in progress refactor

Needs profiling

.vscode/extensions.json

@@ -0,0 +1,8 @@
+{
+    "recommendations": [
+        "matklad.rust-analyzer",
+        "tamasfe.even-better-toml",
+        "serayuzgur.crates",
+        "vadimcn.vscode-lldb",
+    ]
+}

Cargo.toml

@@ -3,6 +3,7 @@ name = "gb"
 version = "0.1.0"
 authors = ["Rekai Musuka <rekai@musuka.dev>"]
 edition = "2018"
+resolver = "2"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
@@ -11,14 +12,11 @@ anyhow = "^1.0"
 bitfield = "^0.13"
 clap = "^2.33"
 gilrs = "^0.8"
-pixels = "^0.3"
-winit = "^0.24"
-winit_input_helper = "^0.9"
-egui = "^0.10"
-egui_wgpu_backend = { git="https://github.com/hasenbanck/egui_wgpu_backend.git", rev="9d03ad345d15d1e44165849b242d3562fdf3e859" }
-egui_winit_platform = { git="https://github.com/hasenbanck/egui_winit_platform.git", rev="17298250e9721e8bf2c1d4a17b3e22777f8cb2e8" }
+pixels = "^0.6"
+winit = "^0.25"
+winit_input_helper = "^0.10"
 rodio = "^0.14"
-crossbeam-channel = "^0.5"
+rtrb = "^0.1.4"
 
 [profile.release]
 debug = true

src/apu.rs

@@ -0,0 +1,938 @@
+use crate::bus::BusIo;
+use crate::emu::SM83_CLOCK_SPEED;
+use gen::SampleProducer;
+use types::ch1::{Sweep, SweepDirection};
+use types::ch3::Volume as Ch3Volume;
+use types::ch4::{CounterWidth, Frequency as Ch4Frequency, PolynomialCounter};
+use types::common::{EnvelopeDirection, FrequencyHigh, SoundDuty, VolumeEnvelope};
+use types::fs::{FrameSequencer, State as FrameSequencerState};
+use types::{ChannelControl, SoundOutput};
+
+pub mod gen;
+mod types;
+
+const SAMPLE_INCREMENT: u64 = gen::SAMPLE_RATE as u64;
+const WAVE_PATTERN_RAM_LEN: usize = 0x10;
+
+#[derive(Default, Debug)]
+pub struct Apu {
+    ctrl: SoundControl,
+    /// Tone & Sweep
+    ch1: Channel1,
+    /// Tone
+    ch2: Channel2,
+    /// Wave
+    ch3: Channel3,
+    /// Noise
+    ch4: Channel4,
+
+    sequencer: FrameSequencer,
+    div_prev: Option<u16>,
+
+    prod: Option<SampleProducer<f32>>,
+    sample_counter: u64,
+}
+
+impl BusIo for Apu {
+    fn read_byte(&self, addr: u16) -> u8 {
+        match addr & 0x00FF {
+            0x10 => self.ch1.sweep(),
+            0x11 => self.ch1.duty(),
+            0x12 => self.ch1.envelope(),
+            0x14 => self.ch1.freq_hi(),
+            0x16 => self.ch2.duty(),
+            0x17 => self.ch2.envelope(),
+            0x19 => self.ch2.freq_hi(),
+            0x1A => self.ch3.dac_enabled(),
+            0x1C => self.ch3.volume(),
+            0x1E => self.ch3.freq_hi(),
+            0x21 => self.ch4.envelope(),
+            0x22 => self.ch4.poly(),
+            0x23 => self.ch4.frequency(),
+            0x24 => self.ctrl.channel(),
+            0x25 => self.ctrl.output(),
+            0x26 => self.ctrl.status(self),
+            0x30..=0x3F => self.ch3.read_byte(addr),
+            _ => {
+                eprintln!("Read 0xFF from unused IO register {:#06X} [APU]", addr);
+                0xFF
+            }
+        }
+    }
+
+    fn write_byte(&mut self, addr: u16, byte: u8) {
+        match addr & 0x00FF {
+            0x10 if self.ctrl.enabled => self.ch1.set_sweep(byte),
+            0x11 if self.ctrl.enabled => self.ch1.set_duty(byte),
+            0x12 if self.ctrl.enabled => self.ch1.set_envelope(byte),
+            0x13 if self.ctrl.enabled => self.ch1.set_freq_lo(byte),
+            0x14 if self.ctrl.enabled => self.ch1.set_freq_hi(byte),
+            0x16 if self.ctrl.enabled => self.ch2.set_duty(byte),
+            0x17 if self.ctrl.enabled => self.ch2.set_envelope(byte),
+            0x18 if self.ctrl.enabled => self.ch2.set_freq_lo(byte),
+            0x19 if self.ctrl.enabled => self.ch2.set_freq_hi(byte),
+            0x1A if self.ctrl.enabled => self.ch3.set_dac_enabled(byte),
+            0x1B if self.ctrl.enabled => self.ch3.set_len(byte),
+            0x1C if self.ctrl.enabled => self.ch3.set_volume(byte),
+            0x1D if self.ctrl.enabled => self.ch3.set_freq_lo(byte),
+            0x1E if self.ctrl.enabled => self.ch3.set_freq_hi(byte),
+            0x20 if self.ctrl.enabled => self.ch4.set_len(byte),
+            0x21 if self.ctrl.enabled => self.ch4.set_envelope(byte),
+            0x22 if self.ctrl.enabled => self.ch4.set_poly(byte),
+            0x23 if self.ctrl.enabled => self.ch4.set_frequency(byte),
+            0x24 if self.ctrl.enabled => self.ctrl.set_channel(byte),
+            0x25 if self.ctrl.enabled => self.ctrl.set_output(byte),
+            0x26 => self.set_status(byte),
+            0x30..=0x3F => self.ch3.write_byte(addr, byte),
+            _ if !self.ctrl.enabled => {}
+            _ => eprintln!(
+                "Wrote {:#04X} to unused IO register {:#06X} [APU]",
+                byte, addr
+            ),
+        }
+    }
+}
+
+impl Apu {
+    pub(crate) fn tick(&mut self, div: u16) {
+        self.sample_counter += SAMPLE_INCREMENT;
+
+        // Frame Sequencer (512Hz)
+        if self.is_falling_edge(12, div) {
+            use FrameSequencerState::*;
+
+            match self.sequencer.state() {
+                Length => self.clock_length(),
+                LengthAndSweep => {
+                    self.clock_length();
+                    self.clock_sweep();
+                }
+                Envelope => self.clock_envelope(),
+                Nothing => {}
+            }
+
+            self.sequencer.next();
+        }
+
+        self.div_prev = Some(div);
+
+        self.ch1.tick();
+        self.ch2.tick();
+        self.ch3.tick();
+        self.ch4.tick();
+
+        if self.sample_counter >= SM83_CLOCK_SPEED {
+            self.sample_counter %= SM83_CLOCK_SPEED;
+
+            if let Some(ref mut prod) = self.prod {
+                if prod.available_blocking() {
+                    // Sample the APU
+
+                    let (left, right) = self.ctrl.out.ch1();
+                    let ch1_left = if left { self.ch1.amplitude() } else { 0.0 };
+                    let ch1_right = if right { self.ch1.amplitude() } else { 0.0 };
+
+                    let (left, right) = self.ctrl.out.ch2();
+                    let ch2_left = if left { self.ch2.amplitude() } else { 0.0 };
+                    let ch2_right = if right { self.ch2.amplitude() } else { 0.0 };
+
+                    let (left, right) = self.ctrl.out.ch3();
+                    let ch3_left = if left { self.ch3.amplitude() } else { 0.0 };
+                    let ch3_right = if right { self.ch3.amplitude() } else { 0.0 };
+
+                    let (left, right) = self.ctrl.out.ch4();
+                    let ch4_left = if left { self.ch4.amplitude() } else { 0.0 };
+                    let ch4_right = if right { self.ch4.amplitude() } else { 0.0 };
+
+                    let left_mixed = (ch1_left + ch2_left + ch3_left + ch4_left) / 4.0;
+                    let right_mixed = (ch1_right + ch2_right + ch3_right + ch4_right) / 4.0;
+
+                    let left_sample = (self.ctrl.channel.left_volume() + 1.0) * left_mixed;
+                    let right_sample = (self.ctrl.channel.right_volume() + 1.0) * right_mixed;
+
+                    prod.push(left_sample)
+                        .and(prod.push(right_sample))
+                        .expect("Add samples to ring buffer");
+                }
+            }
+        }
+    }
+
+    /// 0xFF26 | NR52 - Sound On/Off
+    pub(crate) fn set_status(&mut self, byte: u8) {
+        self.ctrl.enabled = (byte >> 7) & 0x01 == 0x01;
+
+        if self.ctrl.enabled {
+            // Frame Sequencer reset to Step 0
+            self.sequencer.reset();
+
+            // Square Duty units are reset to first step
+            self.ch1.duty_pos = 0;
+            self.ch2.duty_pos = 0;
+
+            // Wave Channel's sample buffer reset to 0
+            self.ch3.offset = 0;
+        }
+
+        if !self.ctrl.enabled {
+            self.reset();
+        } else {
+        }
+    }
+
+    pub fn attach_producer(&mut self, prod: SampleProducer<f32>) {
+        self.prod = Some(prod);
+    }
+
+    fn reset(&mut self) {
+        self.ch1.sweep = Default::default();
+        self.ch1.duty = Default::default();
+        self.ch1.envelope = Default::default();
+        self.ch1.freq_lo = Default::default();
+        self.ch1.freq_hi = Default::default();
+
+        self.ch2.duty = Default::default();
+        self.ch2.envelope = Default::default();
+        self.ch2.freq_lo = Default::default();
+        self.ch2.freq_hi = Default::default();
+
+        self.ch3.dac_enabled = Default::default();
+        self.ch3.len = Default::default();
+        self.ch3.volume = Default::default();
+        self.ch3.freq_lo = Default::default();
+        self.ch3.freq_hi = Default::default();
+
+        self.ch4.len = Default::default();
+        self.ch4.envelope = Default::default();
+        self.ch4.poly = Default::default();
+        self.ch4.freq = Default::default();
+
+        self.ctrl.channel = Default::default();
+        self.ctrl.out = Default::default();
+
+        // Disable the Channels
+        self.ch1.enabled = Default::default();
+        self.ch2.enabled = Default::default();
+        self.ch3.enabled = Default::default();
+        self.ch4.enabled = Default::default();
+    }
+
+    fn process_length(freq_hi: &FrequencyHigh, length_timer: &mut u16, enabled: &mut bool) {
+        if freq_hi.length_disable() && *length_timer > 0 {
+            *length_timer -= 1;
+
+            // Check in this scope ensures (only) the above subtraction
+            // made length_timer 0
+            if *length_timer == 0 {
+                *enabled = false;
+            }
+        }
+    }
+
+    fn ch4_process_length(freq: &Ch4Frequency, length_timer: &mut u16, enabled: &mut bool) {
+        if freq.length_disable() && *length_timer > 0 {
+            *length_timer -= 1;
+
+            // Check in this scope ensures (only) the above subtraction
+            // made length_timer 0
+            if *length_timer == 0 {
+                *enabled = false;
+            }
+        }
+    }
+
+    fn clock_length(&mut self) {
+        Self::process_length(
+            &self.ch1.freq_hi,
+            &mut self.ch1.length_timer,
+            &mut self.ch1.enabled,
+        );
+
+        Self::process_length(
+            &self.ch2.freq_hi,
+            &mut self.ch2.length_timer,
+            &mut self.ch2.enabled,
+        );
+
+        Self::process_length(
+            &self.ch3.freq_hi,
+            &mut self.ch3.length_timer,
+            &mut self.ch3.enabled,
+        );
+
+        Self::ch4_process_length(
+            &self.ch4.freq,
+            &mut self.ch4.length_timer,
+            &mut self.ch4.enabled,
+        );
+    }
+
+    fn clock_sweep(&mut self) {
+        if self.ch1.sweep_timer != 0 {
+            self.ch1.sweep_timer -= 1;
+        }
+
+        if self.ch1.sweep_timer == 0 {
+            let period = self.ch1.sweep.period();
+            self.ch1.sweep_timer = if period == 0 { 8 } else { period };
+
+            if self.ch1.sweep_enabled && period != 0 {
+                let new_freq = self.ch1.calc_sweep_freq();
+
+                if new_freq <= 2047 && self.ch1.sweep.shift_count() != 0 {
+                    self.ch1.set_frequency(new_freq);
+                    self.ch1.shadow_freq = new_freq;
+
+                    let _ = self.ch1.calc_sweep_freq();
+                }
+            }
+        }
+    }
+
+    fn process_envelope(envelope: &VolumeEnvelope, period_timer: &mut u8, current_volume: &mut u8) {
+        use EnvelopeDirection::*;
+
+        if envelope.period() != 0 {
+            if *period_timer != 0 {
+                *period_timer -= 1;
+            }
+
+            if *period_timer == 0 {
+                *period_timer = envelope.period();
+
+                match envelope.direction() {
+                    Decrease if *current_volume > 0x00 => *current_volume -= 1,
+                    Increase if *current_volume < 0x0F => *current_volume += 1,
+                    _ => {}
+                }
+            }
+        }
+    }
+
+    fn clock_envelope(&mut self) {
+        // Channels 1, 2 and 4 have Volume Envelopes
+
+        Self::process_envelope(
+            &self.ch1.envelope,
+            &mut self.ch1.period_timer,
+            &mut self.ch1.current_volume,
+        );
+
+        Self::process_envelope(
+            &self.ch2.envelope,
+            &mut self.ch2.period_timer,
+            &mut self.ch2.current_volume,
+        );
+
+        Self::process_envelope(
+            &self.ch4.envelope,
+            &mut self.ch4.period_timer,
+            &mut self.ch4.current_volume,
+        );
+    }
+
+    fn is_falling_edge(&self, bit: u8, div: u16) -> bool {
+        match self.div_prev {
+            Some(p) => (p >> bit & 0x01) == 0x01 && (div >> bit & 0x01) == 0x00,
+            None => false,
+        }
+    }
+}
+
+#[derive(Debug, Default)]
+pub(crate) struct SoundControl {
+    /// 0xFF24 | NR50 - Channel Control
+    channel: ChannelControl,
+    /// 0xFF25 | NR51 - Selection of Sound output terminal
+    out: SoundOutput,
+
+    enabled: bool,
+}
+
+impl SoundControl {
+    /// 0xFF24 | NR50 - Channel Control
+    pub(crate) fn channel(&self) -> u8 {
+        u8::from(self.channel)
+    }
+
+    /// 0xFF24 | NR50 - Channel Control
+    pub(crate) fn set_channel(&mut self, byte: u8) {
+        if self.enabled {
+            self.channel = byte.into();
+        }
+    }
+
+    /// 0xFF25 | NR51 - Selection of Sound output terminal
+    pub(crate) fn output(&self) -> u8 {
+        u8::from(self.out)
+    }
+
+    /// 0xFF25 | NR51 - Selection of Sound output terminal
+    pub(crate) fn set_output(&mut self, byte: u8) {
+        if self.enabled {
+            self.out = byte.into();
+        }
+    }
+
+    /// 0xFF26 | NR52 - Sound On/Off
+    pub(crate) fn status(&self, apu: &Apu) -> u8 {
+        (self.enabled as u8) << 7
+            | (apu.ch4.enabled as u8) << 3
+            | (apu.ch3.enabled as u8) << 2
+            | (apu.ch2.enabled as u8) << 1
+            | apu.ch1.enabled as u8
+            | 0x70
+    }
+}
+
+#[derive(Debug, Default)]
+pub(crate) struct Channel1 {
+    /// 0xFF10 | NR10 - Channel 1 Sweep Register
+    sweep: Sweep,
+    /// 0xFF11 | NR11 - Channel 1 Sound length / Wave pattern duty
+    duty: SoundDuty,
+    /// 0xFF12 | NR12 - Channel 1 Volume Envelope
+    envelope: VolumeEnvelope,
+    /// 0xFF13 | NR13 - Channel 1 Frequency low (lower 8 bits only)
+    freq_lo: u8,
+    /// 0xFF14 | NR14 - Channel 1 Frequency high
+    freq_hi: FrequencyHigh,
+
+    // Envelope Functionality
+    period_timer: u8,
+    current_volume: u8,
+
+    // Sweep Functionality
+    sweep_timer: u8,
+    shadow_freq: u16,
+    sweep_enabled: bool,
+
+    // Length Functionality
+    length_timer: u16,
+
+    freq_timer: u16,
+    duty_pos: u8,
+
+    enabled: bool,
+}
+
+impl Channel1 {
+    /// 0xFF10 | NR10 - Channel 1 Sweep Register
+    pub(crate) fn sweep(&self) -> u8 {
+        u8::from(self.sweep) | 0x80
+    }
+
+    /// 0xFF10 | NR10 - Channel 1 Sweep Register
+    pub(crate) fn set_sweep(&mut self, byte: u8) {
+        self.sweep = byte.into()
+    }
+
+    /// 0xFF11 | NR11 - Channel 1 Sound length / Wave pattern duty
+    pub(crate) fn duty(&self) -> u8 {
+        u8::from(self.duty) | 0x3F
+    }
+
+    /// 0xFF11 | NR11 - Channel 1 Sound length / Wave pattern duty
+    pub(crate) fn set_duty(&mut self, byte: u8) {
+        self.duty = byte.into();
+        self.length_timer = 64 - self.duty.sound_length() as u16;
+    }
+
+    /// 0xFF12 | NR12 - Channel 1 Volume Envelope
+    pub fn envelope(&self) -> u8 {
+        u8::from(self.envelope)
+    }
+
+    /// 0xFF12 | NR12 - Channel 1 Volume Envelope
+    pub(crate) fn set_envelope(&mut self, byte: u8) {
+        self.envelope = byte.into();
+
+        if !self.is_dac_enabled() {
+            self.enabled = false;
+        }
+    }
+
+    /// 0xFF13 | NR13 - Channel 1 Frequency low (lower 8 bits only)
+    pub(crate) fn set_freq_lo(&mut self, byte: u8) {
+        self.freq_lo = byte;
+    }
+
+    /// 0xFF14 | NR14 - Channel 1 Frequency high
+    pub(crate) fn freq_hi(&self) -> u8 {
+        u8::from(self.freq_hi) | 0xBF
+    }
+
+    /// 0xFF14 | NR14 - Channel 1 Frequency high
+    pub(crate) fn set_freq_hi(&mut self, byte: u8) {
+        self.freq_hi = byte.into();
+
+        // If this bit is set, a trigger event occurs
+        if self.freq_hi.initial() {
+            if self.is_dac_enabled() {
+                self.enabled = true;
+            }
+
+            // Length behaviour during trigger event
+            if self.length_timer == 0 {
+                self.length_timer = 64;
+            }
+
+            // Envelope Behaviour during trigger event
+            self.period_timer = self.envelope.period();
+            self.current_volume = self.envelope.init_vol();
+
+            // Sweep behaviour during trigger event
+            let sweep_period = self.sweep.period();
+            let sweep_shift = self.sweep.shift_count();
+
+            self.shadow_freq = self.frequency();
+            self.sweep_timer = if sweep_period == 0 { 8 } else { sweep_period };
+
+            self.sweep_enabled = sweep_period != 0 || sweep_shift != 0;
+
+            if sweep_shift != 0 {
+                let _ = self.calc_sweep_freq();
+            }
+        }
+    }
+
+    fn tick(&mut self) {
+        if self.freq_timer != 0 {
+            self.freq_timer -= 1;
+        }
+
+        if self.freq_timer == 0 {
+            self.freq_timer = (2048 - self.frequency()) * 4;
+            self.duty_pos = (self.duty_pos + 1) % 8;
+        }
+    }
+
+    fn amplitude(&self) -> f32 {
+        if self.is_dac_enabled() {
+            let sample = self.duty.wave_pattern().amplitude(self.duty_pos) * self.current_volume;
+            let input = if self.enabled { sample } else { 0 };
+
+            (input as f32 / 7.5) - 1.0
+        } else {
+            0.0
+        }
+    }
+
+    fn calc_sweep_freq(&mut self) -> u16 {
+        use SweepDirection::*;
+
+        let shadow_freq_shifted = self.shadow_freq >> self.sweep.shift_count();
+        let new_freq = match self.sweep.direction() {
+            Increase => self.shadow_freq + shadow_freq_shifted,
+            Decrease => self.shadow_freq - shadow_freq_shifted,
+        };
+
+        // Overflow check
+        if new_freq > 2047 {
+            self.enabled = false;
+        }
+
+        new_freq
+    }
+
+    fn set_frequency(&mut self, word: u16) {
+        let freq_bits = word & 0x07FF;
+        self.freq_lo = (freq_bits & 0x00FF) as u8;
+        self.freq_hi
+            .set_freq_bits(((freq_bits & 0x0700) >> 8) as u8);
+    }
+
+    fn frequency(&self) -> u16 {
+        (self.freq_hi.freq_bits() as u16) << 8 | self.freq_lo as u16
+    }
+
+    fn is_dac_enabled(&self) -> bool {
+        self.envelope.0 & 0xF8 != 0x00
+    }
+}
+
+#[derive(Debug, Default)]
+pub(crate) struct Channel2 {
+    /// 0xFF16 | NR21 - Channel 2 Sound length / Wave Pattern Duty
+    duty: SoundDuty,
+    /// 0xFF17 | NR22 - Channel 2 Volume ENvelope
+    envelope: VolumeEnvelope,
+    /// 0xFF18 | NR23 - Channel 2 Frequency low (lower 8 bits only)
+    freq_lo: u8,
+    /// 0xFF19 | NR24 - Channel 2 Frequency high
+    freq_hi: FrequencyHigh,
+
+    // Envelope Functionality
+    period_timer: u8,
+    current_volume: u8,
+
+    // Length Functionality
+    length_timer: u16,
+
+    freq_timer: u16,
+    duty_pos: u8,
+
+    enabled: bool,
+}
+
+impl Channel2 {
+    /// 0xFF16 | NR21 - Channel 2 Sound length / Wave Pattern Duty
+    pub(crate) fn duty(&self) -> u8 {
+        u8::from(self.duty) | 0x3F
+    }
+
+    /// 0xFF16 | NR21 - Channel 2 Sound length / Wave Pattern Duty
+    pub(crate) fn set_duty(&mut self, byte: u8) {
+        self.duty = byte.into();
+        self.length_timer = 64 - self.duty.sound_length() as u16;
+    }
+
+    /// 0xFF17 | NR22 - Channel 2 Volume ENvelope
+    pub(crate) fn envelope(&self) -> u8 {
+        u8::from(self.envelope)
+    }
+
+    /// 0xFF17 | NR22 - Channel 2 Volume ENvelope
+    pub(crate) fn set_envelope(&mut self, byte: u8) {
+        self.envelope = byte.into();
+
+        if !self.is_dac_enabled() {
+            self.enabled = false;
+        }
+    }
+
+    /// 0xFF18 | NR23 - Channel 2 Frequency low (lower 8 bits only)
+    pub(crate) fn set_freq_lo(&mut self, byte: u8) {
+        self.freq_lo = byte;
+    }
+
+    /// 0xFF19 | NR24 - Channel 2 Frequency high
+    pub(crate) fn freq_hi(&self) -> u8 {
+        u8::from(self.freq_hi) | 0xBF
+    }
+
+    /// 0xFF19 | NR24 - Channel 2 Frequency high
+    pub(crate) fn set_freq_hi(&mut self, byte: u8) {
+        self.freq_hi = byte.into();
+
+        if self.freq_hi.initial() {
+            // Envelope behaviour during trigger event
+            self.period_timer = self.envelope.period();
+            self.current_volume = self.envelope.init_vol();
+
+            // Length behaviour during trigger event
+            if self.length_timer == 0 {
+                self.length_timer = 64;
+            }
+
+            if self.is_dac_enabled() {
+                self.enabled = true;
+            }
+        }
+    }
+
+    fn amplitude(&self) -> f32 {
+        if self.is_dac_enabled() {
+            let sample = self.duty.wave_pattern().amplitude(self.duty_pos) * self.current_volume;
+            let input = if self.enabled { sample } else { 0 };
+
+            (input as f32 / 7.5) - 1.0
+        } else {
+            0.0
+        }
+    }
+
+    fn tick(&mut self) {
+        if self.freq_timer != 0 {
+            self.freq_timer -= 1;
+        }
+
+        if self.freq_timer == 0 {
+            self.freq_timer = (2048 - self.frequency()) * 4;
+            self.duty_pos = (self.duty_pos + 1) % 8;
+        }
+    }
+
+    fn frequency(&self) -> u16 {
+        (self.freq_hi.freq_bits() as u16) << 8 | self.freq_lo as u16
+    }
+
+    fn is_dac_enabled(&self) -> bool {
+        self.envelope.0 & 0xF8 != 0x00
+    }
+}
+
+#[derive(Debug, Default)]
+pub(crate) struct Channel3 {
+    /// 0xFF1A | NR30 - Channel 3 Sound on/off
+    dac_enabled: bool,
+    /// 0xFF1B | NR31 - Sound Length
+    len: u8,
+    /// 0xFF1C | NR32 - Channel 3 Volume
+    volume: Ch3Volume,
+    /// 0xFF1D | NR33 - Channel 3 Frequency low (lower 8 bits)
+    freq_lo: u8,
+    /// 0xFF1E | NR34 - Channel 3 Frequency high
+    freq_hi: FrequencyHigh,
+
+    wave_ram: [u8; WAVE_PATTERN_RAM_LEN],
+
+    // Length Functionality
+    length_timer: u16,
+
+    freq_timer: u16,
+    offset: u8,
+
+    enabled: bool,
+}
+
+impl BusIo for Channel3 {
+    fn read_byte(&self, addr: u16) -> u8 {
+        if self.enabled {
+            self.wave_ram[self.offset as usize / 2]
+        } else {
+            self.wave_ram[(addr - Self::WAVE_RAM_START_ADDR) as usize]
+        }
+    }
+
+    fn write_byte(&mut self, addr: u16, byte: u8) {
+        if self.enabled {
+            self.wave_ram[self.offset as usize / 2] = byte;
+        } else {
+            self.wave_ram[(addr - Self::WAVE_RAM_START_ADDR) as usize] = byte;
+        }
+    }
+}
+
+impl Channel3 {
+    const WAVE_RAM_START_ADDR: u16 = 0xFF30;
+
+    /// 0xFF1A | NR30 - Channel 3 Sound on/off
+    pub(crate) fn dac_enabled(&self) -> u8 {
+        ((self.dac_enabled as u8) << 7) | 0x7F
+    }
+
+    /// 0xFF1A | NR30 - Channel 3 Sound on/off
+    pub(crate) fn set_dac_enabled(&mut self, byte: u8) {
+        self.dac_enabled = (byte >> 7) & 0x01 == 0x01;
+
+        if !self.dac_enabled {
+            self.enabled = false;
+        }
+    }
+
+    /// 0xFF1B | NR31 - Sound Length
+    pub(crate) fn set_len(&mut self, byte: u8) {
+        self.len = byte;
+        self.length_timer = 256 - self.len as u16;
+    }
+
+    /// 0xFF1C | NR32 - Channel 3 Volume
+    pub(crate) fn volume(&self) -> u8 {
+        ((self.volume as u8) << 5) | 0x9F
+    }
+
+    /// 0xFF1C | NR32 - Channel 3 Volume
+    pub(crate) fn set_volume(&mut self, byte: u8) {
+        use Ch3Volume::*;
+
+        self.volume = match (byte >> 5) & 0x03 {
+            0b00 => Mute,
+            0b01 => Full,
+            0b10 => Half,
+            0b11 => Quarter,
+            _ => unreachable!("{:#04X} is not a valid value for Channel3Volume", byte),
+        };
+    }
+
+    /// 0xFF1D | NR33 - Channel 3 Frequency low (lower 8 bits)
+    pub(crate) fn set_freq_lo(&mut self, byte: u8) {
+        self.freq_lo = byte;
+    }
+
+    /// 0xFF1E | NR34 - Channel 3 Frequency high
+    pub(crate) fn freq_hi(&self) -> u8 {
+        u8::from(self.freq_hi) | 0xBF
+    }
+
+    /// 0xFF1E | NR34 - Channel 3 Frequency high
+    pub(crate) fn set_freq_hi(&mut self, byte: u8) {
+        self.freq_hi = byte.into();
+
+        if self.freq_hi.initial() {
+            // Length behaviour during trigger event
+            if self.length_timer == 0 {
+                self.length_timer = 256;
+            }
+
+            if self.dac_enabled {
+                self.enabled = true;
+            }
+        }
+    }
+
+    fn tick(&mut self) {
+        if self.freq_timer != 0 {
+            self.freq_timer -= 1;
+        }
+
+        if self.freq_timer == 0 {
+            self.freq_timer = (2048 - self.frequency()) * 2;
+            self.offset = (self.offset + 1) % (WAVE_PATTERN_RAM_LEN * 2) as u8;
+        }
+    }
+
+    fn amplitude(&self) -> f32 {
+        if self.dac_enabled {
+            let sample = self.read_sample(self.offset) >> self.volume.shift_count();
+            let input = if self.enabled { sample } else { 0 };
+
+            (input as f32 / 7.5) - 1.0
+        } else {
+            0.0
+        }
+    }
+
+    fn read_sample(&self, index: u8) -> u8 {
+        let i = (index / 2) as usize;
+
+        if index % 2 == 0 {
+            // We grab the high nibble on even indexes
+            self.wave_ram[i] >> 4
+        } else {
+            // We grab the low nibble on odd indexes
+            self.wave_ram[i] & 0x0F
+        }
+    }
+
+    fn frequency(&self) -> u16 {
+        (self.freq_hi.freq_bits() as u16) << 8 | self.freq_lo as u16
+    }
+}
+
+#[derive(Debug, Default)]
+pub(crate) struct Channel4 {
+    /// 0xFF20 | NR41 - Channel 4 Sound Length
+    len: u8,
+    /// 0xFF21 | NR42 - Channel 4 Volume Envelope
+    envelope: VolumeEnvelope,
+    /// 0xFF22 | NR43 - Chanel 4 Polynomial Counter
+    poly: PolynomialCounter,
+    /// 0xFF23 | NR44 - Channel 4 Counter / Consecutive Selector and Restart
+    freq: Ch4Frequency,
+
+    // Envelope Functionality
+    period_timer: u8,
+    current_volume: u8,
+
+    // Length Functionality
+    length_timer: u16,
+
+    /// Linear Feedback Shift Register (15-bit)
+    lf_shift: u16,
+
+    freq_timer: u16,
+
+    enabled: bool,
+}
+
+impl Channel4 {
+    /// 0xFF20 | NR41 - Channel 4 Sound Length
+    pub(crate) fn set_len(&mut self, byte: u8) {
+        self.len = byte & 0x3F;
+        self.length_timer = 64 - self.len as u16;
+    }
+
+    /// 0xFF21 | NR42 - Channel 4 Volume Envelope
+    pub(crate) fn envelope(&self) -> u8 {
+        u8::from(self.envelope)
+    }
+
+    /// 0xFF21 | NR42 - Channel 4 Volume Envelope
+    pub(crate) fn set_envelope(&mut self, byte: u8) {
+        self.envelope = byte.into();
+
+        if !self.is_dac_enabled() {
+            self.enabled = false;
+        }
+    }
+
+    /// 0xFF22 | NR43 - Chanel 4 Polynomial Counter
+    pub(crate) fn poly(&self) -> u8 {
+        u8::from(self.poly)
+    }
+
+    /// 0xFF22 | NR43 - Chanel 4 Polynomial Counter
+    pub(crate) fn set_poly(&mut self, byte: u8) {
+        self.poly = byte.into();
+    }
+
+    /// 0xFF23 | NR44 - Channel 4 Counter / Consecutive Selector and Restart
+    pub(crate) fn frequency(&self) -> u8 {
+        u8::from(self.freq) | 0xBF
+    }
+
+    /// 0xFF23 | NR44 - Channel 4 Counter / Consecutive Selector and Restart
+    pub(crate) fn set_frequency(&mut self, byte: u8) {
+        self.freq = byte.into();
+
+        if self.freq.initial() {
+            // Envelope behaviour during trigger event
+            self.period_timer = self.envelope.period();
+            self.current_volume = self.envelope.init_vol();
+
+            // Length behaviour during trigger event
+            if self.length_timer == 0 {
+                self.length_timer = 64;
+            }
+
+            // LFSR behaviour during trigger event
+            self.lf_shift = 0x7FFF;
+
+            if self.is_dac_enabled() {
+                self.enabled = true;
+            }
+        }
+    }
+
+    fn tick(&mut self) {
+        if self.freq_timer != 0 {
+            self.freq_timer -= 1;
+        }
+
+        if self.freq_timer == 0 {
+            let divisor = Self::divisor(self.poly.divisor_code()) as u16;
+            self.freq_timer = divisor << self.poly.shift_count();
+
+            let xor_result = (self.lf_shift & 0x01) ^ ((self.lf_shift & 0x02) >> 1);
+            self.lf_shift = (self.lf_shift >> 1) | xor_result << 14;
+
+            if let CounterWidth::Long = self.poly.counter_width() {
+                self.lf_shift = (self.lf_shift & !(0x01 << 6)) | xor_result << 6;
+            }
+        }
+    }
+
+    fn amplitude(&self) -> f32 {
+        if self.is_dac_enabled() {
+            let sample = (!self.lf_shift & 0x01) as u8 * self.current_volume;
+            let input = if self.enabled { sample } else { 0 };
+
+            (input as f32 / 7.5) - 1.0
+        } else {
+            0.0
+        }
+    }
+
+    fn is_dac_enabled(&self) -> bool {
+        self.envelope.0 & 0xF8 != 0x00
+    }
+
+    fn divisor(code: u8) -> u8 {
+        if code == 0 {
+            return 8;
+        }
+
+        code << 4
+    }
+}

