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

@@ -2,20 +2,27 @@
 name = "gb"
 version = "0.1.0"
 authors = ["Rekai Musuka <rekai@musuka.dev>"]
-edition = "2018"
+edition = "2021"
 
 # 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 = "^0.5"
+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);
+        }
+
+        output
+    }
 }
 
-#[derive(Debug, Default)]
+#[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)
     }
 
@@ -463,38 +478,54 @@ 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() {
-            // Envelope Behaviour during trigger event
+            self.enabled = true;
+
+            if self.length_counter == 0 {
+                self.length_counter = 64;
+
+                // 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();
+
             self.shadow_freq = self.frequency();
             self.sweep_timer = if sweep_period == 0 { 8 } else { sweep_period };
 
-            if sweep_period != 0 || sweep_shift != 0 {
+            self.sweep_enabled = sweep_period != 0 || sweep_shift != 0;
-                self.sweep_enabled = true;
-            }
 
             if sweep_shift != 0 {
                 let _ = self.calc_sweep_freq();
             }
 
-            // 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;
-            }
         }
+
+        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) {
@@ -556,7 +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,
@@ -568,7 +599,7 @@ pub(crate) struct Channel2 {
     current_volume: u8,
 
     // Length Functionality
-    length_timer: u16,
+    length_counter: u16,
 
     freq_timer: u16,
     duty_pos: u8,
@@ -585,15 +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();
 
@@ -613,22 +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;
-            }
         }
     }
 
@@ -679,7 +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,
@@ -725,7 +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
@@ -757,18 +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;
         }
     }
 
@@ -827,7 +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,
@@ -841,7 +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
@@ -874,25 +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;
-            }
         }
     }
 
@@ -937,3 +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

@@ -5,33 +5,12 @@ 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);
-}
+    (
-
+        SampleProducer { inner: prod },
-impl<T> Default for AudioSPSC<T> {
+        SampleConsumer { inner: cons },
-    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> {

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

@@ -7,63 +7,30 @@ use crate::ppu::{Ppu, PpuMode};
 use crate::serial::Serial;
 use crate::timer::Timer;
 use crate::work_ram::{VariableWorkRam, WorkRam};
-use std::{fs::File, io::Read};
 
-const BOOT_ROM_SIZE: usize = 0x100;
+pub(crate) const BOOT_SIZE: usize = 0x100;
 
-#[derive(Debug)]
+#[derive(Debug, Default)]
 pub struct Bus {
-    boot: Option<[u8; BOOT_ROM_SIZE]>, // Boot ROM is 256b long
+    boot: Option<[u8; BOOT_SIZE]>, // Boot ROM is 256b long
-    cartridge: 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,
-            cartridge: 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 {
-    pub(crate) fn with_boot(path: &str) -> anyhow::Result<Self> {
+    pub(crate) fn with_boot(rom: [u8; 256]) -> Self {
-        let mut file = File::open(path)?;
+        Self {
-        let mut boot_rom = [0u8; 256];
+            boot: Some(rom),
-
-        file.read_exact(&mut boot_rom)?;
-
-        Ok(Self {
-            boot: Some(boot_rom),
             ..Default::default()
-        })
+        }
-    }
-
-    pub(crate) fn load_cartridge(&mut self, path: &str) -> std::io::Result<()> {
-        self.cartridge = Some(Cartridge::new(path)?);
-        Ok(())
-    }
-
-    pub(crate) fn rom_title(&self) -> Option<&str> {
-        self.cartridge.as_ref()?.title()
     }
 
     #[allow(dead_code)]
@@ -71,13 +38,19 @@ impl Bus {
         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()
@@ -93,7 +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)
@@ -104,16 +77,16 @@ impl Bus {
                     }
                 }
 
