result
+}
+
+const fn powers(short: bool) -> [Transform; 32] {
+    let mut maps = [[0; 15]; 32];
+    let mut bit = 0;
+    while bit < 15 {
+        maps[0][bit] = shift(1 << bit, short);
+        bit += 1;
+    }
+    let mut power = 1;
+    while power < 32 {
+        bit = 0;
+        while bit < 15 {
+            maps[power][bit] = apply(&maps[power - 1], maps[power - 1][bit]);
+            bit += 1;
+        }
+        power += 1;
+    }
+    maps
+}
+
+fn jump(mut state: u16, mut count: u32, short: bool) -> u16 {
+    let maps = &JUMPS[usize::from(short)];
+    let mut power = 0;
+    while count != 0 {
+        if count & 1 != 0 {
+            state = apply(&maps[power], state);
+        }
+        count >>= 1;
+        power += 1;
+    }
+    state
+}
+
+#[cfg(test)]
+mod tests;

crates/gba-core/src/audio/noise/tests.rs

@@ -0,0 +1,172 @@
+//! Original counter references and deterministic clock-batching checks.
+use super::*;
+
+fn start(polynomial: u8) -> Noise {
+    let mut noise = Noise::default();
+    noise.write(1, 0xf0, 0);
+    noise.write(4, polynomial, 0);
+    noise.write(5, 0x80, 0);
+    noise
+}
+
+// Independent bit-array form of the documented Galois counter.
+fn reference(state: u16, short: bool) -> (u16, bool) {
+    let mut bits = [false; 15];
+    for (index, bit) in bits.iter_mut().enumerate() {
+        *bit = state & (1 << index) != 0;
+    }
+    let carry = bits[0];
+    bits.copy_within(1..15, 0);
+    bits[14] = false;
+    if carry {
+        let top = if short { 6 } else { 14 };
+        bits[top] = !bits[top];
+        bits[top - 1] = !bits[top - 1];
+    }
+    let value = bits
+        .iter()
+        .enumerate()
+        .fold(0, |value, (index, &bit)| value | (u16::from(bit) << index));
+    (value, carry)
+}
+
+#[test]
+fn both_counters_visit_every_nonzero_state_before_repeating() {
+    for (short, width) in [(false, 15), (true, 7)] {
+        let initial = 1 << (width - 1);
+        let mut state = initial;
+        let mut seen = vec![false; 1 << width];
+        let mut positive = 0;
+        for index in 0..(1 << width) - 1 {
+            assert_ne!(state, 0);
+            assert!(!seen[usize::from(state)]);
+            seen[usize::from(state)] = true;
+            let (expected, high) = reference(state, short);
+            assert_eq!(shift(state, short), expected);
+            if index < width {
+                assert_eq!(high, index == width - 1);
+            }
+            positive += usize::from(high);
+            state = expected;
+        }
+        assert_eq!(state, initial);
+        assert_eq!(positive, 1 << (width - 1));
+    }
+}
+
+#[test]
+fn jump_maps_match_each_state_and_iterated_random_batches_including_transient_upper_bits() {
+    for short in [false, true] {
+        for state in 0..=0x7fff {
+            assert_eq!(jump(state, 1, short), reference(state, short).0);
+        }
+        let mut random = 0x4183u32;
+        for _ in 0..200 {
+            random = random.wrapping_mul(1664525).wrapping_add(1013904223);
+            let state = (random & 0x7fff) as u16;
+            let count = (random >> 16) & 4095;
+            let mut expected = state;
+            for _ in 0..count {
+                expected = reference(expected, short).0;
+            }
+            assert_eq!(jump(state, count, short), expected);
+            assert_eq!(
+                jump(jump(state, count / 2, short), count - count / 2, short),
+                expected
+            );
+        }
+    }
+}
+
+#[test]
+fn all_divider_shift_width_fields_observe_exact_nominal_edges() {
+    for polynomial in 0..=255u8 {
+        let divisor = [32, 64, 128, 192, 256, 320, 384, 448][usize::from(polynomial & 7)];
+        let period = divisor * (1u32 << (polynomial >> 4));
