chore: prevent bitrot

closes #1

.vscode/extensions.json

@@ -1,8 +1,9 @@
 {
     "recommendations": [
-        "matklad.rust-analyzer",
+        "rust-lang.rust-analyzer",
         "tamasfe.even-better-toml",
         "serayuzgur.crates",
         "vadimcn.vscode-lldb",
+        "donaldhays.rgbds-z80"
     ]
 }

Cargo.toml

@@ -2,21 +2,27 @@
 name = "gb"
 version = "0.1.0"
 authors = ["Rekai Musuka <rekai@musuka.dev>"]
-edition = "2018"
+edition = "2021"
-resolver = "2"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
-anyhow = "^1.0"
+anyhow = "1.0"
-bitfield = "^0.13"
+bitfield = "0.17"
-clap = "^2.33"
+clap = { version = "4.0", features = ["cargo"] }
-gilrs = "^0.8"
+gilrs = "0.11"
-pixels = "^0.6"
+winit = "0.28"
-winit = "^0.25"
+egui = "0.21"
-winit_input_helper = "^0.10"
+egui_wgpu_backend = "0.22"
-rodio = "^0.14"
+egui_winit_platform = "0.18"
-rtrb = "^0.1.4"
+pollster = "0.3"
+rodio = "0.19"
+rtrb = "0.3"
+directories-next = "2.0"
+tracing = "0.1"
+tracing-subscriber = { version = "0.3", features = ["std", "env-filter"] }
+thiserror = "1.0"
+once_cell = "1.10"
 
 [profile.release]
 debug = true

README.md

@@ -1,14 +1,43 @@
 # Rekai's Gameboy Emulator
-[![Build Status](https://ci.paoda.moe/api/badges/paoda/gb/status.svg)](https://ci.paoda.moe/paoda/gb)
 
 ### Status
-* Passes Blargg's cpu_instrs Test
+* From [Blargg Test ROMs](https://github.com/L-P/blargg-test-roms/)
-* Renders Background & Window Tiles 
+    * [x] cpu_instrs
-* Implements a PPU FIFO
+    * [x] instr_timing
+    * [x] mem_timing
+    * [x] mem_timing-2
+    * [ ] dmg_sound (partial) 
+* [x] [dmg-acid2](https://github.com/mattcurrie/dmg-acid2)
+* From [mooneye-gb](https://github.com/Gekkio/mooneye-gb):
+    * Cartridges:
+        * [x] MBC1
+        * [ ] MBC1M
+        * [x] MBC2
+        * [x] MBC5
+* Implements a cycle-accurate PPU FIFO
+    * Doesn't \*exactly\* work just yet
+
+Supports: ROM-only, MBC1, MBC2, MBC3 and MBC5 games.
 
 
-### Notes
+### Compiling
-* [gameboy-logs](https://github.com/wheremyfoodat/Gameboy-logs) suggests that there are still some underlying problems with the cpu implementation
+This project was last successfully built on [Rust 1.64.0](https://github.com/rust-lang/rust/tree/1.64.0)
-* The Sprite FIFO does not work as expected yet
+
-* Sound is neither emulated nor stubbed. Upon writing / reading to a APU related register the emulator will panic. 
+1. `git clone https://github.com/paoda/gb`
-* Code cleanup is pending completion of some minimum viable product of the emulator
+2. `cd gb`
+3. `cargo run --release`
+
+### Controls
+Controls are defined [here](https://github.com/paoda/gb/blob/85940c874460b9cb31bf9b8211bf7dda596d114a/src/joypad.rs#L114)
+
+Key | Button
+--- | ---
+<kbd>X</kbd> | B
+<kbd>Z</kbd> | A
+<kbd>Enter</kbd> | START
+<kbd>Shift</kbd> | SELECT
+
+Then use the Arrow keys for the D-Pad
+
+### Credits
+The Boot ROM found in the `bin/` directory was made by [Optix](https://github.com/Hacktix) over [here](https://github.com/Hacktix/Bootix)

src/apu.rs

@@ -1,3 +1,4 @@
+use crate::apu::gen::SAMPLE_RATE;
 use crate::bus::BusIo;
 use crate::emu::SM83_CLOCK_SPEED;
 use gen::SampleProducer;
@@ -5,8 +6,8 @@ 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::fs::{FrameSequencer, State as FSState};
-use types::{ChannelControl, SoundOutput};
+use types::{ChannelControl, NRx4, SoundOutput};
 
 pub mod gen;
 mod types;
@@ -26,11 +27,13 @@ pub struct Apu {
     /// Noise
     ch4: Channel4,
 
-    sequencer: FrameSequencer,
+    fs: FrameSequencer,
     div_prev: Option<u16>,
 
-    prod: Option<SampleProducer<f32>>,
+    pub(crate) prod: Option<SampleProducer<f32>>,
     sample_counter: u64,
+
+    cap: f32,
 }
 
 impl BusIo for Apu {
@@ -54,7 +57,7 @@ impl BusIo for Apu {
             0x26 => self.ctrl.status(self),
             0x30..=0x3F => self.ch3.read_byte(addr),
             _ => {
-                eprintln!("Read 0xFF from unused IO register {:#06X} [APU]", addr);
+                tracing::warn!("Attempted read from {:#06X}", addr);
                 0xFF
             }
         }
@@ -66,29 +69,26 @@ impl BusIo for Apu {
             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),
+            0x14 if self.ctrl.enabled => self.ch1.set_freq_hi(&self.fs, 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),
+            0x19 if self.ctrl.enabled => self.ch2.set_freq_hi(&self.fs, 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),
+            0x1E if self.ctrl.enabled => self.ch3.set_freq_hi(&self.fs, 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),
+            0x23 if self.ctrl.enabled => self.ch4.set_frequency(&self.fs, 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!(
+            _ => tracing::warn!("Attempted write of {:#04X} to {:#06X}", byte, addr),
-                "Wrote {:#04X} to unused IO register {:#06X} [APU]",
-                byte, addr
-            ),
         }
     }
 }
@@ -99,9 +99,9 @@ impl Apu {
 
         // Frame Sequencer (512Hz)
         if self.is_falling_edge(12, div) {
-            use FrameSequencerState::*;
+            use FSState::*;
 
-            match self.sequencer.state() {
+            match self.fs.state() {
                 Length => self.clock_length(),
                 LengthAndSweep => {
                     self.clock_length();
@@ -111,7 +111,7 @@ impl Apu {
                 Nothing => {}
             }
 
-            self.sequencer.next();
+            self.fs.next();
         }
 
         self.div_prev = Some(div);
@@ -124,25 +124,33 @@ impl Apu {
         if self.sample_counter >= SM83_CLOCK_SPEED {
             self.sample_counter %= SM83_CLOCK_SPEED;
 
-            if let Some(ref mut prod) = self.prod {
+            if let Some(prod) = self.prod.as_mut() {
                 if prod.available_blocking() {
                     // Sample the APU
 
+                    let ch1_amplitude =
+                        Self::high_pass(&mut self.cap, self.ch1.amplitude(), self.ch1.enabled);
                     let (left, right) = self.ctrl.out.ch1();
-                    let ch1_left = if left { self.ch1.amplitude() } else { 0.0 };
+                    let ch1_left = if left { ch1_amplitude } else { 0.0 };
-                    let ch1_right = if right { self.ch1.amplitude() } else { 0.0 };
+                    let ch1_right = if right { ch1_amplitude } else { 0.0 };
 
+                    let ch2_amplitude =
+                        Self::high_pass(&mut self.cap, self.ch2.amplitude(), self.ch2.enabled);
                     let (left, right) = self.ctrl.out.ch2();
-                    let ch2_left = if left { self.ch2.amplitude() } else { 0.0 };
+                    let ch2_left = if left { ch2_amplitude } else { 0.0 };
-                    let ch2_right = if right { self.ch2.amplitude() } else { 0.0 };
+                    let ch2_right = if right { ch2_amplitude } else { 0.0 };
 
+                    let ch3_amplitude =
+                        Self::high_pass(&mut self.cap, self.ch3.amplitude(), self.ch3.enabled);
                     let (left, right) = self.ctrl.out.ch3();
-                    let ch3_left = if left { self.ch3.amplitude() } else { 0.0 };
+                    let ch3_left = if left { ch3_amplitude } else { 0.0 };
-                    let ch3_right = if right { self.ch3.amplitude() } else { 0.0 };
+                    let ch3_right = if right { ch3_amplitude } else { 0.0 };
 
+                    let ch4_amplitude =
+                        Self::high_pass(&mut self.cap, self.ch4.amplitude(), self.ch4.enabled);
                     let (left, right) = self.ctrl.out.ch4();
-                    let ch4_left = if left { self.ch4.amplitude() } else { 0.0 };
+                    let ch4_left = if left { ch4_amplitude } else { 0.0 };
-                    let ch4_right = if right { self.ch4.amplitude() } else { 0.0 };
+                    let ch4_right = if right { 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;
@@ -164,7 +172,7 @@ impl Apu {
 
         if self.ctrl.enabled {
             // Frame Sequencer reset to Step 0
-            self.sequencer.reset();
+            self.fs.reset();
 
             // Square Duty units are reset to first step
             self.ch1.duty_pos = 0;
@@ -180,10 +188,6 @@ impl Apu {
         }
     }
 
-    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();
@@ -207,8 +211,8 @@ impl Apu {
         self.ch4.poly = Default::default();
         self.ch4.freq = Default::default();
 
-        self.ctrl.channel = Default::default();
+        self.ctrl.channel = ChannelControl(0);
-        self.ctrl.out = Default::default();
+        self.ctrl.out = SoundOutput(0);
 
         // Disable the Channels
         self.ch1.enabled = Default::default();
@@ -217,25 +221,13 @@ impl Apu {
         self.ch4.enabled = Default::default();
     }
 
-    fn process_length(freq_hi: &FrequencyHigh, length_timer: &mut u16, enabled: &mut bool) {
+    fn process_length(freq: &impl NRx4, counter: &mut u16, enabled: &mut bool) {
-        if freq_hi.length_disable() && *length_timer > 0 {
+        if freq.length_enable() && *counter > 0 {
-            *length_timer -= 1;
+            *counter -= 1;
 
             // Check in this scope ensures (only) the above subtraction
             // made length_timer 0
-            if *length_timer == 0 {
+            if *counter == 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;
             }
         }
@@ -244,25 +236,25 @@ impl Apu {
     fn clock_length(&mut self) {
         Self::process_length(
             &self.ch1.freq_hi,
-            &mut self.ch1.length_timer,
+            &mut self.ch1.length_counter,
             &mut self.ch1.enabled,
         );
 
         Self::process_length(
             &self.ch2.freq_hi,
-            &mut self.ch2.length_timer,
+            &mut self.ch2.length_counter,
             &mut self.ch2.enabled,
         );
 
         Self::process_length(
             &self.ch3.freq_hi,
-            &mut self.ch3.length_timer,
+            &mut self.ch3.length_counter,
             &mut self.ch3.enabled,
         );
 
-        Self::ch4_process_length(
+        Self::process_length(
             &self.ch4.freq,
-            &mut self.ch4.length_timer,
+            &mut self.ch4.length_counter,
             &mut self.ch4.enabled,
         );
     }
@@ -279,7 +271,7 @@ impl Apu {
             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 {
+                if new_freq <= 0x7FF && self.ch1.sweep.shift_count() != 0 {
                     self.ch1.set_frequency(new_freq);
                     self.ch1.shadow_freq = new_freq;
 
@@ -337,9 +329,22 @@ impl Apu {
             None => false,
         }
     }
+
+    fn high_pass(capacitor: &mut f32, input: f32, enabled: bool) -> f32 {
+        const CHARGE_FACTOR: f32 = 0.999958;
+
+        let mut output = 0.0;
+        if enabled {
+            output = input - *capacitor;
+            *capacitor =
+                input - output * CHARGE_FACTOR.powi(SM83_CLOCK_SPEED as i32 / SAMPLE_RATE as i32);
         }
 
-#[derive(Debug, Default)]
+        output
+    }
+}
+
+#[derive(Debug)]
 pub(crate) struct SoundControl {
     /// 0xFF24 | NR50 - Channel Control
     channel: ChannelControl,
@@ -349,6 +354,16 @@ pub(crate) struct SoundControl {
     enabled: bool,
 }
 
+impl Default for SoundControl {
+    fn default() -> Self {
+        Self {
+            channel: ChannelControl(0),
+            out: SoundOutput(0),
+            enabled: Default::default(),
+        }
+    }
+}
+
 impl SoundControl {
     /// 0xFF24 | NR50 - Channel Control
     pub(crate) fn channel(&self) -> u8 {
@@ -408,7 +423,7 @@ pub(crate) struct Channel1 {
     sweep_enabled: bool,
 
     // Length Functionality
-    length_timer: u16,
+    length_counter: u16,
 
     freq_timer: u16,
     duty_pos: u8,
@@ -435,11 +450,11 @@ impl Channel1 {
     /// 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;
+        self.length_counter = 64 - self.duty.sound_length() as u16;
     }
 
     /// 0xFF12 | NR12 - Channel 1 Volume Envelope
-    pub fn envelope(&self) -> u8 {
+    pub(crate) fn envelope(&self) -> u8 {
         u8::from(self.envelope)
     }
 
@@ -463,25 +478,30 @@ impl Channel1 {
     }
 
     /// 0xFF14 | NR14 - Channel 1 Frequency high
-    pub(crate) fn set_freq_hi(&mut self, byte: u8) {
+    pub(crate) fn set_freq_hi(&mut self, fs: &FrameSequencer, byte: u8) {
-        self.freq_hi = byte.into();
+        let mut new_freq: FrequencyHigh = byte.into();
 
         // If this bit is set, a trigger event occurs
-        if self.freq_hi.initial() {
+        if new_freq.trigger() {
-            if self.is_dac_enabled() {
             self.enabled = true;
-            }
 
-            // Length behaviour during trigger event
+            if self.length_counter == 0 {
-            if self.length_timer == 0 {
+                self.length_counter = 64;
-                self.length_timer = 64;
-            }
 
-            // Envelope Behaviour during trigger event
+                // length_counter was 0 so length evidently wasn't enabled
+                self.freq_hi.set_length_enable(false);
+            };
+
+            // reload freq_timer but last two bits are unmodified
+            self.freq_timer = obscure::square::freq_timer_reload(self.freq_timer, self.frequency());
+
+            // Volume Envelope loaded w/ period
             self.period_timer = self.envelope.period();
+
+            // Channel Volume reloaded
             self.current_volume = self.envelope.init_vol();
 
-            // Sweep behaviour during trigger event
+            // Channel 1 Sweep Behaviour
             let sweep_period = self.sweep.period();
             let sweep_shift = self.sweep.shift_count();
 
@@ -493,7 +513,19 @@ impl Channel1 {
             if sweep_shift != 0 {
                 let _ = self.calc_sweep_freq();
             }
+
+            self.enabled = self.is_dac_enabled();
         }
+
+        obscure::nrx4::length_update(
+            &mut new_freq,
+            fs,
+            &mut self.length_counter,
+            &mut self.enabled,
+            self.freq_hi.length_enable(),
+        );
+
+        self.freq_hi = new_freq;
     }
 
     fn tick(&mut self) {
@@ -555,7 +587,7 @@ impl Channel1 {
 pub(crate) struct Channel2 {
     /// 0xFF16 | NR21 - Channel 2 Sound length / Wave Pattern Duty
     duty: SoundDuty,
-    /// 0xFF17 | NR22 - Channel 2 Volume ENvelope
+    /// 0xFF17 | NR22 - Channel 2 Volume Envelope
     envelope: VolumeEnvelope,
     /// 0xFF18 | NR23 - Channel 2 Frequency low (lower 8 bits only)
     freq_lo: u8,
@@ -567,7 +599,7 @@ pub(crate) struct Channel2 {
     current_volume: u8,
 
     // Length Functionality
-    length_timer: u16,
+    length_counter: u16,
 
     freq_timer: u16,
     duty_pos: u8,
@@ -584,15 +616,15 @@ impl Channel2 {
     /// 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;
+        self.length_counter = 64 - self.duty.sound_length() as u16;
     }
 
-    /// 0xFF17 | NR22 - Channel 2 Volume ENvelope
+    /// 0xFF17 | NR22 - Channel 2 Volume Envelope
     pub(crate) fn envelope(&self) -> u8 {
         u8::from(self.envelope)
     }
 
-    /// 0xFF17 | NR22 - Channel 2 Volume ENvelope
+    /// 0xFF17 | NR22 - Channel 2 Volume Envelope
     pub(crate) fn set_envelope(&mut self, byte: u8) {
         self.envelope = byte.into();
 
@@ -612,22 +644,36 @@ impl Channel2 {
     }
 
     /// 0xFF19 | NR24 - Channel 2 Frequency high
-    pub(crate) fn set_freq_hi(&mut self, byte: u8) {
+    pub(crate) fn set_freq_hi(&mut self, fs: &FrameSequencer, byte: u8) {
+        let prev_le = self.freq_hi.length_enable();
         self.freq_hi = byte.into();
 
-        if self.freq_hi.initial() {
+        obscure::nrx4::length_update(
-            // Envelope behaviour during trigger event
+            &mut self.freq_hi,
+            fs,
+            &mut self.length_counter,
+            &mut self.enabled,
+            prev_le,
+        );
+
+        if self.freq_hi.trigger() {
+            self.enabled = true;
+
+            // Reload length counter if need be
+            if self.length_counter == 0 {
+                self.length_counter = 64;
+            }
+
+            // reload frequency timer
+            self.freq_timer = obscure::square::freq_timer_reload(self.freq_timer, self.frequency());
+
+            // reload envelope
             self.period_timer = self.envelope.period();
+
+            // reload volume
             self.current_volume = self.envelope.init_vol();
 
-            // Length behaviour during trigger event
+            self.enabled = self.is_dac_enabled();
-            if self.length_timer == 0 {
-                self.length_timer = 64;
-            }
-
-            if self.is_dac_enabled() {
-                self.enabled = true;
-            }
         }
     }
 
@@ -678,7 +724,7 @@ pub(crate) struct Channel3 {
     wave_ram: [u8; WAVE_PATTERN_RAM_LEN],
 
     // Length Functionality
-    length_timer: u16,
+    length_counter: u16,
 
     freq_timer: u16,
     offset: u8,
@@ -724,7 +770,7 @@ impl Channel3 {
     /// 0xFF1B | NR31 - Sound Length
     pub(crate) fn set_len(&mut self, byte: u8) {
         self.len = byte;
-        self.length_timer = 256 - self.len as u16;
+        self.length_counter = 256 - self.len as u16;
     }
 
     /// 0xFF1C | NR32 - Channel 3 Volume
@@ -756,18 +802,32 @@ impl Channel3 {
     }
 
     /// 0xFF1E | NR34 - Channel 3 Frequency high
-    pub(crate) fn set_freq_hi(&mut self, byte: u8) {
+    pub(crate) fn set_freq_hi(&mut self, fs: &FrameSequencer, byte: u8) {
+        let prev_le = self.freq_hi.length_enable();
         self.freq_hi = byte.into();
 
-        if self.freq_hi.initial() {
+        obscure::nrx4::length_update(
+            &mut self.freq_hi,
+            fs,
+            &mut self.length_counter,
+            &mut self.enabled,
+            prev_le,
+        );
+
+        if self.freq_hi.trigger() {
+            self.enabled = true;
+
             // Length behaviour during trigger event
-            if self.length_timer == 0 {
+            if self.length_counter == 0 {
-                self.length_timer = 256;
+                self.length_counter = 256;
             }
 
-            if self.dac_enabled {
+            self.freq_timer = (2048 - self.frequency()) * 2;
-                self.enabled = true;
+
-            }
+            // reset wave channel's ptr into wave RAM
+            self.offset = 0;
+
+            self.enabled = self.dac_enabled;
         }
     }
 
@@ -826,7 +886,7 @@ pub(crate) struct Channel4 {
     current_volume: u8,
 
     // Length Functionality
-    length_timer: u16,
+    length_counter: u16,
 
     /// Linear Feedback Shift Register (15-bit)
     lf_shift: u16,
@@ -840,7 +900,7 @@ 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;
+        self.length_counter = 64 - self.len as u16;
     }
 
     /// 0xFF21 | NR42 - Channel 4 Volume Envelope
@@ -873,25 +933,37 @@ impl Channel4 {
     }
 
     /// 0xFF23 | NR44 - Channel 4 Counter / Consecutive Selector and Restart
-    pub(crate) fn set_frequency(&mut self, byte: u8) {
+    pub(crate) fn set_frequency(&mut self, fs: &FrameSequencer, byte: u8) {
+        let prev_le = self.freq.length_enable();
         self.freq = byte.into();
 