-                match self.cartridge.as_ref() {
+                match self.cart.as_ref() {
                     Some(cart) => cart.read_byte(addr),
-                    None => panic!("Tried to read from a non-existent cartridge"),
+                    None => 0xFF,
                 }
             }
             0x8000..=0x9FFF => self.ppu.read_byte(addr), // 8KB Video RAM
-            0xA000..=0xBFFF => match self.cartridge.as_ref() {
+            0xA000..=0xBFFF => match self.cart.as_ref() {
                 // 8KB External RAM
                 Some(cart) => cart.read_byte(addr),
-                None => panic!("Tried to read from a non-existent cartridge"),
+                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
@@ -140,7 +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);
     }
 }
@@ -157,9 +130,9 @@ impl BusIo for Bus {
                     }
                 }
 
-                match self.cartridge.as_ref() {
+                match self.cart.as_ref() {
                     Some(cart) => cart.read_byte(addr),
-                    None => panic!("Tried to read from a non-existent cartridge"),
+                    None => 0xFF,
                 }
             }
             0x8000..=0x9FFF => {
@@ -169,10 +142,10 @@ impl BusIo for Bus {
                     _ => self.ppu.read_byte(addr),
                 }
             }
-            0xA000..=0xBFFF => match self.cartridge.as_ref() {
+            0xA000..=0xBFFF => match self.cart.as_ref() {
                 // 8KB External RAM
                 Some(cart) => cart.read_byte(addr),
-                None => panic!("Tried to read from a non-existent cartridge"),
+                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
@@ -219,7 +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,
@@ -240,8 +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
                     }
                 }
@@ -262,9 +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.cartridge.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 => {
@@ -276,9 +249,8 @@ impl BusIo for Bus {
             }
             0xA000..=0xBFFF => {
                 // 8KB External RAM
-                match self.cartridge.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
@@ -318,7 +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,
@@ -352,13 +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 => {
@@ -380,7 +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();
@@ -410,7 +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/cpu.rs

@@ -1,16 +1,13 @@
-use crate::apu::Apu;
+use crate::bus::{Bus, BusIo, BOOT_SIZE};
-use crate::bus::{Bus, BusIo};
-use crate::instruction::cycle::Cycle;
 use crate::instruction::Instruction;
 use crate::interrupt::{InterruptEnable, InterruptFlag};
-use crate::joypad::Joypad;
+use crate::Cycle;
-use crate::ppu::Ppu;
 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,
@@ -18,31 +15,16 @@ pub struct Cpu {
 }
 
 impl Cpu {
-    pub fn new() -> Self {
+    pub(crate) fn new(rom: [u8; BOOT_SIZE]) -> Self {
         Self {
-            reg: Registers {
+            bus: Bus::with_boot(rom),
-                a: 0x01,
+            reg: Default::default(),
-                b: 0x00,
+            flags: Flags(0),
-                c: 0x13,
+            ime: ImeState::Disabled,
-                d: 0x00,
+            state: State::Execute,
-                e: 0xD8,
-                h: 0x01,
-                l: 0x4D,
-                sp: 0xFFFE,
-                pc: 0x0100,
-            },
-            flags: 0xb0.into(),
-            ..Default::default()
         }
     }
 
-    pub fn boot_new(path: &str) -> anyhow::Result<Self> {
-        Ok(Self {
-            bus: Bus::with_boot(path)?,
-            ..Default::default()
-        })
-    }
-
     pub(crate) fn ime(&self) -> ImeState {
         self.ime
     }
@@ -60,10 +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> {
@@ -72,13 +51,11 @@ impl Cpu {
             _ => None,
         }
     }
+}
 
-    pub fn load_cartridge(&mut self, path: &str) -> std::io::Result<()> {
+impl Default for Cpu {
-        self.bus.load_cartridge(path)
+    fn default() -> Self {
-    }
+        Cpu::new(*include_bytes!("../bin/bootix_dmg.bin"))
-
-    pub fn rom_title(&self) -> Option<&str> {
-        self.bus.rom_title()
     }
 }
 