+        for cycles in [0, period - 1, period, period * 19 + 7] {
+            let mut noise = start(polynomial);
+            noise.advance(cycles);
+            let mut expected = if polynomial & 8 != 0 { 0x40 } else { 0x4000 };
+            let mut high = false;
+            for _ in 0..cycles / period {
+                (expected, high) = reference(expected, polynomial & 8 != 0);
+            }
+            assert_eq!(noise.lfsr, expected);
+            assert_eq!(noise.high, high);
+            assert_eq!(noise.remaining, period - cycles % period);
+            assert_eq!(noise.sample(), if high { 15 } else { -15 });
+        }
+    }
+}
+
+#[test]
+fn maximum_clock_batch_matches_independent_cycle_reduction_and_last_carry() {
+    for (polynomial, width) in [(0, 15), (8, 7)] {
+        let mut noise = start(polynomial);
+        noise.advance(u32::MAX);
+        let edges = u32::MAX / 32;
+        let mut expected = 1 << (width - 1);
+        for _ in 0..(edges - 1) % ((1 << width) - 1) {
+            expected = reference(expected, width == 7).0;
+        }
+        let (state, high) = reference(expected, width == 7);
+        assert_eq!((noise.lfsr, noise.high), (state, high));
+        assert_eq!(noise.remaining, 1);
+    }
+}
+
+#[test]
+fn live_width_and_rate_changes_preserve_counter_and_remaining_interval() {
+    let mut noise = start(0);
+    noise.advance(3 * 32 + 15);
+    assert_eq!(noise.lfsr, 0x800);
+    noise.write(4, 0x19, 0); // Short mode, new period128. Old interval has17 clocks left.
+    let mut expected = noise.lfsr;
+    noise.advance(16);
+    assert_eq!(noise.lfsr, expected);
+    noise.advance(1);
+    expected = reference(expected, true).0;
+    assert_eq!(noise.lfsr, expected); // Upper bits are not truncated or reseeded.
+    assert_eq!(noise.remaining, 128);
+    noise.advance(128 * 1000);
+    for _ in 0..1000 {
+        expected = reference(expected, true).0;
+    }
+    assert_eq!(noise.lfsr, expected);
+    noise.write(4, 0, 0);
+    noise.advance(128 + 32 * 25);
+    for _ in 0..26 {
+        expected = reference(expected, false).0;
+    }
+    assert_eq!(noise.lfsr, expected);
+}
+
+#[test]
+fn trigger_reseeds_counter_reloads_timer_and_resets_held_output() {
+    for (polynomial, width) in [(0, 15), (8, 7)] {
+        let mut noise = start(polynomial);
+        noise.advance(width * 32);
+        assert_eq!(noise.sample(), 15);
+        noise.write(5, 0x80, 0);
+        assert_eq!(noise.lfsr, 1 << (width - 1));
+        assert_eq!(noise.remaining, 32);
+        assert_eq!(noise.sample(), -15);
+        noise.write(1, 0, 0);
+        let before = noise;
+        noise.advance(u32::MAX);
+        assert_eq!(noise, before);
+        assert_eq!(noise.sample(), 0);
+    }
+}
+
+#[test]
+fn zero_cycles_and_arbitrary_splits_preserve_full_noise_state() {
+    let mut bulk = start(8);
+    let mut split = bulk;
+    for cycles in [0, 1, 31, 33, 4096, 1000000, u32::MAX] {
+        bulk.advance(cycles);
+        split.advance(cycles / 2);
+        split.advance(cycles - cycles / 2);
+        assert_eq!(bulk, split);
+    }
+}

crates/gba-core/src/memory/audio_step_tests.rs

@@ -12,7 +12,9 @@ fn sound(memory: &mut Memory, period: u16) {
     memory.write16(0x04000062, 0xf080).unwrap();
     memory.write16(0x04000064, 0x87ff).unwrap();
     memory.write16(0x04000068, 0xf080).unwrap();
-    memory.write16(0x0400006c, 0x87ff).unwrap(); // Both pulses run unrouted; rollback must include them.
+    memory.write16(0x0400006c, 0x87ff).unwrap();
+    memory.write16(0x04000078, 0xf000).unwrap();
+    memory.write16(0x0400007c, 0x8008).unwrap(); // All supported PSG channels run unrouted.
     memory
         .write32(
             TIMER_BASE,
@@ -22,16 +24,22 @@ fn sound(memory: &mut Memory, period: u16) {
 }
 