-        if self.freq.initial() {
+        obscure::nrx4::length_update(
-            // Envelope behaviour during trigger event
+            &mut self.freq,
+            fs,
+            &mut self.length_counter,
+            &mut self.enabled,
+            prev_le,
+        );
+
+        if self.freq.trigger() {
+            self.enabled = true;
+
+            if self.length_counter == 0 {
+                self.length_counter = 64;
+            }
+
+            // FIXME: Frequency Timer reloaded?
+
+            // reload envelope
             self.period_timer = self.envelope.period();
+
+            //reload volume
             self.current_volume = self.envelope.init_vol();
 
-            // Length behaviour during trigger event
+            // LFSR reset
-            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 = self.is_dac_enabled();
-                self.enabled = true;
-            }
         }
     }
 
@@ -936,3 +1008,39 @@ impl Channel4 {
         code << 4
     }
 }
+
+mod obscure {
+    pub(super) mod square {
+        pub(crate) fn freq_timer_reload(freq_timer: u16, frequency: u16) -> u16 {
+            (freq_timer & 0x0003) | (((2048 - frequency) * 4) & 0xFFFC)
+        }
+    }
+
+    pub(super) mod nrx4 {
+        use super::super::{FrameSequencer, NRx4};
+
+        /// Implements the obscure behaviour when writing to NRX4 under certain
+        /// conditions
+        ///
+        /// # Arguments
+        /// * `freq_hi` - mutable reference to a channel's frequency high register
+        /// * `fs` - reference to the APU's frame sequencer
+        /// * `counter` - mutable reference to a channel's internal enabled flag
+        /// * `prev_le` - what length_enable was before NRx4 was written with a new value
+        pub(crate) fn length_update(
+            freq: &mut impl NRx4,
+            fs: &FrameSequencer,
+            counter: &mut u16,
+            enabled: &mut bool,
+            prev_le: bool,
+        ) {
+            if !fs.next_clocks_length() && !prev_le && freq.length_enable() && *counter != 0 {
+                *counter -= 1;
+
+                if *counter == 0 && !freq.trigger() {
+                    *enabled = false;
+                }
+            }
+        }
+    }
+}

src/apu/gen.rs

@@ -5,34 +5,13 @@ 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> {
+pub fn init<T>() -> (SampleProducer<T>, SampleConsumer<T>) {
-    inner: RingBuffer<T>,
+    let (prod, cons) = RingBuffer::new(BUFFER_CAPACITY);
-}
-
-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>,

src/apu/types.rs

@@ -1,5 +1,11 @@
 use bitfield::bitfield;
 
+pub(crate) trait NRx4 {
+    fn trigger(&self) -> bool;
+    fn length_enable(&self) -> bool;
+    fn set_length_enable(&mut self, value: bool);
+}
+
 pub(crate) mod ch1 {
     use super::bitfield;
 
@@ -103,7 +109,7 @@ pub(super) mod ch3 {
 }
 
 pub(super) mod ch4 {
-    use super::bitfield;
+    use super::{bitfield, NRx4};
 
     bitfield! {
         pub struct PolynomialCounter(u8);
@@ -163,7 +169,7 @@ pub(super) mod ch4 {
             match byte & 0x01 {
                 0b00 => Self::Short,
                 0b01 => Self::Long,
-                _ => unreachable!("{:#04X} is not a valid value for CounterWidth"),
+                _ => unreachable!("{:#04X} is not a valid value for CounterWidth", byte),
             }
         }
     }
@@ -177,17 +183,21 @@ pub(super) mod ch4 {
     bitfield! {
         pub struct Frequency(u8);
         impl Debug;
-        _initial, _: 7;
+        _trigger, _: 7;
-        _length_disable, _: 6;
+        _length_enable, _set_length_enable: 6;
     }
 
-    impl Frequency {
+    impl NRx4 for Frequency {
-        pub(crate) fn length_disable(&self) -> bool {
+        fn trigger(&self) -> bool {
-            self._length_disable()
+            self._trigger()
         }
 
-        pub(crate) fn initial(&self) -> bool {
+        fn length_enable(&self) -> bool {
-            self._initial()
+            self._length_enable()
+        }
+
+        fn set_length_enable(&mut self, value: bool) {
+            self._set_length_enable(value);
         }
     }
 
@@ -218,23 +228,27 @@ pub(super) mod ch4 {
 }
 
 pub(super) mod common {
-    use super::bitfield;
+    use super::{bitfield, NRx4};
 
     bitfield! {
         pub struct FrequencyHigh(u8);
         impl Debug;
-        _initial, _: 7;
+        _trigger, _: 7;
-        _length_disable, _: 6;
+        _length_enable, _set_length_enable: 6;
         pub freq_bits, set_freq_bits: 2, 0;
     }
 
-    impl FrequencyHigh {
+    impl NRx4 for FrequencyHigh {
-        pub(crate) fn initial(&self) -> bool {
+        fn trigger(&self) -> bool {
-            self._initial()
+            self._trigger()
         }
 
-        pub(crate) fn length_disable(&self) -> bool {
+        fn length_enable(&self) -> bool {
-            self._length_disable()
+            self._length_enable()
+        }
+
+        fn set_length_enable(&mut self, value: bool) {
+            self._set_length_enable(value);
         }
     }
 
@@ -402,12 +416,6 @@ pub(super) mod common {
         }
     }
 
-    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
@@ -465,12 +473,6 @@ impl Clone for SoundOutput {
     }
 }
 
-impl Default for SoundOutput {
-    fn default() -> Self {
-        Self(0)
-    }
-}
-
 impl From<u8> for SoundOutput {
     fn from(byte: u8) -> Self {
         Self(byte)
@@ -509,12 +511,6 @@ impl Clone for ChannelControl {
     }
 }
 
-impl Default for ChannelControl {
-    fn default() -> Self {
-        Self(0)
-    }
-}
-
 impl From<u8> for ChannelControl {
     fn from(byte: u8) -> Self {
         Self(byte)
@@ -557,6 +553,15 @@ pub(super) mod fs {
             };
         }
 
+        pub(crate) fn next_clocks_length(&self) -> bool {
+            use State::*;
+
+            match self.state {
+                Length | LengthAndSweep => true,
+                Nothing | Envelope => false,
+            }
+        }
+
         pub(crate) fn state(&self) -> State {
             self.state
         }
@@ -567,7 +572,7 @@ pub(super) mod fs {
         }
     }
 
-    #[derive(Debug, Clone, Copy)]
+    #[derive(Debug, Clone, Copy, PartialEq, Eq)]
     pub(crate) enum State {
         Length,
         Nothing,

src/bus.rs

@@ -10,37 +10,19 @@ use crate::work_ram::{VariableWorkRam, WorkRam};
 
 pub(crate) const BOOT_SIZE: usize = 0x100;
 
-#[derive(Debug)]
+#[derive(Debug, Default)]
 pub struct Bus {
     boot: Option<[u8; BOOT_SIZE]>, // Boot ROM is 256b long
-    cart: Option<Cartridge>,
+    pub(crate) cart: Option<Cartridge>,
     pub(crate) ppu: Ppu,
     work_ram: WorkRam,
     var_ram: VariableWorkRam,
-    pub(crate) timer: Timer,
+    timer: Timer,
     int: Interrupt,
     pub(crate) apu: Apu,
     high_ram: HighRam,
     serial: Serial,
-    pub(crate) joypad: Joypad,
+    pub(crate) joyp: Joypad,
-}
-
-impl Default for Bus {
-    fn default() -> Self {
-        Self {
-            boot: None,
-            cart: None,
-            ppu: Default::default(),
-            work_ram: Default::default(),
-            var_ram: Default::default(),
-            timer: Default::default(),
-            int: Default::default(),
-            apu: Default::default(),
-            high_ram: Default::default(),
-            serial: Default::default(),
-            joypad: Default::default(),
-        }
-    }
 }
 
 impl Bus {
@@ -51,26 +33,24 @@ impl Bus {
         }
     }
 
-    pub(crate) fn load_cart(&mut self, rom: Vec<u8>) {
-        self.cart = Some(Cartridge::new(rom));
-    }
-
-    pub(crate) fn cart_title(&self) -> Option<&str> {
-        self.cart.as_ref()?.title()
-    }
-
     #[allow(dead_code)]
     pub(crate) fn boot_mapped(&self) -> bool {
         self.boot.is_some()
     }
 
+    #[inline]
     pub(crate) fn clock(&mut self) {
         self.tick(4);
     }
 
+    #[inline]
     fn tick(&mut self, limit: u8) {
         for _ in 0..limit {
             self.timer.tick();
+
+            if let Some(c) = self.cart.as_mut() {
+                c.tick()
+            }
             self.ppu.tick();
             self.apu.tick(self.timer.divider);
             self.dma_tick()
@@ -86,7 +66,7 @@ impl Bus {
 }
 
 impl Bus {
-    pub fn oam_read_byte(&self, addr: u16) -> u8 {
+    pub(crate) fn oam_read_byte(&self, addr: u16) -> u8 {
         match addr {
             0x0000..=0x7FFF => {
                 // 16KB ROM bank 00 (ends at 0x3FFF)
@@ -99,14 +79,14 @@ impl Bus {
 
                 match self.cart.as_ref() {
                     Some(cart) => cart.read_byte(addr),
-                    None => panic!("Tried to read from a non-existent cartridge"),
+                    None => 0xFF,
                 }
             }
             0x8000..=0x9FFF => self.ppu.read_byte(addr), // 8KB Video RAM
             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"),
+                None => 0xFF,
             },
             0xC000..=0xCFFF => self.work_ram.read_byte(addr), // 4KB Work RAM Bank 0
             0xD000..=0xDFFF => self.var_ram.read_byte(addr),  // 4KB Work RAM Bank 1 -> N
@@ -133,7 +113,7 @@ impl Bus {
         }
     }
 
-    pub fn oam_write_byte(&mut self, addr: u16, byte: u8) {
+    pub(crate) fn oam_write_byte(&mut self, addr: u16, byte: u8) {
         self.ppu.oam.write_byte(addr, byte);
     }
 }
@@ -152,7 +132,7 @@ impl BusIo for Bus {
 
                 match self.cart.as_ref() {
                     Some(cart) => cart.read_byte(addr),
-                    None => panic!("Tried to read from a non-existent cartridge"),
+                    None => 0xFF,
                 }
             }
             0x8000..=0x9FFF => {
@@ -165,7 +145,7 @@ impl BusIo for Bus {
             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"),
+                None => 0xFF,
             },
             0xC000..=0xCFFF => self.work_ram.read_byte(addr), // 4KB Work RAM Bank 0
             0xD000..=0xDFFF => self.var_ram.read_byte(addr),  // 4KB Work RAM Bank 1 -> N
@@ -212,7 +192,7 @@ impl BusIo for Bus {
                 // Every address here starts with 0xFF so we can just check the
                 // low byte to figure out which register it is
                 match addr & 0x00FF {
-                    0x00 => self.joypad.p1,
+                    0x00 => self.joyp.p1,
                     0x01 => self.serial.next,
                     0x02 => self.serial.ctrl.into(),
                     0x04 => (self.timer.divider >> 8) as u8,
@@ -233,8 +213,9 @@ impl BusIo for Bus {
                     0x49 => self.ppu.monochrome.obj_palette_1.into(),
                     0x4A => self.ppu.pos.window_y,
                     0x4B => self.ppu.pos.window_x,
+                    0x4F => 0xFF, // CGB VRAM Bank Select
                     _ => {
-                        eprintln!("Read 0xFF from unused IO register {:#06X}.", addr);
+                        tracing::warn!("Attempted read from {:#06X} on IO", addr);
                         0xFF
                     }
                 }
@@ -255,9 +236,8 @@ impl BusIo for Bus {
             0x0000..=0x7FFF => {
                 // 16KB ROM bank 00 (ends at 0x3FFF)
                 // and 16KB ROM Bank 01 -> NN (switchable via MB)
-                match self.cart.as_mut() {
+                if let Some(cart) = self.cart.as_mut() {
-                    Some(cart) => cart.write_byte(addr, byte),
+                    cart.write_byte(addr, byte);
-                    None => panic!("Tried to write into non-existent cartridge"),
                 }
             }
             0x8000..=0x9FFF => {
@@ -269,9 +249,8 @@ impl BusIo for Bus {
             }
             0xA000..=0xBFFF => {
                 // 8KB External RAM
-                match self.cart.as_mut() {
+                if let Some(cart) = self.cart.as_mut() {
-                    Some(cart) => cart.write_byte(addr, byte),
+                    cart.write_byte(addr, byte);
-                    None => panic!("Tried to write into non-existent cartridge"),
                 }
             }
             0xC000..=0xCFFF => self.work_ram.write_byte(addr, byte), // 4KB Work RAM Bank 0
@@ -311,7 +290,7 @@ impl BusIo for Bus {
                 // Every address here starts with 0xFF so we can just check the
                 // low byte to figure out which register it is
                 match addr & 0x00FF {
-                    0x00 => self.joypad.update(byte),
+                    0x00 => self.joyp.update(byte),
                     0x01 => self.serial.next = byte,
                     0x02 => self.serial.ctrl = byte.into(),
                     0x04 => self.timer.divider = 0x0000,
@@ -345,13 +324,15 @@ impl BusIo for Bus {
                     0x4A => self.ppu.pos.window_y = byte,
                     0x4B => self.ppu.pos.window_x = byte,
                     0x4D => {} // CGB Specific Register
+                    0x4F => {} // CGB VRAM Bank Select
                     0x50 => {
                         // Disable Boot ROM
                         if byte != 0 {
                             self.boot = None;
                         }
                     }
-                    _ => eprintln!("Wrote {:#04X} to unused IO register {:#06X}.", byte, addr),
+                    0x70 => {} // CGB WRAM Bank Select
+                    _ => tracing::warn!("Attempted write of {:#04X} to {:#06X} on IO", byte, addr),
                 };
             }
             0xFF80..=0xFFFE => {
@@ -373,7 +354,7 @@ impl Bus {
         let lcd_stat = self.ppu.int.lcd_stat();
 
         // Read the current interrupt information from the Joypad
-        let joypad = self.joypad.interrupt();
+        let joypad = self.joyp.interrupt();
 
         // Read the current interrupt information from the Timer
         let timer = self.timer.interrupt();
@@ -403,7 +384,7 @@ impl Bus {
         self.ppu.int.set_lcd_stat(lcd_stat);
 
         // Update the Joypad's instance of the following interrupts
-        self.joypad.set_interrupt(joypad);
+        self.joyp.set_interrupt(joypad);
 
         // Update the Timer's instance of the following interrupts
         self.timer.set_interrupt(timer);

src/cartridge.rs

@@ -1,103 +1,131 @@
+use bitfield::bitfield;
+
 use crate::bus::BusIo;
+use crate::emu::SM83_CLOCK_SPEED;
+use crate::Cycle;
 
 const RAM_SIZE_ADDRESS: usize = 0x0149;
 const ROM_SIZE_ADDRESS: usize = 0x0148;
-const MBC_TYPE_ADDRESS: usize = 0x0147;
+const MBC_KIND_ADDRESS: usize = 0x0147;
-const ROM_TITLE_RANGE: std::ops::RangeInclusive<usize> = 0x0134..=0x0143;
+const ROM_TITLE_START: usize = 0x134;
+const ROM_TITLE_MAX_SIZE: usize = 16;
+const ROM_MANUFACTURER_START: usize = 0x13F;
 
-#[derive(Debug, Default)]
+#[derive(Debug)]
 pub(crate) struct Cartridge {
-    memory: Vec<u8>,
+    mem: Vec<u8>,
-    title: Option<String>,
+    pub(crate) title: Option<String>,
-    mbc: Box<dyn MBCIo>,
+    mbc: Box<dyn MbcIo>,
 }
 
 impl Cartridge {
-    pub(crate) fn new(memory: Vec<u8>) -> Self {
+    pub(crate) fn new(mem: Vec<u8>) -> Self {
-        let title = Self::find_title(&memory);
+        let title_mem: &[u8; 16] = mem[ROM_TITLE_START..(ROM_TITLE_START + ROM_TITLE_MAX_SIZE)]
-        eprintln!("Cartridge Title: {:?}", title);
+            .try_into()
+            .expect("coerce slice containing cartridge title from ROM to [u8; 16]");
 
-        Self {
+        let title = Self::detect_title(title_mem);
-            mbc: Self::detect_mbc(&memory),
+        let mbc = Self::detect_mbc(&mem);
-            title,
+        tracing::info!("Title: {:?}", title);
-            memory,
+
-        }
+        Self { mem, title, mbc }
     }
 
-    fn detect_mbc(memory: &[u8]) -> Box<dyn MBCIo> {
+    pub(crate) fn ext_ram(&self) -> Option<&[u8]> {
-        let ram_size = Self::detect_ram_info(memory);
+        self.mbc.ext_ram()
-        let rom_size = Self::detect_rom_info(memory);
+    }
-        let mbc_kind = Self::find_mbc(memory);
+
+    pub(crate) fn ext_ram_mut(&mut self) -> Option<&mut [u8]> {
+        self.mbc.ext_ram_mut()
+    }
+
+    #[inline]
+    pub(crate) fn tick(&mut self) {
+        self.mbc.tick()
+    }
+
+    fn detect_mbc(mem: &[u8]) -> Box<dyn MbcIo> {
+        let ram_size: RamSize = mem[RAM_SIZE_ADDRESS].into();
+        let rom_size: RomSize = mem[ROM_SIZE_ADDRESS].into();
+        let mbc_kind = Self::detect_mbc_kind(mem[MBC_KIND_ADDRESS]);
         let ram_cap = ram_size.capacity();
         let rom_cap = rom_size.capacity();
 
-        eprintln!("Cartridge Ram Size: {} bytes", ram_cap);
+        tracing::info!("RAM size: {} bytes", ram_cap);
-        eprintln!("Cartridge ROM Size: {} bytes", rom_size.capacity());
+        tracing::info!("ROM size: {} bytes", rom_cap);
-        eprintln!("MBC Type: {:?}", mbc_kind);
+        tracing::info!("MBC kind: {:?}", mbc_kind);
 
         match mbc_kind {
-            MBCKind::None => Box::new(NoMBC),
+            MbcKind::None => Box::new(NoMbc),
-            MBCKind::MBC1 => Box::new(MBC1::new(ram_size, rom_size)),
+            MbcKind::Mbc1(hw) => Box::new(Mbc1::new(hw, ram_size, rom_size)),
-            MBCKind::MBC1WithBattery => Box::new(MBC1::new(ram_size, rom_size)), // TODO: Implement Saving
+            MbcKind::Mbc2(hw) => Box::new(Mbc2::new(hw, rom_cap)),
-            MBCKind::MBC2 => Box::new(MBC2::new(rom_cap)),
+            MbcKind::Mbc3(hw @ Mbc3Hardware::Rtc) => Box::new(Mbc3::new(hw, ram_cap)),
-            MBCKind::MBC2WithBattery => Box::new(MBC2::new(rom_cap)), // TODO: Implement Saving
+            MbcKind::Mbc3(hw @ Mbc3Hardware::RtcBatteryRam) => Box::new(Mbc3::new(hw, ram_cap)),
-            MBCKind::MBC3 => Box::new(MBC3::new(ram_cap)),
+            MbcKind::Mbc3(hw @ Mbc3Hardware::BatteryRam) => Box::new(Mbc3::new(hw, ram_cap)),
-            MBCKind::MBC3WithBattery => Box::new(MBC3::new(ram_cap)), // TODO: Implement Saving
+            MbcKind::Mbc5(hw @ Mbc5Hardware::None) => Box::new(Mbc5::new(hw, ram_cap, rom_cap)),
-            MBCKind::MBC5 => Box::new(MBC5::new(ram_cap, rom_cap)),
+            MbcKind::Mbc5(hw @ Mbc5Hardware::BatteryRam) => {
-            MBCKind::MBC5WithBattery => Box::new(MBC5::new(ram_cap, rom_cap)), // TDO: Implement Saving
+                Box::new(Mbc5::new(hw, ram_cap, rom_cap))
+            }
+            kind => todo!("ROMS with {:?} are currently unsupported", kind),
         }
     }
 
-    fn find_title(memory: &[u8]) -> Option<String> {
+    fn detect_title(title_mem: &[u8; ROM_TITLE_MAX_SIZE]) -> Option<String> {
-        let slice = &memory[ROM_TITLE_RANGE];
+        use std::str::from_utf8;
-        let with_nulls = std::str::from_utf8(slice).ok();
-        let trimmed = with_nulls.map(|s| s.trim_matches('\0').trim());
 