@@ -116,7 +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();
@@ -131,13 +108,10 @@ impl Cpu {
             use HaltKind::*;
 
             self.bus.clock();
-
+            return match kind {
-            let elapsed = match kind {
+                ImeEnabled | NonePending => 4,
-                ImeEnabled | NonePending => Cycle::new(4),
                 SomePending => todo!("Implement HALT bug"),
             };
-
-            return elapsed;
         }
 
         let opcode = self.fetch();
@@ -146,11 +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
     }
@@ -167,18 +141,6 @@ impl BusIo for Cpu {
 }
 
 impl Cpu {
-    pub fn ppu(&mut self) -> &Ppu {
-        &self.bus.ppu
-    }
-
-    pub fn apu_mut(&mut self) -> &mut Apu {
-        &mut self.bus.apu
-    }
-
-    pub(crate) fn joypad_mut(&mut self) -> &mut Joypad {
-        &mut self.bus.joypad
-    }
-
     fn handle_ei(&mut self) {
         match self.ime {
             ImeState::EiExecuted => self.ime = ImeState::Pending,
@@ -276,12 +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::*;
@@ -475,12 +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() {
@@ -536,8 +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

@@ -1,73 +1,163 @@
-use crate::cpu::Cpu as SM83;
+use crate::apu::gen::SampleProducer;
-use crate::instruction::cycle::Cycle;
+use crate::bus::BOOT_SIZE;
-use crate::joypad;
+use crate::cartridge::Cartridge;
-use crate::ppu::Ppu;
+use crate::cpu::Cpu;
-use anyhow::Result;
+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 = Cycle::new(456 * 154); // 456 Cycles times 154 scanlines
+pub const CYCLES_IN_FRAME: Cycle = 456 * 154; // 456 Cycles times 154 scanlines
 pub(crate) const SM83_CLOCK_SPEED: u64 = 0x40_0000; // Hz which is 4.194304Mhz
-const DEFAULT_TITLE: &str = "DMG-01 Emulator";
+const DEFAULT_TITLE: &str = "Game Boy Screen";
-const GAMEPAD_ENABLED: bool = true;
 
-pub fn init(boot_path: Option<&str>, rom_path: &str) -> Result<SM83> {
+#[inline]
-    let mut cpu = match boot_path {
+pub fn run_frame(cpu: &mut Cpu, gamepad: &mut Gilrs, key: KeyboardInput) {
-        Some(path) => SM83::boot_new(path)?,
+    run(cpu, gamepad, key, CYCLES_IN_FRAME)
-        None => SM83::new(),
-    };
-
-    eprintln!("Initialized GB Emulator");
-
-    cpu.load_cartridge(rom_path)?;
-    Ok(cpu)
 }
 
-pub fn rom_title(game_boy: &SM83) -> &str {
+#[inline]
-    game_boy.rom_title().unwrap_or(DEFAULT_TITLE)
+pub fn run(cpu: &mut Cpu, gamepad: &mut Gilrs, key: KeyboardInput, cycles: Cycle) {
+    let mut elapsed = 0;
+
+    if let Some(event) = gamepad.next_event() {
+        crate::joypad::handle_gamepad_input(&mut cpu.bus.joyp, event);
+    }
+    crate::joypad::handle_keyboard_input(&mut cpu.bus.joyp, key);
+
+    while elapsed < cycles {
+        elapsed += cpu.step();
+    }
 }
 
-pub fn run(
+pub fn save_and_exit(cpu: &Cpu, control_flow: &mut ControlFlow) {
-    game_boy: &mut SM83,
+    write_save(cpu);
-    gamepad: &mut Gilrs,
+    *control_flow = ControlFlow::Exit;
-    input: &WinitInputHelper,
+}
-    target: Cycle,
-) -> Cycle {
-    let mut elapsed = Cycle::new(0);
 