 #[test]
-fn pulse_status_load_observes_length_expiry_at_bus_completion() {
-    for (index, duty, frequency) in [(0, 0x04000062, 0x04000064), (1, 0x04000068, 0x0400006c)] {
+fn psg_status_load_observes_length_expiry_at_bus_completion() {
+    for (index, duty, frequency) in [
+        (0, 0x04000062, 0x04000064),
+        (1, 0x04000068, 0x0400006c),
+        (3, 0x04000078, 0x0400007c),
+    ] {
         let (cpu, mut memory) = prepared(false, &[0xe5910000], &[(1, SOUNDCNT_X)]);
         sound(&mut memory, 1000);
         memory.write16(duty, 0xf03f).unwrap();
-        memory.write16(frequency, 0xc7ff).unwrap();
+        memory
+            .write16(frequency, if index == 3 { 0xc000 } else { 0xc7ff })
+            .unwrap();
         memory.advance_cycles(32768 - 7);
         let mut machine = Machine::new(cpu, memory);
         machine.step().unwrap();
-        let status = 0x83 ^ (1 << index); // The other channel remains active.
+        let status = 0x8b ^ (1 << index); // The other channels remain active.
         assert_eq!(machine.cpu().registers()[0], status);
         assert_eq!(machine.memory().read16(SOUNDCNT_X).unwrap(), status as u16);
     }
@@ -57,27 +65,47 @@ fn pulse_trigger_reloads_timer_at_store_completion() {
 }
 
 #[test]
-fn successful_dma_trigger_starts_channel2_after_destination_bus_cycles() {
-    let (cpu, mut memory) = prepared(false, &[0xe1a00000], &[]);
+fn noise_trigger_reloads_counter_at_store_completion() {
+    let (cpu, mut memory) = prepared(false, &[0xe5810000], &[(0, 0x8008), (1, 0x0400007c)]);
     sound(&mut memory, 1000);
-    memory.write16(0x04000080, 0x2277).unwrap();
+    memory.write16(0x04000080, 0x8877).unwrap();
     memory.write16(SOUNDCNT_H, 2).unwrap();
-    memory.write8(0x04000069, 0).unwrap();
-    memory.write8(0x04000069, 0xf0).unwrap(); // DAC gate restored, still needs a trigger.
-    memory.write32(0x02000000, 0x87ff).unwrap();
-    memory.write32(DMA_BASE, 0x02000000).unwrap();
-    memory.write32(DMA_BASE + 4, 0x0400006c).unwrap();
-    memory.write32(DMA_BASE + 8, 0x84000001).unwrap();
     let mut machine = Machine::new(cpu, memory);
-    assert_eq!(
-        machine.step().unwrap(),
-        crate::machine::StepKind::Dma { channel: 0 }
-    );
-    assert_eq!(machine.memory().read16(SOUNDCNT_X).unwrap(), 0x83);
-    machine.memory_mut().advance_cycles(15);
-    assert_eq!(machine.memory().audio_level().left, 120);
-    machine.memory_mut().advance_cycles(1);
+    machine.step().unwrap();
+    assert_eq!(machine.cycles(), 7);
+    machine.memory_mut().advance_cycles(7 * 32 - 1);
     assert_eq!(machine.memory().audio_level().left, -120);
+    machine.memory_mut().advance_cycles(1);
+    assert_eq!(machine.memory().audio_level().left, 120);
+}
+
+#[test]
+fn successful_dma_triggers_start_pulse_and_noise_after_destination_bus_cycles() {
+    for (envelope, destination, routing, control, delay, before, after) in [
+        (0x04000069, 0x0400006c, 0x2277, 0x87ff, 15, 120, -120),
+        (0x04000079, 0x0400007c, 0x8877, 0x8008, 223, -120, 120),
+    ] {
+        let (cpu, mut memory) = prepared(false, &[0xe1a00000], &[]);
+        sound(&mut memory, 1000);
+        memory.write16(0x04000080, routing).unwrap();
+        memory.write16(SOUNDCNT_H, 2).unwrap();
+        memory.write8(envelope, 0).unwrap();
+        memory.write8(envelope, 0xf0).unwrap(); // DAC gate restored, still needs a trigger.
+        memory.write32(0x02000000, control).unwrap();
+        memory.write32(DMA_BASE, 0x02000000).unwrap();
+        memory.write32(DMA_BASE + 4, destination).unwrap();
+        memory.write32(DMA_BASE + 8, 0x84000001).unwrap();
+        let mut machine = Machine::new(cpu, memory);
+        assert_eq!(
+            machine.step().unwrap(),
+            crate::machine::StepKind::Dma { channel: 0 }
+        );
+        assert_eq!(machine.memory().read16(SOUNDCNT_X).unwrap(), 0x8b);
+        machine.memory_mut().advance_cycles(delay);
+        assert_eq!(machine.memory().audio_level().left, before);
+        machine.memory_mut().advance_cycles(1);
+        assert_eq!(machine.memory().audio_level().left, after);
+    }
 }
 