-        match trimmed {
+        const ALT_TITLE_LEN: usize = ROM_MANUFACTURER_START - ROM_TITLE_START;
+
+        // ascii byte slie does not have a null-terminator
+        let ascii = match title_mem.iter().position(|b| *b == 0x00) {
+            Some(end) => &title_mem[..end],
+            None => &title_mem[..ROM_TITLE_MAX_SIZE],
+        };
+
+        match from_utf8(ascii).map(str::trim).ok() {
+            None => match from_utf8(&title_mem[..ALT_TITLE_LEN]).map(str::trim).ok() {
                 Some("") | None => None,
-            Some(_) => trimmed.map(String::from),
+                Some(title) => Some(String::from(title)),
+            },
+            Some("") => None,
+            Some(title) => Some(String::from(title)),
         }
     }
 
-    pub(crate) fn title(&self) -> Option<&str> {
+    fn detect_mbc_kind(id: u8) -> MbcKind {
-        self.title.as_deref()
+        use MbcKind::*;
-    }
 
-    fn detect_ram_info(memory: &[u8]) -> RamSize {
+        match id {
-        let id = memory[RAM_SIZE_ADDRESS];
-        id.into()
-    }
-
-    fn detect_rom_info(memory: &[u8]) -> RomSize {
-        let id = memory[ROM_SIZE_ADDRESS];
-        id.into()
-    }
-
-    fn find_mbc(memory: &[u8]) -> MBCKind {
-        use MBCKind::*;
-
-        match memory[MBC_TYPE_ADDRESS] {
             0x00 => None,
-            0x01 | 0x02 => MBC1,
+            0x01 => Mbc1(Mbc1Hardware::None),
-            0x03 => MBC1WithBattery,
+            0x02 => Mbc1(Mbc1Hardware::Ram),
-            0x05 => MBC2,
+            0x03 => Mbc1(Mbc1Hardware::BatteryRam),
-            0x06 => MBC2WithBattery,
+            0x05 => Mbc2(Mbc2Hardware::None),
-            0x19 | 0x1A => MBC5,
+            0x06 => Mbc2(Mbc2Hardware::BatteryRam),
-            0x1B => MBC5WithBattery,
+            0x08 | 0x09 => unimplemented!("NoMBC + RAM and NoMBC + Battery unsupported"),
-            0x13 => MBC3WithBattery,
+            0x0B | 0x0C | 0x0D => unimplemented!("MM01 unsupported"),
-            0x11 | 0x12 => MBC3,
+            0x0F => Mbc3(Mbc3Hardware::Rtc),
-            id => unimplemented!("id {:#04X} is an unsupported MBC", id),
+            0x10 => Mbc3(Mbc3Hardware::RtcBatteryRam),
+            0x11 => Mbc3(Mbc3Hardware::None),
+            0x12 => Mbc3(Mbc3Hardware::Ram),
+            0x13 => Mbc3(Mbc3Hardware::BatteryRam),
+            0x19 => Mbc5(Mbc5Hardware::None),
+            0x1A => Mbc5(Mbc5Hardware::Ram),
+            0x1B => Mbc5(Mbc5Hardware::BatteryRam),
+            0x1C => Mbc5(Mbc5Hardware::Rumble),
+            0x1D => Mbc5(Mbc5Hardware::RumbleRam),
+            0x1E => Mbc5(Mbc5Hardware::RumbleBatteryRam),
+            id => unimplemented!("MBC with code {:#04X} is unsupported", 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],
+            Addr(addr) => self.mem[addr],
-            Value(byte) => byte,
+            Byte(byte) => byte,
         }
     }
 
@@ -107,28 +135,31 @@ impl BusIo for Cartridge {
 }
 
 #[derive(Debug)]
-struct MBC1 {
+struct Mbc1 {
     /// 5-bit number
     rom_bank: u8,
     /// 2-bit number
     ram_bank: u8,
     mode: bool,
     ram_size: RamSize,
-    memory: Vec<u8>,
+    mem: Vec<u8>,
     rom_size: RomSize,
     mem_enabled: bool,
+
+    hw: Mbc1Hardware,
 }
 
-impl MBC1 {
+impl Mbc1 {
-    fn new(ram_size: RamSize, rom_size: RomSize) -> Self {
+    fn new(hw: Mbc1Hardware, ram_size: RamSize, rom_size: RomSize) -> Self {
         Self {
             rom_bank: 0x01,
-            memory: vec![0; ram_size.capacity() as usize],
+            mem: vec![0; ram_size.capacity() as usize],
             ram_size,
             rom_size,
             ram_bank: Default::default(),
             mode: Default::default(),
             mem_enabled: Default::default(),
+            hw,
         }
     }
 
@@ -197,22 +228,34 @@ impl MBC1 {
     }
 }
 
-impl MBCIo for MBC1 {
+impl Savable for Mbc1 {
-    fn handle_read(&self, addr: u16) -> MBCResult {
+    fn ext_ram(&self) -> Option<&[u8]> {
-        use MBCResult::*;
+        match self.hw {
+            Mbc1Hardware::BatteryRam => Some(&self.mem),
+            _ => None,
+        }
+    }
+
+    fn ext_ram_mut(&mut self) -> Option<&mut [u8]> {
+        match self.hw {
+            Mbc1Hardware::BatteryRam => Some(&mut self.mem),
+            _ => None,
+        }
+    }
+}
+
+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)
+                Addr(0x4000 * self.zero_bank() as usize + addr as usize)
             }
-            0x0000..=0x3FFF => Address(addr as usize),
+            0x0000..=0x3FFF => Addr(addr as usize),
-            0x4000..=0x7FFF => {
+            0x4000..=0x7FFF => Addr(0x4000 * self.high_bank() as usize + (addr as usize - 0x4000)),
-                Address(0x4000 * self.high_bank() as usize + (addr as usize - 0x4000))
+            0xA000..=0xBFFF if self.mem_enabled => Byte(self.mem[self.ram_addr(addr)]),
-            }
+            0xA000..=0xBFFF => Byte(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),
         }
     }
@@ -227,9 +270,9 @@ impl MBCIo for MBC1 {
             }
             0x4000..=0x5FFF => self.ram_bank = byte & 0x03,
             0x6000..=0x7FFF => self.mode = (byte & 0x01) == 0x01,
-            0xA000..=0xBFFF if self.mem_enabled && self.ram_size != RamSize::None => {
+            0xA000..=0xBFFF if self.mem_enabled => {
                 let ram_addr = self.ram_addr(addr);
-                self.memory[ram_addr] = byte;
+                self.mem[ram_addr] = byte;
             }
             0xA000..=0xBFFF => {} // Ram isn't enabled, ignored write
             _ => unreachable!("A write to {:#06X} should not be handled by MBC1", addr),
@@ -237,55 +280,198 @@ impl MBCIo for MBC1 {
     }
 }
 
+impl RtcTick for Mbc1 {
+    fn tick(&mut self) {}
+}
+
+#[derive(Debug, Default, Clone, Copy)]
+struct RtClock {
+    /// 6-bit unsigned integer
+    sec: u8,
+    /// 6-bit unsigned integer
+    min: u8,
+    /// 6-bit unsigned integer
+    hr: u8,
+    day_low: u8,
+    day_high: DayHigh,
+
+    cycles: Cycle,
+}
+
+impl RtClock {
+    fn inc_day(&mut self) {
+        // TODO: Figure out order of operations, the brackets are a bit too defensive here
+        let days: u16 = (((self.day_high.ninth() as u16) << 8) | self.day_low as u16) + 1;
+
+        if days > 0x1FF {
+            self.day_high.set_carry(true);
+        }
+
+        self.day_high.set_ninth(((days >> 8) & 0x01) == 0x01);
+        self.day_low = days as u8;
+    }
+}
+
+impl RtcTick for RtClock {
+    fn tick(&mut self) {
+        // This is the sort of situation where you'd want to use a scheduler.
+        if self.day_high.halt() {
+            return;
+        }
+
+        self.cycles += 1;
+
+        if self.cycles >= SM83_CLOCK_SPEED {
+            self.cycles %= SM83_CLOCK_SPEED;
+
+            self.sec += 1;
+
+            if self.sec == 60 {
+                self.sec = 0;
+                self.min += 1;
+            }
+
+            if self.min == 60 {
+                self.min = 0;
+                self.hr += 1;
+            }
+
+            if self.hr == 24 {
+                self.hr = 0;
+                self.inc_day();
+            }
+        }
+    }
+}
+
+trait RtcTick {
+    fn tick(&mut self);
+}
+
+bitfield! {
+    struct DayHigh(u8);
+    impl Debug;
+    _, set_carry: 7;
+    halt, _: 6;
+    ninth, set_ninth: 0;
+}
+
+impl Copy for DayHigh {}
+impl Clone for DayHigh {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl Default for DayHigh {
+    fn default() -> Self {
+        Self(0)
+    }
+}
+
+impl From<u8> for DayHigh {
+    fn from(byte: u8) -> Self {
+        Self(byte)
+    }
+}
+
+impl From<DayHigh> for u8 {
+    fn from(dh: DayHigh) -> Self {
+        dh.0
+    }
+}
+
 #[derive(Debug, Clone, Copy)]
-enum MBC3Device {
+enum Mbc3Device {
-    ExternalRam,
+    Ram,
-    RealTimeClock,
+    Rtc(RtcRegister),
+}
+
+#[derive(Debug, Clone, Copy)]
+enum RtcRegister {
+    Sec,
+    Min,
+    Hr,
+    DayLow,
+    DayHigh,
 }
 
 #[derive(Debug)]
-struct MBC3 {
+struct Mbc3 {
     /// 7-bit Number
     rom_bank: u8,
     /// 2-bit Number
     ram_bank: u8,
 
     devs_enabled: bool,
-    mapped: Option<MBC3Device>,
+    mapped: Option<Mbc3Device>,
-    memory: Vec<u8>,
+    mem: Vec<u8>,
 
     // RTC Data Latch Previous Write
     prev_latch_write: Option<u8>,
+
+    hw: Mbc3Hardware,
+    rtc: RtClock,
+    rtc_latch: Option<RtClock>,
 }
 
-impl MBC3 {
+impl Mbc3 {
-    fn new(ram_cap: usize) -> Self {
+    fn new(hw: Mbc3Hardware, ram_cap: usize) -> Self {
         Self {
-            memory: vec![0; ram_cap],
+            mem: vec![0; ram_cap],
             rom_bank: Default::default(),
             ram_bank: Default::default(),
             devs_enabled: Default::default(),
             mapped: Default::default(),
             prev_latch_write: Default::default(),
+            rtc: Default::default(),
+            rtc_latch: Default::default(),
+            hw,
         }
     }
 }
 
-impl MBCIo for MBC3 {
+impl Savable for Mbc3 {
-    fn handle_read(&self, addr: u16) -> MBCResult {
+    fn ext_ram(&self) -> Option<&[u8]> {
-        use MBCResult::*;
+        match self.hw {
+            Mbc3Hardware::BatteryRam | Mbc3Hardware::RtcBatteryRam => Some(&self.mem),
+            _ => None,
+        }
+    }
+
+    fn ext_ram_mut(&mut self) -> Option<&mut [u8]> {
+        match self.hw {
+            Mbc3Hardware::BatteryRam | Mbc3Hardware::RtcBatteryRam => Some(&mut self.mem),
+            _ => None,
+        }
+    }
+}
+
+impl MbcIo for Mbc3 {
+    fn handle_read(&self, addr: u16) -> MbcResult {
+        use MbcResult::*;
+        use RtcRegister::*;
 
         let res = match addr {
-            0x0000..=0x3FFF => Address(addr as usize),
+            0x0000..=0x3FFF => Addr(addr as usize),
-            0x4000..=0x7FFF => Address(0x4000 * self.rom_bank as usize + (addr as usize - 0x4000)),
+            0x4000..=0x7FFF => Addr(0x4000 * self.rom_bank as usize + (addr as usize - 0x4000)),
             0xA000..=0xBFFF => match self.mapped {
-                Some(MBC3Device::ExternalRam) if self.devs_enabled => {
+                Some(Mbc3Device::Ram) if self.devs_enabled => {
-                    Value(self.memory[0x2000 * self.ram_bank as usize + (addr as usize - 0xA000)])
+                    Byte(self.mem[0x2000 * self.ram_bank as usize + (addr as usize - 0xA000)])
                 }
-                Some(MBC3Device::RealTimeClock) if self.devs_enabled => {
+                Some(Mbc3Device::Rtc(reg)) if self.devs_enabled => Byte(
-                    todo!("Return Latched value of register")
+                    self.rtc_latch
-                }
+                        .as_ref()
-                _ => Value(0xFF),
+                        .map(|rtc| match reg {
+                            Sec => rtc.sec,
+                            Min => rtc.min,
+                            Hr => rtc.hr,
+                            DayLow => rtc.day_low,
+                            DayHigh => rtc.day_high.into(),
+                        })
+                        .unwrap_or(0xFF),
+                ),
+                _ => Byte(0xFF),
             },
             _ => unreachable!("A read from {:#06X} should not be handled by MBC3", addr),
         };
@@ -294,6 +480,8 @@ impl MBCIo for MBC3 {
     }
 
     fn handle_write(&mut self, addr: u16, byte: u8) {
+        use RtcRegister::*;
+
         match addr {
             0x000..=0x1FFF => self.devs_enabled = (byte & 0x0F) == 0x0A, // Enable External RAM and Access to RTC if there is one
             0x2000..=0x3FFF => {
@@ -305,28 +493,39 @@ impl MBCIo for MBC3 {
             0x4000..=0x5FFF => match byte {
                 0x00 | 0x01 | 0x02 | 0x03 => {
                     self.ram_bank = byte & 0x03;
-                    self.mapped = Some(MBC3Device::ExternalRam);
+                    self.mapped = Some(Mbc3Device::Ram);
-                }
-                0x08 | 0x09 | 0x0A | 0x0B | 0x0C => {
-                    self.mapped = Some(MBC3Device::RealTimeClock);
                 }
+                0x08 => self.mapped = Some(Mbc3Device::Rtc(Sec)),
+                0x09 => self.mapped = Some(Mbc3Device::Rtc(Min)),
+                0x0A => self.mapped = Some(Mbc3Device::Rtc(Hr)),
+                0x0B => self.mapped = Some(Mbc3Device::Rtc(DayLow)),
+                0x0C => self.mapped = Some(Mbc3Device::Rtc(DayHigh)),
+
                 _ => {}
             },
             0x6000..=0x7FFF => {
                 if let Some(0x00) = self.prev_latch_write {
                     if byte == 0x01 {
-                        todo!("Perform Data Latch")
+                        self.rtc_latch = Some(self.rtc);
                     }
                 }
                 self.prev_latch_write = Some(byte);
             }
             0xA000..=0xBFFF => match self.mapped {
-                Some(MBC3Device::ExternalRam) if self.devs_enabled => {
+                Some(Mbc3Device::Ram) if self.devs_enabled => {
-                    self.memory[0x2000 * self.ram_bank as usize + (addr as usize - 0xA000)] = byte
+                    self.mem[0x2000 * self.ram_bank as usize + (addr as usize - 0xA000)] = byte
                 }
-                Some(MBC3Device::RealTimeClock) if self.devs_enabled => {
+                Some(Mbc3Device::Rtc(rtc_reg)) if self.devs_enabled => match rtc_reg {
-                    todo!("Write to RTC")
+                    Sec => {
+                        self.rtc.sec = byte & 0x3F;
+                        // Writing to RTC S resets the internal sub-second counter
+                        self.rtc.cycles = 0;
                     }
+                    Min => self.rtc.min = byte & 0x3F,
+                    Hr => self.rtc.hr = byte & 0x1F,
+                    DayLow => self.rtc.day_low = byte,
+                    DayHigh => self.rtc.day_high = (byte & 0xC1).into(),
+                },
                 _ => {}
             },
             _ => unreachable!("A write to {:#06X} should not be handled by MBC3", addr),
@@ -334,27 +533,37 @@ impl MBCIo for MBC3 {
     }
 }
 
+impl RtcTick for Mbc3 {
+    fn tick(&mut self) {
+        if let Mbc3Hardware::RtcBatteryRam | Mbc3Hardware::Rtc = self.hw {
+            self.rtc.tick();
+        }
+    }
+}
+
 #[derive(Debug)]
-struct MBC5 {
+struct Mbc5 {
     /// 9-bit number
     rom_bank: u16,
     /// 4-bit number
     ram_bank: u8,
 
     rom_cap: usize,
-
+    mem: Vec<u8>,
-    memory: Vec<u8>,
     mem_enabled: bool,
+
+    hw: Mbc5Hardware,
 }
 
-impl MBC5 {
+impl Mbc5 {
-    fn new(ram_cap: usize, rom_cap: usize) -> Self {
+    fn new(hw: Mbc5Hardware, ram_cap: usize, rom_cap: usize) -> Self {
         Self {
             rom_bank: 0x01,
-            memory: vec![0; ram_cap],
+            mem: vec![0; ram_cap],
             rom_cap,
             ram_bank: Default::default(),
             mem_enabled: Default::default(),
+            hw,
         }
     }
 
@@ -363,17 +572,33 @@ impl MBC5 {
     }
 }
 
-impl MBCIo for MBC5 {
+impl Savable for Mbc5 {
-    fn handle_read(&self, addr: u16) -> MBCResult {
+    fn ext_ram(&self) -> Option<&[u8]> {
-        use MBCResult::*;
+        match self.hw {
+            Mbc5Hardware::RumbleBatteryRam | Mbc5Hardware::BatteryRam => Some(&self.mem),
+            _ => None,
+        }
+    }
+
+    fn ext_ram_mut(&mut self) -> Option<&mut [u8]> {
+        match self.hw {
+            Mbc5Hardware::RumbleBatteryRam | Mbc5Hardware::BatteryRam => Some(&mut self.mem),
+            _ => None,
+        }
+    }
+}
+
+impl MbcIo for Mbc5 {
+    fn handle_read(&self, addr: u16) -> MbcResult {
+        use MbcResult::*;
 
         match addr {
-            0x0000..=0x3FFF => Address(addr as usize),
+            0x0000..=0x3FFF => Addr(addr as usize),
-            0x4000..=0x7FFF => Address(self.bank_addr(addr)),
+            0x4000..=0x7FFF => Addr(self.bank_addr(addr)),
             0xA000..=0xBFFF if self.mem_enabled => {
-                Value(self.memory[0x2000 * self.ram_bank as usize + (addr as usize - 0xA000)])
+                Byte(self.mem[0x2000 * self.ram_bank as usize + (addr as usize - 0xA000)])
             }
-            0xA000..=0xBFFF => Value(0xFF),
+            0xA000..=0xBFFF => Byte(0xFF),
             _ => unreachable!("A read from {:#06X} should not be handled by MBC5", addr),
         }
     }
@@ -385,7 +610,7 @@ impl MBCIo for MBC5 {
             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;
+                self.mem[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),
@@ -393,25 +618,31 @@ impl MBCIo for MBC5 {
     }
 }
 
-#[derive(Debug)]
+impl RtcTick for Mbc5 {
-struct MBC2 {
+    fn tick(&mut self) {}
-    /// 4-bit number
-    rom_bank: u8,
-    memory: Box<[u8; Self::RAM_SIZE]>,
-    mem_enabled: bool,
-
-    rom_cap: usize,
 }
 
-impl MBC2 {
+#[derive(Debug)]
+struct Mbc2 {
+    /// 4-bit number
+    rom_bank: u8,
+    mem: Box<[u8; Self::RAM_SIZE]>,
+
+    mem_enabled: bool,
+    rom_cap: usize,
+    hw: Mbc2Hardware,
+}
+
+impl Mbc2 {
     const RAM_SIZE: usize = 0x0200;
 
-    fn new(rom_cap: usize) -> Self {
+    fn new(hw: Mbc2Hardware, rom_cap: usize) -> Self {
         Self {
             rom_bank: 0x01,
-            memory: Box::new([0; Self::RAM_SIZE]),
+            mem: Box::new([0; Self::RAM_SIZE]),
-            mem_enabled: Default::default(),
             rom_cap,
+            mem_enabled: Default::default(),
+            hw,
         }
     }
 
@@ -420,18 +651,34 @@ impl MBC2 {
     }
 }
 
-impl MBCIo for MBC2 {
+impl Savable for Mbc2 {
-    fn handle_read(&self, addr: u16) -> MBCResult {
+    fn ext_ram(&self) -> Option<&[u8]> {
-        use MBCResult::*;
+        match self.hw {
+            Mbc2Hardware::BatteryRam => Some(self.mem.as_ref()),
+            Mbc2Hardware::None => None,
+        }
+    }
+
+    fn ext_ram_mut(&mut self) -> Option<&mut [u8]> {
+        match self.hw {
+            Mbc2Hardware::BatteryRam => Some(self.mem.as_mut()),
+            Mbc2Hardware::None => None,
+        }
+    }
+}
+
+impl MbcIo for Mbc2 {
+    fn handle_read(&self, addr: u16) -> MbcResult {
+        use MbcResult::*;
 
         match addr {
-            0x0000..=0x3FFF => Address(addr as usize),
+            0x0000..=0x3FFF => Addr(addr as usize),
-            0x4000..=0x7FFF => Address(self.rom_addr(addr)),
+            0x4000..=0x7FFF => Addr(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)
+                Byte(self.mem[mbc2_addr] | 0xF0)
             }
-            0xA000..=0xBFFF => Value(0xFF),
+            0xA000..=0xBFFF => Byte(0xFF),
             _ => unreachable!("A read from {:#06X} should not be handled by MBC2", addr),
         }
     }
@@ -446,7 +693,7 @@ impl MBCIo for MBC2 {
             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;
+                self.mem[mbc2_addr] = nybble;
             }
             0x4000..=0x7FFF | 0xA000..=0xBFFF => {}
             _ => unreachable!("A write to {:#06X} should not be handled by MBC2", addr),
@@ -454,47 +701,87 @@ impl MBCIo for MBC2 {
     }
 }
 