src/apu/gen.rs

@@ -0,0 +1,101 @@
+use rodio::Source;
+use rtrb::{Consumer, Producer, PushError, RingBuffer};
+
+pub(crate) const SAMPLE_RATE: u32 = 48000; // Hz
+const CHANNEL_COUNT: usize = 2;
+const BUFFER_CAPACITY: usize = 2048 * CHANNEL_COUNT; // # of samples * the # of channels
+
+pub struct AudioSPSC<T> {
+    inner: RingBuffer<T>,
+}
+
+impl<T> Default for AudioSPSC<T> {
+    fn default() -> Self {
+        Self {
+            inner: RingBuffer::new(BUFFER_CAPACITY),
+        }
+    }
+}
+
+impl<T> AudioSPSC<T> {
+    pub fn new(capacity: usize) -> Self {
+        Self {
+            inner: RingBuffer::new(capacity),
+        }
+    }
+
+    pub fn init(self) -> (SampleProducer<T>, SampleConsumer<T>) {
+        let (prod, cons) = self.inner.split();
+
+        (
+            SampleProducer { inner: prod },
+            SampleConsumer { inner: cons },
+        )
+    }
+}
+
+pub struct SampleProducer<T> {
+    inner: Producer<T>,
+}
+
+impl<T> SampleProducer<T> {
+    pub(crate) fn push(&mut self, value: T) -> Result<(), PushError<T>> {
+        self.inner.push(value)
+    }
+
+    #[allow(dead_code)]
+    pub(crate) fn available(&self) -> bool {
+        self.inner.slots() > 2
+    }
+
+    pub(crate) fn available_blocking(&self) -> bool {
+        loop {
+            if self.inner.slots() > 2 {
+                break true;
+            }
+        }
+    }
+}
+
+impl<T> std::fmt::Debug for SampleProducer<T> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct(&format!("SampleProducer<{}>", std::any::type_name::<T>()))
+            .finish_non_exhaustive()
+    }
+}
+
+pub struct SampleConsumer<T> {
+    inner: Consumer<T>,
+}
+
+impl Iterator for SampleConsumer<f32> {
+    type Item = f32;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        // As of 2021-07-28, PopError can only be Empty
+        Some(self.inner.pop().unwrap_or_default())
+    }
+}
+
+impl Source for SampleConsumer<f32> {
+    fn current_frame_len(&self) -> Option<usize> {
+        // A frame changes when the samples rate or
+        // number of channels change. This will never happen, so
+        // we return
+        None
+    }
+
+    fn channels(&self) -> u16 {
+        // The Gameboy supports two channels
+        CHANNEL_COUNT as u16
+    }
+
+    fn sample_rate(&self) -> u32 {
+        SAMPLE_RATE
+    }
+
+    fn total_duration(&self) -> Option<std::time::Duration> {
+        // The duration of this source is infinite
+        None
+    }
+}

src/apu/types.rs

@@ -0,0 +1,577 @@
+use bitfield::bitfield;
+
+pub(crate) mod ch1 {
+    use super::bitfield;
+
+    bitfield! {
+        pub struct Sweep(u8);
+        impl Debug;
+        _period, _: 6, 4;
+        from into SweepDirection, _direction, _: 3, 3;
+        _shift_count, _: 2, 0;
+    }
+
+    impl Sweep {
+        pub(crate) fn period(&self) -> u8 {
+            self._period()
+        }
+
+        pub(crate) fn shift_count(&self) -> u8 {
+            self._shift_count()
+        }
+
+        pub(crate) fn direction(&self) -> SweepDirection {
+            self._direction()
+        }
+    }
+
+    impl Copy for Sweep {}
+    impl Clone for Sweep {
+        fn clone(&self) -> Self {
+            *self
+        }
+    }
+
+    impl Default for Sweep {
+        fn default() -> Self {
+            Self(0)
+        }
+    }
+
+    impl From<u8> for Sweep {
+        fn from(byte: u8) -> Self {
+            Self(byte)
+        }
+    }
+
+    impl From<Sweep> for u8 {
+        fn from(sweep: Sweep) -> Self {
+            sweep.0
+        }
+    }
+
+    #[derive(Debug, Clone, Copy)]
+    pub(crate) enum SweepDirection {
+        Increase = 0,
+        Decrease = 1,
+    }
+
+    impl From<u8> for SweepDirection {
+        fn from(byte: u8) -> Self {
+            match byte & 0x01 {
+                0b00 => Self::Increase,
+                0b01 => Self::Decrease,
+                _ => unreachable!("{:04X} is not a valid value for SweepChange", byte),
+            }
+        }
+    }
+
+    impl From<SweepDirection> for u8 {
+        fn from(change: SweepDirection) -> Self {
+            change as u8
+        }
+    }
+}
+
+pub(super) mod ch3 {
+    #[derive(Debug, Clone, Copy)]
+    pub(crate) enum Volume {
+        Mute = 0,
+        Full = 1,
+        Half = 2,
+        Quarter = 3,
+    }
+
+    impl Volume {
+        pub(crate) fn shift_count(&self) -> u8 {
+            use Volume::*;
+
+            match *self {
+                Mute => 4,
+                Full => 0,
+                Half => 1,
+                Quarter => 2,
+            }
+        }
+    }
+
+    impl Default for Volume {
+        fn default() -> Self {
+            Self::Mute
+        }
+    }
+}
+
+pub(super) mod ch4 {
+    use super::bitfield;
+
+    bitfield! {
+        pub struct PolynomialCounter(u8);
+        impl Debug;
+        _shift_count, _: 7, 4;
+        from into CounterWidth, _counter_width, _: 3, 3;
+        _divisor_code, _: 2, 0;
+    }
+
+    impl PolynomialCounter {
+        pub(crate) fn divisor_code(&self) -> u8 {
+            self._divisor_code()
+        }
+
+        pub(crate) fn counter_width(&self) -> CounterWidth {
+            self._counter_width()
+        }
+
+        pub(crate) fn shift_count(&self) -> u8 {
+            self._shift_count()
+        }
+    }
+
+    impl Copy for PolynomialCounter {}
+    impl Clone for PolynomialCounter {
+        fn clone(&self) -> Self {
+            *self
+        }
+    }
+
+    impl Default for PolynomialCounter {
+        fn default() -> Self {
+            Self(0)
+        }
+    }
+
+    impl From<u8> for PolynomialCounter {
+        fn from(byte: u8) -> Self {
+            Self(byte)
+        }
+    }
+
+    impl From<PolynomialCounter> for u8 {
+        fn from(poly: PolynomialCounter) -> Self {
+            poly.0
+        }
+    }
+
+    #[derive(Debug, Clone, Copy)]
+    pub(crate) enum CounterWidth {
+        Long,  // 15 bits long
+        Short, // 7 bits long
+    }
+
+    impl From<u8> for CounterWidth {
+        fn from(byte: u8) -> Self {
+            match byte & 0x01 {
+                0b00 => Self::Short,
+                0b01 => Self::Long,
+                _ => unreachable!("{:#04X} is not a valid value for CounterWidth"),
+            }
+        }
+    }
+
+    impl From<CounterWidth> for u8 {
+        fn from(counter_width: CounterWidth) -> Self {
+            counter_width as u8
+        }
+    }
+
+    bitfield! {
+        pub struct Frequency(u8);
+        impl Debug;
+        _initial, _: 7;
+        _length_disable, _: 6;
+    }
+
+    impl Frequency {
+        pub(crate) fn length_disable(&self) -> bool {
+            self._length_disable()
+        }
+
+        pub(crate) fn initial(&self) -> bool {
+            self._initial()
+        }
+    }
+
+    impl Copy for Frequency {}
+    impl Clone for Frequency {
+        fn clone(&self) -> Self {
+            *self
+        }
+    }
+
+    impl Default for Frequency {
+        fn default() -> Self {
+            Self(0)
+        }
+    }
+
+    impl From<u8> for Frequency {
+        fn from(byte: u8) -> Self {
+            Self(byte & 0xC0) // Only bits 7 and 6 hold anything of value
+        }
+    }
+
+    impl From<Frequency> for u8 {
+        fn from(state: Frequency) -> Self {
+            state.0 & 0x40 // Only bit 6 holds anything of value
+        }
+    }
+}
+
+pub(super) mod common {
+    use super::bitfield;
+
+    bitfield! {
+        pub struct FrequencyHigh(u8);
+        impl Debug;
+        _initial, _: 7;
+        _length_disable, _: 6;
+        pub freq_bits, set_freq_bits: 2, 0;
+    }
+
+    impl FrequencyHigh {
+        pub(crate) fn initial(&self) -> bool {
+            self._initial()
+        }
+
+        pub(crate) fn length_disable(&self) -> bool {
+            self._length_disable()
+        }
+    }
+
+    impl Copy for FrequencyHigh {}
+    impl Clone for FrequencyHigh {
+        fn clone(&self) -> Self {
+            *self
+        }
+    }
+
+    impl Default for FrequencyHigh {
+        fn default() -> Self {
+            Self(0)
+        }
+    }
+
+    impl From<u8> for FrequencyHigh {
+        fn from(byte: u8) -> Self {
+            Self(byte)
+        }
+    }
+
+    impl From<FrequencyHigh> for u8 {
+        fn from(freq: FrequencyHigh) -> Self {
+            freq.0 & 0x40 // Only bit 6 can be read
+        }
+    }
+
+    bitfield! {
+        pub struct VolumeEnvelope(u8);
+        impl Debug;
+        _init_vol, _: 7, 4;
+        from into EnvelopeDirection, _direction, _: 3, 3;
+        _period, _: 2, 0;
+    }
+
+    impl VolumeEnvelope {
+        pub(crate) fn init_vol(&self) -> u8 {
+            self._init_vol()
+        }
+
+        pub(crate) fn direction(&self) -> EnvelopeDirection {
+            self._direction()
+        }
+
+        pub(crate) fn period(&self) -> u8 {
+            self._period()
+        }
+    }
+
+    impl Copy for VolumeEnvelope {}
+    impl Clone for VolumeEnvelope {
+        fn clone(&self) -> Self {
+            *self
+        }
+    }
+
+    impl Default for VolumeEnvelope {
+        fn default() -> Self {
+            Self(0)
+        }
+    }
+
+    impl From<u8> for VolumeEnvelope {
+        fn from(byte: u8) -> Self {
+            Self(byte)
+        }
+    }
+
+    impl From<VolumeEnvelope> for u8 {
+        fn from(envelope: VolumeEnvelope) -> Self {
+            envelope.0
+        }
+    }
+
+    #[derive(Debug, Clone, Copy)]
+    pub(crate) enum EnvelopeDirection {
+        Decrease = 0,
+        Increase = 1,
+    }
+
+    impl From<u8> for EnvelopeDirection {
+        fn from(byte: u8) -> Self {
+            match byte & 0b01 {
+                0b00 => Self::Decrease,
+                0b01 => Self::Increase,
+                _ => unreachable!("{:#04X} is not a valid value for EnvelopeDirection", byte),
+            }
+        }
+    }
+
+    impl From<EnvelopeDirection> for u8 {
+        fn from(direction: EnvelopeDirection) -> Self {
+            direction as u8
+        }
+    }
+
+    impl Default for EnvelopeDirection {
+        fn default() -> Self {
+            Self::Decrease
+        }
+    }
+
+    bitfield! {
+        pub struct SoundDuty(u8);
+        impl Debug;
+        from into WavePattern, _wave_pattern, _: 7, 6;
+        _sound_length, _: 5, 0;
+    }
+
+    impl SoundDuty {
+        pub(crate) fn wave_pattern(&self) -> WavePattern {
+            self._wave_pattern()
+        }
+
+        pub(crate) fn sound_length(&self) -> u8 {
+            self._sound_length()
+        }
+    }
+
+    impl Copy for SoundDuty {}
+    impl Clone for SoundDuty {
+        fn clone(&self) -> Self {
+            *self
+        }
+    }
+
+    impl Default for SoundDuty {
+        fn default() -> Self {
+            Self(0)
+        }
+    }
+
+    impl From<u8> for SoundDuty {
+        fn from(byte: u8) -> Self {
+            Self(byte)
+        }
+    }
+
+    impl From<SoundDuty> for u8 {
+        fn from(duty: SoundDuty) -> Self {
+            duty.0 & 0xC0 // Only bits 7 and 6 can be read
+        }
+    }
+
+    #[derive(Debug, Clone, Copy)]
+    pub(crate) enum WavePattern {
+        OneEighth = 0,     // 12.5% ( _-------_-------_------- )
+        OneQuarter = 1,    // 25%   ( __------__------__------ )
+        OneHalf = 2,       // 50%   ( ____----____----____---- ) (normal)
+        ThreeQuarters = 3, // 75%   ( ______--______--______-- )
+    }
+
+    impl WavePattern {
+        pub(crate) const fn amplitude(&self, index: u8) -> u8 {
+            use WavePattern::*;
+            let i = 7 - index; // an index of 0 should get the highest bit
+
+            match *self {
+                OneEighth => (0b00000001 >> i) & 0x01,
+                OneQuarter => (0b10000001 >> i) & 0x01,
+                OneHalf => (0b10000111 >> i) & 0x01,
+                ThreeQuarters => (0b01111110 >> i) & 0x01,
+            }
+        }
+    }
+
+    impl Default for WavePattern {
+        fn default() -> Self {
+            Self::OneEighth // Rationale: OneEighth is 0x00
+        }
+    }
+
+    impl From<WavePattern> for u8 {
+        fn from(pattern: WavePattern) -> Self {
+            pattern as Self
+        }
+    }
+
+    impl From<u8> for WavePattern {
+        fn from(byte: u8) -> Self {
+            match byte & 0b11 {
+                0b00 => Self::OneEighth,
+                0b01 => Self::OneQuarter,
+                0b10 => Self::OneHalf,
+                0b11 => Self::ThreeQuarters,
+                _ => unreachable!("{:#04X} is not a valid value for WavePattern", byte),
+            }
+        }
+    }
+}
+
+bitfield! {
+    pub struct SoundOutput(u8);
+    impl Debug;
+    pub ch4_left, _: 7;
+    pub ch3_left, _: 6;
+    pub ch2_left, _: 5;
+    pub ch1_left, _: 4;
+    pub ch4_right, _: 3;
+    pub ch3_right, _: 2;
+    pub ch2_right, _: 1;
+    pub ch1_right, _: 0;
+}
+
+impl SoundOutput {
+    pub(super) fn ch1(&self) -> (bool, bool) {
+        (self.ch1_left(), self.ch1_right())
+    }
+
+    pub(super) fn ch2(&self) -> (bool, bool) {
+        (self.ch2_left(), self.ch2_right())
+    }
+
+    pub(super) fn ch3(&self) -> (bool, bool) {
+        (self.ch3_left(), self.ch3_right())
+    }
+
+    pub(super) fn ch4(&self) -> (bool, bool) {
+        (self.ch4_left(), self.ch4_right())
+    }
+}
+
+impl Copy for SoundOutput {}
+impl Clone for SoundOutput {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl Default for SoundOutput {
+    fn default() -> Self {
+        Self(0)
+    }
+}
+
+impl From<u8> for SoundOutput {
+    fn from(byte: u8) -> Self {
+        Self(byte)
+    }
+}
+
+impl From<SoundOutput> for u8 {
+    fn from(output: SoundOutput) -> Self {
+        output.0
+    }
+}
+
+bitfield! {
+    pub struct ChannelControl(u8);
+    impl Debug;
+    vin_left, _: 7;
+    _left_volume, _: 6, 4;
+    vin_right, _: 3;
+    _right_volume, _: 2, 0;
+}
+
+impl ChannelControl {
+    pub(crate) fn left_volume(&self) -> f32 {
+        self._left_volume() as f32
+    }
+
+    pub(crate) fn right_volume(&self) -> f32 {
+        self._right_volume() as f32
+    }
+}
+
+impl Copy for ChannelControl {}
+impl Clone for ChannelControl {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl Default for ChannelControl {
+    fn default() -> Self {
+        Self(0)
+    }
+}
+
+impl From<u8> for ChannelControl {
+    fn from(byte: u8) -> Self {
+        Self(byte)
+    }
+}
+
+impl From<ChannelControl> for u8 {
+    fn from(ctrl: ChannelControl) -> Self {
+        ctrl.0
+    }
+}
+
+pub(super) mod fs {
+    #[derive(Debug)]
+    pub(crate) struct FrameSequencer {
+        step: u8,
+        state: State,
+    }
+
+    impl Default for FrameSequencer {
+        fn default() -> Self {
+            Self {
+                step: Default::default(),
+                state: State::Length,
+            }
+        }
+    }
+
+    impl FrameSequencer {
+        pub(crate) fn next(&mut self) {
+            use State::*;
+
+            self.step = (self.step + 1) % 8;
+            self.state = match self.step {
+                1 | 3 | 5 => Nothing,
+                0 | 4 => Length,
+                2 | 6 => LengthAndSweep,
+                7 => Envelope,
+                _ => unreachable!("Step {} is invalid for the Frame Sequencer", self.step),
+            };
+        }
+
+        pub(crate) fn state(&self) -> State {
+            self.state
+        }
+
+        pub(crate) fn reset(&mut self) {
+            self.step = Default::default();
+            self.state = State::Length;
+        }
+    }
+
+    #[derive(Debug, Clone, Copy)]
+    pub(crate) enum State {
+        Length,
+        Nothing,
+        LengthAndSweep,
+        Envelope,
+    }
+}

