fix(apu): incremental improvements to APU accuracy

src/apu.rs

@@ -64,28 +64,29 @@ impl BusIo for Apu {
 
     fn write_byte(&mut self, addr: u16, byte: u8) {
         match addr & 0x00FF {
-            0x10 => self.ch1.set_sweep(byte),
+            0x10 if self.ctrl.enabled => self.ch1.set_sweep(byte),
-            0x11 => self.ch1.set_duty(byte),
+            0x11 if self.ctrl.enabled => self.ch1.set_duty(byte),
-            0x12 => self.ch1.set_envelope(byte),
+            0x12 if self.ctrl.enabled => self.ch1.set_envelope(byte),
-            0x13 => self.ch1.set_freq_lo(byte),
+            0x13 if self.ctrl.enabled => self.ch1.set_freq_lo(byte),
-            0x14 => self.ch1.set_freq_hi(byte),
+            0x14 if self.ctrl.enabled => self.ch1.set_freq_hi(byte),
-            0x16 => self.ch2.set_duty(byte),
+            0x16 if self.ctrl.enabled => self.ch2.set_duty(byte),
-            0x17 => self.ch2.set_envelope(byte),
+            0x17 if self.ctrl.enabled => self.ch2.set_envelope(byte),
-            0x18 => self.ch2.set_freq_lo(byte),
+            0x18 if self.ctrl.enabled => self.ch2.set_freq_lo(byte),
-            0x19 => self.ch2.set_freq_hi(byte),
+            0x19 if self.ctrl.enabled => self.ch2.set_freq_hi(byte),
-            0x1A => self.ch3.set_enabled(byte),
+            0x1A if self.ctrl.enabled => self.ch3.set_enabled(byte),
-            0x1B => self.ch3.set_len(byte),
+            0x1B if self.ctrl.enabled => self.ch3.set_len(byte),
-            0x1C => self.ch3.set_volume(byte),
+            0x1C if self.ctrl.enabled => self.ch3.set_volume(byte),
-            0x1D => self.ch3.set_freq_lo(byte),
+            0x1D if self.ctrl.enabled => self.ch3.set_freq_lo(byte),
-            0x1E => self.ch3.set_freq_hi(byte),
+            0x1E if self.ctrl.enabled => self.ch3.set_freq_hi(byte),
-            0x20 => self.ch4.set_len(byte),
+            0x20 if self.ctrl.enabled => self.ch4.set_len(byte),
-            0x21 => self.ch4.set_envelope(byte),
+            0x21 if self.ctrl.enabled => self.ch4.set_envelope(byte),
-            0x22 => self.ch4.set_poly(byte),
+            0x22 if self.ctrl.enabled => self.ch4.set_poly(byte),
-            0x23 => self.ch4.set_freq_data(byte),
+            0x23 if self.ctrl.enabled => self.ch4.set_frequency(byte),
-            0x24 => self.ctrl.set_channel(byte),
+            0x24 if self.ctrl.enabled => self.ctrl.set_channel(byte),
-            0x25 => self.ctrl.set_output(byte),
+            0x25 if self.ctrl.enabled => self.ctrl.set_output(byte),
             0x26 => self.set_status(byte),
             0x30..=0x3F => self.ch3.write_byte(addr, byte),
+            _ if !self.ctrl.enabled => {}
             _ => eprintln!(
                 "Wrote {:#04X} to unused IO register {:#06X} [APU]",
                 byte, addr
@@ -135,7 +136,7 @@ impl Apu {
             self.sample_counter %= SM83_CLOCK_SPEED;
 
             if let Some(ref mut prod) = self.prod {
-                if prod.two_available() {
+                if prod.available_block() {
                     // Sample the APU
                     let ch1_amplitude = self.ch1.amplitude();
                     let ch1_left = self.ctrl.output.ch1_left() as u8 as f32 * ch1_amplitude;
@@ -172,14 +173,21 @@ impl Apu {
     pub(crate) fn set_status(&mut self, byte: u8) {
         self.ctrl.enabled = (byte >> 7) & 0x01 == 0x01;
 