 #[test]
@@ -146,6 +174,29 @@ fn failed_dma_source_and_destination_restore_audio_after_speculative_timer_edges
     }
 }
 
+#[test]
+fn arm_block_store_rejects_later_reserved_ratio_before_committing_noise_trigger() {
+    let (cpu, mut memory) = prepared(
+        false,
+        &[0xe8a0000e],
+        &[(0, 0x04000078), (1, 0xf000), (2, 0x8008), (3, 0x00030000)],
+    ); // STMIA r0!,{r1-r3}: noise configuration/trigger, then reserved PSG ratio.
+    memory.write16(SOUNDCNT_X, 0x80).unwrap();
+    let audio = memory.io.audio;
+    let mut machine = Machine::new(cpu, memory);
+    let cpu = machine.cpu().clone();
+    for _ in 0..2 {
+        assert!(machine
+            .step()
+            .unwrap_err()
+            .to_string()
+            .contains("reserved PSG volume selection"));
+        assert_eq!(machine.memory().io.audio, audio);
+        assert_eq!(machine.cpu(), &cpu);
+        assert_eq!(machine.cycles(), 0);
+    }
+}
+
 #[test]
 fn arm_block_store_rejects_later_wave_trigger_before_committing_channel2_trigger() {
     let (cpu, mut memory) = prepared(

crates/gba-core/tests/bios/ram_reset.rs

@@ -20,7 +20,7 @@ const REGIONS: [(u32, u32, u32); 5] = [
 const MARKER: u32 = 0xa55a_c33c;
 
 #[test]
-fn sound_reset_disables_running_pulses_only_when_selected() {
+fn sound_reset_disables_running_psg_channels_only_when_selected() {
     use gba_core::io::SOUNDCNT_X;
     for thumb in [false, true] {
         for flags in [0, 0x40] {
@@ -31,10 +31,12 @@ fn sound_reset_disables_running_pulses_only_when_selected() {
             bus.write16(0x04000064, 0x87ff).unwrap();
             bus.write16(0x04000068, 0xf080).unwrap();
             bus.write16(0x0400006c, 0x87fe).unwrap();
+            bus.write16(0x04000078, 0xf000).unwrap();
+            bus.write16(0x0400007c, 0x8000).unwrap();
             complete(&mut machine, return_pc);
             assert_eq!(
                 machine.memory().read16(SOUNDCNT_X).unwrap(),
-                if flags == 0 { 0x83 } else { 0 }
+                if flags == 0 { 0x8b } else { 0 }
             );
         }
     }

crates/gba-core/tests/direct_sound.rs

@@ -167,15 +167,13 @@ fn wave_banks_and_idle_channel_masks_preserve_explicit_limits() {
     bus.write16(0x04000070, 0x40).unwrap();
     assert_eq!(bus.read32(WAVE_RAM).unwrap(), 0);
     bus.write32(WAVE_RAM, 0xaabbccdd).unwrap();
-    for address in [0x04000074, 0x0400007c] {
-        let previous = bus.read16(address).unwrap();
-        assert!(bus
-            .write16(address, 0xffff)
-            .unwrap_err()
-            .to_string()
-            .contains("PSG channel trigger"));
-        assert_eq!(bus.read16(address).unwrap(), previous);
-    }
+    let previous = bus.read16(0x04000074).unwrap();
+    assert!(bus
+        .write16(0x04000074, 0xffff)
+        .unwrap_err()
+        .to_string()
+        .contains("PSG channel trigger"));
+    assert_eq!(bus.read16(0x04000074).unwrap(), previous);
     bus.write16(SOUNDCNT_X, 0).unwrap();
     assert_eq!(bus.read32(WAVE_RAM).unwrap(), 0x11223344);
     assert_eq!(bus.read16(0x04000062).unwrap(), 0);