src/bus.rs

@@ -1,26 +1,25 @@
+use crate::apu::Apu;
 use crate::cartridge::Cartridge;
 use crate::high_ram::HighRam;
 use crate::interrupt::{Interrupt, InterruptFlag};
 use crate::joypad::Joypad;
 use crate::ppu::{Ppu, PpuMode};
 use crate::serial::Serial;
-use crate::sound::{AudioSender, Sound};
 use crate::timer::Timer;
 use crate::work_ram::{VariableWorkRam, WorkRam};
-use std::{fs::File, io::Read};
 
-const BOOT_ROM_SIZE: usize = 0x100;
+pub(crate) const BOOT_SIZE: usize = 0x100;
 
-#[derive(Debug, Clone)]
+#[derive(Debug)]
 pub struct Bus {
-    boot: Option<[u8; BOOT_ROM_SIZE]>, // Boot ROM is 256b long
-    cartridge: Option<Cartridge>,
-    pub ppu: Ppu,
+    boot: Option<[u8; BOOT_SIZE]>, // Boot ROM is 256b long
+    cart: Option<Cartridge>,
+    pub(crate) ppu: Ppu,
     work_ram: WorkRam,
     var_ram: VariableWorkRam,
     pub(crate) timer: Timer,
     int: Interrupt,
-    snd: Sound,
+    pub(crate) apu: Apu,
     high_ram: HighRam,
     serial: Serial,
     pub(crate) joypad: Joypad,
@@ -30,13 +29,13 @@ impl Default for Bus {
     fn default() -> Self {
         Self {
             boot: None,
-            cartridge: None,
+            cart: None,
             ppu: Default::default(),
             work_ram: Default::default(),
             var_ram: Default::default(),
             timer: Default::default(),
             int: Default::default(),
-            snd: Default::default(),
+            apu: Default::default(),
             high_ram: Default::default(),
             serial: Default::default(),
             joypad: Default::default(),
@@ -45,48 +44,41 @@ impl Default for Bus {
 }
 
 impl Bus {
-    pub(crate) fn with_boot(path: &str) -> anyhow::Result<Self> {
-        let mut file = File::open(path)?;
-        let mut boot_rom = [0u8; 256];
-
-        file.read_exact(&mut boot_rom)?;
-
-        Ok(Self {
-            boot: Some(boot_rom),
+    pub(crate) fn with_boot(rom: [u8; 256]) -> Self {
+        Self {
+            boot: Some(rom),
             ..Default::default()
-        })
+        }
     }
 
-    pub(crate) fn load_cartridge(&mut self, path: &str) -> std::io::Result<()> {
-        self.cartridge = Some(Cartridge::new(path)?);
-        Ok(())
+    pub(crate) fn load_cart(&mut self, rom: Vec<u8>) {
+        self.cart = Some(Cartridge::new(rom));
     }
 
-    pub(crate) fn rom_title(&self) -> Option<&str> {
-        self.cartridge.as_ref()?.title()
+    pub(crate) fn cart_title(&self) -> Option<&str> {
+        self.cart.as_ref()?.title()
     }
 
-    pub(crate) fn pass_audio_src(&mut self, sender: AudioSender<f32>) {
-        self.snd.set_audio_src(sender)
-    }
-
-    pub(crate) fn is_full(&self) -> bool {
-        self.snd.is_full()
-    }
-
-    pub(crate) fn flush_samples(&mut self) {
-        self.snd.flush_samples()
+    #[allow(dead_code)]
+    pub(crate) fn boot_mapped(&self) -> bool {
+        self.boot.is_some()
     }
 
     pub(crate) fn clock(&mut self) {
-        self.ppu.clock();
-        self.timer.clock();
-        self.snd.clock(self.timer.divider);
-        self.clock_dma();
+        self.tick(4);
     }
 
-    fn clock_dma(&mut self) {
-        if let Some((src_addr, dest_addr)) = self.ppu.dma.clock() {
+    fn tick(&mut self, limit: u8) {
+        for _ in 0..limit {
+            self.timer.tick();
+            self.ppu.tick();
+            self.apu.tick(self.timer.divider);
+            self.dma_tick()
+        }
+    }
+
+    fn dma_tick(&mut self) {
+        if let Some((src_addr, dest_addr)) = self.ppu.dma.tick() {
             let byte = self.oam_read_byte(src_addr);
             self.oam_write_byte(dest_addr, byte);
         }
@@ -105,13 +97,13 @@ impl Bus {
                     }
                 }
 
-                match self.cartridge.as_ref() {
+                match self.cart.as_ref() {
                     Some(cart) => cart.read_byte(addr),
                     None => panic!("Tried to read from a non-existent cartridge"),
                 }
             }
             0x8000..=0x9FFF => self.ppu.read_byte(addr), // 8KB Video RAM
-            0xA000..=0xBFFF => match self.cartridge.as_ref() {
+            0xA000..=0xBFFF => match self.cart.as_ref() {
                 // 8KB External RAM
                 Some(cart) => cart.read_byte(addr),
                 None => panic!("Tried to read from a non-existent cartridge"),
@@ -158,7 +150,7 @@ impl BusIo for Bus {
                     }
                 }
 
-                match self.cartridge.as_ref() {
+                match self.cart.as_ref() {
                     Some(cart) => cart.read_byte(addr),
                     None => panic!("Tried to read from a non-existent cartridge"),
                 }
@@ -170,7 +162,7 @@ impl BusIo for Bus {
                     _ => self.ppu.read_byte(addr),
                 }
             }
-            0xA000..=0xBFFF => match self.cartridge.as_ref() {
+            0xA000..=0xBFFF => match self.cart.as_ref() {
                 // 8KB External RAM
                 Some(cart) => cart.read_byte(addr),
                 None => panic!("Tried to read from a non-existent cartridge"),
@@ -224,29 +216,11 @@ impl BusIo for Bus {
                     0x01 => self.serial.next,
                     0x02 => self.serial.ctrl.into(),
                     0x04 => (self.timer.divider >> 8) as u8,
-                    0x05 => self.timer.counter,
+                    0x05 => self.timer.tima(),
                     0x06 => self.timer.modulo,
                     0x07 => self.timer.ctrl.into(),
                     0x0F => self.interrupt_flag().into(),
-                    0x10 => self.snd.ch1.sweep.into(),
-                    0x11 => self.snd.ch1.duty(),
-                    0x12 => self.snd.ch1.envelope.into(),
-                    0x14 => self.snd.ch1.freq_hi(),
-                    0x16 => self.snd.ch2.duty(),
-                    0x17 => self.snd.ch2.envelope.into(),
-                    0x19 => self.snd.ch2.freq_hi(),
-                    0x1A => self.snd.ch3.enabled(),
-                    0x1B => self.snd.ch3.len(),
-                    0x1C => self.snd.ch3.volume(),
-                    0x1E => self.snd.ch3.freq_hi(),
-                    0x20 => self.snd.ch4.len(),
-                    0x21 => self.snd.ch4.envelope.into(),
-                    0x22 => self.snd.ch4.poly.into(),
-                    0x23 => self.snd.ch4.freq_data(),
-                    0x24 => self.snd.ctrl.channel.into(),
-                    0x25 => self.snd.ctrl.output.into(),
-                    0x26 => self.snd.ctrl.status(&self.snd),
-                    0x30..=0x3F => self.snd.ch3.wave_ram[addr as usize - 0xFF30],
+                    0x10..=0x3F => self.apu.read_byte(addr),
                     0x40 => self.ppu.ctrl.into(),
                     0x41 => self.ppu.stat.into(),
                     0x42 => self.ppu.pos.scroll_y,
@@ -259,8 +233,10 @@ impl BusIo for Bus {
                     0x49 => self.ppu.monochrome.obj_palette_1.into(),
                     0x4A => self.ppu.pos.window_y,
                     0x4B => self.ppu.pos.window_x,
-                    0x4D => 0xFF, // CGB Specific Register
-                    _ => unimplemented!("Unable to read {:#06X} in I/O Registers", addr),
+                    _ => {
+                        eprintln!("Read 0xFF from unused IO register {:#06X}.", addr);
+                        0xFF
+                    }
                 }
             }
             0xFF80..=0xFFFE => {
@@ -279,7 +255,7 @@ impl BusIo for Bus {
             0x0000..=0x7FFF => {
                 // 16KB ROM bank 00 (ends at 0x3FFF)
                 // and 16KB ROM Bank 01 -> NN (switchable via MB)
-                match self.cartridge.as_mut() {
+                match self.cart.as_mut() {
                     Some(cart) => cart.write_byte(addr, byte),
                     None => panic!("Tried to write into non-existent cartridge"),
                 }
@@ -293,7 +269,7 @@ impl BusIo for Bus {
             }
             0xA000..=0xBFFF => {
                 // 8KB External RAM
-                match self.cartridge.as_mut() {
+                match self.cart.as_mut() {
                     Some(cart) => cart.write_byte(addr, byte),
                     None => panic!("Tried to write into non-existent cartridge"),
                 }
@@ -328,7 +304,7 @@ impl BusIo for Bus {
                     _ => {}
                 }
             }
-            0xFEA0..=0xFEFF => {} // TODO: As far as I know, writes to here do nothing.
+            0xFEA0..=0xFEFF => {} // FIXME: As far as I know, writes to here do nothing.
             0xFF00..=0xFF7F => {
                 // IO Registers
 
@@ -339,32 +315,11 @@ impl BusIo for Bus {
                     0x01 => self.serial.next = byte,
                     0x02 => self.serial.ctrl = byte.into(),
                     0x04 => self.timer.divider = 0x0000,
-                    0x05 => self.timer.counter = byte,
+                    0x05 => self.timer.set_tima(byte),
                     0x06 => self.timer.modulo = byte,
                     0x07 => self.timer.ctrl = byte.into(),
                     0x0F => self.set_interrupt_flag(byte),
-                    0x10 => self.snd.ch1.sweep = byte.into(),
-                    0x11 => self.snd.ch1.set_duty(byte),
-                    0x12 => self.snd.ch1.envelope = byte.into(),
-                    0x13 => self.snd.ch1.set_freq_lo(byte),
-                    0x14 => self.snd.ch1.set_freq_hi(byte),
-                    0x16 => self.snd.ch2.set_duty(byte),
-                    0x17 => self.snd.ch2.envelope = byte.into(),
-                    0x18 => self.snd.ch2.set_freq_lo(byte),
-                    0x19 => self.snd.ch2.set_freq_hi(byte),
-                    0x1A => self.snd.ch3.set_enabled(byte),
-                    0x1B => self.snd.ch3.set_len(byte),
-                    0x1C => self.snd.ch3.set_volume(byte),
-                    0x1D => self.snd.ch3.set_freq_lo(byte),
-                    0x1E => self.snd.ch3.set_freq_hi(byte),
-                    0x20 => self.snd.ch4.set_len(byte),
-                    0x21 => self.snd.ch4.envelope = byte.into(),
-                    0x22 => self.snd.ch4.poly = byte.into(),
-                    0x23 => self.snd.ch4.set_freq_data(byte),
-                    0x24 => self.snd.ctrl.channel = byte.into(),
-                    0x25 => self.snd.ctrl.output = byte.into(),
-                    0x26 => self.snd.ctrl.set_status(byte), // FIXME: Should we control which bytes are written to here?
-                    0x30..=0x3F => self.snd.ch3.wave_ram[addr as usize - 0xFF30] = byte,
+                    0x10..=0x3F => self.apu.write_byte(addr, byte),
                     0x40 => self.ppu.ctrl = byte.into(),
                     0x41 => self.ppu.stat.update(byte),
                     0x42 => self.ppu.pos.scroll_y = byte,
@@ -396,8 +351,7 @@ impl BusIo for Bus {
                             self.boot = None;
                         }
                     }
-                    0x7F => {} // Tetris tries to write to this non-existent IO Address
-                    _ => unimplemented!("Unable to write to {:#06X} in I/O Registers", addr),
+                    _ => eprintln!("Wrote {:#04X} to unused IO register {:#06X}.", byte, addr),
                 };
             }
             0xFF80..=0xFFFE => {
@@ -412,17 +366,6 @@ impl BusIo for Bus {
     }
 }
 
-impl Bus {
-    pub(crate) fn read_word(&self, addr: u16) -> u16 {
-        (self.read_byte(addr + 1) as u16) << 8 | self.read_byte(addr) as u16
-    }
-
-    pub(crate) fn write_word(&mut self, addr: u16, word: u16) {
-        self.write_byte(addr + 1, (word >> 8) as u8);
-        self.write_byte(addr, (word & 0x00FF) as u8);
-    }
-}
-
 impl Bus {
     fn interrupt_flag(&self) -> InterruptFlag {
         // Read the current interrupt information from the PPU

src/cartridge.rs

@@ -1,137 +1,99 @@
-use std::fs::File;
-use std::io::{self, Read};
-use std::path::Path;
-
 use crate::bus::BusIo;
 
 const RAM_SIZE_ADDRESS: usize = 0x0149;
 const ROM_SIZE_ADDRESS: usize = 0x0148;
 const MBC_TYPE_ADDRESS: usize = 0x0147;
+const ROM_TITLE_RANGE: std::ops::RangeInclusive<usize> = 0x0134..=0x0143;
 
-#[derive(Debug, Clone, Default)]
+#[derive(Debug, Default)]
 pub(crate) struct Cartridge {
     memory: Vec<u8>,
     title: Option<String>,
-    mbc: Box<dyn MemoryBankController>,
+    mbc: Box<dyn MBCIo>,
 }
 
 impl Cartridge {
-    pub(crate) fn new<P: AsRef<Path> + ?Sized>(path: &P) -> io::Result<Self> {
-        let mut memory = vec![];
-        let mut rom = File::open(path)?;
-        rom.read_to_end(&mut memory)?;
-
+    pub(crate) fn new(memory: Vec<u8>) -> Self {
         let title = Self::find_title(&memory);
         eprintln!("Cartridge Title: {:?}", title);
 
-        Ok(Self {
+        Self {
             mbc: Self::detect_mbc(&memory),
             title,
             memory,
-        })
+        }
     }
 
-    fn detect_mbc(memory: &[u8]) -> Box<dyn MemoryBankController> {
-        let ram_size = Self::find_ram_size(memory);
-        let bank_count = Self::find_bank_count(memory);
+    fn detect_mbc(memory: &[u8]) -> Box<dyn MBCIo> {
+        let ram_size = Self::detect_ram_info(memory);
+        let rom_size = Self::detect_rom_info(memory);
         let mbc_kind = Self::find_mbc(memory);
-        let ram_byte_count = ram_size.len();
+        let ram_cap = ram_size.capacity();
+        let rom_cap = rom_size.capacity();
 