-        if !self.ctrl.enabled {
+        if self.ctrl.enabled {
-            self.reset();
+            // Frame Sequencer reset to Step 0
+            self.frame_seq_state = Default::default();
+
+            // Square Duty units are reset to first step
+            self.ch1.duty_pos = 0;
+            self.ch2.duty_pos = 0;
+
+            // Wave Channel's sample buffer reset to 0
         }
 
-        self.ch1.enabled = self.ctrl.enabled;
+        if !self.ctrl.enabled {
-        self.ch2.enabled = self.ctrl.enabled;
+            self.reset();
-        self.ch3.enabled = self.ctrl.enabled;
+        } else {
-        self.ch4.enabled = self.ctrl.enabled;
+        }
     }
 
     fn reset(&mut self) {
@@ -188,15 +196,18 @@ impl Apu {
         self.ch1.sweep = Default::default();
         self.ch1.duty = Default::default();
         self.ch1.envelope = Default::default();
-        self.ch1.freq_hi = Default::default(); // FIXME: What about frequency low?
+        self.ch1.freq_lo = Default::default();
+        self.ch1.freq_hi = Default::default();
 
         self.ch2.duty = Default::default();
         self.ch2.envelope = Default::default();
+        self.ch2.freq_lo = Default::default();
         self.ch2.freq_hi = Default::default();
 
         self.ch3.enabled = Default::default();
         self.ch3.len = Default::default();
         self.ch3.volume = Default::default();
+        self.ch3.freq_lo = Default::default();
         self.ch3.freq_hi = Default::default();
 
         self.ch4.len = Default::default();
@@ -204,15 +215,17 @@ impl Apu {
         self.ch4.poly = Default::default();
         self.ch4.freq = Default::default();
 
-        self.ch2 = Default::default();
-        self.ch3 = Default::default();
-        self.ch4 = Default::default();
         self.ctrl.channel = Default::default();
         self.ctrl.output = Default::default();
+
+        // Disable the rest of the channels
+        self.ch1.enabled = Default::default();
+        self.ch2.enabled = Default::default();
+        self.ch4.enabled = Default::default();
     }
 
     fn clock_length(freq_hi: &FrequencyHigh, length_timer: &mut u16, enabled: &mut bool) {
-        if freq_hi.idk() && *length_timer > 0 {
+        if freq_hi.length_disable() && *length_timer > 0 {
             *length_timer -= 1;
 
             // Check in this scope ensures (only) the above subtraction
@@ -223,8 +236,8 @@ impl Apu {
         }
     }
 
-    fn clock_length_ch4(freq_data: &Ch4Frequency, length_timer: &mut u16, enabled: &mut bool) {
+    fn clock_length_ch4(freq: &Ch4Frequency, length_timer: &mut u16, enabled: &mut bool) {
-        if freq_data.idk() && *length_timer > 0 {
+        if freq.length_disable() && *length_timer > 0 {
             *length_timer -= 1;
 
             // Check in this scope ensures (only) the above subtraction
@@ -262,23 +275,20 @@ impl Apu {
     }
 
     fn handle_sweep(&mut self) {
-        if self.ch1.sweep_timer > 0 {
+        if self.ch1.sweep_timer != 0 {
             self.ch1.sweep_timer -= 1;
         }
 
         if self.ch1.sweep_timer == 0 {
-            self.ch1.sweep_timer = if self.ch1.sweep.period() != 0 {
+            let period = self.ch1.sweep.period();
-                self.ch1.sweep.period()
+            self.ch1.sweep_timer = if period == 0 { 8 } else { period };
-            } else {
-                8
-            };
 