crates/gba-core/tests/noise_sound.rs

@@ -0,0 +1,263 @@
+//! Original public-bus regressions for noise channel 4. No game audio or assets.
+use gba_core::{
+    audio::StereoLevel,
+    cpu::Cpu,
+    io::{FIFO_A, HALTCNT, IF, SOUNDBIAS, SOUNDCNT_H, SOUNDCNT_X, TIMER_BASE},
+    machine::Machine,
+    memory::{Memory, ROM_START},
+};
+
+const ENVELOPE: u32 = 0x04000078;
+const CONTROL: u32 = 0x0400007c;
+const MIX: u32 = 0x04000080;
+const SEQUENCE: u32 = 32768;
+
+fn bus() -> Memory {
+    let mut bus = Memory::new(0xeafffffeu32.to_le_bytes().to_vec()).unwrap();
+    bus.write16(SOUNDCNT_X, 0x80).unwrap();
+    bus.write16(SOUNDBIAS, 0x200).unwrap();
+    bus.write16(MIX, 0x8877).unwrap();
+    bus.write16(SOUNDCNT_H, 2).unwrap();
+    bus
+}
+
+fn start(bus: &mut Memory, short: bool) {
+    bus.write16(ENVELOPE, 0xf000).unwrap();
+    bus.write16(CONTROL, if short { 0x8008 } else { 0x8000 })
+        .unwrap();
+}
+
+fn stereo(value: i16) -> StereoLevel {
+    StereoLevel {
+        left: value,
+        right: value,
+    }
+}
+
+fn reference_level(cycles: u32, short: bool) -> i16 {
+    let edges = cycles / 32;
+    if edges == 0 {
+        return -120;
+    }
+    let (mut state, mask, period) = if short {
+        (0x40u16, 0x60, 127)
+    } else {
+        (0x4000, 0x6000, 32767)
+    };
+    for _ in 0..(edges - 1) % period {
+        state = (state >> 1) ^ if state & 1 != 0 { mask } else { 0 };
+    }
+    if state & 1 != 0 {
+        120
+    } else {
+        -120
+    }
+}
+
+#[test]
+fn register_masks_padding_readonly_status_and_first_carry_in_both_widths() {
+    for (short, width) in [(false, 15), (true, 7)] {
+        let mut bus = bus();
+        bus.write32(ENVELOPE, 0xfffff0ff).unwrap(); // Unused bits/halfword cannot affect control.
+        assert_eq!(bus.read32(ENVELOPE).unwrap(), 0xf000);
+        assert_eq!(bus.read16(CONTROL).unwrap(), 0);
+        bus.write8(CONTROL, if short { 8 } else { 0 }).unwrap();
+        bus.write8(CONTROL + 1, 0xff).unwrap();
+        assert_eq!(
+            bus.read32(CONTROL).unwrap(),
+            if short { 0x4008 } else { 0x4000 }
+        );
+        assert_eq!(bus.read32(SOUNDCNT_X).unwrap(), 0x88);
+        bus.write16(SOUNDCNT_X, 0x8f).unwrap();
+        assert_eq!(bus.read16(SOUNDCNT_X).unwrap(), 0x88);
+        assert_eq!(bus.audio_level(), stereo(-120));
+        bus.advance_cycles(width * 32 - 1);
+        assert_eq!(bus.audio_level(), stereo(-120));
+        bus.advance_cycles(1);
+        assert_eq!(bus.audio_level(), stereo(120));
+        assert_eq!(bus.read16(IF).unwrap(), 0);
+    }
+}
+
+#[test]
+fn length_expiry_and_extra_clock_reload_do_not_need_a_general_timer() {
+    let mut bus = bus();
+    bus.write16(ENVELOPE, 0xf03e).unwrap();
+    bus.write16(CONTROL, 0xc000).unwrap();
+    bus.advance_cycles(SEQUENCE);
+    assert_eq!(bus.read16(SOUNDCNT_X).unwrap(), 0x88);
+    bus.advance_cycles(SEQUENCE * 2);
+    assert_eq!(bus.read16(SOUNDCNT_X).unwrap(), 0x80);
+    assert_eq!(bus.audio_level(), stereo(0));
+    bus.write16(CONTROL, 0xc000).unwrap(); // Next step3: empty length reloads63.
+    bus.advance_cycles(SEQUENCE * 125);