-    if GAMEPAD_ENABLED {
+#[inline]
-        if let Some(event) = gamepad.next_event() {
+pub fn pixel_buf(cpu: &Cpu) -> &[u8; GB_HEIGHT * GB_WIDTH * 4] {
-            joypad::handle_gamepad_input(game_boy.joypad_mut(), event);
+    use crate::ppu::Device;
+    cpu.bus.ppu.frame_buf.get(Device::Host)
+}
+
+pub fn from_boot_rom<P: AsRef<Path>>(path: P) -> std::io::Result<Cpu> {
+    Ok(Cpu::new(read_boot(path)?))
+}
+
+pub fn read_game_rom<P: AsRef<Path>>(cpu: &mut Cpu, path: P) -> std::io::Result<()> {
+    cpu.bus.cart = Some(Cartridge::new(std::fs::read(path.as_ref())?));
+    Ok(())
+}
+
+pub fn set_audio_prod(cpu: &mut Cpu, prod: SampleProducer<f32>) {
+    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 fn load_save(cpu: &mut Cpu) {
+    match cpu.bus.cart.as_mut() {
+        Some(cart) => match read_save_from_file(cart) {
+            Ok(path) => tracing::info!("Loaded save from {:?}", path),
+            Err(err @ SaveError::NotApplicable) => tracing::warn!("Unable to load save: {}", err),
+            Err(err @ SaveError::DiffSize) => tracing::error!("Unable to load save: {}", err),
+            Err(SaveError::Io(err)) => match err.kind() {
+                std::io::ErrorKind::NotFound => tracing::warn!("Save not found"),
+                _ => tracing::error!("{:?}", err),
+            },
+        },
+        None => tracing::error!("No cartridge is currently present"),
+    }
+}
+
+fn write_save_to_file(cart: &Cartridge) -> Result<PathBuf, SaveError> {
+    match cart.title.as_ref().zip(cart.ext_ram()) {
+        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),
     }
-
-    joypad::handle_keyboard_input(game_boy.joypad_mut(), input);
-    while elapsed < target {
-        elapsed += game_boy.step();
-    }
-
-    elapsed
 }
 
-pub fn run_frame(game_boy: &mut SM83, gamepad: &mut Gilrs, input: &WinitInputHelper) -> Cycle {
+fn read_save_from_file(cart: &mut Cartridge) -> Result<PathBuf, SaveError> {
-    let mut elapsed = Cycle::new(0);
+    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");
 
-    if GAMEPAD_ENABLED {
+            let mut file = File::open(&save_path)?;
-        if let Some(event) = gamepad.next_event() {
+            let mut memory = Vec::new();
-            joypad::handle_gamepad_input(game_boy.joypad_mut(), event);
+            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),
     }
-
-    joypad::handle_keyboard_input(game_boy.joypad_mut(), input);
-    while elapsed < CYCLES_IN_FRAME {
-        elapsed += game_boy.step();
-    }
-
-    elapsed
 }
 
-pub fn draw(ppu: &Ppu, frame: &mut [u8]) {
+fn read_boot<P: AsRef<Path>>(path: P) -> std::io::Result<[u8; BOOT_SIZE]> {
-    ppu.copy_to_gui(frame);
+    let mut buf = [0; BOOT_SIZE];
+    let mut file = File::open(path.as_ref())?;
+
+    file.read_exact(&mut buf)?;
+    Ok(buf)
+}
+
+fn data_path() -> Option<PathBuf> {
+    match directories_next::ProjectDirs::from("dev", "musuka", crate_name!()) {
+        Some(dirs) => {
+            let data_local = dirs.data_local_dir();
+            std::fs::create_dir_all(data_local).ok()?;
+            Some(data_local.to_path_buf())
+        }
+        None => None,
+    }
+}
+
+#[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/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

@@ -1,5 +1,7 @@
-pub use apu::gen::AudioSPSC;
+#![allow(clippy::derivable_impls)] // Will remove this if bitfield-rs allows default impls
-pub use instruction::cycle::Cycle;
+
+pub use apu::gen::init as spsc_init;
+pub type Cycle = u64;
 
 pub const GB_WIDTH: usize = 160;
 pub const GB_HEIGHT: usize = 144;
@@ -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