-        eprintln!("Cartridge Ram Size: {} bytes", ram_size.len());
-        eprintln!("Cartridge ROM Size: {} bytes", bank_count.size());
+        eprintln!("Cartridge Ram Size: {} bytes", ram_cap);
+        eprintln!("Cartridge ROM Size: {} bytes", rom_size.capacity());
         eprintln!("MBC Type: {:?}", mbc_kind);
 
         match mbc_kind {
-            MbcKind::None => Box::new(NoMbc {}),
-            MbcKind::Mbc1 => {
-                let mbc = Mbc1 {
-                    ram_size,
-                    ram: vec![0; ram_byte_count as usize],
-                    bank_count,
-                    ..Default::default()
-                };
-
-                Box::new(mbc)
-            }
-            MbcKind::Mbc1WithBattery => {
-                // TODO: Implement Saving
-                let mbc = Mbc1 {
-                    ram_size,
-                    ram: vec![0; ram_byte_count as usize],
-                    bank_count,
-                    ..Default::default()
-                };
-
-                Box::new(mbc)
-            }
-            MbcKind::Mbc3WithBattery => {
-                // TODO: Implement Saving
-                let mbc = MBC3 {
-                    ram_size,
-                    ram: vec![0; ram_byte_count as usize],
-                    ..Default::default()
-                };
-
-                Box::new(mbc)
-            }
-            MbcKind::Mbc3 => {
-                let mbc = MBC3 {
-                    ram_size,
-                    ram: vec![0; ram_byte_count as usize],
-                    ..Default::default()
-                };
-
-                Box::new(mbc)
-            }
-            MbcKind::Mbc5 => todo!("Implement MBC5"),
+            MBCKind::None => Box::new(NoMBC),
+            MBCKind::MBC1 => Box::new(MBC1::new(ram_size, rom_size)),
+            MBCKind::MBC1WithBattery => Box::new(MBC1::new(ram_size, rom_size)), // TODO: Implement Saving
+            MBCKind::MBC2 => Box::new(MBC2::new(rom_cap)),
+            MBCKind::MBC2WithBattery => Box::new(MBC2::new(rom_cap)), // TODO: Implement Saving
+            MBCKind::MBC3 => Box::new(MBC3::new(ram_cap)),
+            MBCKind::MBC3WithBattery => Box::new(MBC3::new(ram_cap)), // TODO: Implement Saving
+            MBCKind::MBC5 => Box::new(MBC5::new(ram_cap, rom_cap)),
+            MBCKind::MBC5WithBattery => Box::new(MBC5::new(ram_cap, rom_cap)), // TDO: Implement Saving
         }
     }
 
     fn find_title(memory: &[u8]) -> Option<String> {
-        // FIXME: Get rid of magic values and handle cases
-        // where 0x134..0x143 reads past the length of the
-        // string
+        let slice = &memory[ROM_TITLE_RANGE];
+        let with_nulls = std::str::from_utf8(slice).ok();
+        let trimmed = with_nulls.map(|s| s.trim_matches('\0').trim());
 
-        let slice = &memory[0x134..0x143];
-
-        let str_with_nulls = std::str::from_utf8(slice).ok();
-        str_with_nulls.map(|s| s.trim_matches('\0').to_string())
+        match trimmed {
+            Some("") | None => None,
+            Some(_) => trimmed.map(String::from),
+        }
     }
 
     pub(crate) fn title(&self) -> Option<&str> {
         self.title.as_deref()
     }
 
-    fn find_ram_size(memory: &[u8]) -> RamSize {
+    fn detect_ram_info(memory: &[u8]) -> RamSize {
         let id = memory[RAM_SIZE_ADDRESS];
         id.into()
     }
 
-    fn find_bank_count(memory: &[u8]) -> BankCount {
+    fn detect_rom_info(memory: &[u8]) -> RomSize {
         let id = memory[ROM_SIZE_ADDRESS];
         id.into()
     }
 
-    fn find_mbc(memory: &[u8]) -> MbcKind {
-        let id = memory[MBC_TYPE_ADDRESS];
+    fn find_mbc(memory: &[u8]) -> MBCKind {
+        use MBCKind::*;
 
-        // TODO: Refactor this to match the other enums in this module
-        match id {
-            0x00 => MbcKind::None,
-            0x01 => MbcKind::Mbc1,
-            0x02 => MbcKind::Mbc1,
-            0x03 => MbcKind::Mbc1WithBattery,
-            0x19 => MbcKind::Mbc5,
-            0x13 => MbcKind::Mbc3WithBattery,
-            0x11 => MbcKind::Mbc3,
-            _ => unimplemented!("id {:#04X} is an unsupported MBC", id),
+        match memory[MBC_TYPE_ADDRESS] {
+            0x00 => None,
+            0x01 | 0x02 => MBC1,
+            0x03 => MBC1WithBattery,
+            0x05 => MBC2,
+            0x06 => MBC2WithBattery,
+            0x19 | 0x1A => MBC5,
+            0x1B => MBC5WithBattery,
+            0x13 => MBC3WithBattery,
+            0x11 | 0x12 => MBC3,
+            id => unimplemented!("id {:#04X} is an unsupported MBC", id),
         }
     }
 }
 
 impl BusIo for Cartridge {
     fn read_byte(&self, addr: u16) -> u8 {
-        use MbcResult::*;
+        use MBCResult::*;
 
         match self.mbc.handle_read(addr) {
             Address(addr) => self.memory[addr],
@@ -144,92 +106,90 @@ impl BusIo for Cartridge {
     }
 }
 
-#[derive(Debug, Clone)]
-struct Mbc1 {
+#[derive(Debug)]
+struct MBC1 {
     /// 5-bit number
     rom_bank: u8,
     /// 2-bit number
     ram_bank: u8,
     mode: bool,
     ram_size: RamSize,
-    ram: Vec<u8>,
-    bank_count: BankCount,
-    ram_enabled: bool,
+    memory: Vec<u8>,
+    rom_size: RomSize,
+    mem_enabled: bool,
 }
 
-impl Default for Mbc1 {
-    fn default() -> Self {
+impl MBC1 {
+    fn new(ram_size: RamSize, rom_size: RomSize) -> Self {
         Self {
             rom_bank: 0x01,
+            memory: vec![0; ram_size.capacity() as usize],
+            ram_size,
+            rom_size,
             ram_bank: Default::default(),
             mode: Default::default(),
-            ram_size: Default::default(),
-            ram: Default::default(),
-            bank_count: Default::default(),
-            ram_enabled: Default::default(),
+            mem_enabled: Default::default(),
         }
     }
-}
 
-impl Mbc1 {
     fn zero_bank(&self) -> u8 {
-        use BankCount::*;
+        use RomSize::*;
 
-        match self.bank_count {
+        match self.rom_size {
             None | Four | Eight | Sixteen | ThirtyTwo => 0x00,
             SixtyFour => (self.ram_bank & 0x01) << 5,
-            OneHundredTwentyEight => (self.ram_bank & 0x03) << 5,
-            _ => unreachable!("{:?} is not a valid MBC1 BankCount", self.bank_count),
+            OneTwentyEight => (self.ram_bank & 0x03) << 5,
+            _ => unreachable!("{:?} is not a valid MBC1 BankCount", self.rom_size),
         }
     }
 
     fn _mbcm_zero_bank(&self) -> u8 {
-        use BankCount::*;
+        use RomSize::*;
 
-        match self.bank_count {
+        match self.rom_size {
             None | Four | Eight | Sixteen | ThirtyTwo => 0x00,
             SixtyFour => (self.ram_bank & 0x03) << 4,
-            OneHundredTwentyEight => (self.ram_bank & 0x03) << 5,
-            _ => unreachable!("{:?} is not a valid MBC1 BankCount", self.bank_count),
+            OneTwentyEight => (self.ram_bank & 0x03) << 5,
+            _ => unreachable!("{:?} is not a valid MBC1 BankCount", self.rom_size),
         }
     }
 
     fn high_bank(&self) -> u8 {
-        use BankCount::*;
+        use RomSize::*;
 
         let base = self.rom_bank & self.rom_size_mask();
 
-        match self.bank_count {
+        match self.rom_size {
             None | Four | Eight | Sixteen | ThirtyTwo => base,
             SixtyFour => base & !(0x01 << 5) | ((self.ram_bank & 0x01) << 5),
-            OneHundredTwentyEight => base & !(0x03 << 5) | ((self.ram_bank & 0x03) << 5),
-            _ => unreachable!("{:?} is not a valid MBC1 BankCount", self.bank_count),
+            OneTwentyEight => base & !(0x03 << 5) | ((self.ram_bank & 0x03) << 5),
+            _ => unreachable!("{:?} is not a valid MBC1 BankCount", self.rom_size),
         }
     }
 
     fn rom_size_mask(&self) -> u8 {
-        use BankCount::*;
+        use RomSize::*;
 
-        match self.bank_count {
+        match self.rom_size {
             None => 0b00000001,
             Four => 0b00000011,
             Eight => 0b00000111,
             Sixteen => 0b00001111,
-            ThirtyTwo | SixtyFour | OneHundredTwentyEight => 0b00011111,
-            _ => unreachable!("{:?} is not a valid MBC1 BankCount", self.bank_count),
+            ThirtyTwo | SixtyFour | OneTwentyEight => 0b00011111,
+            _ => unreachable!("{:?} is not a valid MBC1 BankCount", self.rom_size),
         }
     }
 
-    fn ram_addr(&self, addr: u16) -> u16 {
+    fn ram_addr(&self, addr: u16) -> usize {
         use RamSize::*;
 
         match self.ram_size {
-            _2KB | _8KB => (addr - 0xA000) % self.ram_size.len() as u16,
-            _32KB => {
+            Unused | One => (addr as usize - 0xA000) % self.ram_size.capacity(),
+            Four => {
                 if self.mode {
-                    0x2000 * self.ram_bank as u16 + (addr - 0xA000)
+                    0x2000 * self.ram_bank as usize + (addr as usize - 0xA000)
                 } else {
-                    addr - 0xA000
+                    addr as usize - 0xA000
                 }
             }
             _ => unreachable!("RAM size can not be greater than 32KB on MBC1"),
@@ -237,49 +197,39 @@ impl Mbc1 {
     }
 }
 
-impl MemoryBankController for Mbc1 {
-    fn handle_read(&self, addr: u16) -> MbcResult {
-        use MbcResult::*;
+impl MBCIo for MBC1 {
+    fn handle_read(&self, addr: u16) -> MBCResult {
+        use MBCResult::*;
 
         match addr {
-            0x0000..=0x3FFF => {
-                if self.mode {
-                    Address(0x4000 * self.zero_bank() as usize + addr as usize)
-                } else {
-                    Address(addr as usize)
-                }
+            0x0000..=0x3FFF if self.mode => {
+                Address(0x4000 * self.zero_bank() as usize + addr as usize)
             }
+            0x0000..=0x3FFF => Address(addr as usize),
             0x4000..=0x7FFF => {
                 Address(0x4000 * self.high_bank() as usize + (addr as usize - 0x4000))
             }
-            0xA000..=0xBFFF => {
-                if self.ram_enabled {
-                    Value(self.ram[self.ram_addr(addr) as usize])
-                } else {
-                    Value(0xFF)
-                }
+            0xA000..=0xBFFF if self.mem_enabled && self.ram_size != RamSize::None => {
+                Value(self.memory[self.ram_addr(addr)])
             }
+            0xA000..=0xBFFF => Value(0xFF),
             _ => unreachable!("A read from {:#06X} should not be handled by MBC1", addr),
         }
     }
 
     fn handle_write(&mut self, addr: u16, byte: u8) {
         match addr {
-            0x0000..=0x1FFF => self.ram_enabled = (byte & 0x0F) == 0x0A,
+            0x0000..=0x1FFF => self.mem_enabled = (byte & 0x0F) == 0x0A,
             0x2000..=0x3FFF => {
-                self.rom_bank = if byte == 0x00 {
-                    0x01
-                } else {
-                    byte & self.rom_size_mask()
-                };
-
-                self.rom_bank &= 0x1F;
+                let value = byte & 0x1F;
+                let masked_value = byte & self.rom_size_mask();
+                self.rom_bank = if value == 0 { 0x01 } else { masked_value };
             }
             0x4000..=0x5FFF => self.ram_bank = byte & 0x03,
             0x6000..=0x7FFF => self.mode = (byte & 0x01) == 0x01,
-            0xA000..=0xBFFF if self.ram_enabled => {
-                let ram_addr = self.ram_addr(addr) as usize;
-                self.ram[ram_addr] = byte;
+            0xA000..=0xBFFF if self.mem_enabled && self.ram_size != RamSize::None => {
+                let ram_addr = self.ram_addr(addr);
+                self.memory[ram_addr] = byte;
             }
             0xA000..=0xBFFF => {} // Ram isn't enabled, ignored write
             _ => unreachable!("A write to {:#06X} should not be handled by MBC1", addr),
@@ -293,32 +243,47 @@ enum MBC3Device {
     RealTimeClock,
 }
 
-#[derive(Debug, Clone, Default)]
+#[derive(Debug)]
 struct MBC3 {
     /// 7-bit Number
     rom_bank: u8,
     /// 2-bit Number
     ram_bank: u8,
 
-    devices_enabled: bool,
-    currently_mapped: Option<MBC3Device>,
-    ram_size: RamSize,
-    ram: Vec<u8>,
+    devs_enabled: bool,
+    mapped: Option<MBC3Device>,
+    memory: Vec<u8>,
+
+    // RTC Data Latch Previous Write
+    prev_latch_write: Option<u8>,
 }
 
-impl MemoryBankController for MBC3 {
-    fn handle_read(&self, addr: u16) -> MbcResult {
-        use MbcResult::*;
+impl MBC3 {
+    fn new(ram_cap: usize) -> Self {
+        Self {
+            memory: vec![0; ram_cap],
+            rom_bank: Default::default(),
+            ram_bank: Default::default(),
+            devs_enabled: Default::default(),
+            mapped: Default::default(),
+            prev_latch_write: Default::default(),
+        }
+    }
+}
+
+impl MBCIo for MBC3 {
+    fn handle_read(&self, addr: u16) -> MBCResult {
+        use MBCResult::*;
 
         let res = match addr {
             0x0000..=0x3FFF => Address(addr as usize),
             0x4000..=0x7FFF => Address(0x4000 * self.rom_bank as usize + (addr as usize - 0x4000)),
-            0xA000..=0xBFFF => match self.currently_mapped {
-                Some(MBC3Device::ExternalRam) if self.devices_enabled => {
-                    Value(self.ram[0x2000 * self.ram_bank as usize + (addr as usize - 0xA000)])
+            0xA000..=0xBFFF => match self.mapped {
+                Some(MBC3Device::ExternalRam) if self.devs_enabled => {
+                    Value(self.memory[0x2000 * self.ram_bank as usize + (addr as usize - 0xA000)])
                 }
-                Some(MBC3Device::RealTimeClock) if self.devices_enabled => {
-                    unimplemented!("Reading from MBC3 RTC is currently unsupported")
+                Some(MBC3Device::RealTimeClock) if self.devs_enabled => {
+                    todo!("Return Latched value of register")
                 }
                 _ => Value(0xFF),
             },
@@ -330,29 +295,37 @@ impl MemoryBankController for MBC3 {
 
     fn handle_write(&mut self, addr: u16, byte: u8) {
         match addr {
-            0x000..=0x1FFF => self.devices_enabled = (byte & 0x0F) == 0x0A, // Enable External RAM and Access to RTC if there is one
-            0x2000..=0x3FFF => match byte {
-                0x00 => self.rom_bank = 0x01,
-                byte => self.rom_bank = byte & 0x7F,
-            },
+            0x000..=0x1FFF => self.devs_enabled = (byte & 0x0F) == 0x0A, // Enable External RAM and Access to RTC if there is one
+            0x2000..=0x3FFF => {
+                self.rom_bank = match byte {
+                    0x00 => 0x01,
+                    byte => byte & 0x7F,
+                }
+            }
             0x4000..=0x5FFF => match byte {
                 0x00 | 0x01 | 0x02 | 0x03 => {
                     self.ram_bank = byte & 0x03;
-                    self.currently_mapped = Some(MBC3Device::ExternalRam);
+                    self.mapped = Some(MBC3Device::ExternalRam);
                 }
                 0x08 | 0x09 | 0x0A | 0x0B | 0x0C => {
-                    self.currently_mapped = Some(MBC3Device::RealTimeClock);
-                    unimplemented!("RTC in MBC3 is currently unimplemented")
+                    self.mapped = Some(MBC3Device::RealTimeClock);
                 }
                 _ => {}
             },
-            0x6000..=0x7FFF => unimplemented!("RTC Data Latch is currently unimplemented"),
-            0xA000..=0xBFFF => match self.currently_mapped {
-                Some(MBC3Device::ExternalRam) if self.devices_enabled => {
-                    self.ram[0x2000 * self.ram_bank as usize + (addr as usize - 0xA000)] = byte
+            0x6000..=0x7FFF => {
+                if let Some(0x00) = self.prev_latch_write {
+                    if byte == 0x01 {
+                        todo!("Perform Data Latch")
+                    }
                 }
-                Some(MBC3Device::RealTimeClock) if self.devices_enabled => {
-                    unimplemented!("Writing to MBC3 RTC is currently unsupported")
+                self.prev_latch_write = Some(byte);
+            }
+            0xA000..=0xBFFF => match self.mapped {
+                Some(MBC3Device::ExternalRam) if self.devs_enabled => {
+                    self.memory[0x2000 * self.ram_bank as usize + (addr as usize - 0xA000)] = byte
+                }
+                Some(MBC3Device::RealTimeClock) if self.devs_enabled => {
+                    todo!("Write to RTC")
                 }
                 _ => {}
             },
@@ -361,12 +334,132 @@ impl MemoryBankController for MBC3 {
     }
 }
 
-#[derive(Debug, Clone, Copy)]
-struct NoMbc {}
+#[derive(Debug)]
+struct MBC5 {
+    /// 9-bit number
+    rom_bank: u16,
+    /// 4-bit number
+    ram_bank: u8,
 
-impl MemoryBankController for NoMbc {
-    fn handle_read(&self, addr: u16) -> MbcResult {
-        MbcResult::Address(addr as usize)
+    rom_cap: usize,
+
+    memory: Vec<u8>,
+    mem_enabled: bool,
+}
+
+impl MBC5 {
+    fn new(ram_cap: usize, rom_cap: usize) -> Self {
+        Self {
+            rom_bank: 0x01,
+            memory: vec![0; ram_cap],
+            rom_cap,
+            ram_bank: Default::default(),
+            mem_enabled: Default::default(),
+        }
+    }
+
+    fn bank_addr(&self, addr: u16) -> usize {
+        (0x4000 * self.rom_bank as usize + (addr as usize - 0x4000)) % self.rom_cap
+    }
+}
+
+impl MBCIo for MBC5 {
+    fn handle_read(&self, addr: u16) -> MBCResult {
+        use MBCResult::*;
+
+        match addr {
+            0x0000..=0x3FFF => Address(addr as usize),
+            0x4000..=0x7FFF => Address(self.bank_addr(addr)),
+            0xA000..=0xBFFF if self.mem_enabled => {
+                Value(self.memory[0x2000 * self.ram_bank as usize + (addr as usize - 0xA000)])
+            }
+            0xA000..=0xBFFF => Value(0xFF),
+            _ => unreachable!("A read from {:#06X} should not be handled by MBC5", addr),
+        }
+    }
+
+    fn handle_write(&mut self, addr: u16, byte: u8) {
+        match addr {
+            0x0000..=0x1FFF => self.mem_enabled = (byte & 0x0F) == 0x0A,
+            0x2000..=0x2FFF => self.rom_bank = (self.rom_bank & 0x0100) | byte as u16,
+            0x3000..=0x3FFF => self.rom_bank = (self.rom_bank & 0x00FF) | (byte as u16 & 0x01) << 8,
+            0x4000..=0x5FFF => self.ram_bank = byte & 0x0F,
+            0xA000..=0xBFFF if self.mem_enabled => {
+                self.memory[0x2000 * self.ram_bank as usize + (addr as usize - 0xA000)] = byte;
+            }
+            0xA000..=0xBFFF => {}
+            _ => unreachable!("A write to {:#06X} should not be handled by MBC5", addr),
+        }
+    }
+}
+
+#[derive(Debug)]
+struct MBC2 {
+    /// 4-bit number
+    rom_bank: u8,
+    memory: Box<[u8; Self::RAM_SIZE]>,
+    mem_enabled: bool,
+
+    rom_cap: usize,
+}
+
+impl MBC2 {
+    const RAM_SIZE: usize = 0x0200;
+
+    fn new(rom_cap: usize) -> Self {
+        Self {
+            rom_bank: 0x01,
+            memory: Box::new([0; Self::RAM_SIZE]),
+            mem_enabled: Default::default(),
+            rom_cap,
+        }
+    }
+
+    fn rom_addr(&self, addr: u16) -> usize {
+        (0x4000 * self.rom_bank as usize + (addr as usize - 0x4000)) % self.rom_cap
+    }
+}
+
+impl MBCIo for MBC2 {
+    fn handle_read(&self, addr: u16) -> MBCResult {
+        use MBCResult::*;
+
+        match addr {
+            0x0000..=0x3FFF => Address(addr as usize),
+            0x4000..=0x7FFF => Address(self.rom_addr(addr)),
+            0xA000..=0xBFFF if self.mem_enabled => {
+                let mbc2_addr = addr as usize & (Self::RAM_SIZE - 1);
+                Value(self.memory[mbc2_addr] | 0xF0)
+            }
+            0xA000..=0xBFFF => Value(0xFF),
+            _ => unreachable!("A read from {:#06X} should not be handled by MBC2", addr),
+        }
+    }
+
+    fn handle_write(&mut self, addr: u16, byte: u8) {
+        let nybble = byte & 0x0F;
+
+        match addr {
+            0x0000..=0x3FFF if addr >> 8 & 0x01 == 0x01 => {
+                self.rom_bank = if nybble == 0x00 { 0x01 } else { nybble };
+            }
+            0x0000..=0x3FFF => self.mem_enabled = nybble == 0x0A,
+            0xA000..=0xBFFF if self.mem_enabled => {
+                let mbc2_addr = addr as usize & (Self::RAM_SIZE - 1);
+                self.memory[mbc2_addr] = nybble;
+            }
+            0x4000..=0x7FFF | 0xA000..=0xBFFF => {}
+            _ => unreachable!("A write to {:#06X} should not be handled by MBC2", addr),
+        }
+    }
+}
+
+#[derive(Debug)]
+struct NoMBC;
+
+impl MBCIo for NoMBC {
+    fn handle_read(&self, addr: u16) -> MBCResult {
+        MBCResult::Address(addr as usize)
     }
 
     fn handle_write(&mut self, _addr: u16, _byte: u8) {
@@ -374,67 +467,57 @@ impl MemoryBankController for NoMbc {
     }
 }
 
-trait MemoryBankController: CloneMbc {
-    fn handle_read(&self, addr: u16) -> MbcResult;
+trait MBCIo {
+    fn handle_read(&self, addr: u16) -> MBCResult;
     fn handle_write(&mut self, addr: u16, byte: u8);
 }
 