+impl RtcTick for Mbc2 {
+    fn tick(&mut self) {}
+}
+
 #[derive(Debug)]
-struct NoMBC;
+struct NoMbc;
 
-impl MBCIo for NoMBC {
+impl Savable for NoMbc {
-    fn handle_read(&self, addr: u16) -> MBCResult {
+    fn ext_ram(&self) -> Option<&[u8]> {
-        MBCResult::Address(addr as usize)
+        None
     }
 
-    fn handle_write(&mut self, _addr: u16, _byte: u8) {
+    fn ext_ram_mut(&mut self) -> Option<&mut [u8]> {
-        // eprintln!("Tried to write {:#04X} to a read-only cartridge", byte);
+        None
     }
 }
 
-trait MBCIo {
+impl MbcIo for NoMbc {
-    fn handle_read(&self, addr: u16) -> MBCResult;
+    fn handle_read(&self, addr: u16) -> MbcResult {
+        MbcResult::Addr(addr as usize)
+    }
+
+    fn handle_write(&mut self, _: u16, byte: u8) {
+        tracing::warn!("Attempted write of {:#04X} to cartridge w/out MBC", byte);
+    }
+}
+
+impl RtcTick for NoMbc {
+    fn tick(&mut self) {}
+}
+
+trait MbcIo: Savable + RtcTick {
+    fn handle_read(&self, addr: u16) -> MbcResult;
     fn handle_write(&mut self, addr: u16, byte: u8);
 }
 
 #[derive(Debug, Clone, Copy)]
-enum MBCResult {
+enum MbcResult {
-    Address(usize),
+    Addr(usize),
-    Value(u8),
+    Byte(u8),
 }
 
 #[derive(Debug, Clone, Copy)]
-enum MBCKind {
+enum MbcKind {
     None,
-    MBC1,
+    Mbc1(Mbc1Hardware),
-    MBC1WithBattery,
+    Mbc2(Mbc2Hardware),
-    MBC2,
+    Mbc3(Mbc3Hardware),
-    MBC2WithBattery,
+    Mbc5(Mbc5Hardware),
-    MBC3,
-    MBC3WithBattery,
-    MBC5,
-    MBC5WithBattery,
 }
 
-impl Default for MBCKind {
+#[derive(Debug, Clone, Copy)]
-    fn default() -> Self {
+enum Mbc1Hardware {
-        Self::None
+    None,
+    Ram,
+    BatteryRam,
 }
+
+#[derive(Debug, Clone, Copy)]
+enum Mbc2Hardware {
+    None,
+    BatteryRam,
+}
+
+#[derive(Debug, Clone, Copy)]
+enum Mbc3Hardware {
+    Rtc,
+    RtcBatteryRam,
+    None,
+    Ram,
+    BatteryRam,
+}
+
+#[derive(Debug, Clone, Copy)]
+enum Mbc5Hardware {
+    None,
+    Ram,
+    BatteryRam,
+    Rumble,
+    RumbleRam,
+    RumbleBatteryRam,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq)]
@@ -522,12 +809,6 @@ impl RamSize {
     }
 }
 
-impl Default for RamSize {
-    fn default() -> Self {
-        Self::None
-    }
-}
-
 impl From<u8> for RamSize {
     fn from(byte: u8) -> Self {
         use RamSize::*;
@@ -596,14 +877,90 @@ impl From<u8> for RomSize {
     }
 }
 
-impl std::fmt::Debug for Box<dyn MBCIo> {
+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 Default for Box<dyn MBCIo> {
+trait Savable {
-    fn default() -> Self {
+    fn ext_ram(&self) -> Option<&[u8]>;
-        Box::new(NoMBC)
+    fn ext_ram_mut(&mut self) -> Option<&mut [u8]>;
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Cartridge;
+
+    #[test]
+    fn empty_rom_title() {
+        let title = [
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00,
+        ];
+
+        assert_eq!(None, Cartridge::detect_title(&title));
+    }
+
+    #[test]
+    fn normal_rom_title() {
+        let title = [
+            0x50, 0x4F, 0x4B, 0x45, 0x4D, 0x4F, 0x4E, 0x20, 0x42, 0x4C, 0x55, 0x45, 0x00, 0x00,
+            0x00, 0x00,
+        ];
+
+        assert_eq!(
+            Some(String::from("POKEMON BLUE")),
+            Cartridge::detect_title(&title)
+        );
+    }
+
+    #[test]
+    fn extra_spaces_title() {
+        let title = [
+            0x54, 0x4f, 0x4b, 0x49, 0x4d, 0x45, 0x4b, 0x49, 0x20, 0x43, 0x55, 0x4c, 0x20, 0x20, 0,
+            0,
+        ];
+
+        assert_eq!(
+            Some(String::from("TOKIMEKI CUL")),
+            Cartridge::detect_title(&title)
+        )
+    }
+
+    #[test]
+    fn long_title() {
+        let title = [
+            0x54, 0x4f, 0x4b, 0x49, 0x4d, 0x45, 0x4b, 0x49, 0x20, 0x43, 0x55, 0x4c, 0x54, 0x55,
+            0x52, 0x45,
+        ];
+
+        assert_eq!(
+            Some(String::from("TOKIMEKI CULTURE")),
+            Cartridge::detect_title(&title),
+        );
+    }
+
+    #[test]
+    fn publisher_code_and_title() {
+        let title: [u8; 16] = [
+            0x47, 0x52, 0x41, 0x4E, 0x44, 0x20, 0x54, 0x48, 0x45, 0x46, 0x54, 0x41, 0x4F, 0x41,
+            0x45, 0x80,
+        ];
+
+        assert_eq!(
+            Some(String::from("GRAND THEFT")),
+            Cartridge::detect_title(&title)
+        );
+    }
+
+    #[test]
+    fn all_whitespace_title() {
+        let title = [
+            0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00,
+        ];
+
+        assert_eq!(None, Cartridge::detect_title(&title));
     }
 }

src/cpu.rs

@@ -5,9 +5,9 @@ use crate::Cycle;
 use bitfield::bitfield;
 use std::fmt::{Display, Formatter, Result as FmtResult};
 
-#[derive(Debug, Default)]
+#[derive(Debug)]
 pub struct Cpu {
-    pub bus: Bus,
+    pub(crate) bus: Bus,
     reg: Registers,
     flags: Flags,
     ime: ImeState,
@@ -15,28 +15,13 @@ pub struct Cpu {
 }
 
 impl Cpu {
-    pub(crate) fn without_boot() -> Self {
+    pub(crate) fn new(rom: [u8; BOOT_SIZE]) -> Self {
-        Self {
-            reg: Registers {
-                a: 0x01,
-                b: 0x00,
-                c: 0x13,
-                d: 0x00,
-                e: 0xD8,
-                h: 0x01,
-                l: 0x4D,
-                sp: 0xFFFE,
-                pc: 0x0100,
-            },
-            flags: 0xb0.into(),
-            ..Default::default()
-        }
-    }
-
-    pub(crate) fn with_boot(rom: [u8; BOOT_SIZE]) -> Self {
         Self {
             bus: Bus::with_boot(rom),
-            ..Default::default()
+            reg: Default::default(),
+            flags: Flags(0),
+            ime: ImeState::Disabled,
+            state: State::Execute,
         }
     }
 
@@ -57,10 +42,7 @@ impl Cpu {
     }
 
     pub(crate) fn is_halted(&self) -> bool {
-        match self.state {
+        matches!(self.state, State::Halt(_))
-            State::Halt(_) => true,
-            _ => false,
-        }
     }
 
     pub(crate) fn halt_kind(&self) -> Option<HaltKind> {
@@ -71,6 +53,12 @@ impl Cpu {
     }
 }
 
+impl Default for Cpu {
+    fn default() -> Self {
+        Cpu::new(*include_bytes!("../bin/bootix_dmg.bin"))
+    }
+}
+
 impl Cpu {
     /// Fetch an [Instruction] from the memory bus
     /// (4 cycles)
@@ -105,7 +93,7 @@ impl Cpu {
     /// routine.
     ///
     /// Handle HALT and interrupts.
-    pub fn step(&mut self) -> Cycle {
+    pub(crate) fn step(&mut self) -> Cycle {
         // Log instructions
         // if self.reg.pc > 0xFF {
         //     let out = std::io::stdout();
@@ -120,13 +108,10 @@ impl Cpu {
             use HaltKind::*;
 
             self.bus.clock();
-
+            return match kind {
-            let elapsed = match kind {
                 ImeEnabled | NonePending => 4,
                 SomePending => todo!("Implement HALT bug"),
             };
-
-            return elapsed;
         }
 
         let opcode = self.fetch();
@@ -135,11 +120,11 @@ impl Cpu {
         self.handle_ei();
 
         // For use in Blargg's Test ROMs
-        if self.read_byte(0xFF02) == 0x81 {
+        // if self.read_byte(0xFF02) == 0x81 {
-            let c = self.read_byte(0xFF01) as char;
+        //     let c = self.read_byte(0xFF01) as char;
-            self.write_byte(0xFF02, 0x00);
+        //     self.write_byte(0xFF02, 0x00);
-            eprint!("{}", c);
+        //     eprint!("{}", c);
-        }
+        // }
 
         elapsed
     }
@@ -156,14 +141,6 @@ impl BusIo for Cpu {
 }
 
 impl Cpu {
-    pub(crate) fn bus(&self) -> &Bus {
-        &self.bus
-    }
-
-    pub(crate) fn bus_mut(&mut self) -> &mut Bus {
-        &mut self.bus
-    }
-
     fn handle_ei(&mut self) {
         match self.ime {
             ImeState::EiExecuted => self.ime = ImeState::Pending,
@@ -261,12 +238,6 @@ enum State {
     // Stop,
 }
 
-impl Default for State {
-    fn default() -> Self {
-        Self::Execute
-    }
-}
-
 impl Cpu {
     pub(crate) fn set_register(&mut self, register: Register, value: u8) {
         use Register::*;
@@ -460,12 +431,6 @@ impl Clone for Flags {
     }
 }
 
-impl Default for Flags {
-    fn default() -> Self {
-        Self(0)
-    }
-}
-
 impl Display for Flags {
     fn fmt(&self, f: &mut Formatter<'_>) -> FmtResult {
         if self.z() {
@@ -521,8 +486,38 @@ pub(crate) enum ImeState {
     Enabled,
 }
 
-impl Default for ImeState {
+pub(crate) mod dbg {
-    fn default() -> Self {
+    use super::{Cpu, ImeState, RegisterPair};
-        Self::Disabled
+
+    pub(crate) fn flags(cpu: &Cpu) -> u8 {
+        cpu.flags.into()
+    }
+
+    pub(crate) fn af(cpu: &Cpu) -> u16 {
+        cpu.register_pair(RegisterPair::AF)
+    }
+
+    pub(crate) fn bc(cpu: &Cpu) -> u16 {
+        cpu.register_pair(RegisterPair::BC)
+    }
+
+    pub(crate) fn de(cpu: &Cpu) -> u16 {
+        cpu.register_pair(RegisterPair::DE)
+    }
+
+    pub(crate) fn hl(cpu: &Cpu) -> u16 {
+        cpu.register_pair(RegisterPair::HL)
+    }
+
+    pub(crate) fn sp(cpu: &Cpu) -> u16 {
+        cpu.register_pair(RegisterPair::SP)
+    }
+
+    pub(crate) fn pc(cpu: &Cpu) -> u16 {
+        cpu.register_pair(RegisterPair::PC)
+    }
+
+    pub(crate) fn ime(cpu: &Cpu) -> bool {
+        matches!(cpu.ime, ImeState::Enabled)
     }
 }

src/emu.rs

@@ -1,121 +1,163 @@
 use crate::apu::gen::SampleProducer;
+use crate::bus::BOOT_SIZE;
+use crate::cartridge::Cartridge;
 use crate::cpu::Cpu;
-use crate::joypad::{self, Joypad};
 use crate::{Cycle, GB_HEIGHT, GB_WIDTH};
+use clap::crate_name;
 use gilrs::Gilrs;
+use std::fs::File;
+use std::io::{Read, Write};
+use std::path::{Path, PathBuf};
 use std::time::Duration;
-use winit_input_helper::WinitInputHelper;
+use thiserror::Error;
+use winit::event::KeyboardInput;
+use winit::event_loop::ControlFlow;
 
 pub const SM83_CYCLE_TIME: Duration = Duration::from_nanos(1_000_000_000 / SM83_CLOCK_SPEED);
 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 DEFAULT_TITLE: &str = "Game Boy Screen";
 
-pub fn run_frame(emu: &mut Emulator, gamepad: &mut Gilrs, key: &WinitInputHelper) -> Cycle {
+#[inline]
+pub fn run_frame(cpu: &mut Cpu, gamepad: &mut Gilrs, key: KeyboardInput) {
+    run(cpu, gamepad, key, CYCLES_IN_FRAME)
+}
+
+#[inline]
+pub fn run(cpu: &mut Cpu, gamepad: &mut Gilrs, key: KeyboardInput, cycles: Cycle) {
     let mut elapsed = 0;
 
     if let Some(event) = gamepad.next_event() {
-        joypad::handle_gamepad_input(emu.joyp_mut(), event);
+        crate::joypad::handle_gamepad_input(&mut cpu.bus.joyp, event);
     }
+    crate::joypad::handle_keyboard_input(&mut cpu.bus.joyp, key);
 
-    joypad::handle_keyboard_input(emu.joyp_mut(), key);
+    while elapsed < cycles {
-    while elapsed < CYCLES_IN_FRAME {
+        elapsed += cpu.step();
-        elapsed += emu.step();
-    }
-
-    elapsed
-}
-
-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(),
     }
 }
 
-    fn step(&mut self) -> Cycle {
+pub fn save_and_exit(cpu: &Cpu, control_flow: &mut ControlFlow) {
-        self.cpu.step()
+    write_save(cpu);
+    *control_flow = ControlFlow::Exit;
 }
 
-    fn load_cart(&mut self, rom: Vec<u8>) {
+#[inline]
-        self.cpu.bus_mut().load_cart(rom)
+pub fn pixel_buf(cpu: &Cpu) -> &[u8; GB_HEIGHT * GB_WIDTH * 4] {
+    use crate::ppu::Device;
+    cpu.bus.ppu.frame_buf.get(Device::Host)
 }
 
-    fn joyp_mut(&mut self) -> &mut Joypad {
+pub fn from_boot_rom<P: AsRef<Path>>(path: P) -> std::io::Result<Cpu> {
-        &mut self.cpu.bus_mut().joypad
+    Ok(Cpu::new(read_boot(path)?))
 }
 
-    pub fn set_prod(&mut self, prod: SampleProducer<f32>) {
+pub fn read_game_rom<P: AsRef<Path>>(cpu: &mut Cpu, path: P) -> std::io::Result<()> {
-        self.cpu.bus_mut().apu.attach_producer(prod)
+    cpu.bus.cart = Some(Cartridge::new(std::fs::read(path.as_ref())?));
+    Ok(())
 }
 
-    pub fn title(&self) -> &str {
+pub fn set_audio_prod(cpu: &mut Cpu, prod: SampleProducer<f32>) {
-        self.cpu.bus().cart_title().unwrap_or(DEFAULT_TITLE)
+    cpu.bus.apu.prod = Some(prod);
+}
+
+pub fn rom_title(cpu: &Cpu) -> &str {
+    cpu.bus
+        .cart
+        .as_ref()
+        .and_then(|c| c.title.as_deref())
+        .unwrap_or(DEFAULT_TITLE)
+}
+
+pub fn write_save(cpu: &Cpu) {
+    match cpu.bus.cart.as_ref() {
+        Some(cart) => match write_save_to_file(cart) {
+            Ok(path) => tracing::info!("Wrote to save at {:?}", path),
+            Err(err @ SaveError::NotApplicable) => tracing::warn!("Unable to Save: {:?}", err),
+            Err(SaveError::DiffSize) => unreachable!(),
+            Err(SaveError::Io(err)) => tracing::error!("{:?}", err),
+        },
+        None => tracing::error!("No cartridge is currently present"),
     }
 }
 
-pub mod build {
+pub fn load_save(cpu: &mut Cpu) {
-    use std::fs::File;
+    match cpu.bus.cart.as_mut() {
-    use std::io::{Read, Result};
+        Some(cart) => match read_save_from_file(cart) {
-    use std::path::Path;
+            Ok(path) => tracing::info!("Loaded save from {:?}", path),
-
+            Err(err @ SaveError::NotApplicable) => tracing::warn!("Unable to load save: {}", err),
-    use crate::bus::BOOT_SIZE;
+            Err(err @ SaveError::DiffSize) => tracing::error!("Unable to load save: {}", err),
-    use crate::cpu::Cpu;
+            Err(SaveError::Io(err)) => match err.kind() {
-
+                std::io::ErrorKind::NotFound => tracing::warn!("Save not found"),
-    use super::Emulator;
+                _ => tracing::error!("{:?}", err),
-
+            },
-    #[derive(Debug, Default)]
+        },
-    pub struct EmulatorBuilder {
+        None => tracing::error!("No cartridge is currently present"),
-        boot: Option<[u8; BOOT_SIZE]>,
+    }
-        cart: Option<Vec<u8>>,
 }
 
-    impl EmulatorBuilder {
+fn write_save_to_file(cart: &Cartridge) -> Result<PathBuf, SaveError> {
-        pub fn new() -> Self {
+    match cart.title.as_ref().zip(cart.ext_ram()) {
-            Default::default()
+        Some((title, ram)) => {
+            let mut save_path = data_path().unwrap_or_else(|| PathBuf::from("."));
+            save_path.push(title);
+            save_path.set_extension("sav");
+
+            let mut file = File::create(&save_path)?;
+            file.write_all(ram)?;
+            Ok(save_path)
+        }
+        None => Err(SaveError::NotApplicable),
+    }
 }
 
-        pub fn with_boot<P: AsRef<Path>>(mut self, path: P) -> Result<Self> {
+fn read_save_from_file(cart: &mut Cartridge) -> Result<PathBuf, SaveError> {
+    match cart.title.clone().zip(cart.ext_ram_mut()) {
+        Some((title, ext_ram)) => {
+            let mut save_path = data_path().unwrap_or_else(|| PathBuf::from("."));
+            save_path.push(title);
+            save_path.set_extension("sav");
+
+            let mut file = File::open(&save_path)?;
+            let mut memory = Vec::new();
+            file.read_to_end(&mut memory)?;
+
+            if ext_ram.len() != memory.len() {
+                return Err(SaveError::DiffSize);
+            }
+
+            ext_ram.copy_from_slice(&memory);
+            Ok(save_path)
+        }
+        None => Err(SaveError::NotApplicable),
+    }
+}
+
+fn read_boot<P: AsRef<Path>>(path: P) -> std::io::Result<[u8; BOOT_SIZE]> {
+    let mut buf = [0; BOOT_SIZE];
     let mut file = File::open(path.as_ref())?;
 
-            let mut buf = [0x00; BOOT_SIZE];
     file.read_exact(&mut buf)?;
-
+    Ok(buf)
-            self.boot = Some(buf);
-            Ok(self)
 }
 