@@ -1,160 +1,126 @@
-use anyhow::{anyhow, Result};
+use std::path::PathBuf;
-use clap::{crate_authors, crate_description, crate_name, crate_version, App, Arg};
+use std::time::Instant;
-use gb::{AudioSPSC, Cycle, GB_HEIGHT, GB_WIDTH};
+
-use gilrs::Gilrs;
+use clap::{arg, command, value_parser};
-use pixels::{PixelsBuilder, SurfaceTexture};
+use gb::gui::{EmuMode, Gui};
-use rodio::{OutputStream, Sink};
+use gb::{emu, gui};
-use std::time::{Duration, Instant};
+use gilrs::Gilrs;
-use winit::dpi::LogicalSize;
+use rodio::{OutputStream, Sink};
-use winit::event::{Event, VirtualKeyCode};
+use tracing_subscriber::EnvFilter;
-use winit::event_loop::{ControlFlow, EventLoop};
+use winit::event::{Event, WindowEvent};
-use winit::window::{Window, WindowBuilder};
+use winit::event_loop::{EventLoop, EventLoopBuilder};
-use winit_input_helper::WinitInputHelper;
 
-const WINDOW_SCALE: f64 = 2.0;
 const AUDIO_ENABLED: bool = true;
 
-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 rom_path = m
+    // Set up subscriber
-        .value_of("rom")
+    if std::env::var("RUST_LOG").is_err() {
-        .expect("Required value 'rom' was provided");
+        std::env::set_var("RUST_LOG", "gb=info");
+    }
 
-    let mut game_boy =
+    tracing_subscriber::fmt::fmt()
-        gb::emu::init(m.value_of("boot"), rom_path).expect("Initialize DMG-01 Emulator");
+        .with_env_filter(EnvFilter::from_default_env())
-    let rom_title = gb::emu::rom_title(&game_boy);
+        .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 sink = Sink::try_new(&stream_handle)?;
+            let s = Sink::try_new(&stream_handle).expect("create sink from audio stream handle");
-        sink.append(cons);
+            s.append(cons);
-        game_boy.apu_mut().attach_producer(prod);
+            s.set_volume(0.1);
+            s
+        };
 
+        emu::set_audio_prod(&mut cpu, prod);
+
+        tracing::info!("Spawn Audio Thread");
         std::thread::spawn(move || {
             sink.sleep_until_end();
         });
     }
 
-    let mut start = Instant::now();
+    // Set up state for the Immediate-mode GUI
-    let frame_time = Duration::from_secs_f64(1.0 / 59.73); // 59.73 Hz on Host
+    let event_loop: EventLoop<Event<()>> = EventLoopBuilder::with_user_event().build();
-    let mut cycle_count: Cycle = Default::default();
+
+    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);
+                    }
+                    _ => {}
+                };
+
+                // Input has been consumed, reset it
+                last_key = gui::unused_key();
+
+                app.request_redraw();
             }
-        }
+            Event::RedrawRequested(..) => {
-
+                app.update_time(start_time.elapsed().as_secs_f64());
-        if input.update(&event) {
+                app.paint(&cpu);
-            if input.key_pressed(VirtualKeyCode::Escape) || input.quit() {
-                *control_flow = ControlFlow::Exit;
-                return;
-            }
-
-            if let Some(size) = input.window_resized() {
-                pixels.resize_surface(size.width, size.height);
-            }
-
-            let mut diff = Instant::now() - start;
-            while diff.subsec_nanos() < frame_time.subsec_nanos() {
-                if cycle_count < gb::emu::CYCLES_IN_FRAME {
-                    cycle_count += gb::emu::run_frame(&mut game_boy, &mut gamepad, &input);
-                }
-
-                diff = Instant::now() - start;
-            }
-            start = Instant::now();
-
-            if cycle_count >= gb::emu::CYCLES_IN_FRAME {
-                cycle_count %= gb::emu::CYCLES_IN_FRAME;
-
-                gb::emu::draw(game_boy.ppu(), pixels.get_frame());
-                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,
+                _ => {}
+            },
+            _ => {}
         }
     });
 }
-
-#[cfg(not(windows))]
-fn create_window(event_loop: &EventLoop<()>, title: &str) -> Result<Window> {
-    let size = LogicalSize::new(
-        (GB_WIDTH as f64) * WINDOW_SCALE,
-        (GB_HEIGHT as f64) * WINDOW_SCALE,
-    );
-    Ok(WindowBuilder::new()
-        .with_title(title)
-        .with_inner_size(size)
-        .with_min_inner_size(size)
-        .with_resizable(true)
-        .with_decorations(true)
-        .with_transparent(false)
-        .build(event_loop)?)
-}
-
-#[cfg(windows)]
-fn create_window(event_loop: &EventLoop<()>, title: &str) -> Result<Window> {
-    use winit::platform::windows::WindowBuilderExtWindows;
-
-    let size = LogicalSize::new(
-        (GB_WIDTH as f64) * WINDOW_SCALE,
-        (GB_HEIGHT as f64) * WINDOW_SCALE,
-    );
-    Ok(WindowBuilder::new()
-        .with_title(title)
-        .with_inner_size(size)
-        .with_min_inner_size(size)
-        .with_resizable(true)
-        .with_decorations(true)
-        .with_transparent(false)
-        .with_drag_and_drop(false)
-        .build(event_loop)?)
-}