-trait CloneMbc {
-    fn clone_mbc(&self) -> Box<dyn MemoryBankController>;
-}
-
-impl<T> CloneMbc for T
-where
-    T: MemoryBankController + Clone + 'static,
-{
-    fn clone_mbc<'a>(&self) -> Box<dyn MemoryBankController> {
-        Box::new(self.clone())
-    }
-}
-
 #[derive(Debug, Clone, Copy)]
-enum MbcResult {
+enum MBCResult {
     Address(usize),
     Value(u8),
 }
 
 #[derive(Debug, Clone, Copy)]
-enum MbcKind {
+enum MBCKind {
     None,
-    Mbc1,
-    Mbc1WithBattery,
-    Mbc5,
-    Mbc3WithBattery,
-    Mbc3,
+    MBC1,
+    MBC1WithBattery,
+    MBC2,
+    MBC2WithBattery,
+    MBC3,
+    MBC3WithBattery,
+    MBC5,
+    MBC5WithBattery,
 }
 
-impl Default for MbcKind {
+impl Default for MBCKind {
     fn default() -> Self {
         Self::None
     }
 }
 
-#[derive(Debug, Clone, Copy)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 enum RamSize {
     None = 0x00,
-    _2KB = 0x01,
-    _8KB = 0x02,
-    _32KB = 0x03,  // Split into 4 RAM banks
-    _128KB = 0x04, // Split into 16 RAM banks
-    _64KB = 0x05,  // Split into 8 RAm Banks
+    Unused = 0x01,
+    One = 0x02,
+    Four = 0x03,
+    Sixteen = 0x04,
+    Eight = 0x05,
 }
 
 impl RamSize {
-    fn len(&self) -> u32 {
+    fn capacity(&self) -> usize {
         use RamSize::*;
 
         match *self {
             None => 0,
-            _2KB => 2_048,
-            _8KB => 8_192,
-            _32KB => 32_768,
-            _128KB => 131_072,
-            _64KB => 65_536,
+            Unused => 0x800,
+            One => 0x2000,
+            Four => 0x8000,
+            Sixteen => 0x20000,
+            Eight => 0x10000,
         }
     }
 }
@@ -451,63 +534,49 @@ impl From<u8> for RamSize {
 
         match byte {
             0x00 => None,
-            0x01 => _2KB,
-            0x02 => _8KB,
-            0x03 => _32KB,
-            0x04 => _128KB,
-            0x05 => _64KB,
+            0x01 => Unused,
+            0x02 => One,
+            0x03 => Four,
+            0x04 => Sixteen,
+            0x05 => Eight,
             _ => unreachable!("{:#04X} is not a valid value for RAMSize", byte),
         }
     }
 }
 
 #[derive(Debug, Clone, Copy)]
-enum BankCount {
-    None = 0x00,                  // 32KB (also called Two)
-    Four = 0x01,                  // 64KB
-    Eight = 0x02,                 // 128KB
-    Sixteen = 0x03,               // 256KB
-    ThirtyTwo = 0x04,             // 512KB
-    SixtyFour = 0x05,             // 1MB
-    OneHundredTwentyEight = 0x06, // 2MB
-    TwoHundredFiftySix = 0x07,    // 4MB
-    FiveHundredTwelve = 0x08,     // 8MB
-    SeventyTwo = 0x52,            // 1.1MB
-    Eighty = 0x53,                // 1.2MB
-    NinetySix = 0x54,             // 1.5MB
+enum RomSize {
+    None = 0x00,              // 32KB (also called Two)
+    Four = 0x01,              // 64KB
+    Eight = 0x02,             // 128KB
+    Sixteen = 0x03,           // 256KB
+    ThirtyTwo = 0x04,         // 512KB
+    SixtyFour = 0x05,         // 1MB
+    OneTwentyEight = 0x06,    // 2MB
+    TwoFiftySix = 0x07,       // 4MB
+    FiveHundredTwelve = 0x08, // 8MB
+    SeventyTwo = 0x52,        // 1.1MB
+    Eighty = 0x53,            // 1.2MB
+    NinetySix = 0x54,         // 1.5MB
 }
 
-impl Default for BankCount {
-    fn default() -> Self {
-        Self::None
-    }
-}
-
-impl BankCount {
+impl RomSize {
     // https://hacktix.github.io/GBEDG/mbcs/#rom-size
-    fn size(self) -> u32 {
-        use BankCount::*;
+    fn capacity(&self) -> usize {
+        use RomSize::*;
 
         match self {
-            None => 32_768,
-            Four => 65_536,
-            Eight => 131_072,
-            Sixteen => 262_144,
-            ThirtyTwo => 524_288,
-            SixtyFour => 1_048_576,
-            OneHundredTwentyEight => 2_097_152,
-            TwoHundredFiftySix => 4_194_304,
-            FiveHundredTwelve => 8_388_608,
-            SeventyTwo => 1_179_648,
-            Eighty => 1_310_720,
-            NinetySix => 1_572_864,
+            SeventyTwo => 0x120000,
+            Eighty => 0x140000,
+            NinetySix => 0x180000,
+            _ => 0x8000 << *self as u8,
         }
     }
 }
 
-impl From<u8> for BankCount {
+impl From<u8> for RomSize {
     fn from(byte: u8) -> Self {
-        use BankCount::*;
+        use RomSize::*;
 
         match byte {
             0x00 => None,
@@ -516,8 +585,8 @@ impl From<u8> for BankCount {
             0x03 => Sixteen,
             0x04 => ThirtyTwo,
             0x05 => SixtyFour,
-            0x06 => OneHundredTwentyEight,
-            0x07 => TwoHundredFiftySix,
+            0x06 => OneTwentyEight,
+            0x07 => TwoFiftySix,
             0x08 => FiveHundredTwelve,
             0x52 => SeventyTwo,
             0x53 => Eighty,
@@ -527,20 +596,14 @@ impl From<u8> for BankCount {
     }
 }
 
-impl std::fmt::Debug for Box<dyn MemoryBankController> {
+impl std::fmt::Debug for Box<dyn MBCIo> {
     fn fmt(&self, _f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         todo!("Implement Debug for Box<dyn MBC> Trait Object");
     }
 }
 
-impl std::clone::Clone for Box<dyn MemoryBankController> {
-    fn clone(&self) -> Self {
-        self.clone_mbc()
-    }
-}
-
-impl Default for Box<dyn MemoryBankController> {
+impl Default for Box<dyn MBCIo> {
     fn default() -> Self {
-        Box::new(Mbc1::default())
+        Box::new(NoMBC)
     }
 }

src/cpu.rs

@@ -1,26 +1,21 @@
-use crate::bus::{Bus, BusIo};
-use crate::instruction::{Cycle, Instruction};
+use crate::bus::{Bus, BusIo, BOOT_SIZE};
+use crate::instruction::Instruction;
 use crate::interrupt::{InterruptEnable, InterruptFlag};
-use crate::joypad::Joypad;
-use crate::ppu::Ppu;
-use crate::sound::AudioSender;
-use crate::timer::Timer;
+use crate::Cycle;
 use bitfield::bitfield;
 use std::fmt::{Display, Formatter, Result as FmtResult};
 
-#[derive(Debug, Clone, Default)]
+#[derive(Debug, Default)]
 pub struct Cpu {
     pub bus: Bus,
     reg: Registers,
     flags: Flags,
     ime: ImeState,
-    // TODO: Merge halted and state properties
-    halted: Option<HaltState>,
     state: State,
 }
 
 impl Cpu {
-    pub fn new() -> Self {
+    pub(crate) fn without_boot() -> Self {
         Self {
             reg: Registers {
                 a: 0x01,
@@ -38,15 +33,11 @@ impl Cpu {
         }
     }
 
-    pub fn boot_new(path: &str) -> anyhow::Result<Self> {
-        Ok(Self {
-            bus: Bus::with_boot(path)?,
+    pub(crate) fn with_boot(rom: [u8; BOOT_SIZE]) -> Self {
+        Self {
+            bus: Bus::with_boot(rom),
             ..Default::default()
-        })
-    }
-
-    pub fn set_audio_src(&mut self, sender: AudioSender<f32>) {
-        self.bus.pass_audio_src(sender)
+        }
     }
 
     pub(crate) fn ime(&self) -> ImeState {
@@ -57,88 +48,100 @@ impl Cpu {
         self.ime = state;
     }
 
-    pub(crate) fn halt(&mut self, state: HaltState) {
-        self.halted = Some(state);
+    pub(crate) fn halt_cpu(&mut self, kind: HaltKind) {
+        self.state = State::Halt(kind);
     }
 
-    fn resume(&mut self) {
-        self.halted = None;
+    fn resume_execution(&mut self) {
+        self.state = State::Execute;
     }
 
-    pub(crate) fn halted(&self) -> Option<HaltState> {
-        self.halted
+    pub(crate) fn is_halted(&self) -> bool {
+        match self.state {
+            State::Halt(_) => true,
+            _ => false,
+        }
     }
 
-    fn inc_pc(&mut self) {
-        self.reg.pc += 1;
-    }
-
-    pub fn load_cartridge(&mut self, path: &str) -> std::io::Result<()> {
-        self.bus.load_cartridge(path)
-    }
-
-    pub fn rom_title(&self) -> Option<&str> {
-        self.bus.rom_title()
+    pub(crate) fn halt_kind(&self) -> Option<HaltKind> {
+        match self.state {
+            State::Halt(kind) => Some(kind),
+            _ => None,
+        }
     }
 }
 
 impl Cpu {
-    fn fetch(&self) -> u8 {
-        self.bus.read_byte(self.reg.pc)
+    /// Fetch an [Instruction] from the memory bus
+    /// (4 cycles)
+    fn fetch(&mut self) -> u8 {
+        let byte = self.read_byte(self.reg.pc);
+        self.bus.clock();
+        self.reg.pc += 1;
+        byte
     }
 
+    /// Decode a byte into an [SM83](Cpu) [Instruction]
+    ///
+    /// If opcode == 0xCB, then decoding costs 4 cycles.
+    /// Otherwise, decoding is free
     pub(crate) fn decode(&mut self, opcode: u8) -> Instruction {
-        Instruction::from_byte(self, opcode)
+        if opcode == 0xCB {
+            Instruction::decode(self.fetch(), true)
+        } else {
+            Instruction::decode(opcode, false)
+        }
     }
 
+    /// Execute an [Instruction].
+    ///
+    /// The amount of cycles necessary to execute an instruction range from
+    /// 0 to 20 T-cycles
     fn execute(&mut self, instruction: Instruction) -> Cycle {
         Instruction::execute(self, instruction)
     }
 
+    /// Perform the [`Cpu::fetch()`] [`Cpu::decode()`] [`Cpu::execute()`]
+    /// routine.
+    ///
+    /// Handle HALT and interrupts.
     pub fn step(&mut self) -> Cycle {
-        let cycles = match self.halted() {
-            Some(state) => {
-                use HaltState::*;
+        // Log instructions
+        // if self.reg.pc > 0xFF {
+        //     let out = std::io::stdout();
+        //     let _ = self._print_logs(out.lock());
+        // }
 
-                match state {
-                    ImeEnabled | NonePending => Cycle::new(4),
-                    SomePending => todo!("Implement HALT bug"),
-                }
-            }
-            None => {
-                let opcode = self.fetch();
-                self.inc_pc();
-
-                let instr = self.decode(opcode);
-
-                // let out = std::io::stdout();
-                // let _ = self._debug_log(out.lock(), &instr);
-
-                let cycles = self.execute(instr);
-
-                self.check_ime();
-
-                cycles
-            }
-        };
-
-        // TODO: With how we currently handle audio
-        // this --while being correct-- incurs a performance penalty
-        // as our emu is audio-bound.
-        let mut elapsed = 0x00;
-        let pending: u32 = cycles.into();
-        while elapsed < pending {
-            if !self.bus.is_full() {
-                self.bus.clock();
-                elapsed += 1;
-            } else {
-                self.bus.flush_samples();
-            }
+        if let Some(elapsed) = self.handle_interrupt() {
+            return elapsed;
         }
 
-        self.handle_interrupts();
+        if let Some(kind) = self.halt_kind() {
+            use HaltKind::*;
 
-        cycles
+            self.bus.clock();
+
+            let elapsed = match kind {
+                ImeEnabled | NonePending => 4,
+                SomePending => todo!("Implement HALT bug"),
+            };
+
+            return elapsed;
+        }
+
+        let opcode = self.fetch();
+        let instr = self.decode(opcode);
+        let elapsed = self.execute(instr);
+        self.handle_ei();
+
+        // For use in Blargg's Test ROMs
+        if self.read_byte(0xFF02) == 0x81 {
+            let c = self.read_byte(0xFF01) as char;
+            self.write_byte(0xFF02, 0x00);
+            eprint!("{}", c);
+        }
+
+        elapsed
     }
 }
 
@@ -153,124 +156,108 @@ impl BusIo for Cpu {
 }
 
 impl Cpu {
-    pub(crate) fn read_imm_byte(&mut self, addr: u16) -> u8 {
-        self.inc_pc(); // NB: the addr read in the line below will be equal to PC - 1 after this function call
-        self.bus.read_byte(addr)
+    pub(crate) fn bus(&self) -> &Bus {
+        &self.bus
     }
 
-    pub(crate) fn read_imm_word(&mut self, addr: u16) -> u16 {
-        self.inc_pc();
-        self.inc_pc(); // NB: the addr read in the line below will be equal to PC - 2 after this function call
-        self.bus.read_word(addr)
+    pub(crate) fn bus_mut(&mut self) -> &mut Bus {
+        &mut self.bus
     }
 
-    pub(crate) fn write_word(&mut self, addr: u16, word: u16) {
-        self.bus.write_word(addr, word)
-    }
-}
-
-impl Cpu {
-    pub fn ppu(&mut self) -> &Ppu {
-        &self.bus.ppu
-    }
-
-    pub(crate) fn joypad_mut(&mut self) -> &mut Joypad {
-        &mut self.bus.joypad
-    }
-
-    pub(crate) fn timer(&self) -> &Timer {
-        &self.bus.timer
-    }
-
-    fn check_ime(&mut self) {
+    fn handle_ei(&mut self) {
         match self.ime {
-            ImeState::Pending => {
-                // This is within the context of the EI instruction, we need to not update EI until the end of the
-                // next executed Instruction
-                self.ime = ImeState::PendingEnd;
-            }
-            ImeState::PendingEnd => {
-                // The Instruction after EI has now been executed, so we want to enable the IME flag here
-                self.ime = ImeState::Enabled;
-            }
-            ImeState::Disabled | ImeState::Enabled => {} // Do Nothing
+            ImeState::EiExecuted => self.ime = ImeState::Pending,
+            ImeState::Pending => self.ime = ImeState::Enabled,
+            ImeState::Disabled | ImeState::Enabled => {}
         }
     }
 
-    fn handle_interrupts(&mut self) {
-        let req = self.read_byte(0xFF0F);
-        let enabled = self.read_byte(0xFFFF);
+    pub(crate) fn int_request(&self) -> u8 {
+        self.read_byte(0xFF0F)
+    }
 
-        if self.halted.is_some() {
+    pub(crate) fn int_enable(&self) -> u8 {
+        self.read_byte(0xFFFF)
+    }
+
+    fn handle_interrupt(&mut self) -> Option<Cycle> {
+        let irq = self.int_request();
+        let enable = self.int_enable();
+
+        // TODO: Ensure that this behaviour is correct
+        if self.is_halted() {
             // When we're here either a HALT with IME set or
             // a HALT with IME not set and No pending Interrupts was called
 
-            if req & enabled != 0 {
+            if irq & enable != 0 {
                 // The if self.ime() below correctly follows the "resuming from HALT" behaviour so
                 // nothing actually needs to be added here. This is just documentation
                 // since it's a bit weird why nothing is being done
 
-                self.resume()
+                self.resume_execution();
             }
         }
 
-        if let ImeState::Enabled = self.ime() {
-            let mut req: InterruptFlag = req.into();
-            let enabled: InterruptEnable = enabled.into();
+        match self.ime() {
+            ImeState::Enabled => {
+                let mut irq: InterruptFlag = irq.into();
+                let enable: InterruptEnable = enable.into();
 
-            let vector = if req.vblank() && enabled.vblank() {
-                // Handle VBlank Interrupt
-                req.set_vblank(false);
+                let rst_vector = if irq.vblank() && enable.vblank() {
+                    // Handle VBlank Interrupt
+                    irq.set_vblank(false);
 
-                // INT 40h
-                Some(0x40)
-            } else if req.lcd_stat() && enabled.lcd_stat() {
-                // Handle LCD STAT Interrupt
-                req.set_lcd_stat(false);
+                    // INT 40h
+                    Some(0x40)
+                } else if irq.lcd_stat() && enable.lcd_stat() {
+                    // Handle LCD STAT Interrupt
+                    irq.set_lcd_stat(false);
 
-                // INT 48h
-                Some(0x48)
-            } else if req.timer() && enabled.timer() {
-                // Handle Timer Interrupt
-                req.set_timer(false);
+                    // INT 48h
+                    Some(0x48)
+                } else if irq.timer() && enable.timer() {
+                    // Handle Timer Interrupt
+                    irq.set_timer(false);
 
-                // INT 50h
-                Some(0x50)
-            } else if req.serial() && enabled.serial() {
-                // Handle Serial Interrupt
-                req.set_serial(false);
+                    // INT 50h
+                    Some(0x50)
+                } else if irq.serial() && enable.serial() {
+                    // Handle Serial Interrupt
+                    irq.set_serial(false);
 
-                // INT 58h
-                Some(0x58)
-            } else if req.joypad() && enabled.joypad() {
-                // Handle Joypad Interrupt
-                req.set_joypad(false);
+                    // INT 58h
+                    Some(0x58)
+                } else if irq.joypad() && enable.joypad() {
+                    // Handle Joypad Interrupt
+                    irq.set_joypad(false);
 
-                // INT 60h
-                Some(0x60)
-            } else {
-                None
-            };
+                    // INT 60h
+                    Some(0x60)
+                } else {
+                    None
+                };
 
-            let _ = match vector {
-                Some(address) => {
-                    //  Write the Changes to 0xFF0F and 0xFFFF registers
-                    self.write_byte(0xFF0F, req.into());
+                match rst_vector {
+                    Some(vector) => {
+                        //  Write the Changes to 0xFF0F and 0xFFFF registers
+                        self.write_byte(0xFF0F, irq.into());
 
-                    // Disable all future interrupts
-                    self.set_ime(ImeState::Disabled);
-                    Instruction::reset(self, address)
+                        // Disable all future interrupts
+                        self.set_ime(ImeState::Disabled);
+                        Some(Instruction::reset(self, vector))
+                    }
+                    None => None,
                 }
-                None => Cycle::new(0), // NO Interrupts were enabled and / or requested
-            };
+            }
+            _ => None,
         }
     }
 }
 
-#[derive(Debug, Copy, Clone)]
+#[derive(Debug, Clone, Copy)]
 enum State {
     Execute,
-    // Halt,
+    Halt(HaltKind),
     // Stop,
 }
 
@@ -292,7 +279,6 @@ impl Cpu {
             E => self.reg.e = value,
             H => self.reg.h = value,
             L => self.reg.l = value,
-            Flag => self.flags = value.into(),
         }
     }
 
@@ -307,7 +293,6 @@ impl Cpu {
             E => self.reg.e,
             H => self.reg.h,
             L => self.reg.l,
-            Flag => self.flags.into(),
         }
     }
 
@@ -356,13 +341,20 @@ impl Cpu {
         &self.flags
     }
 
+    pub(crate) fn update_flags(&mut self, z: bool, n: bool, h: bool, c: bool) {
+        self.flags.set_z(z);
+        self.flags.set_n(n);
+        self.flags.set_h(h);
+        self.flags.set_c(c);
+    }
+
     pub(crate) fn set_flags(&mut self, flags: Flags) {
         self.flags = flags;
     }
 }
 
 impl Cpu {
-    fn _debug_log(&self, mut w: impl std::io::Write, instr: &Instruction) -> std::io::Result<()> {
+    fn _print_debug(&self, mut w: impl std::io::Write) -> std::io::Result<()> {
         write!(w, "A: {:02X} ", self.reg.a)?;
         write!(w, "F: {:02X} ", u8::from(self.flags))?;
         write!(w, "B: {:02X} ", self.reg.b)?;
@@ -376,31 +368,40 @@ impl Cpu {
         write!(w, "({:02X} ", self.read_byte(self.reg.pc))?;
         write!(w, "{:02X} ", self.read_byte(self.reg.pc + 1))?;
         write!(w, "{:02X} ", self.read_byte(self.reg.pc + 2))?;
-        write!(w, "{:02X}) ", self.read_byte(self.reg.pc + 3))?;
-        writeln!(w, "| {:?}", instr)?;
+        write!(w, "{:02X})", self.read_byte(self.reg.pc + 3))?;
+        writeln!(w, "| {:?}", self._dbg_instr())?;
         w.flush()
     }
 