-        pub fn with_cart<P: AsRef<Path>>(mut self, path: P) -> Result<Self> {
+fn data_path() -> Option<PathBuf> {
-            let mut file = File::open(path.as_ref())?;
+    match directories_next::ProjectDirs::from("dev", "musuka", crate_name!()) {
-
+        Some(dirs) => {
-            let mut buf = Vec::new();
+            let data_local = dirs.data_local_dir();
-            file.read_to_end(&mut buf)?;
+            std::fs::create_dir_all(data_local).ok()?;
-
+            Some(data_local.to_path_buf())
-            self.cart = Some(buf);
-            Ok(self)
         }
-
+        None => None,
-        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
     }
 }
+
+#[derive(Debug, Error)]
+pub enum SaveError {
+    #[error("cartridge lacks title and/or external ram")]
+    NotApplicable,
+    #[error(transparent)]
+    Io(#[from] std::io::Error),
+    #[error("save file size differs from external ram")]
+    DiffSize,
 }

src/gui.rs

@@ -0,0 +1,472 @@
+use egui::{Context, TextureId};
+use egui_wgpu_backend::{wgpu, RenderPass, ScreenDescriptor};
+use egui_winit_platform::Platform;
+use wgpu::{
+    Adapter, CommandEncoder, Device, Extent3d, FilterMode, Instance, Queue, RequestDeviceError,
+    Surface, SurfaceCapabilities, SurfaceConfiguration, Texture, TextureView,
+    TextureViewDescriptor,
+};
+
+use winit::dpi::PhysicalSize;
+use winit::error::OsError;
+use winit::event::{ElementState, Event, KeyboardInput};
+use winit::event_loop::{ControlFlow, EventLoop};
+use winit::window::Window;
+
+use crate::cpu::Cpu;
+use crate::{emu, GB_HEIGHT, GB_WIDTH};
+
+const EGUI_DIMENSIONS: (usize, usize) = (1280, 720);
+const FILTER_MODE: FilterMode = FilterMode::Nearest;
+const WINDOW_TITLE: &str = "DMG-01 Emulator";
+
+const SCALE: f32 = 3.0;
+
+pub struct Gui {
+    pub should_quit: bool,
+    pub title: String,
+    pub mode: EmuMode,
+
+    window: Window,
+    platform: Platform,
+
+    surface: Surface,
+    device: Device,
+    queue: Queue,
+    surface_config: SurfaceConfiguration,
+    render_pass: RenderPass,
+
+    texture: Texture,
+    texture_size: Extent3d,
+    texture_id: TextureId,
+}
+
+impl Gui {
+    pub fn new<T>(title: String, event_loop: &EventLoop<T>, cpu: &Cpu) -> Self {
+        use wgpu::InstanceDescriptor;
+
+        let window = build_window(event_loop).expect("build window");
+
+        let instance = Instance::new(InstanceDescriptor::default());
+        let surface = unsafe {
+            instance
+                .create_surface(&window)
+                .expect("create wgpu instance surface")
+        };
+
+        let adapter = request_adapter(&instance, &surface).expect("request adaptor");
+        let (device, queue) = request_device(&adapter).expect("request device");
+
+        let capabilities = surface.get_capabilities(&adapter);
+
+        let surface_config = surface_config(&window, capabilities);
+        surface.configure(&device, &surface_config);
+        let platform = platform(&window);
+        let mut render_pass = RenderPass::new(&device, surface_config.format, 1);
+
+        let texture_size = texture_size();
+        let texture = create_texture(&device, texture_size);
+        write_to_texture(&queue, &texture, emu::pixel_buf(cpu), texture_size);
+
+        let view = texture.create_view(&TextureViewDescriptor::default());
+        let texture_id = expose_texture_to_egui(&mut render_pass, &device, &view);
+
+        Self {
+            should_quit: Default::default(),
+            title,
+            mode: EmuMode::Running,
+
+            window,
+            platform,
+
+            surface,
+            device,
+            queue,
+            surface_config,
+            render_pass,
+
+            texture,
+            texture_size,
+            texture_id,
+        }
+    }
+
+    pub fn maybe_quit(&self, cpu: &Cpu, control_flow: &mut ControlFlow) {
+        if self.should_quit {
+            emu::save_and_exit(cpu, control_flow);
+        }
+    }
+
+    pub fn request_redraw(&self) {
+        self.window.request_redraw();
+    }
+
+    pub fn handle_event<T>(&mut self, event: &Event<T>) {
+        self.platform.handle_event(event);
+    }
+
+    pub fn update_time(&mut self, elapsed_seconds: f64) {
+        self.platform.update_time(elapsed_seconds);
+    }
+
+    pub fn resize(&mut self, size: PhysicalSize<u32>) {
+        // See: https://github.com/rust-windowing/winit/issues/208
+        if size.width > 0 && size.height > 0 {
+            self.surface_config.width = size.width;
+            self.surface_config.height = size.height;
+            self.surface.configure(&self.device, &self.surface_config);
+        }
+    }
+
+    pub fn paint(&mut self, cpu: &Cpu) {
+        use wgpu::{Color, SurfaceError};
+
+        let data = emu::pixel_buf(cpu);
+        write_to_texture(&self.queue, &self.texture, data, self.texture_size);
+
+        let output_frame = match self.surface.get_current_texture() {
+            Ok(frame) => frame,
+            Err(SurfaceError::Outdated) => return, // Occurs on minimization on Windows
+            Err(e) => {
+                eprintln!("Dropped frame with error: {}", e);
+                return;
+            }
+        };
+
+        let output_view = output_frame
+            .texture
+            .create_view(&TextureViewDescriptor::default());
+
+        // Begin to draw Egui components
+        self.platform.begin_frame();
+        self.draw_egui(cpu, &self.platform.context(), self.texture_id);
+
+        // End the UI frame. We could now handle the output and draw the UI with the backend.
+        let full_output = self.platform.end_frame(Some(&self.window));
+        let paint_jobs = self.platform.context().tessellate(full_output.shapes);
+
+        let mut encoder = create_command_encoder(&self.device);
+        let screen_descriptor = create_screen_descriptor(&self.window, &self.surface_config);
+        let tdelta = full_output.textures_delta;
+        // Upload all resources for the GPU.
+        self.render_pass
+            .add_textures(&self.device, &self.queue, &tdelta)
+            .expect("add texture ok");
+        self.render_pass
+            .update_buffers(&self.device, &self.queue, &paint_jobs, &screen_descriptor);
+
+        // Record all render passes.
+        self.render_pass
+            .execute(
+                &mut encoder,
+                &output_view,
+                &paint_jobs,
+                &screen_descriptor,
+                Some(Color::BLACK),
+            )
+            .expect("execute render pass");
+
+        // Submit the commands.
+        self.queue.submit(std::iter::once(encoder.finish()));
+
+        // Redraw egui
+        output_frame.present();
+
+        self.render_pass
+            .remove_textures(tdelta)
+            .expect("remove texture delta");
+    }
+
+    #[inline]
+    pub fn draw_egui(&mut self, cpu: &Cpu, ctx: &Context, texture_id: TextureId) {
+        use crate::{cpu, instruction, ppu};
+
+        fn selectable_text(ui: &mut egui::Ui, mut text: &str) -> egui::Response {
+            ui.add(egui::TextEdit::multiline(&mut text).code_editor())
+        }
+
+        egui::TopBottomPanel::top("top_panel").show(ctx, |ui| {
+            ui.menu_button("File", |ui| {
+                if ui.button("Quit").clicked() {
+                    self.should_quit = true;
+                }
+            });
+
+            egui::Window::new(&self.title).show(ctx, |ui| {
+                ui.image(
+                    texture_id,
+                    [GB_WIDTH as f32 * SCALE, GB_HEIGHT as f32 * SCALE],
+                );
+            });
+
+            egui::Window::new("Disassembly").show(ctx, |ui| {
+                selectable_text(ui, &instruction::dbg::tmp_disasm(cpu, 20));
+            });
+
+            egui::Window::new("Settings").show(ctx, |ui| {
+                egui::ComboBox::from_label("Emulation Mode")
+                    .selected_text(format!("{:?}", self.mode))
+                    .show_ui(ui, |ui| {
+                        ui.selectable_value(&mut self.mode, EmuMode::Running, "Running");
+                        ui.selectable_value(&mut self.mode, EmuMode::Stopped, "Stopped");
+                        ui.selectable_value(&mut self.mode, EmuMode::StepFrame, "Step Frame");
+                        ui.selectable_value(&mut self.mode, EmuMode::Step, "Step");
+                    })
+            });
+
+            egui::Window::new("GB Info").show(ctx, |ui| {
+                ui.horizontal(|ui| {
+                    ui.vertical(|ui| {
+                        ui.heading("CPU");
+
+                        ui.monospace(format!("AF: {:#06X}", cpu::dbg::af(cpu)));
+                        ui.monospace(format!("BC: {:#06X}", cpu::dbg::bc(cpu)));
+                        ui.monospace(format!("DE: {:#06X}", cpu::dbg::de(cpu)));
+                        ui.monospace(format!("HL: {:#06X}", cpu::dbg::hl(cpu)));
+                        ui.add_space(10.0);
+                        ui.monospace(format!("SP: {:#06X}", cpu::dbg::sp(cpu)));
+                        ui.monospace(format!("PC: {:#06X}", cpu::dbg::pc(cpu)));
+                    });
+
+                    ui.vertical(|ui| {
+                        let ppu = &cpu.bus.ppu;
+
+                        ui.heading("PPU");
+
+                        ui.monospace(format!("LY: {}", ppu::dbg::ly(ppu)));
+                        ui.horizontal(|ui| {
+                            ui.monospace(format!("SCX: {}", ppu::dbg::scx(ppu)));
+                            ui.monospace(format!("SCY: {}", ppu::dbg::scy(ppu)));
+                        });
+                        ui.horizontal(|ui| {
+                            ui.monospace(format!("WX: {}", ppu::dbg::wx(ppu)));
+                            ui.monospace(format!("WY: {}", ppu::dbg::wy(ppu)));
+                        });
+
+                        ui.monospace(format!(
+                            "Mode: {:?} {}",
+                            ppu::dbg::mode(ppu),
+                            ppu::dbg::dot(ppu)
+                        ))
+                    });
+                });
+
+                ui.add_space(10.0);
+
+                let _ = ui.selectable_label(cpu::dbg::ime(cpu), "IME");
+                ui.horizontal(|ui| {
+                    let irq = cpu.int_request();
+
+                    ui.label("IRQ:");
+                    let _ = ui.selectable_label(irq & 0b01 == 0x01, "VBlank");
+                    let _ = ui.selectable_label(irq >> 1 & 0x01 == 0x01, "LCD STAT");
+                    let _ = ui.selectable_label(irq >> 2 & 0x01 == 0x01, "Timer");
+                    let _ = ui.selectable_label(irq >> 3 & 0x01 == 0x01, "Serial");
+                    let _ = ui.selectable_label(irq >> 4 & 0x01 == 0x01, "Joypad");
+                });
+
+                ui.horizontal(|ui| {
+                    let ie = cpu.int_enable();
+
+                    // TODO: Reimplement this
+                    // let r_len = ctx.fonts().glyph_width(egui::TextStyle::Body, 'R');
+                    // let e_len = ctx.fonts().glyph_width(egui::TextStyle::Body, 'E');
+                    // let q_len = ctx.fonts().glyph_width(egui::TextStyle::Body, 'Q');
+
+                    ui.label("IE:");
+                    // ui.add_space(q_len - (e_len - r_len));
+                    let _ = ui.selectable_label(ie & 0b01 == 0x01, "VBlank");
+                    let _ = ui.selectable_label(ie >> 1 & 0x01 == 0x01, "LCD STAT");
+                    let _ = ui.selectable_label(ie >> 2 & 0x01 == 0x01, "Timer");
+                    let _ = ui.selectable_label(ie >> 3 & 0x01 == 0x01, "Serial");
+                    let _ = ui.selectable_label(ie >> 4 & 0x01 == 0x01, "Joypad");
+                });
+
+                ui.add_space(10.0);
+
+                ui.horizontal(|ui| {
+                    let flags = cpu::dbg::flags(cpu);
+
+                    let _ = ui.selectable_label((flags >> 7 & 0x01) == 0x01, "Zero");
+                    let _ = ui.selectable_label((flags >> 6 & 0x01) == 0x01, "Negative");
+                    let _ = ui.selectable_label((flags >> 5 & 0x01) == 0x01, "Half-Carry");
+                    let _ = ui.selectable_label((flags >> 4 & 0x01) == 0x01, "Carry");
+                })
+            })
+        });
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum EmuMode {
+    Running,
+    StepFrame,
+    Stopped,
+    Step,
+}
+
+/// To avoid using an [Option<KeyboardInput>] to keep track of user input from winit,
+/// we can use a "default" value. However, in order for this to work the chosen "default"
+/// value must be an **unused** key, so that it is ignored by the emulator.
+pub fn unused_key() -> KeyboardInput {
+    #![allow(deprecated)]
+
+    KeyboardInput {
+        scancode: Default::default(),
+        state: ElementState::Released,
+        virtual_keycode: Default::default(),
+        modifiers: Default::default(), // this argument is deprecated
+    }
+}
+
+fn build_window<T>(event_loop: &EventLoop<T>) -> Result<Window, OsError> {
+    use winit::window::WindowBuilder;
+
+    WindowBuilder::new()
+        .with_decorations(true)
+        .with_resizable(true)
+        .with_transparent(false)
+        .with_title(WINDOW_TITLE)
+        .with_inner_size(PhysicalSize {
+            width: EGUI_DIMENSIONS.0 as f32,
+            height: EGUI_DIMENSIONS.1 as f32,
+        })
+        .build(event_loop)
+}
+
+fn request_adapter(instance: &Instance, surface: &Surface) -> Option<Adapter> {
+    use wgpu::{PowerPreference, RequestAdapterOptions};
+
+    pollster::block_on(instance.request_adapter(&RequestAdapterOptions {
+        power_preference: PowerPreference::HighPerformance,
+        compatible_surface: Some(surface),
+        force_fallback_adapter: false, // TODO: What do I want to do with this?
+    }))
+}
+
+fn request_device(adapter: &Adapter) -> Result<(Device, Queue), RequestDeviceError> {
+    use wgpu::{DeviceDescriptor, Features, Limits};
+
+    pollster::block_on(adapter.request_device(
+        &DeviceDescriptor {
+            features: Features::default(),
+            limits: Limits::default(),
+            label: None,
+        },
+        None,
+    ))
+}
+
+fn surface_config(window: &Window, capabilities: SurfaceCapabilities) -> SurfaceConfiguration {
+    use egui_wgpu_backend::wgpu::{PresentMode, TextureFormat, TextureUsages};
+
+    let size = window.inner_size();
+    SurfaceConfiguration {
+        usage: TextureUsages::RENDER_ATTACHMENT,
+        format: capabilities.formats[0],
+        width: size.width as u32,
+        height: size.height as u32,
+        present_mode: PresentMode::Fifo,
+        alpha_mode: capabilities.alpha_modes[0],
+        view_formats: vec![TextureFormat::Rgba8UnormSrgb],
+    }
+}
+
+fn platform(window: &Window) -> Platform {
+    use egui::FontDefinitions;
+    use egui_winit_platform::PlatformDescriptor;
+
+    let size = window.inner_size();
+    Platform::new(PlatformDescriptor {
+        physical_width: size.width as u32,
+        physical_height: size.height as u32,
+        scale_factor: window.scale_factor(),
+        font_definitions: FontDefinitions::default(),
+        ..Default::default()
+    })
+}
+
+fn texture_size() -> Extent3d {
+    Extent3d {
+        width: GB_WIDTH as u32,
+        height: GB_HEIGHT as u32,
+        ..Default::default()
+    }
+}
+
+fn create_texture(device: &Device, size: Extent3d) -> Texture {
+    use wgpu::{TextureDescriptor, TextureDimension, TextureFormat, TextureUsages};
+
+    device.create_texture(&TextureDescriptor {
+        size,
+        mip_level_count: 1,
+        sample_count: 1,
+        dimension: TextureDimension::D2,
+        format: TextureFormat::Bgra8Unorm,
+        usage: TextureUsages::COPY_DST | TextureUsages::TEXTURE_BINDING,
+        label: Some("gb_pixel_buffer"),
+        view_formats: &[TextureFormat::Bgra8Unorm],
+    })
+}
+
+#[inline]
+fn write_to_texture(
+    queue: &Queue,
+    texture: &Texture,
+    data: &[u8; GB_WIDTH * 4 * GB_HEIGHT],
+    size: Extent3d,
+) {
+    use std::num::NonZeroU32;
+    use wgpu::{ImageCopyTexture, ImageDataLayout, Origin3d, TextureAspect};
+
+    queue.write_texture(
+        ImageCopyTexture {
+            texture,
+            mip_level: 0,
+            origin: Origin3d::ZERO,
+            aspect: TextureAspect::All,
+        },
+        data,
+        ImageDataLayout {
+            offset: 0,
+            bytes_per_row: NonZeroU32::new(4 * GB_WIDTH as u32),
+            rows_per_image: NonZeroU32::new(GB_HEIGHT as u32),
+        },
+        size,
+    );
+}
+
+fn expose_texture_to_egui(
+    render_pass: &mut RenderPass,
+    device: &Device,
+    view: &TextureView,
+) -> TextureId {
+    render_pass.egui_texture_from_wgpu_texture(device, view, FILTER_MODE)
+}
+
+#[inline]
+fn create_command_encoder(device: &Device) -> CommandEncoder {
+    use wgpu::CommandEncoderDescriptor;
+
+    device.create_command_encoder(&CommandEncoderDescriptor {
+        label: Some("encoder"),
+    })
+}
+
+#[inline]
+fn create_screen_descriptor(window: &Window, config: &SurfaceConfiguration) -> ScreenDescriptor {
+    ScreenDescriptor {
+        physical_width: config.width,
+        physical_height: config.height,
+        scale_factor: window.scale_factor() as f32,
+    }
+}
+
+pub mod kbd {
+    use winit::event::{ElementState, KeyboardInput, VirtualKeyCode};
+
+    pub fn space_released(input: &KeyboardInput) -> bool {
+        let keycode = input.virtual_keycode;
+        matches!(input.state, ElementState::Released if keycode == Some(VirtualKeyCode::Space))
+    }
+}

src/high_ram.rs

@@ -17,11 +17,11 @@ impl Default for HighRam {
 }
 
 impl BusIo for HighRam {
-    fn write_byte(&mut self, addr: u16, byte: u8) {
-        self.buf[addr as usize - HIGH_RAM_START_ADDRESS] = byte;
-    }
-
     fn read_byte(&self, addr: u16) -> u8 {
         self.buf[addr as usize - HIGH_RAM_START_ADDRESS]
     }
+
+    fn write_byte(&mut self, addr: u16, byte: u8) {
+        self.buf[addr as usize - HIGH_RAM_START_ADDRESS] = byte;
+    }
 }

src/instruction.rs

@@ -1,6 +1,6 @@
 use self::add::{Source as AddSource, Target as AddTarget};
 use self::alu::Source as AluSource;
-use self::jump::{JumpCondition, JumpLocation};
+use self::jump::{JpCond, JpLoc};
 use self::load::{Source as LDSource, Target as LDTarget};
 use self::table::{
     alu_imm_instr, alu_reg_instr, flag_instr, group1, group2, group3, jump_cond, prefix_alu,
@@ -16,7 +16,7 @@ use crate::Cycle;
 pub(crate) enum Instruction {
     NOP,
     STOP,
-    JR(JumpCondition),
+    JR(JpCond),
     LD(LDTarget, LDSource),
     ADD(AddTarget, AddSource),
     LDHL,
@@ -38,13 +38,13 @@ pub(crate) enum Instruction {
     XOR(AluSource),
     OR(AluSource),
     CP(AluSource),
-    RET(JumpCondition),
+    RET(JpCond),
     POP(Group3RegisterPair),
     RETI,
-    JP(JumpCondition, JumpLocation),
+    JP(JpCond, JpLoc),
     DI,
     EI,
-    CALL(JumpCondition),
+    CALL(JpCond),
     PUSH(Group3RegisterPair),
     RST(u8),
     RLC(Register),
@@ -58,6 +58,7 @@ pub(crate) enum Instruction {
     BIT(u8, Register),
     RES(u8, Register),
     SET(u8, Register),
+    Invalid,
 }
 
 impl std::fmt::Debug for Instruction {
@@ -109,6 +110,7 @@ impl std::fmt::Debug for Instruction {
             BIT(b, r) => write!(f, "BIT {}, {:?}", b, r),
             RES(b, r) => write!(f, "RES {}, {:?}", b, r),
             SET(b, r) => write!(f, "SET {}, {:?}", b, r),
+            Invalid => f.write_str("???"),
         }
     }
 }
@@ -116,7 +118,7 @@ impl std::fmt::Debug for Instruction {
 impl Instruction {
     pub(crate) fn execute(cpu: &mut Cpu, instruction: Self) -> Cycle {
         match instruction {
-            Instruction::NOP => (4),
+            Instruction::NOP => 4,
             Instruction::LD(target, src) => match (target, src) {
                 (LDTarget::IndirectImmediateWord, LDSource::SP) => {
                     // LD (u16), SP | Store stack pointer in byte at 16-bit register
@@ -293,7 +295,7 @@ impl Instruction {
                 let addr = pc.wrapping_add(byte as u16);
 
                 match cond {
-                    JumpCondition::NotZero => {
+                    JpCond::NotZero => {
                         if !flags.z() {
                             Self::jump(cpu, addr);
                             12
@@ -301,7 +303,7 @@ impl Instruction {
                             8
                         }
                     }
-                    JumpCondition::Zero => {
+                    JpCond::Zero => {
                         if flags.z() {
                             Self::jump(cpu, addr);
                             12
@@ -309,7 +311,7 @@ impl Instruction {
                             8
                         }
                     }
-                    JumpCondition::NotCarry => {
+                    JpCond::NotCarry => {
                         if !flags.c() {
                             Self::jump(cpu, addr);
                             12
@@ -317,7 +319,7 @@ impl Instruction {
                             8
                         }
                     }
-                    JumpCondition::Carry => {
+                    JpCond::Carry => {
                         if flags.c() {
                             Self::jump(cpu, addr);
                             12
@@ -325,7 +327,7 @@ impl Instruction {
                             8
                         }
                     }
-                    JumpCondition::Always => {
+                    JpCond::Always => {
                         Self::jump(cpu, addr);
                         12
                     }
@@ -334,7 +336,6 @@ impl Instruction {
             Instruction::ADD(target, src) => match (target, src) {
                 (AddTarget::HL, AddSource::Group1(pair)) => {
                     // ADD HL, r16 | Add 16-bit register to HL
-                    // FIXME: Memory Timings are not properly emulated for this instruction
                     use Group1RegisterPair::*;
                     let mut flags: Flags = *cpu.flags();
 