+    assert_eq!(bus.read16(SOUNDCNT_X).unwrap(), 0x88);
+    bus.advance_cycles(SEQUENCE);
+    assert_eq!(bus.read16(SOUNDCNT_X).unwrap(), 0x80);
+}
+
+#[test]
+fn envelope_changes_at_shared_step7_and_saturation_does_not_clear_status() {
+    let mut bus = bus();
+    bus.advance_cycles(SEQUENCE * 6);
+    bus.write16(ENVELOPE, 0x3900).unwrap();
+    bus.write16(CONTROL, 0x8008).unwrap();
+    bus.advance_cycles(SEQUENCE * 2 - 1);
+    assert_eq!(bus.audio_level().left.abs(), 24);
+    bus.advance_cycles(1);
+    assert_eq!(bus.audio_level().left.abs(), 32);
+    bus.write16(ENVELOPE, 0xc100).unwrap();
+    bus.write16(CONTROL, 0x8008).unwrap();
+    bus.advance_cycles(SEQUENCE * 8 * 20);
+    assert_eq!(bus.read16(SOUNDCNT_X).unwrap(), 0x88);
+    assert_eq!(bus.audio_level(), stereo(0));
+}
+
+#[test]
+fn master_off_and_dac_gate_prevent_activation_until_reconfigured_and_triggered() {
+    let mut bus = bus();
+    start(&mut bus, false);
+    bus.write8(ENVELOPE + 1, 0).unwrap();
+    assert_eq!(bus.read16(SOUNDCNT_X).unwrap(), 0x80);
+    assert_eq!(bus.audio_level(), stereo(0));
+    bus.write8(ENVELOPE + 1, 0xf0).unwrap();
+    assert_eq!(bus.read16(SOUNDCNT_X).unwrap(), 0x80);
+    bus.write16(CONTROL, 0x8000).unwrap();
+    assert_eq!(bus.read16(SOUNDCNT_X).unwrap(), 0x88);
+    bus.write16(SOUNDCNT_X, 0).unwrap();
+    bus.write32(ENVELOPE, u32::MAX).unwrap();
+    bus.write32(CONTROL, u32::MAX).unwrap();
+    assert_eq!(bus.read32(ENVELOPE).unwrap(), 0);
+    assert_eq!(bus.read32(CONTROL).unwrap(), 0);
+    bus.write16(SOUNDCNT_X, 0x80).unwrap();
+    bus.write16(CONTROL, 0x8000).unwrap();
+    assert_eq!(bus.read16(SOUNDCNT_X).unwrap(), 0x80);
+}
+
+#[test]
+fn independent_routing_gain_ratios_and_sum_before_rounding_include_noise() {
+    let mut bus = bus();
+    start(&mut bus, false);
+    for (ratio, expected) in [(0, -30), (1, -60), (2, -120)] {
+        bus.write16(SOUNDCNT_H, ratio).unwrap();
+        assert_eq!(bus.audio_level(), stereo(expected));
+    }
+    bus.write16(MIX, 0x8070).unwrap();
+    assert_eq!(
+        bus.audio_level(),
+        StereoLevel {
+            left: -120,
+            right: 0
+        }
+    );
+    bus.write16(MIX, 0x0800).unwrap();
+    assert_eq!(
+        bus.audio_level(),
+        StereoLevel {
+            left: 0,
+            right: -15
+        }
+    );
+    bus.write16(MIX, 0x9900).unwrap();
+    bus.write16(SOUNDCNT_H, 0).unwrap();
+    bus.write16(0x04000062, 0x2080).unwrap(); // Pulse1 +2.
+    bus.write16(0x04000064, 0x8000).unwrap();
+    bus.write16(ENVELOPE, 0x1000).unwrap(); // Noise -1.
+    bus.write16(CONTROL, 0x8000).unwrap();
+    assert_eq!(bus.audio_level(), stereo(0)); // Sum1 >>2, not separately rounded0 + -1.
+}
+
+#[test]
+fn all_supported_sources_mix_before_bias_clipping_and_noise_mute_does_not_stop_it() {
+    for positive in [false, true] {
+        let mut bus = bus();
+        bus.write16(MIX, 0x9977).unwrap(); // Pulse1 and noise, both sides.
+        bus.write16(SOUNDCNT_H, 0x0306).unwrap(); // Also Direct Sound A.
+        bus.write16(0x04000062, if positive { 0xf080 } else { 0xf000 })
+            .unwrap();
+        bus.write16(0x04000064, 0x8000).unwrap();