-    fn _log_state(&self, mut writer: impl std::io::Write) -> std::io::Result<()> {
-        write!(writer, "A: {:02X} ", self.reg.a)?;
-        write!(writer, "F: {:02X} ", u8::from(self.flags))?;
-        write!(writer, "B: {:02X} ", self.reg.b)?;
-        write!(writer, "C: {:02X} ", self.reg.c)?;
-        write!(writer, "D: {:02X} ", self.reg.d)?;
-        write!(writer, "E: {:02X} ", self.reg.e)?;
-        write!(writer, "H: {:02X} ", self.reg.h)?;
-        write!(writer, "L: {:02X} ", self.reg.l)?;
-        write!(writer, "SP: {:04X} ", self.reg.sp)?;
-        write!(writer, "PC: 00:{:04X} ", self.reg.pc)?;
-        write!(writer, "({:02X} ", self.read_byte(self.reg.pc))?;
-        write!(writer, "{:02X} ", self.read_byte(self.reg.pc + 1))?;
-        write!(writer, "{:02X} ", self.read_byte(self.reg.pc + 2))?;
-        writeln!(writer, "{:02X})", self.read_byte(self.reg.pc + 3))?;
-        writer.flush()
+    fn _print_logs(&self, mut w: impl std::io::Write) -> std::io::Result<()> {
+        write!(w, "A: {:02X} ", self.reg.a)?;
+        write!(w, "F: {:02X} ", u8::from(self.flags))?;
+        write!(w, "B: {:02X} ", self.reg.b)?;
+        write!(w, "C: {:02X} ", self.reg.c)?;
+        write!(w, "D: {:02X} ", self.reg.d)?;
+        write!(w, "E: {:02X} ", self.reg.e)?;
+        write!(w, "H: {:02X} ", self.reg.h)?;
+        write!(w, "L: {:02X} ", self.reg.l)?;
+        write!(w, "SP: {:04X} ", self.reg.sp)?;
+        write!(w, "PC: 00:{:04X} ", self.reg.pc)?;
+        write!(w, "({:02X} ", self.read_byte(self.reg.pc))?;
+        write!(w, "{:02X} ", self.read_byte(self.reg.pc + 1))?;
+        write!(w, "{:02X} ", self.read_byte(self.reg.pc + 2))?;
+        writeln!(w, "{:02X})", self.read_byte(self.reg.pc + 3))?;
+        w.flush()
+    }
+
+    fn _dbg_instr(&self) -> Instruction {
+        let byte = self.read_byte(self.reg.pc);
+        if byte == 0xCB {
+            Instruction::decode(self.read_byte(self.reg.pc + 1), true)
+        } else {
+            Instruction::decode(byte, false)
+        }
     }
 }
 
-#[derive(Debug, Copy, Clone)]
+#[derive(Debug, Clone, Copy)]
 pub(crate) enum Register {
     A,
     B,
@@ -409,10 +410,9 @@ pub(crate) enum Register {
     E,
     H,
     L,
-    Flag,
 }
 
-#[derive(Debug, Copy, Clone)]
+#[derive(Debug, Clone, Copy)]
 pub(crate) enum RegisterPair {
     AF,
     BC,
@@ -422,7 +422,7 @@ pub(crate) enum RegisterPair {
     PC,
 }
 
-#[derive(Debug, Copy, Clone, Default)]
+#[derive(Debug, Default)]
 struct Registers {
     a: u8,
     b: u8,
@@ -507,7 +507,7 @@ impl From<u8> for Flags {
 }
 
 #[derive(Debug, Clone, Copy)]
-pub(crate) enum HaltState {
+pub(crate) enum HaltKind {
     ImeEnabled,
     NonePending,
     SomePending,
@@ -516,8 +516,8 @@ pub(crate) enum HaltState {
 #[derive(Debug, Clone, Copy)]
 pub(crate) enum ImeState {
     Disabled,
+    EiExecuted,
     Pending,
-    PendingEnd,
     Enabled,
 }
 

src/emu.rs

@@ -1,60 +1,121 @@
-use crate::cpu::Cpu as SM83;
-use crate::instruction::Cycle;
-use crate::joypad;
-use crate::ppu::Ppu;
-use anyhow::Result;
+use crate::apu::gen::SampleProducer;
+use crate::cpu::Cpu;
+use crate::joypad::{self, Joypad};
+use crate::{Cycle, GB_HEIGHT, GB_WIDTH};
 use gilrs::Gilrs;
 use std::time::Duration;
 use winit_input_helper::WinitInputHelper;
 
 pub const SM83_CYCLE_TIME: Duration = Duration::from_nanos(1_000_000_000 / SM83_CLOCK_SPEED);
-pub const CYCLES_IN_FRAME: Cycle = Cycle::new(456 * 154); // 456 Cycles times 154 scanlines
+pub const CYCLES_IN_FRAME: Cycle = 456 * 154; // 456 Cycles times 154 scanlines
 pub(crate) const SM83_CLOCK_SPEED: u64 = 0x40_0000; // Hz which is 4.194304Mhz
 const DEFAULT_TITLE: &str = "DMG-01 Emulator";
-const GAMEPAD_ENABLED: bool = false;
 
-pub fn init(boot_path: Option<&str>, rom_path: &str) -> Result<SM83> {
-    let mut cpu = match boot_path {
-        Some(path) => SM83::boot_new(path)?,
-        None => SM83::new(),
-    };
+pub fn run_frame(emu: &mut Emulator, gamepad: &mut Gilrs, key: &WinitInputHelper) -> Cycle {
+    let mut elapsed = 0;
 
-    eprintln!("Initialized GB Emulator");
+    if let Some(event) = gamepad.next_event() {
+        joypad::handle_gamepad_input(emu.joyp_mut(), event);
+    }
 
-    cpu.load_cartridge(rom_path)?;
-    Ok(cpu)
-}
-
-pub fn rom_title(game_boy: &SM83) -> &str {
-    game_boy.rom_title().unwrap_or(DEFAULT_TITLE)
-}
-
-pub fn run(
-    game_boy: &mut SM83,
-    gamepad: &mut Gilrs,
-    input: &WinitInputHelper,
-    pending: Cycle,
-) -> Cycle {
-    let mut elapsed = Cycle::new(0);
-
-    while elapsed < pending {
-        elapsed += run_unsynced(game_boy, gamepad, input);
+    joypad::handle_keyboard_input(emu.joyp_mut(), key);
+    while elapsed < CYCLES_IN_FRAME {
+        elapsed += emu.step();
     }
 
     elapsed
 }
 
-pub fn run_unsynced(game_boy: &mut SM83, gamepad: &mut Gilrs, input: &WinitInputHelper) -> Cycle {
-    if GAMEPAD_ENABLED {
-        if let Some(event) = gamepad.next_event() {
-            joypad::handle_gamepad_input(game_boy.joypad_mut(), event);
+pub fn draw_frame(emu: &Emulator, buf: &mut [u8; GB_HEIGHT * GB_WIDTH * 4]) {
+    buf.copy_from_slice(emu.cpu.bus().ppu.frame_buf());
+}
+
+pub struct Emulator {
+    cpu: Cpu,
+    timestamp: Cycle,
+}
+
+impl Emulator {
+    fn new(cpu: Cpu) -> Self {
+        Self {
+            cpu,
+            timestamp: Default::default(),
         }
     }
 
-    joypad::handle_keyboard_input(game_boy.joypad_mut(), input);
-    game_boy.step()
+    fn step(&mut self) -> Cycle {
+        self.cpu.step()
+    }
+
+    fn load_cart(&mut self, rom: Vec<u8>) {
+        self.cpu.bus_mut().load_cart(rom)
+    }
+
+    fn joyp_mut(&mut self) -> &mut Joypad {
+        &mut self.cpu.bus_mut().joypad
+    }
+
+    pub fn set_prod(&mut self, prod: SampleProducer<f32>) {
+        self.cpu.bus_mut().apu.attach_producer(prod)
+    }
+
+    pub fn title(&self) -> &str {
+        self.cpu.bus().cart_title().unwrap_or(DEFAULT_TITLE)
+    }
 }
 
-pub fn draw(ppu: &Ppu, frame: &mut [u8]) {
-    ppu.copy_to_gui(frame);
+pub mod build {
+    use std::fs::File;
+    use std::io::{Read, Result};
+    use std::path::Path;
+
+    use crate::bus::BOOT_SIZE;
+    use crate::cpu::Cpu;
+
+    use super::Emulator;
+
+    #[derive(Debug, Default)]
+    pub struct EmulatorBuilder {
+        boot: Option<[u8; BOOT_SIZE]>,
+        cart: Option<Vec<u8>>,
+    }
+
+    impl EmulatorBuilder {
+        pub fn new() -> Self {
+            Default::default()
+        }
+
+        pub fn with_boot<P: AsRef<Path>>(mut self, path: P) -> Result<Self> {
+            let mut file = File::open(path.as_ref())?;
+
+            let mut buf = [0x00; BOOT_SIZE];
+            file.read_exact(&mut buf)?;
+
+            self.boot = Some(buf);
+            Ok(self)
+        }
+
+        pub fn with_cart<P: AsRef<Path>>(mut self, path: P) -> Result<Self> {
+            let mut file = File::open(path.as_ref())?;
+
+            let mut buf = Vec::new();
+            file.read_to_end(&mut buf)?;
+
+            self.cart = Some(buf);
+            Ok(self)
+        }
+
+        pub fn finish(mut self) -> Emulator {
+            let mut emu = Emulator::new(match self.boot {
+                Some(rom) => Cpu::with_boot(rom),
+                None => Cpu::without_boot(),
+            });
+
+            if let Some(rom) = self.cart.take() {
+                emu.load_cart(rom)
+            }
+
+            emu
+        }
+    }
 }

src/gui.rs

@@ -1,321 +0,0 @@
-use crate::bus::BusIo;
-use crate::cpu::{Cpu as SM83, Register, RegisterPair};
-use egui::{ClippedMesh, FontDefinitions};
-use egui_wgpu_backend::{RenderPass, ScreenDescriptor};
-use egui_winit_platform::{Platform, PlatformDescriptor};
-use pixels::{wgpu, PixelsContext};
-use std::time::Instant;
-use wgpu::TextureFormat::Bgra8UnormSrgb;
-
-// Boilerplate code from: https://github.com/parasyte/pixels/blob/0.3.0/examples/egui-winit/src/gui.rs
-
-/// Manages all state required for rendering egui over `Pixels`.
-pub struct Egui {
-    // State for egui.
-    start_time: Instant,
-    platform: Platform,
-    screen_desc: ScreenDescriptor,
-    render_pass: RenderPass,
-    paint_jobs: Vec<ClippedMesh>,
-
-    pub config: Configuration,
-
-    show_flags: bool,
-    show_cpu_info: bool,
-    show_registers: bool,
-    show_int: bool,
-    show_timer: bool,
-}
-
-impl Egui {
-    /// Create egui.
-    pub fn new(width: u32, height: u32, scale_factor: f64, context: &PixelsContext) -> Self {
-        let platform = Platform::new(PlatformDescriptor {
-            physical_width: width,
-            physical_height: height,
-            scale_factor,
-            font_definitions: FontDefinitions::default(),
-            style: Default::default(),
-        });
-
-        let screen_desc = ScreenDescriptor {
-            physical_width: width,
-            physical_height: height,
-            scale_factor: scale_factor as f32,
-        };
-
-        let render_pass = RenderPass::new(&context.device, Bgra8UnormSrgb);
-
-        Self {
-            start_time: Instant::now(),
-            platform,
-            screen_desc,
-            render_pass,
-            paint_jobs: Vec::new(),
-            config: Default::default(),
-            show_flags: false,
-            show_cpu_info: false,
-            show_registers: false,
-            show_int: false,
-            show_timer: false,
-        }
-    }
-
-    /// Handle input events from the window manager.
-    pub fn handle_event(&mut self, event: &winit::event::Event<'_, ()>) {
-        self.platform.handle_event(event);
-    }
-
-    /// Resize egui.
-    pub fn resize(&mut self, width: u32, height: u32) {
-        self.screen_desc.physical_width = width;
-        self.screen_desc.physical_height = height;
-    }
-
-    /// Update scaling factor.
-    pub fn scale_factor(&mut self, scale_factor: f64) {
-        self.screen_desc.scale_factor = scale_factor as f32;
-    }
-
-    /// Prepare egui.
-    pub fn prepare(&mut self, game_boy: &SM83) {
-        self.platform
-            .update_time(self.start_time.elapsed().as_secs_f64());
-
-        // Begin the egui frame.
-        self.platform.begin_frame();
-
-        // Draw the demo application.
-        self.ui(&self.platform.context(), game_boy);
-
-        // End the egui frame and create all paint jobs to prepare for rendering.
-        let (_output, paint_commands) = self.platform.end_frame();
-        self.paint_jobs = self.platform.context().tessellate(paint_commands);
-    }
-
-    /// Create the UI using egui.
-    fn ui(&mut self, ctx: &egui::CtxRef, game_boy: &SM83) {
-        egui::TopPanel::top("menubar_container").show(ctx, |ui| {
-            egui::menu::bar(ui, |ui| {
-                egui::menu::menu(ui, "File", |ui| {
-                    if ui.button("Configuration").clicked() {
-                        self.config.show = true;
-                    }
-                });
-
-                egui::menu::menu(ui, "Status", |ui| {
-                    if ui.button("Flags").clicked() {
-                        self.show_flags = true;
-                    }
-
-                    if ui.button("CPU Information").clicked() {
-                        self.show_cpu_info = true;
-                    }
-
-                    if ui.button("Registers").clicked() {
-                        self.show_registers = true;
-                    }
-
-                    if ui.button("Interrupts").clicked() {
-                        self.show_int = true;
-                    }
-
-                    if ui.button("Timer").clicked() {
-                        self.show_timer = true;
-                    }
-                });
-            });
-        });
-
-        egui::Window::new("Cpu Flags")
-            .open(&mut self.show_flags)
-            .show(ctx, |ui| {
-                let flags = game_boy.flags();
-
-                ui.horizontal(|ui| {
-                    let _ = ui.selectable_label(flags.z(), "Zero");
-                    let _ = ui.selectable_label(flags.n(), "Negative");
-                    let _ = ui.selectable_label(flags.h(), "Half-Carry");
-                    let _ = ui.selectable_label(flags.c(), "Carry");
-                });
-            });
-
-        egui::Window::new("Timer")
-            .open(&mut self.show_timer)
-            .show(ctx, |ui| {
-                let timer = game_boy.timer();
-
-                ui.horizontal(|ui| {
-                    ui.label("DIV");
-                    ui.monospace(format!("{:#06X}", timer.divider));
-                });
-
-                ui.horizontal(|ui| {
-                    ui.label("TIMA");
-                    ui.monospace(format!("{:#04X}", timer.counter));
-                });
-
-                ui.horizontal(|ui| {
-                    ui.label("TMA");
-                    ui.monospace(format!("{:#04X}", timer.modulo));
-                });
-
-                ui.horizontal(|ui| {
-                    ui.label("TAC");
-                    ui.monospace(format!("{:?}", timer.ctrl));
-                });
-            });
-
-        egui::Window::new("Registers")
-            .open(&mut self.show_registers)
-            .show(ctx, |ui| {
-                ui.horizontal(|ui| {
-                    ui.label("A");
-                    ui.monospace(format!("{:#04X}", game_boy.register(Register::A)));
-
-                    ui.label("F");
-                    ui.monospace(format!("{:#04X}", game_boy.register(Register::Flag)));
-                });
-
-                ui.horizontal(|ui| {
-                    ui.label("B");
-                    ui.monospace(format!("{:#04X}", game_boy.register(Register::B)));
-
-                    ui.label("C");
-                    ui.monospace(format!("{:#04X}", game_boy.register(Register::C)));
-                });
-
-                ui.horizontal(|ui| {
-                    ui.label("D");
-                    ui.monospace(format!("{:#04X}", game_boy.register(Register::D)));
-
-                    ui.label("E");
-                    ui.monospace(format!("{:#04X}", game_boy.register(Register::E)));
-                });
-
-                ui.horizontal(|ui| {
-                    ui.label("H");
-                    ui.monospace(format!("{:#04X}", game_boy.register(Register::H)));
-
-                    ui.label("L");
-                    ui.monospace(format!("{:#04X}", game_boy.register(Register::L)));
-                });
-
-                ui.horizontal(|ui| {
-                    ui.label("AF");
-                    ui.monospace(format!("{:#06X}", game_boy.register_pair(RegisterPair::AF)));
-                });
-
-                ui.horizontal(|ui| {
-                    ui.label("BC");
-                    ui.monospace(format!("{:#06X}", game_boy.register_pair(RegisterPair::BC)));
-                });
-
-                ui.horizontal(|ui| {
-                    ui.label("DE");
-                    ui.monospace(format!("{:#06X}", game_boy.register_pair(RegisterPair::DE)));
-                });
-
-                ui.horizontal(|ui| {
-                    ui.label("HL");
-                    ui.monospace(format!("{:#06X}", game_boy.register_pair(RegisterPair::HL)));
-                });
-            });
-
-        egui::Window::new("Cpu Information")
-            .open(&mut self.show_cpu_info)
-            .show(ctx, |ui| {
-                ui.horizontal(|ui| {
-                    ui.label("PC");
-                    ui.monospace(format!("{:#06X}", game_boy.register_pair(RegisterPair::PC)));
-                });
-
-                ui.horizontal(|ui| {
-                    ui.label("SP");
-                    ui.monospace(format!("{:#06X}", game_boy.register_pair(RegisterPair::SP)));
-                });
-
-                ui.horizontal(|ui| {
-                    ui.label("IME");
-                    ui.label(format!("{:?}", game_boy.ime()));
-                });
-
-                ui.horizontal(|ui| {
-                    ui.label("HALT");
-                    ui.label(format!("{:?}", game_boy.halted()));
-                });
-            });
-
-        egui::Window::new("IRQ Information")
-            .open(&mut self.show_int)
-            .show(ctx, |ui| {
-                let req = game_boy.read_byte(0xFF0F);
-                let enabled = game_boy.read_byte(0xFFFF);
-
-                ui.heading("Interrupt Requests");
-                ui.horizontal(|ui| {
-                    let _ = ui.selectable_label(req & 0x01 == 0x01, "VBLANK");
-                    let _ = ui.selectable_label((req >> 1) & 0x01 == 0x01, "LCD STAT");
-                    let _ = ui.selectable_label((req >> 2) & 0x01 == 0x01, "TIMER");
-                    let _ = ui.selectable_label((req >> 3) & 0x01 == 0x01, "SERIAL");
-                    let _ = ui.selectable_label((req >> 4) & 0x01 == 0x01, "JOYPAD");
-                });
-
-                ui.heading("Interrupt Enable");
-                ui.horizontal(|ui| {
-                    let _ = ui.selectable_label(enabled & 0x01 == 0x01, "VBLANK");
-                    let _ = ui.selectable_label((enabled >> 1) & 0x01 == 0x01, "LCD STAT");
-                    let _ = ui.selectable_label((enabled >> 2) & 0x01 == 0x01, "TIMER");
-                    let _ = ui.selectable_label((enabled >> 3) & 0x01 == 0x01, "SERIAL");
-                    let _ = ui.selectable_label((enabled >> 4) & 0x01 == 0x01, "JOYPAD");
-                });
-            });
-
-        egui::Window::new("Configuration")
-            .open(&mut self.config.show)
-            .show(ctx, |_ui| {});
-    }
-
-    /// Render egui.
-    pub fn render(
-        &mut self,
-        encoder: &mut wgpu::CommandEncoder,
-        render_target: &wgpu::TextureView,
-        context: &PixelsContext,
-    ) {
-        // Upload all resources to the GPU.
-        self.render_pass.update_texture(
-            &context.device,
-            &context.queue,
-            &self.platform.context().texture(),
-        );
-        self.render_pass
-            .update_user_textures(&context.device, &context.queue);
-        self.render_pass.update_buffers(
-            &context.device,
-            &context.queue,
-            &self.paint_jobs,
-            &self.screen_desc,
-        );
-
-        // Record all render passes.
-        self.render_pass.execute(
-            encoder,
-            render_target,
-            &self.paint_jobs,
-            &self.screen_desc,
-            None,
-        );
-    }
-}
-
-pub struct Configuration {
-    /// Show Configuration egui menu
-    show: bool,
-}
-
-impl Default for Configuration {
-    fn default() -> Self {
-        Self { show: false }
-    }
-}

src/interrupt.rs

@@ -1,6 +1,6 @@
 use bitfield::bitfield;
 
-#[derive(Debug, Clone, Copy, Default)]
+#[derive(Debug, Default)]
 pub(crate) struct Interrupt {
     pub(crate) flag: InterruptFlag,
     pub(crate) enable: InterruptEnable,

src/joypad.rs

@@ -1,7 +1,7 @@
 use gilrs::{Button, Event as GamepadEvent, EventType as GamepadEventType};
 use winit_input_helper::WinitInputHelper;
 
-#[derive(Debug, Clone, Copy)]
+#[derive(Debug)]
 pub struct Joypad {
     /// 0xFF00 | P1/JOYP - Player 1 Joypad
     pub(crate) p1: u8,
@@ -48,7 +48,7 @@ impl Joypad {
     }
 }
 
-#[derive(Debug, Clone, Copy, Default)]
+#[derive(Debug, Default)]
 struct JoypadState {
     // Direction Row
     dpad_down: ButtonEvent,
@@ -120,49 +120,57 @@ pub fn handle_keyboard_input(pad: &mut Joypad, input: &WinitInputHelper) {
 
     if input.key_pressed(VirtualKeyCode::Down) {
         state.dpad_down.update(true, irq);
-    } else if input.key_released(VirtualKeyCode::Down) {
+    }
+    if input.key_released(VirtualKeyCode::Down) {
         state.dpad_down.update(false, irq);
     }
 
     if input.key_pressed(VirtualKeyCode::Up) {
         state.dpad_up.update(true, irq);
-    } else if input.key_released(VirtualKeyCode::Up) {
+    }
+    if input.key_released(VirtualKeyCode::Up) {
         state.dpad_up.update(false, irq);
     }
 
     if input.key_pressed(VirtualKeyCode::Left) {
         state.dpad_left.update(true, irq);
-    } else if input.key_released(VirtualKeyCode::Left) {
+    }
+    if input.key_released(VirtualKeyCode::Left) {
         state.dpad_left.update(false, irq);
     }
 
     if input.key_pressed(VirtualKeyCode::Right) {
         state.dpad_right.update(true, irq);
-    } else if input.key_released(VirtualKeyCode::Right) {
+    }
+    if input.key_released(VirtualKeyCode::Right) {
         state.dpad_right.update(false, irq);
     }
 
     if input.key_pressed(VirtualKeyCode::T) {
         state.start.update(true, irq);
-    } else if input.key_released(VirtualKeyCode::T) {
+    }
+    if input.key_released(VirtualKeyCode::T) {
         state.start.update(false, irq);
     }
 
     if input.key_pressed(VirtualKeyCode::Y) {
         state.select.update(true, irq);
-    } else if input.key_released(VirtualKeyCode::Y) {
+    }
+    if input.key_released(VirtualKeyCode::Y) {
         state.select.update(false, irq);
     }
 
     if input.key_pressed(VirtualKeyCode::Z) {
         state.south.update(true, irq);
-    } else if input.key_released(VirtualKeyCode::Z) {
+    }
+    if input.key_released(VirtualKeyCode::Z) {
         state.south.update(false, irq);
     }
 
     if input.key_pressed(VirtualKeyCode::X) {
         state.east.update(true, irq);
-    } else if input.key_released(VirtualKeyCode::X) {
+    }
+    if input.key_released(VirtualKeyCode::X) {
         state.east.update(false, irq);
     }
 }

src/lib.rs

@@ -1,21 +1,19 @@
-pub use gui::Egui;
-pub use instruction::Cycle;
-pub use sound::AudioMPSC;
+pub use apu::gen::AudioSPSC;
+pub type Cycle = u64;
 
 pub const GB_WIDTH: usize = 160;
 pub const GB_HEIGHT: usize = 144;
 