@@ -342,10 +343,11 @@ impl Instruction {
                         BC | DE | HL | SP => {
                             let left = cpu.register_pair(RegisterPair::HL);
                             let right = cpu.register_pair(pair.as_register_pair());
-                            cpu.set_register_pair(
+                            let result = Self::add_u16(left, right, &mut flags);
-                                RegisterPair::HL,
+
-                                Self::add_u16(left, right, &mut flags),
+                            cpu.set_register(CpuRegister::L, result as u8);
-                            );
+                            cpu.bus.clock();
+                            cpu.set_register(CpuRegister::H, (result >> 8) as u8);
                         }
                     }
                     cpu.set_flags(flags);
@@ -361,12 +363,12 @@ impl Instruction {
                     let (cycles, sum) = match reg {
                         B | C | D | E | H | L | A => {
                             let right = cpu.register(reg.cpu_register());
-                            ((4), Self::add(left, right, &mut flags))
+                            (4, Self::add(left, right, &mut flags))
                         }
                         IndirectHL => {
                             let addr = cpu.register_pair(RegisterPair::HL);
                             let right = Self::read_byte(&mut cpu.bus, addr);
-                            ((8), Self::add(left, right, &mut flags))
+                            (8, Self::add(left, right, &mut flags))
                         }
                     };
 
@@ -376,12 +378,15 @@ impl Instruction {
                 }
                 (AddTarget::SP, AddSource::ImmediateSignedByte) => {
                     // ADD SP, i8 | Add i8 to stack pointer
-                    // FIXME: Memory Timings are not properly emulated for this instruction
                     let mut flags: Flags = *cpu.flags();
 
                     let left = cpu.register_pair(RegisterPair::SP);
                     let sum = Self::add_u16_i8(left, Self::imm_byte(cpu) as i8, &mut flags);
+                    cpu.bus.clock(); // internal
+
                     cpu.set_register_pair(RegisterPair::SP, sum);
+                    cpu.bus.clock();
+
                     cpu.set_flags(flags);
                     16
                 }
@@ -423,7 +428,6 @@ impl Instruction {
                     AllRegisters::Group1(pair) => {
                         // INC r16 | Increment 16-bit register
                         // Note: No flags are set with this version of the INC instruction
-                        // FIXME: Memory Timings are not properly emulated for this instruction
                         use Group1RegisterPair::*;
 
                         match pair {
@@ -431,6 +435,7 @@ impl Instruction {
                                 let pair = pair.as_register_pair();
                                 let left = cpu.register_pair(pair);
                                 cpu.set_register_pair(pair, left.wrapping_add(1));
+                                cpu.bus.clock(); // internal
                             }
                         }
                         8
@@ -462,7 +467,6 @@ impl Instruction {
                     }
                     AllRegisters::Group1(pair) => {
                         // DEC r16 | Decrement Register Pair
-                        // FIXME: Memory Timings are not properly emulated for this instruction
                         use Group1RegisterPair::*;
 
                         match pair {
@@ -470,6 +474,7 @@ impl Instruction {
                                 let pair = pair.as_register_pair();
                                 let left = cpu.register_pair(pair);
                                 cpu.set_register_pair(pair, left.wrapping_sub(1));
+                                cpu.bus.clock(); // internal
                             }
                         };
                         8
@@ -610,13 +615,13 @@ impl Instruction {
                         B | C | D | E | H | L | A => {
                             let right = cpu.register(reg.cpu_register());
                             let sum = Self::add_with_carry_bit(left, right, flags.c(), &mut flags);
-                            ((4), sum)
+                            (4, sum)
                         }
                         IndirectHL => {
                             let addr = cpu.register_pair(RegisterPair::HL);
                             let right = Self::read_byte(&mut cpu.bus, addr);
                             let sum = Self::add_with_carry_bit(left, right, flags.c(), &mut flags);
-                            ((8), sum)
+                            (8, sum)
                         }
                     };
                     cpu.set_register(CpuRegister::A, sum);
@@ -646,12 +651,12 @@ impl Instruction {
                     let (cycles, diff) = match reg {
                         B | C | D | E | H | L | A => {
                             let right = cpu.register(reg.cpu_register());
-                            ((4), Self::sub(left, right, &mut flags))
+                            (4, Self::sub(left, right, &mut flags))
                         }
                         IndirectHL => {
                             let addr = cpu.register_pair(RegisterPair::HL);
                             let right = Self::read_byte(&mut cpu.bus, addr);
-                            ((8), Self::sub(left, right, &mut flags))
+                            (8, Self::sub(left, right, &mut flags))
                         }
                     };
                     cpu.set_register(CpuRegister::A, diff);
@@ -681,13 +686,13 @@ impl Instruction {
                         B | C | D | E | H | L | A => {
                             let right = cpu.register(reg.cpu_register());
                             let diff = Self::sub_with_carry(left, right, flags.c(), &mut flags);
-                            ((4), diff)
+                            (4, diff)
                         }
                         IndirectHL => {
                             let addr = cpu.register_pair(RegisterPair::HL);
                             let right = Self::read_byte(&mut cpu.bus, addr);
                             let diff = Self::sub_with_carry(left, right, flags.c(), &mut flags);
-                            ((8), diff)
+                            (8, diff)
                         }
                     };
                     cpu.set_register(CpuRegister::A, diff);
@@ -717,7 +722,7 @@ impl Instruction {
                         IndirectHL => {
                             let addr = cpu.register_pair(RegisterPair::HL);
                             let right = Self::read_byte(&mut cpu.bus, addr);
-                            ((8), left & right)
+                            (8, left & right)
                         }
                     };
                     cpu.set_register(CpuRegister::A, acc);
@@ -743,7 +748,7 @@ impl Instruction {
                         IndirectHL => {
                             let addr = cpu.register_pair(RegisterPair::HL);
                             let right = Self::read_byte(&mut cpu.bus, addr);
-                            ((8), left ^ right)
+                            (8, left ^ right)
                         }
                     };
                     cpu.set_register(CpuRegister::A, acc);
@@ -769,7 +774,7 @@ impl Instruction {
                         IndirectHL => {
                             let addr = cpu.register_pair(RegisterPair::HL);
                             let right = Self::read_byte(&mut cpu.bus, addr);
-                            ((8), left | right)
+                            (8, left | right)
                         }
                     };
                     cpu.set_register(CpuRegister::A, acc);
@@ -834,7 +839,7 @@ impl Instruction {
                 let flags: Flags = *cpu.flags();
 
                 match cond {
-                    JumpCondition::NotZero => {
+                    JpCond::NotZero => {
                         cpu.bus.clock(); // internal branch decision
 
                         if !flags.z() {
@@ -845,7 +850,7 @@ impl Instruction {
                             8
                         }
                     }
-                    JumpCondition::Zero => {
+                    JpCond::Zero => {
                         cpu.bus.clock(); // internal branch decision
 
                         if flags.z() {
@@ -856,7 +861,7 @@ impl Instruction {
                             8
                         }
                     }
-                    JumpCondition::NotCarry => {
+                    JpCond::NotCarry => {
                         cpu.bus.clock(); // internal branch decision
 
                         if !flags.c() {
@@ -867,7 +872,7 @@ impl Instruction {
                             8
                         }
                     }
-                    JumpCondition::Carry => {
+                    JpCond::Carry => {
                         cpu.bus.clock(); // internal branch decision
 
                         if flags.c() {
@@ -878,7 +883,7 @@ impl Instruction {
                             8
                         }
                     }
-                    JumpCondition::Always => {
+                    JpCond::Always => {
                         let addr = Self::pop(cpu);
                         Self::jump(cpu, addr);
                         16
@@ -905,13 +910,13 @@ impl Instruction {
                 16
             }
             Instruction::JP(cond, location) => match location {
-                JumpLocation::HL => {
+                JpLoc::HL => {
                     // JP HL | Store HL in program counter
                     let right = cpu.register_pair(RegisterPair::HL);
                     cpu.set_register_pair(RegisterPair::PC, right);
                     4
                 }
-                JumpLocation::ImmediateWord => {
+                JpLoc::ImmediateWord => {
                     // JP cond u16 | Store u16 in program counter if condition is true
                     // JP u16      | Store u16 in program counter
                     let flags: Flags = *cpu.flags();
@@ -919,7 +924,7 @@ impl Instruction {
                     let addr = Self::imm_word(cpu);
 
                     match cond {
-                        JumpCondition::NotZero => {
+                        JpCond::NotZero => {
                             if !flags.z() {
                                 Self::jump(cpu, addr);
                                 16
@@ -927,7 +932,7 @@ impl Instruction {
                                 12
                             }
                         }
-                        JumpCondition::Zero => {
+                        JpCond::Zero => {
                             if flags.z() {
                                 Self::jump(cpu, addr);
                                 16
@@ -935,7 +940,7 @@ impl Instruction {
                                 12
                             }
                         }
-                        JumpCondition::NotCarry => {
+                        JpCond::NotCarry => {
                             if !flags.c() {
                                 Self::jump(cpu, addr);
                                 16
@@ -943,7 +948,7 @@ impl Instruction {
                                 12
                             }
                         }
-                        JumpCondition::Carry => {
+                        JpCond::Carry => {
                             if flags.c() {
                                 Self::jump(cpu, addr);
                                 16
@@ -951,7 +956,7 @@ impl Instruction {
                                 12
                             }
                         }
-                        JumpCondition::Always => {
+                        JpCond::Always => {
                             Self::jump(cpu, addr);
                             16
                         }
@@ -977,7 +982,7 @@ impl Instruction {
                 let return_addr = cpu.register_pair(RegisterPair::PC);
 
                 match cond {
-                    JumpCondition::NotZero => {
+                    JpCond::NotZero => {
                         if !flags.z() {
                             cpu.bus.clock(); // internal branch decision
                             Self::push(cpu, return_addr);
@@ -987,7 +992,7 @@ impl Instruction {
                             12
                         }
                     }
-                    JumpCondition::Zero => {
+                    JpCond::Zero => {
                         if flags.z() {
                             cpu.bus.clock(); // internal branch decision
                             Self::push(cpu, return_addr);
@@ -997,7 +1002,7 @@ impl Instruction {
                             12
                         }
                     }
-                    JumpCondition::NotCarry => {
+                    JpCond::NotCarry => {
                         if !flags.c() {
                             cpu.bus.clock(); // internal branch decision
                             Self::push(cpu, return_addr);
@@ -1007,7 +1012,7 @@ impl Instruction {
                             12
                         }
                     }
-                    JumpCondition::Carry => {
+                    JpCond::Carry => {
                         if flags.c() {
                             cpu.bus.clock(); // internal branch decision
                             Self::push(cpu, return_addr);
@@ -1017,7 +1022,7 @@ impl Instruction {
                             12
                         }
                     }
-                    JumpCondition::Always => {
+                    JpCond::Always => {
                         cpu.bus.clock(); // internal branch decision
                         Self::push(cpu, return_addr);
                         cpu.set_register_pair(RegisterPair::PC, addr);
@@ -1053,14 +1058,14 @@ impl Instruction {
                         let byte = cpu.register(reg);
                         let rotated = byte.rotate_left(1);
                         cpu.set_register(reg, rotated);
-                        ((8), byte >> 7, rotated)
+                        (8, byte >> 7, rotated)
                     }
                     IndirectHL => {
                         let addr = cpu.register_pair(RegisterPair::HL);
                         let byte = Self::read_byte(&mut cpu.bus, addr);
                         let rotated = byte.rotate_left(1);
                         Self::write_byte(&mut cpu.bus, addr, rotated);
-                        ((16), byte >> 7, rotated)
+                        (16, byte >> 7, rotated)
                     }
                 };
                 cpu.update_flags(rotated == 0, false, false, most_sgfnt == 0x01);
@@ -1076,14 +1081,14 @@ impl Instruction {
                         let byte = cpu.register(reg);
                         let rotated = byte.rotate_right(1);
                         cpu.set_register(reg, rotated);
-                        ((8), byte & 0x01, rotated)
+                        (8, byte & 0x01, rotated)
                     }
                     IndirectHL => {
                         let addr = cpu.register_pair(RegisterPair::HL);
                         let byte = Self::read_byte(&mut cpu.bus, addr);
                         let rotated = byte.rotate_right(1);
                         Self::write_byte(&mut cpu.bus, addr, rotated);
-                        ((16), byte & 0x01, rotated)
+                        (16, byte & 0x01, rotated)
                     }
                 };
                 cpu.update_flags(rotated == 0, false, false, least_sgfnt == 0x01);
@@ -1101,14 +1106,14 @@ impl Instruction {
                         let byte = cpu.register(reg);
                         let (rotated, carry) = Self::rl_thru_carry(byte, flags.c());
                         cpu.set_register(reg, rotated);
-                        ((8), rotated, carry)
+                        (8, rotated, carry)
                     }
                     IndirectHL => {
                         let addr = cpu.register_pair(RegisterPair::HL);
                         let byte = Self::read_byte(&mut cpu.bus, addr);
                         let (rotated, carry) = Self::rl_thru_carry(byte, flags.c());
                         Self::write_byte(&mut cpu.bus, addr, rotated);
-                        ((16), rotated, carry)
+                        (16, rotated, carry)
                     }
                 };
                 cpu.update_flags(rotated == 0, false, false, carry);
@@ -1126,14 +1131,14 @@ impl Instruction {
                         let byte = cpu.register(reg);
                         let (rotated, carry) = Self::rr_thru_carry(byte, flags.c());
                         cpu.set_register(reg, rotated);
-                        ((8), rotated, carry)
+                        (8, rotated, carry)
                     }
                     IndirectHL => {
                         let addr = cpu.register_pair(RegisterPair::HL);
                         let byte = Self::read_byte(&mut cpu.bus, addr);
                         let (rotated, carry) = Self::rr_thru_carry(byte, flags.c());
                         Self::write_byte(&mut cpu.bus, addr, rotated);
-                        ((16), rotated, carry)
+                        (16, rotated, carry)
                     }
                 };
                 cpu.update_flags(rotated == 0, false, false, carry);
@@ -1149,14 +1154,14 @@ impl Instruction {
                         let byte = cpu.register(reg);
                         let shifted = byte << 1;
                         cpu.set_register(reg, shifted);
-                        ((8), (byte >> 7) & 0x01, shifted)
+                        (8, (byte >> 7) & 0x01, shifted)
                     }
                     IndirectHL => {
                         let addr = cpu.register_pair(RegisterPair::HL);
                         let byte = Self::read_byte(&mut cpu.bus, addr);
                         let shifted = byte << 1;
                         Self::write_byte(&mut cpu.bus, addr, shifted);
-                        ((16), (byte >> 7) & 0x01, shifted)
+                        (16, (byte >> 7) & 0x01, shifted)
                     }
                 };
                 cpu.update_flags(shifted == 0, false, false, most_sgfnt == 0x01);
@@ -1172,14 +1177,14 @@ impl Instruction {
                         let byte = cpu.register(reg);
                         let shifted = ((byte >> 7) & 0x01) << 7 | byte >> 1;
                         cpu.set_register(reg, shifted);
-                        ((8), byte & 0x01, shifted)
+                        (8, byte & 0x01, shifted)
                     }
                     IndirectHL => {
                         let addr = cpu.register_pair(RegisterPair::HL);
                         let byte = Self::read_byte(&mut cpu.bus, addr);
                         let shifted = ((byte >> 7) & 0x01) << 7 | byte >> 1;
                         Self::write_byte(&mut cpu.bus, addr, shifted);
-                        ((16), byte & 0x01, shifted)
+                        (16, byte & 0x01, shifted)
                     }
                 };
                 cpu.update_flags(shifted == 0, false, false, least_sgfnt == 0x01);
@@ -1194,14 +1199,14 @@ impl Instruction {
                         let reg = reg.cpu_register();
                         let swapped = Self::swap_bits(cpu.register(reg));
                         cpu.set_register(reg, swapped);
-                        ((8), swapped)
+                        (8, swapped)
                     }
 
                     IndirectHL => {
                         let addr = cpu.register_pair(RegisterPair::HL);
                         let swapped = Self::swap_bits(Self::read_byte(&mut cpu.bus, addr));
                         Self::write_byte(&mut cpu.bus, addr, swapped);
-                        ((16), swapped)
+                        (16, swapped)
                     }
                 };
                 cpu.update_flags(swapped == 0, false, false, false);
@@ -1217,14 +1222,14 @@ impl Instruction {
                         let byte = cpu.register(reg);
                         let shifted = byte >> 1;
                         cpu.set_register(reg, shifted);
-                        ((8), byte & 0x01, shifted)
+                        (8, byte & 0x01, shifted)
                     }
                     IndirectHL => {
                         let addr = cpu.register_pair(RegisterPair::HL);
                         let byte = Self::read_byte(&mut cpu.bus, addr);
                         let shifted = byte >> 1;
                         Self::write_byte(&mut cpu.bus, addr, shifted);
-                        ((16), byte & 0x01, shifted)
+                        (16, byte & 0x01, shifted)
                     }
                 };
                 cpu.update_flags(shift_reg == 0, false, false, least_sgfnt == 0x01);
@@ -1239,12 +1244,12 @@ impl Instruction {
                     B | C | D | E | H | L | A => {
                         let reg = reg.cpu_register();
                         let byte = cpu.register(reg);
-                        ((8), ((byte >> bit) & 0x01) == 0x01)
+                        (8, ((byte >> bit) & 0x01) == 0x01)
                     }
                     IndirectHL => {
                         let addr = cpu.register_pair(RegisterPair::HL);
                         let byte = Self::read_byte(&mut cpu.bus, addr);
-                        ((12), ((byte >> bit) & 0x01) == 0x01)
+                        (12, ((byte >> bit) & 0x01) == 0x01)
                     }
                 };
                 flags.set_z(!is_set);
@@ -1291,6 +1296,7 @@ impl Instruction {
                     }
                 }
             }
+            Instruction::Invalid => panic!("Attempted to execute invalid instruction"),
         }
     }
 
@@ -1554,7 +1560,7 @@ impl Instruction {
             // STOP
             0o020 => STOP,
             // JR i8
-            0o030 => JR(JumpCondition::Always),
+            0o030 => JR(JpCond::Always),
             // JR cond i8
             0o040 | 0o050 | 0o060 | 0o070 => JR(jump_cond((byte >> 3) & 0x03)),
             // LD r16, u16
@@ -1619,16 +1625,16 @@ impl Instruction {
             // POP r16
             0o301 | 0o321 | 0o341 | 0o361 => POP(group3((byte >> 4) & 0x03)),
             // RET
-            0o311 => RET(JumpCondition::Always),
+            0o311 => RET(JpCond::Always),
             // RETI
             0o331 => RETI,
             // JP HL
-            0o351 => JP(JumpCondition::Always, JumpLocation::HL),
+            0o351 => JP(JpCond::Always, JpLoc::HL),
             // LD SP, HL
             0o371 => LD(LDTarget::SP, LDSource::HL),
             // JP cond u16
             0o302 | 0o312 | 0o322 | 0o332 => {
-                JP(jump_cond((byte >> 3) & 0x03), JumpLocation::ImmediateWord)
+                JP(jump_cond((byte >> 3) & 0x03), JpLoc::ImmediateWord)
             }
             // LD (0xFF00 + C), A
             0o342 => LD(LDTarget::IoWithC, LDSource::A),
@@ -1639,9 +1645,7 @@ impl Instruction {
             // LD A, (u16)
             0o372 => LD(LDTarget::A, LDSource::IndirectImmediateWord),
             // JP u16
-            0o303 => JP(JumpCondition::Always, JumpLocation::ImmediateWord),
+            0o303 => JP(JpCond::Always, JpLoc::ImmediateWord),
-            // 0xCB Prefix
-            0o313 => unreachable!("{:#04X} should be handled by the prefixed decoder", byte),
             // DI
             0o363 => DI,
             // EI
@@ -1650,12 +1654,14 @@ impl Instruction {
             0o304 | 0o314 | 0o324 | 0o334 => CALL(jump_cond((byte >> 3) & 0x03)),
             // PUSH r16
             0o305 | 0o325 | 0o345 | 0o365 => PUSH(group3((byte >> 4) & 0x03)),
-            0o315 => CALL(JumpCondition::Always),
+            // CALL u16
+            0o315 => CALL(JpCond::Always),
+            // ADD, ADC, SUB, SBC, AND, XOR, OR, and CP
             0o306 | 0o316 | 0o326 | 0o336 | 0o346 | 0o356 | 0o366 | 0o376 => {
                 alu_imm_instr((byte >> 3) & 0x07)
             }
             0o307 | 0o317 | 0o327 | 0o337 | 0o347 | 0o357 | 0o367 | 0o377 => RST(byte & 0b00111000),
-            _ => panic!("{:#04X} is an illegal opcode", byte),
+            _ => Invalid, // 0xCB is 0o313
         }
     }
 