src/ppu/dma.rs

@@ -1,6 +1,6 @@
-use crate::instruction::cycle::Cycle;
+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 {
@@ -56,7 +66,7 @@ impl DirectMemoryAccess {
     }
 
     fn reset(&mut self) {
-        self.cycle = Cycle::new(0);
+        self.cycle = 0;
         self.state = DmaState::Disabled;
         self.start.0 = None;
     }
@@ -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/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

@@ -21,19 +21,6 @@ impl Timer {
         use State::*;
         use TimerSpeed::*;
 
-        if let TIMAOverflow(step) | AbortedTIMAOverflow(step) = self.state {
-            if step < 4 {
-                self.state = TIMAOverflow(step + 1);
-                return;
-            }
-
-            if self.state == TIMAOverflow(step) {
-                self.counter = self.modulo;
-                self.interrupt = true;
-            }
-            self.state = Normal;
-        }
-
         self.divider = self.divider.wrapping_add(1);
 
         // Get Bit Position
@@ -55,6 +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
@@ -67,14 +65,12 @@ impl Timer {
         use State::*;
 
         match self.state {
-            Normal => self.counter = byte,
+            Normal | AbortedTIMAOverflow(_) => self.counter = byte,
             TIMAOverflow(step) => {
-                if step < 4 {
+                self.counter = byte;
-                    self.counter = byte;
+                self.state = AbortedTIMAOverflow(step);
-                    self.state = AbortedTIMAOverflow(step);
-                }
             }
-            AbortedTIMAOverflow(_) => self.counter = byte,
+            LoadTIMA => { /* Ignored */ }
         }
     }
 
@@ -94,12 +90,23 @@ impl Timer {
             self.state = State::TIMAOverflow(0);
         }
     }
+
+    fn next(&mut self) {
+        use State::*;
+
+        self.state = match self.state {
+            Normal | LoadTIMA | AbortedTIMAOverflow(3) => Normal,
+            TIMAOverflow(3) => LoadTIMA,
+            AbortedTIMAOverflow(step) => AbortedTIMAOverflow(step + 1),
+            TIMAOverflow(step) => TIMAOverflow(step + 1),
+        }
+    }
 }
 
 impl Default for Timer {
     fn default() -> Self {
         Self {
-            ctrl: Default::default(),
+            ctrl: TimerControl(0),
             counter: 0,
             modulo: 0,
             divider: 0,
@@ -150,12 +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)
@@ -173,4 +174,5 @@ enum State {
     TIMAOverflow(u8),
     AbortedTIMAOverflow(u8),
     Normal,
+    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;
+    }
 }