+mod apu;
 mod bus;
 mod cartridge;
 mod cpu;
 pub mod emu;
-mod gui;
 mod high_ram;
 mod instruction;
 mod interrupt;
 mod joypad;
 mod ppu;
 mod serial;
-mod sound;
 mod timer;
 mod work_ram;

src/main.rs

@@ -1,17 +1,20 @@
+use std::convert::TryInto;
+
 use anyhow::{anyhow, Result};
 use clap::{crate_authors, crate_description, crate_name, crate_version, App, Arg};
-use gb::{AudioMPSC, Cycle, Egui, GB_HEIGHT, GB_WIDTH};
+use gb::emu::build::EmulatorBuilder;
+use gb::{AudioSPSC, Cycle, GB_HEIGHT, GB_WIDTH};
 use gilrs::Gilrs;
-use pixels::{Pixels, SurfaceTexture};
+use pixels::{PixelsBuilder, SurfaceTexture};
 use rodio::{OutputStream, Sink};
-use std::time::Instant;
-use winit::dpi::LogicalSize;
+use winit::dpi::{LogicalSize, PhysicalSize};
 use winit::event::{Event, VirtualKeyCode};
 use winit::event_loop::{ControlFlow, EventLoop};
 use winit::window::{Window, WindowBuilder};
 use winit_input_helper::WinitInputHelper;
 
-const SCALE: f64 = 2.0;
+const WINDOW_SCALE: usize = 3;
+const AUDIO_ENABLED: bool = false;
 
 fn main() -> Result<()> {
     let app = App::new(crate_name!())
@@ -38,68 +41,58 @@ fn main() -> Result<()> {
         )
         .get_matches();
 
-    // `rom` is a required value in every situation so this will
-    // always exist.
-    let rom_path = m
-        .value_of("rom")
-        .expect("Required value 'rom' was provided");
+    let mut emu_build =
+        EmulatorBuilder::new().with_cart(m.value_of("rom").expect("ROM path provided"))?;
 
-    let mut game_boy =
-        gb::emu::init(m.value_of("boot"), rom_path).expect("Initialized DMG-01 Emulator");
-    let cartridge_title = gb::emu::rom_title(&game_boy);
+    if let Some(path) = m.value_of("boot") {
+        emu_build = emu_build.with_boot(path)?;
+    }
 
-    // Initialize Gamepad Support
-    let mut gamepad = Gilrs::new().expect("Initialized Gilrs for Controller Input");
+    let mut emu = emu_build.finish();
+    let rom_title = emu.title();
+
+    let mut gamepad = Gilrs::new().expect("Initialize Controller Support");
 
     // Initialize GUI
     let event_loop = EventLoop::new();
     let mut input = WinitInputHelper::new();
-    let window = create_window(&event_loop, cartridge_title)?;
+    let window = create_window(&event_loop, rom_title)?;
 
-    let (mut pixels, mut egui) = {
+    let mut pixels = {
         let size = window.inner_size();
-        let scale_factor = window.scale_factor();
         let surface_texture = SurfaceTexture::new(size.width, size.height, &window);
-        let pixels = Pixels::new(GB_WIDTH as u32, GB_HEIGHT as u32, surface_texture)?;
-        let egui = Egui::new(size.width, size.height, scale_factor, pixels.context());
 
-        (pixels, egui)
+        PixelsBuilder::new(GB_WIDTH as u32, GB_HEIGHT as u32, surface_texture)
+            .enable_vsync(false)
+            .build()?
     };
 
-    let (send, recv) = AudioMPSC::new();
-    game_boy.set_audio_src(send);
-
     // Initialize Audio
     let (_stream, stream_handle) = OutputStream::try_default().expect("Initialized Audio");
-    let sink = Sink::try_new(&stream_handle).expect("Initialize Audio Sink");
 
-    std::thread::spawn(move || {
-        sink.append(recv);
+    if AUDIO_ENABLED {
+        let spsc: AudioSPSC<f32> = Default::default();
+        let (prod, cons) = spsc.init();
+        let sink = {
+            let s = Sink::try_new(&stream_handle)?;
+            s.append(cons);
+            s.set_volume(0.1);
+            s
+        };
 
-        sink.sleep_until_end();
-    });
+        emu.set_prod(prod);
+
+        std::thread::spawn(move || {
+            sink.sleep_until_end();
+        });
+    }
 
-    let mut now = Instant::now();
     let mut cycle_count: Cycle = Default::default();
 
     event_loop.run(move |event, _, control_flow| {
-        // Update egui
-        egui.handle_event(&event);
-
         if let Event::RedrawRequested(_) = event {
-            // Prepare egui
-            egui.prepare(&game_boy);
-
-            // Render everything together
-            let render_result = pixels.render_with(|encoder, target, ctx| {
-                // Render the texture
-                ctx.scaling_renderer.render(encoder, target);
-
-                // Render egui
-                egui.render(encoder, target, ctx);
-            });
-
-            if render_result
+            if pixels
+                .render()
                 .map_err(|e| anyhow!("pixels.render() failed: {}", e))
                 .is_err()
             {
@@ -114,26 +107,21 @@ fn main() -> Result<()> {
                 return;
             }
 
-            if let Some(scale_factor) = input.scale_factor() {
-                egui.scale_factor(scale_factor);
-            }
-
             if let Some(size) = input.window_resized() {
                 pixels.resize_surface(size.width, size.height);
-                egui.resize(size.width, size.height);
             }
 
-            let delta = now.elapsed().subsec_nanos();
-            now = Instant::now();
-
-            let pending = Cycle::new(delta / gb::emu::SM83_CYCLE_TIME.subsec_nanos());
-            cycle_count += gb::emu::run(&mut game_boy, &mut gamepad, &input, pending);
+            cycle_count += gb::emu::run_frame(&mut emu, &mut gamepad, &input);
 
             if cycle_count >= gb::emu::CYCLES_IN_FRAME {
-                // Draw Frame
-                cycle_count = Cycle::new(0);
+                cycle_count %= gb::emu::CYCLES_IN_FRAME;
 
-                gb::emu::draw(game_boy.ppu(), pixels.get_frame());
+                let buf: &mut [u8; GB_WIDTH * GB_HEIGHT * 4] = pixels
+                    .get_frame()
+                    .try_into()
+                    .expect("Size of Pixel Buffer is GB_WIDTH * GB_HEIGHT * 4");
+
+                gb::emu::draw_frame(&emu, buf);
                 window.request_redraw();
             }
         }
@@ -141,14 +129,16 @@ fn main() -> Result<()> {
 }
 
 fn create_window(event_loop: &EventLoop<()>, title: &str) -> Result<Window> {
-    let size = LogicalSize::new((GB_WIDTH as f64) * SCALE, (GB_HEIGHT as f64) * SCALE);
+    let logical = LogicalSize::new(GB_WIDTH as f64, GB_HEIGHT as f64);
+    let physical = PhysicalSize::new(
+        (GB_WIDTH * WINDOW_SCALE) as f32,
+        (GB_HEIGHT * WINDOW_SCALE) as f32,
+    );
+
     Ok(WindowBuilder::new()
         .with_title(title)
-        .with_inner_size(size)
-        .with_min_inner_size(size)
+        .with_min_inner_size(logical)
+        .with_inner_size(physical)
         .with_resizable(true)
-        .with_decorations(true)
-        .with_transparent(false)
-        // .with_drag_and_drop(false) // OleInitialize failed error if this is set to true
         .build(event_loop)?)
 }

src/ppu.rs

@@ -1,5 +1,5 @@
 use crate::bus::BusIo;
-use crate::instruction::Cycle;
+use crate::Cycle;
 use crate::GB_HEIGHT;
 use crate::GB_WIDTH;
 use dma::DirectMemoryAccess;
@@ -8,7 +8,7 @@ use std::convert::TryInto;
 pub(crate) use types::PpuMode;
 use types::{
     BackgroundPalette, GrayShade, LCDControl, LCDStatus, ObjectFlags, ObjectPalette,
-    ObjectPaletteId, ObjectSize, Pixels, RenderPriority, TileDataAddress,
+    ObjectPaletteKind, ObjectSize, Pixels, RenderPriority, TileDataAddress,
 };
 
 mod dma;
@@ -33,7 +33,7 @@ const LIGHT_GRAY: [u8; 4] = 0xAEBA89FFu32.to_be_bytes();
 const DARK_GRAY: [u8; 4] = 0x5E6745FFu32.to_be_bytes();
 const BLACK: [u8; 4] = 0x202020FFu32.to_be_bytes();
 
-#[derive(Debug, Clone)]
+#[derive(Debug)]
 pub struct Ppu {
     pub(crate) int: Interrupt,
     /// 0xFF40 | LCDC - LCD Control
@@ -47,10 +47,14 @@ pub struct Ppu {
     pub(crate) dma: DirectMemoryAccess,
     scan_state: OamScanState,
     fetch: PixelFetcher,
-    fifo: FifoRenderer,
+    fifo: PixelFifo,
     obj_buffer: ObjectBuffer,
     frame_buf: Box<[u8; GB_WIDTH * GB_HEIGHT * 4]>,
     window_stat: WindowStatus,
+
+    scanline_start: bool,
+    to_discard: u8,
+
     x_pos: u8,
     cycle: Cycle,
 }
@@ -66,7 +70,7 @@ impl BusIo for Ppu {
 }
 
 impl Ppu {
-    pub(crate) fn clock(&mut self) {
+    pub(crate) fn tick(&mut self) {
         self.cycle += 1;
 
         if !self.ctrl.lcd_enabled() {
@@ -75,7 +79,7 @@ impl Ppu {
 
         match self.stat.mode() {
             PpuMode::OamScan => {
-                if self.cycle >= 80.into() {
+                if self.cycle >= 80 {
                     self.stat.set_mode(PpuMode::Drawing);
                 }
 
@@ -84,7 +88,7 @@ impl Ppu {
             PpuMode::Drawing => {
                 if self.ctrl.lcd_enabled() {
                     // Only Draw when the LCD Is Enabled
-                    self.draw(self.cycle.into());
+                    self.draw(self.cycle);
                 } else {
                     self.reset();
                 }
@@ -105,6 +109,8 @@ impl Ppu {
                     }
 
                     self.x_pos = 0;
+                    self.scanline_start = true;
+                    self.to_discard = 0;
 
                     self.fetch.hblank_reset();
                     self.window_stat.hblank_reset();
@@ -119,7 +125,7 @@ impl Ppu {
             PpuMode::HBlank => {
                 // This mode will always end at 456 cycles
 
-                if self.cycle >= 456.into() {
+                if self.cycle >= 456 {
                     self.cycle %= 456;
                     self.pos.line_y += 1;
 
@@ -161,7 +167,7 @@ impl Ppu {
                 }
             }
             PpuMode::VBlank => {
-                if self.cycle > 456.into() {
+                if self.cycle > 456 {
                     self.cycle %= 456;
                     self.pos.line_y += 1;
 
@@ -222,23 +228,26 @@ impl Ppu {
         self.scan_state.next();
     }
 
-    fn draw(&mut self, _cycle: u32) {
+    fn draw(&mut self, _cycle: Cycle) {
         use FetcherState::*;
 
-        let iter = &mut self.obj_buffer.iter();
+        let mut iter = self.obj_buffer.iter_mut();
+        let default = &mut None;
 
         let obj_attr = loop {
-            match iter.flatten().next() {
-                Some(attr) => {
-                    if attr.x <= (self.x_pos + 8) {
-                        self.fetch.back.reset();
-                        self.fetch.back.pause();
-                        self.fifo.pause();
+            match iter.next() {
+                Some(attr_opt) => {
+                    if let Some(attr) = attr_opt {
+                        if attr.x <= (self.x_pos + 8) {
+                            self.fetch.back.reset();
+                            self.fetch.back.pause();
+                            self.fifo.pause();
 
-                        break Some(*attr);
+                            break attr_opt;
+                        }
                     }
                 }
-                None => break None,
+                None => break default,
             }
         };
 
@@ -246,10 +255,10 @@ impl Ppu {
             match self.fetch.obj.state {
                 TileNumber => {
                     self.fetch.obj.tile.with_id(attr.tile_index);
-                    self.fetch.obj.next(ToLowByteSleep);
+                    self.fetch.obj.next(SleepOne);
                 }
-                ToLowByteSleep => self.fetch.obj.next(TileLowByte),
-                TileLowByte => {
+                SleepOne => self.fetch.obj.next(TileLow),
+                TileLow => {
                     let obj_size = self.ctrl.obj_size();
 
                     let addr = PixelFetcher::get_obj_addr(&attr, &self.pos, obj_size);
@@ -257,10 +266,10 @@ impl Ppu {
                     let byte = self.read_byte(addr);
                     self.fetch.obj.tile.with_low_byte(byte);
 
-                    self.fetch.obj.next(ToHighByteSleep);
+                    self.fetch.obj.next(SleepTwo);
                 }
-                ToHighByteSleep => self.fetch.obj.next(TileHighByte),
-                TileHighByte => {
+                SleepTwo => self.fetch.obj.next(TileHigh),
+                TileHigh => {
                     let obj_size = self.ctrl.obj_size();
 
                     let addr = PixelFetcher::get_obj_addr(&attr, &self.pos, obj_size);
@@ -268,10 +277,10 @@ impl Ppu {
                     let byte = self.read_byte(addr + 1);
                     self.fetch.obj.tile.with_high_byte(byte);
 
-                    self.fetch.obj.next(ToFifoSleep);
+                    self.fetch.obj.next(SleepThree);
                 }
-                ToFifoSleep => self.fetch.obj.next(SendToFifoOne),
-                SendToFifoOne => {
+                SleepThree => self.fetch.obj.next(ToFifoOne),
+                ToFifoOne => {
                     // Load into Fifo
                     let (high, low) = self
                         .fetch
@@ -282,25 +291,21 @@ impl Ppu {
 
                     let tbpp = Pixels::from_bytes(high, low);
 
-                    let palette = match attr.flags.palette() {
-                        ObjectPaletteId::Zero => self.monochrome.obj_palette_0,
-                        ObjectPaletteId::One => self.monochrome.obj_palette_1,
-                    };
-
-                    let end = Pixels::PIXEL_COUNT - self.fifo.obj.len();
-                    let start = Pixels::PIXEL_COUNT - end;
-
+                    let palette_kind = attr.flags.palette();
                     let x_flip = attr.flags.x_flip();
 
-                    for i in start..Pixels::PIXEL_COUNT {
+                    let pixel_count = (attr.x - self.x_pos) as usize;
+                    let start = self.fifo.obj.len();
+
+                    for i in start..pixel_count {
                         let x = if x_flip { 7 - i } else { i };
 
                         let priority = attr.flags.priority();
-                        let shade = palette.shade(tbpp.shade_id(x));
+                        let shade_id = tbpp.shade_id(x);
 
-                        let fifo_info = ObjectFifoInfo {
-                            shade,
-                            palette,
+                        let fifo_info = ObjPixelProperty {
+                            shade_id,
+                            palette_kind,
                             priority,
                         };
 
@@ -309,11 +314,11 @@ impl Ppu {
 
                     self.fetch.back.resume();
                     self.fifo.resume();
-                    self.obj_buffer.remove(&attr);
+                    let _ = std::mem::take(obj_attr);
 
-                    self.fetch.obj.next(SendToFifoTwo);
+                    self.fetch.obj.next(ToFifoTwo);
                 }
-                SendToFifoTwo => self.fetch.obj.reset(),
+                ToFifoTwo => self.fetch.obj.reset(),
             }
         }
 
@@ -343,34 +348,32 @@ impl Ppu {
                     self.fetch.back.tile.with_id(id);
 
                     // Move on to the Next state in 2 T-cycles
-                    self.fetch.back.next(ToLowByteSleep);
+                    self.fetch.back.next(SleepOne);
                 }
-                ToLowByteSleep => self.fetch.back.next(TileLowByte),
-                TileLowByte => {
+                SleepOne => self.fetch.back.next(TileLow),
+                TileLow => {
                     let addr = self.fetch.bg_byte_addr(&self.ctrl, &self.pos);
 
                     let low = self.read_byte(addr);
                     self.fetch.back.tile.with_low_byte(low);
 
-                    self.fetch.back.next(ToHighByteSleep);
+                    self.fetch.back.next(SleepTwo);
                 }
-                ToHighByteSleep => self.fetch.back.next(TileHighByte),
-                TileHighByte => {
+                SleepTwo => self.fetch.back.next(TileHigh),
+                TileHigh => {
                     let addr = self.fetch.bg_byte_addr(&self.ctrl, &self.pos);
 
                     let high = self.read_byte(addr + 1);
                     self.fetch.back.tile.with_high_byte(high);
 
-                    self.fetch.back.next(ToFifoSleep);
+                    self.fetch.back.next(SleepThree);
                 }
-                ToFifoSleep => self.fetch.back.next(SendToFifoOne),
-                SendToFifoOne => {
-                    self.fetch.back.next(SendToFifoTwo);
+                SleepThree => self.fetch.back.next(ToFifoOne),
+                ToFifoOne => {
+                    self.fetch.back.next(ToFifoTwo);
                 }
-                SendToFifoTwo => {
-                    let palette = &self.monochrome.bg_palette;
-
-                    if let Ok(()) = self.fetch.send_to_fifo(&mut self.fifo, palette) {
+                ToFifoTwo => {
+                    if let Ok(()) = self.fetch.send_to_fifo(&mut self.fifo) {
                         self.fetch.x_pos += 1;
                         self.fetch.back.next(TileNumber);
                         self.fetch.back.tile = Default::default();
@@ -380,50 +383,26 @@ impl Ppu {
         }
 
         if self.fifo.is_enabled() {
-            use RenderPriority::*;
+            if self.x_pos == 0 && self.scanline_start {
+                self.to_discard = self.pos.scroll_x % 8;
+                self.scanline_start = false;
+            }
 
-            if self.x_pos == 0 && !self.fifo.back.is_empty() {
-                let to_discard = self.pos.scroll_x % 8;
+            if self.to_discard > 0 && !self.fifo.back.is_empty() {
+                let _ = self.fifo.back.pop_front();
+                self.to_discard -= 1;
 
-                for _ in 0..to_discard {
-                    let _ = self.fifo.back.pop_front();
-                }
+                // Delay the PPU by one cycle
+                return;
             }
 
             // Handle Background Pixel and Sprite FIFO
-            let bg_enabled = self.ctrl.bg_win_enabled();
-            let obj_enabled = self.ctrl.obj_enabled();
-            let i0_colour = self.monochrome.bg_palette.i0_colour();
-
-            // FIXME: Is this the correct behaviour
-            let rgba_opt = self.fifo.back.pop_front().map(|bg_info| {
-                let bg_shade = if bg_enabled { bg_info.shade } else { i0_colour };
-
-                match self.fifo.obj.pop_front() {
-                    Some(obj_info) => {
-                        if obj_enabled {
-                            match (obj_info.shade, obj_info.priority) {
-                                (Some(obj_shade), BackgroundAndWindow) => match bg_info.shade {
-                                    GrayShade::White => obj_shade.into_rgba(),
-                                    _ => bg_shade.into_rgba(),
-                                },
-                                (Some(obj_shade), Object) => obj_shade.into_rgba(),
-                                (None, _) => bg_shade.into_rgba(),
-                            }
-                        } else {
-                            bg_shade.into_rgba()
-                        }
-                    }
-                    None => bg_shade.into_rgba(),
-                }
-            });
-
-            if let Some(rgba) = rgba_opt.as_ref() {
+            if let Some(rgba) = self.clock_fifo().map(GrayShade::into_rgba) {
                 let y = self.pos.line_y as usize;
                 let x = self.x_pos as usize;
 
                 let i = (GB_WIDTH * 4) * y + (x * 4);
-                self.frame_buf[i..(i + rgba.len())].copy_from_slice(rgba);
+                self.frame_buf[i..(i + rgba.len())].copy_from_slice(&rgba);
 
                 self.x_pos += 1;
             }
@@ -439,8 +418,51 @@ impl Ppu {
         self.frame_buf.swap_with_slice(&mut blank);
     }
 
-    pub fn copy_to_gui(&self, frame: &mut [u8]) {
-        frame.copy_from_slice(self.frame_buf.as_ref());
+    pub(crate) fn frame_buf(&self) -> &[u8; GB_HEIGHT * GB_WIDTH * 4] {
+        &self.frame_buf
+    }
+
+    fn clock_fifo(&mut self) -> Option<GrayShade> {
+        use ObjectPaletteKind::*;
+        use RenderPriority::*;
+
+        let obj_palette_0 = &self.monochrome.obj_palette_0;
+        let obj_palette_1 = &self.monochrome.obj_palette_1;
+
+        match self.fifo.back.pop_front() {
+            Some(bg_pixel) => match self.fifo.obj.pop_front() {
+                Some(obj_pixel) if self.ctrl.obj_enabled() => match obj_pixel.priority {
+                    Object | BackgroundAndWindow if obj_pixel.shade_id == 0 => {
+                        Some(self.bg_pixel(bg_pixel.shade_id))
+                    }
+                    BackgroundAndWindow if bg_pixel.shade_id != 0 => {
+                        Some(self.bg_pixel(bg_pixel.shade_id))
+                    }
+                    Object | BackgroundAndWindow => {
+                        let maybe_sprite = match obj_pixel.palette_kind {
+                            Zero => obj_palette_0.shade(obj_pixel.shade_id),
+                            One => obj_palette_1.shade(obj_pixel.shade_id),
+                        };
+
+                        let sprite = maybe_sprite
+                            .expect("Sprite w/ a colour id of 0 has already been handled");
+                        Some(sprite)
+                    }
+                },
+                _ => Some(self.bg_pixel(bg_pixel.shade_id)),
+            },
+            None => None,
+        }
+    }
+
+    fn bg_pixel(&self, shade_id: u8) -> GrayShade {
+        let bg_palette = &self.monochrome.bg_palette;
+
+        if self.ctrl.bg_win_enabled() {
+            bg_palette.shade(shade_id)
+        } else {
+            bg_palette.shade(0)
+        }
     }
 }
 