@@ -1678,7 +1684,7 @@ impl Instruction {
 mod jump {
 
     #[derive(Clone, Copy)]
-    pub(crate) enum JumpCondition {
+    pub(crate) enum JpCond {
         Always,
         NotZero,
         Zero,
@@ -1686,9 +1692,9 @@ mod jump {
         Carry,
     }
 
-    impl std::fmt::Debug for JumpCondition {
+    impl std::fmt::Debug for JpCond {
         fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-            use JumpCondition::*;
+            use JpCond::*;
 
             match self {
                 Always => f.write_str(""),
@@ -1701,14 +1707,14 @@ mod jump {
     }
 
     #[derive(Clone, Copy)]
-    pub(crate) enum JumpLocation {
+    pub(crate) enum JpLoc {
         HL,
         ImmediateWord,
     }
 
-    impl std::fmt::Debug for JumpLocation {
+    impl std::fmt::Debug for JpLoc {
         fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-            use JumpLocation::*;
+            use JpLoc::*;
 
             match *self {
                 HL => f.write_str("HL"),
@@ -1868,7 +1874,7 @@ mod load {
 mod table {
     use super::add::{Source as AddSource, Target as AddTarget};
     use super::alu::Source as AluSource;
-    use super::{Instruction, JumpCondition};
+    use super::{Instruction, JpCond};
     use crate::cpu::{Register as CpuRegister, RegisterPair};
 
     #[derive(Clone, Copy)]
@@ -1893,7 +1899,7 @@ mod table {
     }
 
     impl Group1RegisterPair {
-        pub fn as_register_pair(&self) -> RegisterPair {
+        pub(crate) fn as_register_pair(&self) -> RegisterPair {
             use Group1RegisterPair::*;
 
             match self {
@@ -1927,7 +1933,7 @@ mod table {
     }
 
     impl Group2RegisterPair {
-        pub fn as_register_pair(&self) -> RegisterPair {
+        pub(crate) fn as_register_pair(&self) -> RegisterPair {
             use Group2RegisterPair::*;
 
             match self {
@@ -1961,7 +1967,7 @@ mod table {
     }
 
     impl Group3RegisterPair {
-        pub fn as_register_pair(&self) -> RegisterPair {
+        pub(crate) fn as_register_pair(&self) -> RegisterPair {
             use Group3RegisterPair::*;
 
             match self {
@@ -2003,7 +2009,7 @@ mod table {
     }
 
     impl Register {
-        pub fn cpu_register(&self) -> CpuRegister {
+        pub(crate) fn cpu_register(&self) -> CpuRegister {
             use Register::*;
 
             match self {
@@ -2067,12 +2073,12 @@ mod table {
             0b101 => L,
             0b110 => IndirectHL,
             0b111 => A,
-            _ => unreachable!("{:#04X} is not a valid Register"),
+            _ => unreachable!("{:#04X} is not a valid Register", code),
         }
     }
 
-    pub(crate) fn jump_cond(code: u8) -> JumpCondition {
+    pub(crate) fn jump_cond(code: u8) -> JpCond {
-        use JumpCondition::*;
+        use JpCond::*;
 
         match code {
             0b00 => NotZero,
@@ -2147,3 +2153,268 @@ mod table {
         }
     }
 }
+
+pub(crate) mod dbg {
+    use std::borrow::Cow;
+
+    use super::add::{Source as AddSource, Target as AddTarget};
+    use super::alu::Source as AluSource;
+    use super::jump::{JpCond, JpLoc};
+    use super::load::{Source as LDSource, Target as LDTarget};
+    use super::{AllRegisters, BusIo, Cpu, Instruction, RegisterPair};
+
+    pub(crate) fn tmp_disasm(cpu: &Cpu, limit: u8) -> String {
+        let mut sm83_asm = String::new();
+        let mut pc = cpu.register_pair(RegisterPair::PC);
+
+        for _ in 0..limit {
+            let opcode = cpu.read_byte(pc);
+            pc += 1;
+
+            let maybe_instr = if opcode == 0xCB {
+                let opcode = cpu.read_byte(pc);
+                pc += 1;
+
+                Instruction::prefixed(opcode)
+            } else {
+                Instruction::unprefixed(opcode)
+            };
+
+            match maybe_instr {
+                Instruction::Invalid => {}
+                instr => {
+                    let asm = format!("{:04X} {:?}\n", pc - 1, instr);
+                    sm83_asm.push_str(&asm);
+                    pc += delta::pc_inc_count(instr)
+                }
+            }
+        }
+
+        sm83_asm
+    }
+
+    pub(crate) fn new_disasm(cpu: &Cpu, limit: u8) -> String {
+        let mut assembly = String::new();
+        let mut pc = cpu.register_pair(RegisterPair::PC);
+
+        for _ in 0..limit {
+            let opcode = cpu.read_byte(pc);
+            pc += 1;
+
+            let maybe_instr = if opcode == 0xCB {
+                let opcode = cpu.read_byte(pc);
+                pc += 1;
+
+                Instruction::prefixed(opcode)
+            } else {
+                Instruction::unprefixed(opcode)
+            };
+
+            match maybe_instr {
+                Instruction::Invalid => {}
+                instr => {
+                    let output = format!("${:04X} {}\n", pc - 1, disasm(cpu, pc, instr));
+                    assembly.push_str(&output);
+                    pc += delta::pc_inc_count(instr);
+                }
+            }
+        }
+
+        assembly
+    }
+
+    // TODO: It might be better if I pass in a mutable writer instead of usnig a String
+    fn disasm(cpu: &Cpu, pc: u16, instr: Instruction) -> Cow<str> {
+        use Instruction::*;
+
+        let imm_byte = cpu.read_byte(pc + 1);
+        let imm_word = (cpu.read_byte(pc + 2) as u16) << 8 | imm_byte as u16;
+
+        match instr {
+            // Unprefixed Instructions
+            NOP => "NOP".into(),
+            LD(LDTarget::IndirectImmediateWord, LDSource::SP) => {
+                format!("LD ({:#06X}), SP", imm_word).into()
+            }
+            STOP => "STOP".into(),
+            JR(JpCond::Always) => format!("JR {}", imm_byte as i8).into(),
+            JR(cond) => format!("JR {:?} {}", cond, imm_byte as i8).into(),
+            LD(LDTarget::Group1(rp), LDSource::ImmediateWord) => {
+                format!("LD {:?} {:#06X}", rp, imm_word).into()
+            }
+            ADD(AddTarget::HL, AddSource::Group1(rp)) => format!("ADD HL, {:?}", rp).into(),
+            LD(LDTarget::IndirectGroup2(rp), LDSource::A) => format!("LD ({:?}), A", rp).into(),
+            LD(LDTarget::A, LDSource::IndirectGroup2(rp)) => format!("LD A, ({:?})", rp).into(),
+            INC(AllRegisters::Group1(rp)) => format!("INC {:?}", rp).into(),
+            DEC(AllRegisters::Group1(rp)) => format!("DEC {:?}", rp).into(),
+            INC(AllRegisters::Register(reg)) => format!("INC {:?}", reg).into(),
+            DEC(AllRegisters::Register(reg)) => format!("DEC {:?}", reg).into(),
+            LD(LDTarget::Register(reg), LDSource::ImmediateByte) => {
+                format!("LD {:?}, {:#04X}", reg, imm_byte).into()
+            }
+            RLCA => "RLCA".into(),
+            RRCA => "RRCA".into(),
+            RLA => "RLA".into(),
+            RRA => "RRA".into(),
+            DAA => "DAA".into(),
+            CPL => "CPL".into(),
+            SCF => "SCF".into(),
+            CCF => "CCF".into(),
+            HALT => "HALT".into(),
+            LD(LDTarget::Register(left), LDSource::Register(right)) => {
+                format!("LD {:?}, {:?}", left, right).into()
+            }
+            ADD(AddTarget::A, AddSource::Register(reg)) => format!("ADD A, {:?}", reg).into(),
+            ADC(AluSource::Register(reg)) => format!("ADC {:?}", reg).into(),
+            SUB(AluSource::Register(reg)) => format!("SUB {:?}", reg).into(),
+            SBC(AluSource::Register(reg)) => format!("SBC {:?}", reg).into(),
+            AND(AluSource::Register(reg)) => format!("AND {:?}", reg).into(),
+            XOR(AluSource::Register(reg)) => format!("XOR {:?}", reg).into(),
+            OR(AluSource::Register(reg)) => format!("OR {:?}", reg).into(),
+            CP(AluSource::Register(reg)) => format!("CP {:?}", reg).into(),
+            RET(JpCond::Always) => "RET".into(),
+            RET(cond) => format!("RET {:?}", cond).into(),
+            LD(LDTarget::IoWithImmediateOffset, LDSource::A) => {
+                format!("LD ({:#06X}) , A", 0xFF00 + imm_byte as u16).into()
+            }
+            ADD(AddTarget::SP, AddSource::ImmediateSignedByte) => {
+                format!("ADD SP, {}", imm_byte as i8).into()
+            }
+            LD(LDTarget::A, LDSource::IoWithImmediateOffset) => {
+                format!("LD A, ({:#06X})", 0xFF00 + imm_byte as u16).into()
+            }
+            LDHL => format!("LD HL, SP + {}", imm_byte as i8).into(),
+            POP(rp) => format!("POP {:?}", rp).into(),
+            RETI => "RETI".into(),
+            JP(JpCond::Always, JpLoc::HL) => "JP HL".into(),
+            LD(LDTarget::SP, LDSource::HL) => "LD SP, HL".into(),
+            JP(JpCond::Always, JpLoc::ImmediateWord) => format!("JP {:#06X}", imm_word).into(),
+            JP(cond, JpLoc::ImmediateWord) => format!("JP {:?} {:#06X}", cond, imm_word).into(),
+            LD(LDTarget::IoWithC, LDSource::A) => "LD (0xFF00 + C), A".into(),
+            LD(LDTarget::IndirectImmediateWord, LDSource::A) => {
+                format!("LD ({:#06X}), A", imm_byte).into()
+            }
+            LD(LDTarget::A, LDSource::IoWithC) => "LD A, (0xFF00 + C)".into(),
+            LD(LDTarget::A, LDSource::IndirectImmediateWord) => {
+                format!("LD A, ({:#06X})", imm_word).into()
+            }
+            DI => "DI".into(),
+            EI => "EI".into(),
+            CALL(cond) => format!("CALL {:?} {:#06X}", cond, imm_word).into(),
+            PUSH(rp) => format!("PUSH {:?}", rp).into(),
+            ADD(AddTarget::A, AddSource::ImmediateByte) => {
+                format!("ADD A, {:#04X}", imm_byte).into()
+            }
+            ADC(AluSource::ImmediateByte) => format!("ADC {:#04X}", imm_byte).into(),
+            SUB(AluSource::ImmediateByte) => format!("SUB {:#04X}", imm_byte).into(),
+            SBC(AluSource::ImmediateByte) => format!("SBC {:#04X}", imm_byte).into(),
+            AND(AluSource::ImmediateByte) => format!("AND {:#04X}", imm_byte).into(),
+            XOR(AluSource::ImmediateByte) => format!("XOR {:#04X}", imm_byte).into(),
+            OR(AluSource::ImmediateByte) => format!("OR {:#04X}", imm_byte).into(),
+            CP(AluSource::ImmediateByte) => format!("CP {:#04X}", imm_byte).into(),
+            RST(v) => format!("RST {:#04X}", v).into(),
+
+            // Prefixed Instructions
+            RLC(reg) => format!("RLC {:?}", reg).into(),
+            RRC(reg) => format!("RRC {:?}", reg).into(),
+            RL(reg) => format!("RL {:?}", reg).into(),
+            RR(reg) => format!("RR {:?}", reg).into(),
+            SLA(reg) => format!("SLA {:?}", reg).into(),
+            SRA(reg) => format!("SRA {:?}", reg).into(),
+            SWAP(reg) => format!("SWAP {:?}", reg).into(),
+            SRL(reg) => format!("SRL {:?}", reg).into(),
+            BIT(bit, reg) => format!("BIT {}, {:?}", bit, reg).into(),
+            RES(bit, reg) => format!("RES {}, {:?}", bit, reg).into(),
+            SET(bit, reg) => format!("SET {}, {:?}", bit, reg).into(),
+            _ => unreachable!("{:?} is an illegal instruction", instr),
+        }
+    }
+
+    mod delta {
+        use super::super::add::{Source as AddSource, Target as AddTarget};
+        use super::super::alu::Source as AluSource;
+        use super::super::jump::{JpCond, JpLoc};
+        use super::super::load::{Source as LDSource, Target as LDTarget};
+        use super::super::{AllRegisters, Instruction};
+
+        pub(super) fn pc_inc_count(instr: Instruction) -> u16 {
+            use Instruction::*;
+
+            match instr {
+                // Unprefixed
+                NOP => 0,
+                LD(LDTarget::IndirectImmediateWord, LDSource::SP) => 2,
+                STOP => 0,
+                JR(_) => 1,
+                LD(LDTarget::Group1(_), LDSource::ImmediateWord) => 2,
+                ADD(AddTarget::HL, AddSource::Group1(_)) => 0,
+                LD(LDTarget::IndirectGroup2(_), LDSource::A) => 0,
+                LD(LDTarget::A, LDSource::IndirectGroup2(_)) => 0,
+                INC(AllRegisters::Group1(_)) => 0,
+                DEC(AllRegisters::Group1(_)) => 0,
+                INC(AllRegisters::Register(_)) => 0,
+                DEC(AllRegisters::Register(_)) => 0,
+                LD(LDTarget::Register(_), LDSource::ImmediateByte) => 1,
+                RLCA => 0,
+                RRCA => 0,
+                RLA => 0,
+                RRA => 0,
+                DAA => 0,
+                CPL => 0,
+                SCF => 0,
+                CCF => 0,
+                HALT => 0,
+                LD(LDTarget::Register(_), LDSource::Register(_)) => 0,
+                ADD(AddTarget::A, AddSource::Register(_)) => 0,
+                ADC(AluSource::Register(_)) => 0,
+                SUB(AluSource::Register(_)) => 0,
+                SBC(AluSource::Register(_)) => 0,
+                AND(AluSource::Register(_)) => 0,
+                XOR(AluSource::Register(_)) => 0,
+                OR(AluSource::Register(_)) => 0,
+                CP(AluSource::Register(_)) => 0,
+                RET(_) => 0,
+                LD(LDTarget::IoWithImmediateOffset, LDSource::A) => 1,
+                ADD(AddTarget::SP, AddSource::ImmediateSignedByte) => 1,
+                LD(LDTarget::A, LDSource::IoWithImmediateOffset) => 1,
+                LDHL => 1,
+                POP(_) => 0,
+                RETI => 0,
+                JP(JpCond::Always, JpLoc::HL) => 0,
+                LD(LDTarget::SP, LDSource::HL) => 0,
+                JP(_, JpLoc::ImmediateWord) => 2,
+                LD(LDTarget::IoWithC, LDSource::A) => 0,
+                LD(LDTarget::IndirectImmediateWord, LDSource::A) => 2,
+                LD(LDTarget::A, LDSource::IoWithC) => 0,
+                LD(LDTarget::A, LDSource::IndirectImmediateWord) => 2,
+                DI => 0,
+                EI => 0,
+                CALL(_) => 0,
+                PUSH(_) => 0,
+                ADD(AddTarget::A, AddSource::ImmediateByte) => 1,
+                ADC(AluSource::ImmediateByte) => 1,
+                SUB(AluSource::ImmediateByte) => 1,
+                SBC(AluSource::ImmediateByte) => 1,
+                AND(AluSource::ImmediateByte) => 1,
+                XOR(AluSource::ImmediateByte) => 1,
+                OR(AluSource::ImmediateByte) => 1,
+                CP(AluSource::ImmediateByte) => 1,
+                RST(_) => 0,
+
+                // Prefixed
+                RLC(_) => 0,
+                RRC(_) => 0,
+                RL(_) => 0,
+                RR(_) => 0,
+                SLA(_) => 0,
+                SRA(_) => 0,
+                SWAP(_) => 0,
+                SRL(_) => 0,
+                BIT(_, _) => 0,
+                RES(_, _) => 0,
+                SET(_, _) => 0,
+                _ => unreachable!("{:?} is an illegal instruction", instr),
+            }
+        }
+    }
+}

src/interrupt.rs

@@ -1,11 +1,20 @@
 use bitfield::bitfield;
 
-#[derive(Debug, Default)]
+#[derive(Debug)]
 pub(crate) struct Interrupt {
     pub(crate) flag: InterruptFlag,
     pub(crate) enable: InterruptEnable,
 }
 
+impl Default for Interrupt {
+    fn default() -> Self {
+        Self {
+            flag: InterruptFlag(0),
+            enable: InterruptEnable(0),
+        }
+    }
+}
+
 bitfield! {
    pub struct InterruptEnable(u8);
     impl Debug;
@@ -23,12 +32,6 @@ impl Clone for InterruptEnable {
     }
 }
 
-impl Default for InterruptEnable {
-    fn default() -> Self {
-        Self(0)
-    }
-}
-
 impl From<u8> for InterruptEnable {
     fn from(byte: u8) -> Self {
         Self(byte)
@@ -58,12 +61,6 @@ impl Clone for InterruptFlag {
     }
 }
 
-impl Default for InterruptFlag {
-    fn default() -> Self {
-        Self(0)
-    }
-}
-
 impl From<u8> for InterruptFlag {
     fn from(byte: u8) -> Self {
         Self(byte)

src/joypad.rs

@@ -1,5 +1,5 @@
 use gilrs::{Button, Event as GamepadEvent, EventType as GamepadEventType};
-use winit_input_helper::WinitInputHelper;
+use winit::event::{ElementState, KeyboardInput, VirtualKeyCode};
 
 #[derive(Debug)]
 pub struct Joypad {
@@ -110,72 +110,39 @@ impl ButtonEvent {
     }
 }
 
-pub fn handle_keyboard_input(pad: &mut Joypad, input: &WinitInputHelper) {
+#[inline]
-    use winit::event::VirtualKeyCode;
+pub(crate) fn handle_keyboard_input(pad: &mut Joypad, key: KeyboardInput) {
-
-    // TODO: What do I have to do to get a match statement here?
-
     let state = &mut pad.ext;
     let irq = &mut pad.interrupt;
 