+        start(&mut bus, true);
+        if positive {
+            bus.advance_cycles(7 * 32);
+        }
+        bus.write32(FIFO_A, if positive { 0x7f7f7f7f } else { 0x80808080 })
+            .unwrap();
+        bus.write32(TIMER_BASE, 0x0080ffff).unwrap();
+        bus.advance_cycles(1);
+        assert_eq!(bus.audio_level(), stereo(if positive { 511 } else { -512 }));
+        bus.write16(MIX, 0).unwrap();
+        assert_eq!(bus.read16(SOUNDCNT_X).unwrap(), 0x89);
+        assert_eq!(bus.audio_level(), stereo(if positive { 508 } else { -512 }));
+    }
+}
+
+#[test]
+fn noise_status_and_length_are_independent_of_both_pulses() {
+    let mut bus = bus();
+    for (duty, frequency) in [(0x04000062, 0x04000064), (0x04000068, 0x0400006c)] {
+        bus.write16(duty, 0xf080).unwrap();
+        bus.write16(frequency, 0x87ff).unwrap();
+    }
+    bus.write16(ENVELOPE, 0xf03f).unwrap();
+    bus.write16(CONTROL, 0xc000).unwrap();
+    assert_eq!(bus.read16(SOUNDCNT_X).unwrap(), 0x8b);
+    bus.advance_cycles(SEQUENCE);
+    assert_eq!(bus.read16(SOUNDCNT_X).unwrap(), 0x83);
+    assert_eq!(bus.audio_level(), stereo(0)); // Pulses are not routed here.
+}
+
+#[test]
+fn halt_keeps_noise_and_modulation_running_but_stop_freezes_them() {
+    for stop in [false, true] {
+        let mut bus = bus();
+        start(&mut bus, true);
+        bus.write8(HALTCNT, if stop { 0x80 } else { 0 }).unwrap();
+        let mut machine = Machine::new(Cpu::new(ROM_START), bus);
+        let before = machine.cpu().clone();
+        machine.step().unwrap();
+        assert_eq!(machine.cpu(), &before);
+        assert_eq!(
+            machine.memory().audio_level(),
+            stereo(reference_level(machine.cycles() as u32, true))
+        );
+        if stop {
+            machine.memory_mut().advance_cycles(u32::MAX);
+            assert_eq!(machine.cycles(), 0);
+            assert_eq!(machine.memory().audio_level(), stereo(-120));
+        } else {
+            assert!(machine.cycles() > 0);
+        }
+    }
+}
+
+#[test]
+fn batched_single_clock_and_row_capture_paths_agree_and_maximum_batches_are_bounded() {
+    let mut bulk = bus();
+    let mut rows = bus();
+    let mut small = bus();
+    rows.set_scanline_rendering(true);
+    for bus in [&mut bulk, &mut rows, &mut small] {
+        bus.write16(ENVELOPE, 0xc100).unwrap();
+        bus.write16(CONTROL, 0x8008).unwrap();
+    }
+    for cycles in [1, 31, 4096, 32768, 262145] {
+        bulk.advance_cycles(cycles);
+        rows.advance_cycles(cycles);
+        for _ in 0..cycles {
+            small.advance_cycles(1);
+        }
+        assert_eq!(bulk.audio_level(), small.audio_level());
+        assert_eq!(bulk.audio_level(), rows.audio_level());
+        assert_eq!(
+            bulk.read16(SOUNDCNT_X).unwrap(),
+            rows.read16(SOUNDCNT_X).unwrap()
+        );
+    }
+    for short in [false, true] {
+        let mut bus = bus();
+        start(&mut bus, short);
+        bus.advance_cycles(u32::MAX);
+        assert_eq!(bus.audio_level(), stereo(reference_level(u32::MAX, short)));
+        assert_eq!(bus.read16(SOUNDCNT_X).unwrap(), 0x88);
+    }
+}