@@ -448,7 +470,7 @@ impl Default for Ppu {
     fn default() -> Self {
         Self {
             vram: Box::new([0u8; VRAM_SIZE]),
-            cycle: Cycle::new(0),
+            cycle: Default::default(),
             frame_buf: Box::new([0; GB_WIDTH * GB_HEIGHT * 4]),
             int: Default::default(),
             ctrl: Default::default(),
@@ -463,11 +485,13 @@ impl Default for Ppu {
             window_stat: Default::default(),
             dma: Default::default(),
             x_pos: 0,
+            scanline_start: true,
+            to_discard: Default::default(),
         }
     }
 }
 
-#[derive(Debug, Clone, Copy, Default)]
+#[derive(Debug, Default)]
 pub(crate) struct Interrupt {
     _vblank: bool,
     _lcd_stat: bool,
@@ -491,7 +515,7 @@ impl Interrupt {
     }
 }
 
-#[derive(Debug, Clone, Copy, Default)]
+#[derive(Debug, Default)]
 pub(crate) struct ScreenPosition {
     /// 0xFF42 | SCY - Scroll Y
     pub(crate) scroll_y: u8,
@@ -507,7 +531,7 @@ pub(crate) struct ScreenPosition {
     pub(crate) window_x: u8,
 }
 
-#[derive(Debug, Clone, Copy, Default)]
+#[derive(Debug, Default)]
 pub(crate) struct Monochrome {
     /// 0xFF47 | BGP - Background Palette Data
     pub(crate) bg_palette: BackgroundPalette,
@@ -517,7 +541,7 @@ pub(crate) struct Monochrome {
     pub(crate) obj_palette_1: ObjectPalette,
 }
 
-#[derive(Debug, Clone)]
+#[derive(Debug)]
 pub(crate) struct ObjectAttributeTable {
     buf: Box<[u8; OAM_SIZE]>,
 }
@@ -584,75 +608,42 @@ impl<'a> From<&'a [u8; 4]> for ObjectAttribute {
     }
 }
 
-#[derive(Debug, Clone, Copy)]
+#[derive(Debug)]
 struct ObjectBuffer {
-    buf: [Option<ObjectAttribute>; OBJECT_LIMIT],
+    inner: [Option<ObjectAttribute>; OBJECT_LIMIT],
     len: usize,
 }
 
-impl ObjectBuffer {
-    fn iter(&self) -> std::slice::Iter<'_, Option<ObjectAttribute>> {
-        self.into_iter()
-    }
-}
-
-impl<'a> IntoIterator for &'a ObjectBuffer {
-    type Item = &'a Option<ObjectAttribute>;
-
-    type IntoIter = std::slice::Iter<'a, Option<ObjectAttribute>>;
-
-    fn into_iter(self) -> Self::IntoIter {
-        self.buf.iter()
-    }
-}
-
-impl<'a> IntoIterator for &'a mut ObjectBuffer {
-    type Item = &'a Option<ObjectAttribute>;
-
-    type IntoIter = std::slice::Iter<'a, Option<ObjectAttribute>>;
-
-    fn into_iter(self) -> Self::IntoIter {
-        self.buf.iter()
-    }
-}
-
 impl ObjectBuffer {
     fn is_full(&self) -> bool {
         self.len == OBJECT_LIMIT
     }
 
     fn clear(&mut self) {
-        self.buf = [Default::default(); 10];
+        self.inner = [Default::default(); 10];
         self.len = 0;
     }
 
     fn add(&mut self, attr: ObjectAttribute) {
-        self.buf[self.len] = Some(attr);
+        self.inner[self.len] = Some(attr);
         self.len += 1;
     }
 
-    fn remove(&mut self, attr: &ObjectAttribute) {
-        let maybe_index = self.buf.iter().position(|maybe_attr| match maybe_attr {
-            Some(other_attr) => attr == other_attr,
-            None => false,
-        });
-
-        if let Some(i) = maybe_index {
-            self.buf[i] = None;
-        }
+    fn iter_mut(&mut self) -> std::slice::IterMut<'_, Option<ObjectAttribute>> {
+        self.inner.iter_mut()
     }
 }
 
 impl Default for ObjectBuffer {
     fn default() -> Self {
         Self {
-            buf: [Default::default(); OBJECT_LIMIT],
+            inner: [Default::default(); OBJECT_LIMIT],
             len: Default::default(),
         }
     }
 }
 
-#[derive(Debug, Clone, Copy, Default)]
+#[derive(Debug, Default)]
 struct PixelFetcher {
     x_pos: u8,
     back: BackgroundFetcher,
@@ -721,7 +712,7 @@ impl PixelFetcher {
         tile_data_addr + (offset * 2)
     }
 
-    fn send_to_fifo(&self, fifo: &mut FifoRenderer, palette: &BackgroundPalette) -> Result<(), ()> {
+    fn send_to_fifo(&self, fifo: &mut PixelFifo) -> Result<(), ()> {
         if !fifo.back.is_empty() {
             return Err(());
         }
@@ -735,9 +726,9 @@ impl PixelFetcher {
         let tbpp = Pixels::from_bytes(high, low);
 
         for x in 0..Pixels::PIXEL_COUNT {
-            let shade = palette.shade(tbpp.shade_id(x));
+            let shade_id = tbpp.shade_id(x);
 
-            let fifo_info = BackgroundFifoInfo { shade };
+            let fifo_info = BgPixelProperty { shade_id };
             fifo.back.push_back(fifo_info);
         }
 
@@ -771,7 +762,7 @@ trait Fetcher {
     fn hblank_reset(&mut self);
 }
 
-#[derive(Debug, Clone, Copy)]
+#[derive(Debug)]
 struct BackgroundFetcher {
     state: FetcherState,
     tile: TileBuilder,
@@ -837,7 +828,7 @@ impl Default for BackgroundFetcher {
     }
 }
 
-#[derive(Debug, Clone, Copy, Default)]
+#[derive(Debug, Default)]
 struct ObjectFetcher {
     state: FetcherState,
     tile: TileBuilder,
@@ -859,7 +850,7 @@ impl Fetcher for ObjectFetcher {
     }
 }
 
-#[derive(Debug, Clone, Copy, Default)]
+#[derive(Debug, Default)]
 struct WindowLineCounter {
     count: u8,
 }
@@ -881,13 +872,13 @@ impl WindowLineCounter {
 #[derive(Debug, Clone, Copy)]
 enum FetcherState {
     TileNumber,
-    ToLowByteSleep,
-    TileLowByte,
-    ToHighByteSleep,
-    TileHighByte,
-    ToFifoSleep,
-    SendToFifoOne,
-    SendToFifoTwo,
+    SleepOne,
+    TileLow,
+    SleepTwo,
+    TileHigh,
+    SleepThree,
+    ToFifoOne,
+    ToFifoTwo,
 }
 
 impl Default for FetcherState {
@@ -896,28 +887,28 @@ impl Default for FetcherState {
     }
 }
 
-#[derive(Debug, Clone, Copy, Default)]
-struct BackgroundFifoInfo {
-    shade: GrayShade,
+#[derive(Debug, Default)]
+struct BgPixelProperty {
+    shade_id: u8,
 }
 
-#[derive(Debug, Clone, Copy, Default)]
-struct ObjectFifoInfo {
-    shade: Option<GrayShade>,
-    palette: ObjectPalette,
+#[derive(Debug, Default)]
+struct ObjPixelProperty {
+    shade_id: u8,
+    palette_kind: ObjectPaletteKind,
     priority: RenderPriority,
 }
 
 // FIXME: Fifo Registers have a known size. Are heap allocations
 // really necessary here?
-#[derive(Debug, Clone)]
-struct FifoRenderer {
-    back: VecDeque<BackgroundFifoInfo>,
-    obj: VecDeque<ObjectFifoInfo>,
+#[derive(Debug)]
+struct PixelFifo {
+    back: VecDeque<BgPixelProperty>,
+    obj: VecDeque<ObjPixelProperty>,
     enabled: bool,
 }
 
-impl FifoRenderer {
+impl PixelFifo {
     fn is_enabled(&self) -> bool {
         self.enabled
     }
@@ -931,7 +922,7 @@ impl FifoRenderer {
     }
 }
 
-impl Default for FifoRenderer {
+impl Default for PixelFifo {
     fn default() -> Self {
         Self {
             back: VecDeque::with_capacity(8),
@@ -941,7 +932,7 @@ impl Default for FifoRenderer {
     }
 }
 
-#[derive(Debug, Clone, Copy, Default)]
+#[derive(Debug, Default)]
 struct TileBuilder {
     id: Option<u8>,
     low: Option<u8>,
@@ -966,7 +957,7 @@ impl TileBuilder {
     }
 }
 
-#[derive(Debug, Clone, Copy, Default)]
+#[derive(Debug, Default)]
 struct OamScanState {
     count: u8,
     mode: OamScanMode,
@@ -987,8 +978,8 @@ impl OamScanState {
         self.count
     }
 
-    fn mode(&self) -> OamScanMode {
-        self.mode
+    fn mode(&self) -> &OamScanMode {
+        &self.mode
     }
 
     fn next(&mut self) {
@@ -1013,7 +1004,7 @@ impl Default for OamScanMode {
     }
 }
 
-#[derive(Debug, Clone, Copy, Default)]
+#[derive(Debug, Default)]
 struct WindowStatus {
     /// This will be true if WY == LY at any point in the frame thus far
     coincidence: bool,

src/ppu/dma.rs

@@ -1,6 +1,6 @@
-use crate::instruction::Cycle;
+use crate::Cycle;
 
-#[derive(Debug, Default, Clone)]
+#[derive(Debug, Default)]
 pub(crate) struct DirectMemoryAccess {
     pub(crate) state: DmaState,
     cycle: Cycle,
@@ -9,7 +9,7 @@ pub(crate) struct DirectMemoryAccess {
 }
 
 impl DirectMemoryAccess {
-    pub(crate) fn clock(&mut self) -> Option<(u16, u16)> {
+    pub(crate) fn tick(&mut self) -> Option<(u16, u16)> {
         match self.state {
             DmaState::Pending => {
                 self.cycle += 1;
@@ -28,7 +28,7 @@ impl DirectMemoryAccess {
 
                 let src_addr = self
                     .start
-                    .addr
+                    .0
                     .as_mut()
                     .expect("Source Address present during DMA Transfer");
 
@@ -56,9 +56,9 @@ impl DirectMemoryAccess {
     }
 
     fn reset(&mut self) {
-        self.cycle = Cycle::new(0);
+        self.cycle = 0;
         self.state = DmaState::Disabled;
-        self.start.addr = None;
+        self.start.0 = None;
     }
 }
 
@@ -75,26 +75,21 @@ impl Default for DmaState {
     }
 }
 
-#[derive(Debug, Default, Clone, Copy)]
-pub(crate) struct DmaAddress {
-    /// The current *source* address of the DMA Transfer
-    ///
-    /// NB: Will be None if no DMA Transfer is in progress
-    addr: Option<u16>,
-}
+#[derive(Debug, Clone, Copy, Default)]
+pub(crate) struct DmaAddress(Option<u16>);
 
 impl DmaAddress {
     pub(crate) fn update(&mut self, byte: u8, state: &mut DmaState) {
         let start = (byte as u16) << 8;
 
-        self.addr = Some(start);
+        self.0 = Some(start);
         *state = DmaState::Pending;
     }
 }
 
 impl From<DmaAddress> for u8 {
     fn from(ctrl: DmaAddress) -> Self {
-        match ctrl.addr {
+        match ctrl.0 {
             Some(addr) => (addr >> 8) as u8,
             None => 0xFF, // TODO: What garbage value should be here?
         }

src/ppu/types.rs

@@ -332,7 +332,7 @@ bitfield! {
     pub from into RenderPriority, priority, set_priority: 7, 7;
     pub y_flip, set_y_flip: 6;
     pub x_flip, set_x_flip: 5;
-    pub from into ObjectPaletteId, palette, set_palette: 4, 4;
+    pub from into ObjectPaletteKind, palette, set_palette: 4, 4;
 }
 
 impl Eq for ObjectFlags {}
@@ -368,23 +368,29 @@ impl Default for ObjectFlags {
 }
 
 #[derive(Debug, Clone, Copy)]
-pub enum ObjectPaletteId {
+pub enum ObjectPaletteKind {
     Zero = 0,
     One = 1,
 }
 
-impl From<u8> for ObjectPaletteId {
+impl Default for ObjectPaletteKind {
+    fn default() -> Self {
+        Self::Zero
+    }
+}
+
+impl From<u8> for ObjectPaletteKind {
     fn from(byte: u8) -> Self {
         match byte & 0b01 {
-            0b00 => ObjectPaletteId::Zero,
-            0b01 => ObjectPaletteId::One,
+            0b00 => ObjectPaletteKind::Zero,
+            0b01 => ObjectPaletteKind::One,
             _ => unreachable!("{:#04X} is not a valid value for BgPaletteNumber", byte),
         }
     }
 }
 
-impl From<ObjectPaletteId> for u8 {
-    fn from(palette_num: ObjectPaletteId) -> Self {
+impl From<ObjectPaletteKind> for u8 {
+    fn from(palette_num: ObjectPaletteKind) -> Self {
         palette_num as u8
     }
 }

src/scheduler.rs

@@ -0,0 +1,79 @@
+use crate::Cycle;
+use std::collections::BinaryHeap;
+
+#[derive(Debug)]
+pub(crate) struct Scheduler {
+    timestamp: Cycle,
+    queue: BinaryHeap<Event>,
+}
+
+impl Scheduler {
+    pub(crate) fn init() -> Self {
+        let mut scheduler = Self {
+            timestamp: Default::default(),
+            queue: Default::default(),
+        };
+
+        scheduler.push(Event {
+            kind: EventKind::TimestampOverflow,
+            timestamp: Cycle::MAX,
+            cb: |_delay| panic!("Reached Cycle::MAX"),
+        });
+
+        scheduler
+    }
+
+    pub(crate) fn push(&mut self, event: Event) {
+        self.queue.push(event);
+    }
+
+    pub(crate) fn step(&mut self, cycles: Cycle) {
+        self.timestamp += cycles;
+
+        loop {
+            let should_pop = match self.queue.peek() {
+                Some(event) => self.timestamp >= event.timestamp,
+                None => false,
+            };
+
+            if !should_pop {
+                break;
+            }
+
+            let event = self.queue.pop().expect("Pop Event from Scheduler Queue");
+
+            (event.cb)(self.timestamp - event.timestamp);
+        }
+    }
+}
+
+#[derive(Debug)]
+pub(crate) struct Event {
+    kind: EventKind,
+    cb: fn(Cycle),
+    pub(crate) timestamp: Cycle,
+}
+
+impl Eq for Event {}
+impl PartialEq for Event {
+    fn eq(&self, other: &Self) -> bool {
+        self.kind == other.kind && self.timestamp == other.timestamp
+    }
+}
+
+impl PartialOrd for Event {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Ord for Event {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.timestamp.cmp(&other.timestamp)
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) enum EventKind {
+    TimestampOverflow,
+}

src/serial.rs

@@ -1,6 +1,6 @@
 use bitfield::bitfield;
 
-#[derive(Debug, Clone, Copy, Default)]
+#[derive(Debug, Default)]
 pub(crate) struct Serial {
     /// 0xFF01 | SB - Serial Transfer Data
     pub(crate) next: u8,

src/timer.rs

@@ -1,23 +1,37 @@
 use bitfield::bitfield;
 
-#[derive(Debug, Clone, Copy)]
+#[derive(Debug)]
 pub(crate) struct Timer {
     /// 0xFF07 | TAC - Timer Control
     pub(crate) ctrl: TimerControl,
     /// 0xFF05 | TIMA - Timer Counter
-    pub(crate) counter: u8,
+    counter: u8,
     /// 0xFF06 | TMA - Timer Modulo
     pub(crate) modulo: u8,
     /// 0xFF04 | DIV - Divider Register
     pub(crate) divider: u16,
-    prev_and_result: Option<u8>,
+
+    and_result: Option<u8>,
     interrupt: bool,
+    state: State,
 }
 
 impl Timer {
-    pub(crate) fn clock(&mut self) {
+    pub(crate) fn tick(&mut self) {
+        use State::*;
         use TimerSpeed::*;
 
+        match self.state {
+            TIMAOverflow(_) | AbortedTIMAOverflow(_) => self.next(),
+            LoadTMA => {
+                self.counter = self.modulo;
+                self.interrupt = true;
+
+                self.next();
+            }
+            Normal => {}
+        }
+
         self.divider = self.divider.wrapping_add(1);
 
         // Get Bit Position
@@ -29,27 +43,34 @@ impl Timer {
         };
 
         let bit = (self.divider >> bit) as u8 & 0x01;
-        let timer_enable = self.ctrl.enabled() as u8;
-        let and_result = bit & timer_enable;
+        let new_result = bit & self.ctrl.enabled() as u8;
 
-        if let Some(0x01) = self.prev_and_result {
-            if and_result == 0x00 {
+        if let Some(0x01) = self.and_result {
+            if new_result == 0x00 {
                 // Falling Edge, increase TIMA Register
-                self.increment_tima();
+                self.inc_counter();
             }
         }
 
-        self.prev_and_result = Some(and_result);
+        self.and_result = Some(new_result);
     }
 
-    fn increment_tima(&mut self) {
-        let (result, did_overflow) = self.counter.overflowing_add(1);
+    /// 0xFF05 | TIMA - Timer Counter
+    pub(crate) fn tima(&self) -> u8 {
+        self.counter
+    }
 
-        self.counter = if did_overflow {
-            self.interrupt = true;
-            self.modulo
-        } else {
-            result
+    /// 0xFF05 | TIMA - Timer Counter
+    pub(crate) fn set_tima(&mut self, byte: u8) {
+        use State::*;
+
+        match self.state {
+            Normal | AbortedTIMAOverflow(_) => self.counter = byte,
+            TIMAOverflow(step) => {
+                self.counter = byte;
+                self.state = AbortedTIMAOverflow(step);
+            }
+            LoadTMA => {}
         }
     }
 
@@ -60,6 +81,27 @@ impl Timer {
     pub(crate) fn set_interrupt(&mut self, value: bool) {
         self.interrupt = value;
     }
+
+    fn inc_counter(&mut self) {
+        let (sum, did_overflow) = self.counter.overflowing_add(1);
+        self.counter = if did_overflow { 0 } else { sum };
+
+        if did_overflow {
+            self.state = State::TIMAOverflow(0);
+        }
+    }
+
+    fn next(&mut self) {
+        use State::*;
+
+        self.state = match self.state {
+            Normal | LoadTMA => Normal,
+            AbortedTIMAOverflow(4) => Normal,
+            TIMAOverflow(4) => LoadTMA,
+            AbortedTIMAOverflow(step) => AbortedTIMAOverflow(step + 1),
+            TIMAOverflow(step) => TIMAOverflow(step + 1),
+        }
+    }
 }
 
 impl Default for Timer {
@@ -70,7 +112,8 @@ impl Default for Timer {
             modulo: 0,
             divider: 0,
             interrupt: false,
-            prev_and_result: None,
+            and_result: None,
+            state: State::Normal,
         }
     }
 }
@@ -132,3 +175,11 @@ impl From<TimerControl> for u8 {
         ctrl.0
     }
 }
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum State {
+    TIMAOverflow(u8),
+    AbortedTIMAOverflow(u8),
+    Normal,
+    LoadTMA,
+}

src/work_ram.rs

@@ -5,7 +5,7 @@ const VARIABLE_WORK_RAM_SIZE: usize = WORK_RAM_SIZE;
 const WORK_RAM_START_ADDRESS: usize = 0xC000;
 const VARIABLE_WORK_RAM_START_ADDRESS: usize = 0xD000;
 
-#[derive(Debug, Clone)]
+#[derive(Debug)]
 pub(crate) struct WorkRam {
     bank: Box<[u8; WORK_RAM_SIZE]>,
 }
@@ -28,48 +28,25 @@ impl Default for WorkRam {
     }
 }
 
-#[derive(Debug, Clone, Copy)]
-enum BankNumber {
-    One = 1,
-    Two = 2,
-    Three = 3,
-    Four = 4,
-    Five = 5,
-    Six = 6,
-    Seven = 7,
-}
-
-#[derive(Debug, Clone)]
+#[derive(Debug)]
 pub(crate) struct VariableWorkRam {
-    current: BankNumber,
-    bank_n: Box<[[u8; VARIABLE_WORK_RAM_SIZE]; 7]>, // 4K for Variable amount of Banks (Banks 1 -> 7) in Game Boy Colour
+    buf: Box<[u8; VARIABLE_WORK_RAM_SIZE]>, // 4K for Variable amount of Banks (Banks 1 -> 7) in Game Boy Colour
 }
 
 impl Default for VariableWorkRam {
     fn default() -> Self {
         Self {
-            current: BankNumber::One,
-            bank_n: Box::new([[0u8; VARIABLE_WORK_RAM_SIZE]; 7]),
+            buf: Box::new([0u8; VARIABLE_WORK_RAM_SIZE]),
         }
     }
 }
 
-impl VariableWorkRam {
-    fn set_current_bank(&mut self, bank: BankNumber) {
-        self.current = bank;
-    }
-
-    fn get_current_bank(&self) -> BankNumber {
-        self.current
-    }
-}
-
 impl BusIo for VariableWorkRam {
     fn write_byte(&mut self, addr: u16, byte: u8) {
-        self.bank_n[self.current as usize][addr as usize - VARIABLE_WORK_RAM_START_ADDRESS] = byte;
+        self.buf[addr as usize - VARIABLE_WORK_RAM_START_ADDRESS] = byte;
     }
 
     fn read_byte(&self, addr: u16) -> u8 {
-        self.bank_n[self.current as usize][addr as usize - VARIABLE_WORK_RAM_START_ADDRESS]
+        self.buf[addr as usize - VARIABLE_WORK_RAM_START_ADDRESS]
     }
 }