-    if input.key_pressed(VirtualKeyCode::Down) {
+    match key.state {
-        state.dpad_down.update(true, irq);
+        ElementState::Pressed => match key.virtual_keycode {
-    }
+            Some(VirtualKeyCode::Down) => state.dpad_down.update(true, irq),
-    if input.key_released(VirtualKeyCode::Down) {
+            Some(VirtualKeyCode::Up) => state.dpad_up.update(true, irq),
-        state.dpad_down.update(false, irq);
+            Some(VirtualKeyCode::Left) => state.dpad_left.update(true, irq),
-    }
+            Some(VirtualKeyCode::Right) => state.dpad_right.update(true, irq),
-
+            Some(VirtualKeyCode::Return) => state.start.update(true, irq),
-    if input.key_pressed(VirtualKeyCode::Up) {
+            Some(VirtualKeyCode::RShift) => state.select.update(true, irq),
-        state.dpad_up.update(true, irq);
+            Some(VirtualKeyCode::Z) => state.south.update(true, irq),
-    }
+            Some(VirtualKeyCode::X) => state.east.update(true, irq),
-    if input.key_released(VirtualKeyCode::Up) {
+            None | Some(_) => {}
-        state.dpad_up.update(false, irq);
+        },
-    }
+        ElementState::Released => match key.virtual_keycode {
-
+            Some(VirtualKeyCode::Down) => state.dpad_down.update(false, irq),
-    if input.key_pressed(VirtualKeyCode::Left) {
+            Some(VirtualKeyCode::Up) => state.dpad_up.update(false, irq),
-        state.dpad_left.update(true, irq);
+            Some(VirtualKeyCode::Left) => state.dpad_left.update(false, irq),
-    }
+            Some(VirtualKeyCode::Right) => state.dpad_right.update(false, irq),
-    if input.key_released(VirtualKeyCode::Left) {
+            Some(VirtualKeyCode::Return) => state.start.update(false, irq),
-        state.dpad_left.update(false, irq);
+            Some(VirtualKeyCode::RShift) => state.select.update(false, irq),
-    }
+            Some(VirtualKeyCode::Z) => state.south.update(false, irq),
-
+            Some(VirtualKeyCode::X) => state.east.update(false, irq),
-    if input.key_pressed(VirtualKeyCode::Right) {
+            None | Some(_) => {}
-        state.dpad_right.update(true, irq);
+        },
-    }
-    if input.key_released(VirtualKeyCode::Right) {
-        state.dpad_right.update(false, irq);
-    }
-
-    if input.key_pressed(VirtualKeyCode::T) {
-        state.start.update(true, irq);
-    }
-    if input.key_released(VirtualKeyCode::T) {
-        state.start.update(false, irq);
-    }
-
-    if input.key_pressed(VirtualKeyCode::Y) {
-        state.select.update(true, irq);
-    }
-    if input.key_released(VirtualKeyCode::Y) {
-        state.select.update(false, irq);
-    }
-
-    if input.key_pressed(VirtualKeyCode::Z) {
-        state.south.update(true, irq);
-    }
-    if input.key_released(VirtualKeyCode::Z) {
-        state.south.update(false, irq);
-    }
-
-    if input.key_pressed(VirtualKeyCode::X) {
-        state.east.update(true, irq);
-    }
-    if input.key_released(VirtualKeyCode::X) {
-        state.east.update(false, irq);
     }
 }
 
-pub fn handle_gamepad_input(pad: &mut Joypad, event: GamepadEvent) {
+#[inline]
+pub(crate) fn handle_gamepad_input(pad: &mut Joypad, event: GamepadEvent) {
     use Button::*;
     use GamepadEventType::*;
 

src/lib.rs

@@ -1,4 +1,6 @@
-pub use apu::gen::AudioSPSC;
+#![allow(clippy::derivable_impls)] // Will remove this if bitfield-rs allows default impls
+
+pub use apu::gen::init as spsc_init;
 pub type Cycle = u64;
 
 pub const GB_WIDTH: usize = 160;
@@ -9,6 +11,7 @@ mod bus;
 mod cartridge;
 mod cpu;
 pub mod emu;
+pub mod gui;
 mod high_ram;
 mod instruction;
 mod interrupt;

src/main.rs

@@ -1,144 +1,126 @@
-use std::convert::TryInto;
+use std::path::PathBuf;
+use std::time::Instant;
 
-use anyhow::{anyhow, Result};
+use clap::{arg, command, value_parser};
-use clap::{crate_authors, crate_description, crate_name, crate_version, App, Arg};
+use gb::gui::{EmuMode, Gui};
-use gb::emu::build::EmulatorBuilder;
+use gb::{emu, gui};
-use gb::{AudioSPSC, Cycle, GB_HEIGHT, GB_WIDTH};
 use gilrs::Gilrs;
-use pixels::{PixelsBuilder, SurfaceTexture};
 use rodio::{OutputStream, Sink};
-use winit::dpi::{LogicalSize, PhysicalSize};
+use tracing_subscriber::EnvFilter;
-use winit::event::{Event, VirtualKeyCode};
+use winit::event::{Event, WindowEvent};
-use winit::event_loop::{ControlFlow, EventLoop};
+use winit::event_loop::{EventLoop, EventLoopBuilder};
-use winit::window::{Window, WindowBuilder};
-use winit_input_helper::WinitInputHelper;
 
-const WINDOW_SCALE: usize = 3;
+const AUDIO_ENABLED: bool = true;
-const AUDIO_ENABLED: bool = false;
 
-fn main() -> Result<()> {
+fn main() {
-    let app = App::new(crate_name!())
+    let m = command!()
-        .version(crate_version!())
-        .author(crate_authors!())
-        .about(crate_description!());
-
-    let m = app
         .arg(
-            Arg::with_name("rom")
+            arg!(-b --boot <FILE> "path to boot ROM")
-                .value_name("ROM_FILE")
+                .required(false)
-                .takes_value(true)
+                .value_parser(value_parser!(PathBuf)),
-                .required(true)
-                .index(1)
-                .help("Path to the Game ROM"),
         )
         .arg(
-            Arg::with_name("boot")
+            arg!([ROM_FILE] "path to game ROM")
-                .short("b")
+                .required(true)
-                .long("boot")
+                .value_parser(value_parser!(PathBuf)),
-                .value_name("FILE")
-                .takes_value(true)
-                .help("Path to Boot ROM"),
         )
         .get_matches();
 
-    let mut emu_build =
+    // Set up subscriber
-        EmulatorBuilder::new().with_cart(m.value_of("rom").expect("ROM path provided"))?;
+    if std::env::var("RUST_LOG").is_err() {
-
+        std::env::set_var("RUST_LOG", "gb=info");
-    if let Some(path) = m.value_of("boot") {
-        emu_build = emu_build.with_boot(path)?;
     }
 
-    let mut emu = emu_build.finish();
+    tracing_subscriber::fmt::fmt()
-    let rom_title = emu.title();
+        .with_env_filter(EnvFilter::from_default_env())
+        .init();
 
-    let mut gamepad = Gilrs::new().expect("Initialize Controller Support");
+    // Init CPU
-
+    let mut cpu = match m.get_one::<PathBuf>("boot") {
-    // Initialize GUI
+        Some(path) => {
-    let event_loop = EventLoop::new();
+            tracing::info!("User-provided boot ROM");
-    let mut input = WinitInputHelper::new();
+            emu::from_boot_rom(path).expect("initialize emulator with custom boot rom")
-    let window = create_window(&event_loop, rom_title)?;
+        }
-
+        None => {
-    let mut pixels = {
+            tracing::info!("Built-in boot ROM");
-        let size = window.inner_size();
+            Default::default()
-        let surface_texture = SurfaceTexture::new(size.width, size.height, &window);
+        }
-
-        PixelsBuilder::new(GB_WIDTH as u32, GB_HEIGHT as u32, surface_texture)
-            .enable_vsync(false)
-            .build()?
     };
 
+    // Load ROM if filepath was provided
+    if let Some(path) = m.get_one::<PathBuf>("ROM_FILE") {
+        tracing::info!("User-provided cartridge ROM");
+        emu::read_game_rom(&mut cpu, path).expect("read game rom from path");
+    }
+    emu::load_save(&mut cpu);
+    let rom_title = emu::rom_title(&cpu).to_string();
+
+    tracing::info!("Initialize Gamepad");
+    let mut gamepad = Gilrs::new().expect("Initialize Controller Support");
+
     // Initialize Audio
     let (_stream, stream_handle) = OutputStream::try_default().expect("Initialized Audio");
 
     if AUDIO_ENABLED {
-        let spsc: AudioSPSC<f32> = Default::default();
+        let (prod, cons) = gb::spsc_init();
-        let (prod, cons) = spsc.init();
         let sink = {
-            let s = Sink::try_new(&stream_handle)?;
+            let s = Sink::try_new(&stream_handle).expect("create sink from audio stream handle");
             s.append(cons);
             s.set_volume(0.1);
             s
         };
 
-        emu.set_prod(prod);
+        emu::set_audio_prod(&mut cpu, prod);
 
+        tracing::info!("Spawn Audio Thread");
         std::thread::spawn(move || {
             sink.sleep_until_end();
         });
     }
 
-    let mut cycle_count: Cycle = Default::default();
+    // Set up state for the Immediate-mode GUI
+    let event_loop: EventLoop<Event<()>> = EventLoopBuilder::with_user_event().build();
+
+    let mut app = Gui::new(rom_title, &event_loop, &cpu);
+    let mut last_key = gui::unused_key(); // TODO: Fix this awful impl
+
+    // used for egui animations
+    let start_time = Instant::now();
 
     event_loop.run(move |event, _, control_flow| {
-        if let Event::RedrawRequested(_) = event {
+        app.handle_event(&event);
-            if pixels
+
-                .render()
+        match event {
-                .map_err(|e| anyhow!("pixels.render() failed: {}", e))
+            Event::MainEventsCleared => {
-                .is_err()
+                app.maybe_quit(&cpu, control_flow);
-            {
+
-                *control_flow = ControlFlow::Exit;
+                match app.mode {
-                return;
+                    EmuMode::Running => emu::run_frame(&mut cpu, &mut gamepad, last_key),
+                    EmuMode::StepFrame if gui::kbd::space_released(&last_key) => {
+                        emu::run_frame(&mut cpu, &mut gamepad, last_key)
                     }
+                    EmuMode::Step if gui::kbd::space_released(&last_key) => {
+                        emu::run(&mut cpu, &mut gamepad, last_key, 4);
                     }
+                    _ => {}
+                };
 
-        if input.update(&event) {
+                // Input has been consumed, reset it
-            if input.key_pressed(VirtualKeyCode::Escape) || input.quit() {
+                last_key = gui::unused_key();
-                *control_flow = ControlFlow::Exit;
+
-                return;
+                app.request_redraw();
             }
-
+            Event::RedrawRequested(..) => {
-            if let Some(size) = input.window_resized() {
+                app.update_time(start_time.elapsed().as_secs_f64());
-                pixels.resize_surface(size.width, size.height);
+                app.paint(&cpu);
-            }
-
-            cycle_count += gb::emu::run_frame(&mut emu, &mut gamepad, &input);
-
-            if cycle_count >= gb::emu::CYCLES_IN_FRAME {
-                cycle_count %= gb::emu::CYCLES_IN_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();
             }
+            Event::WindowEvent { event, .. } => match event {
+                WindowEvent::Resized(size) => app.resize(size),
+                WindowEvent::CloseRequested => emu::save_and_exit(&cpu, control_flow),
+                WindowEvent::KeyboardInput { input, .. } => last_key = input,
+                _ => {}
+            },
+            _ => {}
         }
     });
 }
-
-fn create_window(event_loop: &EventLoop<()>, title: &str) -> Result<Window> {
-    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_min_inner_size(logical)
-        .with_inner_size(physical)
-        .with_resizable(true)
-        .build(event_loop)?)
-}

src/ppu/dma.rs

@@ -1,6 +1,6 @@
 use crate::Cycle;
 
-#[derive(Debug, Default)]
+#[derive(Debug)]
 pub(crate) struct DirectMemoryAccess {
     pub(crate) state: DmaState,
     cycle: Cycle,
@@ -8,6 +8,16 @@ pub(crate) struct DirectMemoryAccess {
     pub(crate) start: DmaAddress,
 }
 
+impl Default for DirectMemoryAccess {
+    fn default() -> Self {
+        Self {
+            state: DmaState::Disabled,
+            cycle: Default::default(),
+            start: Default::default(),
+        }
+    }
+}
+
 impl DirectMemoryAccess {
     pub(crate) fn tick(&mut self) -> Option<(u16, u16)> {
         match self.state {
@@ -69,12 +79,6 @@ pub(crate) enum DmaState {
     Transferring,
 }
 
-impl Default for DmaState {
-    fn default() -> Self {
-        Self::Disabled
-    }
-}
-
 #[derive(Debug, Clone, Copy, Default)]
 pub(crate) struct DmaAddress(Option<u16>);
 

src/ppu/types.rs

@@ -29,12 +29,6 @@ impl Clone for LCDStatus {
     }
 }
 
-impl Default for LCDStatus {
-    fn default() -> Self {
-        Self(0x80) // bit 7 is always 1
-    }
-}
-
 impl From<LCDStatus> for u8 {
     fn from(status: LCDStatus) -> Self {
         status.0
@@ -67,12 +61,6 @@ impl From<PpuMode> for u8 {
     }
 }
 
-impl Default for PpuMode {
-    fn default() -> Self {
-        Self::HBlank
-    }
-}
-
 bitfield! {
     pub struct LCDControl(u8);
     impl Debug;
@@ -93,12 +81,6 @@ impl Clone for LCDControl {
     }
 }
 
-impl Default for LCDControl {
-    fn default() -> Self {
-        Self(0)
-    }
-}
-
 impl From<u8> for LCDControl {
     fn from(byte: u8) -> Self {
         Self(byte)
@@ -142,12 +124,6 @@ impl From<TileMapAddress> for u8 {
     }
 }
 
-impl Default for TileMapAddress {
-    fn default() -> Self {
-        Self::X9800
-    }
-}
-
 #[derive(Debug, Clone, Copy)]
 pub enum TileDataAddress {
     X8800 = 0,
@@ -170,12 +146,6 @@ impl From<TileDataAddress> for u8 {
     }
 }
 
-impl Default for TileDataAddress {
-    fn default() -> Self {
-        Self::X8800
-    }
-}
-
 #[derive(Debug, Clone, Copy)]
 pub enum ObjectSize {
     Eight = 0,
@@ -183,7 +153,7 @@ pub enum ObjectSize {
 }
 
 impl ObjectSize {
-    pub(crate) fn as_u8(&self) -> u8 {
+    pub(crate) fn size(&self) -> u8 {
         use ObjectSize::*;
 
         match self {
@@ -209,12 +179,6 @@ impl From<ObjectSize> for u8 {
     }
 }
 
-impl Default for ObjectSize {
-    fn default() -> Self {
-        Self::Eight
-    }
-}
-
 bitfield! {
     pub struct BackgroundPalette(u8);
     impl Debug;
@@ -243,12 +207,6 @@ impl Clone for BackgroundPalette {
     }
 }
 
-impl Default for BackgroundPalette {
-    fn default() -> Self {
-        Self(0)
-    }
-}
-
 impl From<u8> for BackgroundPalette {
     fn from(byte: u8) -> Self {
         Self(byte)
@@ -288,12 +246,6 @@ impl Clone for ObjectPalette {
     }
 }
 
-impl Default for ObjectPalette {
-    fn default() -> Self {
-        Self(0)
-    }
-}
-
 impl From<u8> for ObjectPalette {
     fn from(byte: u8) -> Self {
         Self(byte)
@@ -361,24 +313,12 @@ impl From<ObjectFlags> for u8 {
     }
 }
 
-impl Default for ObjectFlags {
-    fn default() -> Self {
-        Self(0)
-    }
-}
-
 #[derive(Debug, Clone, Copy)]
 pub enum ObjectPaletteKind {
     Zero = 0,
     One = 1,
 }
 
-impl Default for ObjectPaletteKind {
-    fn default() -> Self {
-        Self::Zero
-    }
-}
-
 impl From<u8> for ObjectPaletteKind {
     fn from(byte: u8) -> Self {
         match byte & 0b01 {
@@ -395,7 +335,7 @@ impl From<ObjectPaletteKind> for u8 {
     }
 }
 
-#[derive(Debug, Clone, Copy, PartialEq)]
+#[derive(Debug, Clone, Copy, Eq, PartialEq)]
 pub enum RenderPriority {
     Object = 0,
     BackgroundAndWindow = 1,
@@ -417,12 +357,6 @@ impl From<RenderPriority> for u8 {
     }
 }
 
-impl Default for RenderPriority {
-    fn default() -> Self {
-        Self::Object
-    }
-}
-
 #[derive(Debug, Clone, Copy)]
 pub enum GrayShade {
     White = 0,
@@ -442,12 +376,6 @@ impl GrayShade {
     }
 }
 
-impl Default for GrayShade {
-    fn default() -> Self {
-        Self::White
-    }
-}
-
 impl From<u8> for GrayShade {
     fn from(byte: u8) -> Self {
         match byte & 0b11 {

src/scheduler.rs

@@ -1,79 +0,0 @@
-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, Default)]
+#[derive(Debug)]
 pub(crate) struct Serial {
     /// 0xFF01 | SB - Serial Transfer Data
     pub(crate) next: u8,
@@ -8,6 +8,15 @@ pub(crate) struct Serial {
     pub(crate) ctrl: SerialControl,
 }
 
+impl Default for Serial {
+    fn default() -> Self {
+        Self {
+            next: Default::default(),
+            ctrl: SerialControl(0),
+        }
+    }
+}
+
 bitfield! {
     pub struct SerialControl(u8);
     impl Debug;
@@ -23,12 +32,6 @@ impl Clone for SerialControl {
     }
 }
 
-impl Default for SerialControl {
-    fn default() -> Self {
-        Self(0)
-    }
-}
-
 impl From<u8> for SerialControl {
     fn from(byte: u8) -> Self {
         Self(byte)
@@ -47,12 +50,6 @@ enum ShiftClock {
     Internal = 1,
 }
 
-impl Default for ShiftClock {
-    fn default() -> Self {
-        Self::External
-    }
-}
-
 impl From<u8> for ShiftClock {
     fn from(byte: u8) -> Self {
         match byte & 0b01 {
@@ -69,12 +66,6 @@ enum ClockSpeed {
     Fast = 1,
 }
 
-impl Default for ClockSpeed {
-    fn default() -> Self {
-        Self::Normal
-    }
-}
-
 impl From<u8> for ClockSpeed {
     fn from(byte: u8) -> Self {
         match byte & 0b01 {

src/timer.rs

@@ -21,17 +21,6 @@ impl Timer {
         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
@@ -53,6 +42,17 @@ impl Timer {
         }
 
         self.and_result = Some(new_result);
+
+        match self.state {
+            TIMAOverflow(_) | AbortedTIMAOverflow(_) => self.next(),
+            LoadTIMA => {
+                self.counter = self.modulo;
+                self.interrupt = true;
+
+                self.next();
+            }
+            Normal => {}
+        }
     }
 
     /// 0xFF05 | TIMA - Timer Counter
@@ -70,7 +70,7 @@ impl Timer {
                 self.counter = byte;
                 self.state = AbortedTIMAOverflow(step);
             }
-            LoadTMA => {}
+            LoadTIMA => { /* Ignored */ }
         }
     }
 
@@ -95,9 +95,8 @@ impl Timer {
         use State::*;
 
         self.state = match self.state {
-            Normal | LoadTMA => Normal,
+            Normal | LoadTIMA | AbortedTIMAOverflow(3) => Normal,
-            AbortedTIMAOverflow(4) => Normal,
+            TIMAOverflow(3) => LoadTIMA,
-            TIMAOverflow(4) => LoadTMA,
             AbortedTIMAOverflow(step) => AbortedTIMAOverflow(step + 1),
             TIMAOverflow(step) => TIMAOverflow(step + 1),
         }
@@ -107,7 +106,7 @@ impl Timer {
 impl Default for Timer {
     fn default() -> Self {
         Self {
-            ctrl: Default::default(),
+            ctrl: TimerControl(0),
             counter: 0,
             modulo: 0,
             divider: 0,
@@ -158,12 +157,6 @@ impl Clone for TimerControl {
     }
 }
 
-impl Default for TimerControl {
-    fn default() -> Self {
-        Self(0)
-    }
-}
-
 impl From<u8> for TimerControl {
     fn from(byte: u8) -> Self {
         Self(byte)
@@ -181,5 +174,5 @@ enum State {
     TIMAOverflow(u8),
     AbortedTIMAOverflow(u8),
     Normal,
-    LoadTMA,
+    LoadTIMA,
 }

src/work_ram.rs

@@ -11,13 +11,13 @@ pub(crate) struct WorkRam {
 }
 
 impl BusIo for WorkRam {
-    fn write_byte(&mut self, addr: u16, byte: u8) {
-        self.bank[addr as usize - WORK_RAM_START_ADDRESS] = byte;
-    }
-
     fn read_byte(&self, addr: u16) -> u8 {
         self.bank[addr as usize - WORK_RAM_START_ADDRESS]
     }
+
+    fn write_byte(&mut self, addr: u16, byte: u8) {
+        self.bank[addr as usize - WORK_RAM_START_ADDRESS] = byte;
+    }
 }
 
 impl Default for WorkRam {
@@ -42,11 +42,11 @@ impl Default for VariableWorkRam {
 }
 
 impl BusIo for VariableWorkRam {
-    fn write_byte(&mut self, addr: u16, byte: u8) {
-        self.buf[addr as usize - VARIABLE_WORK_RAM_START_ADDRESS] = byte;
-    }
-
     fn read_byte(&self, addr: u16) -> u8 {
         self.buf[addr as usize - VARIABLE_WORK_RAM_START_ADDRESS]
     }
+
+    fn write_byte(&mut self, addr: u16, byte: u8) {
+        self.buf[addr as usize - VARIABLE_WORK_RAM_START_ADDRESS] = byte;
+    }
 }