-            if self.ch1.sweep_enabled && self.ch1.sweep.period() != 0 {
+            if self.ch1.sweep_enabled && period != 0 {
                 let new_freq = self.ch1.calc_sweep_freq();
 
                 if new_freq <= 2047 && self.ch1.sweep.shift_count() != 0 {
                     self.ch1.set_frequency(new_freq);
-                    self.ch1.shadow_freq = new_freq & 0x07FF;
+                    self.ch1.shadow_freq = new_freq;
 
                     let _ = self.ch1.calc_sweep_freq();
                 }
@@ -371,6 +381,7 @@ impl SoundControl {
             | (apu.ch3.enabled as u8) << 2
             | (apu.ch2.enabled as u8) << 1
             | apu.ch1.enabled as u8
+            | 0x70
     }
 }
 
@@ -431,10 +442,8 @@ impl Channel1 {
 
     /// 0xFF10 | NR10 - Channel 1 Sweep Register
     pub(crate) fn set_sweep(&mut self, byte: u8) {
-        if self.enabled {
         self.sweep = byte.into()
     }
-    }
 
     /// 0xFF11 | NR11 - Channel 1 Sound length / Wave pattern duty
     pub(crate) fn duty(&self) -> u8 {
@@ -443,11 +452,9 @@ impl Channel1 {
 
     /// 0xFF11 | NR11 - Channel 1 Sound length / Wave pattern duty
     pub(crate) fn set_duty(&mut self, byte: u8) {
-        if self.enabled {
         self.duty = byte.into();
         self.length_timer = 64 - self.duty.sound_length() as u16;
     }
-    }
 
     /// 0xFF12 | NR12 - Channel 1 Volume Envelope
     pub fn envelope(&self) -> u8 {
@@ -456,17 +463,13 @@ impl Channel1 {
 
     /// 0xFF12 | NR12 - Channel 1 Volume Envelope
     pub(crate) fn set_envelope(&mut self, byte: u8) {
-        if self.enabled {
         self.envelope = byte.into()
     }
-    }
 
     /// 0xFF13 | NR13 - Channel 1 Frequency low (lower 8 bits only)
     pub(crate) fn set_freq_lo(&mut self, byte: u8) {
-        if self.enabled {
         self.freq_lo = byte;
     }
-    }
 
     /// 0xFF14 | NR14 - Channel 1 Frequency high
     pub(crate) fn freq_hi(&self) -> u8 {
@@ -475,7 +478,6 @@ impl Channel1 {
 
     /// 0xFF14 | NR14 - Channel 1 Frequency high
     pub(crate) fn set_freq_hi(&mut self, byte: u8) {
-        if self.enabled {
         self.freq_hi = byte.into();
 
         // If this bit is set, a trigger event occurs
@@ -485,18 +487,16 @@ impl Channel1 {
             self.current_volume = self.envelope.init_vol();
 
             // Sweep behaviour during trigger event
-                self.shadow_freq = self.frequency() & 0x07FF; // Mask should be redundant
+            let sweep_period = self.sweep.period();
-                self.sweep_timer = if self.sweep.period() != 0 {
+            let sweep_shift = self.sweep.shift_count();
-                    self.sweep.period()
+            self.shadow_freq = self.frequency();
-                } else {
+            self.sweep_timer = if sweep_period == 0 { 8 } else { sweep_period };
-                    8
-                };
 
-                if self.sweep.period() != 0 || self.sweep.shift_count() != 0 {
+            if sweep_period != 0 || sweep_shift != 0 {
                 self.sweep_enabled = true;
             }
 
-                if self.sweep.shift_count() != 0 {
+            if sweep_shift != 0 {
                 let _ = self.calc_sweep_freq();
             }
 
@@ -504,17 +504,18 @@ impl Channel1 {
             if self.length_timer == 0 {
                 self.length_timer = 64;
             }
-            }
+
+            self.enabled = true;
         }
     }
 
     fn calc_sweep_freq(&mut self) -> u16 {
         use SweepDirection::*;
-        let shifted_shadow_freq = self.shadow_freq >> self.sweep.shift_count();
 
+        let shadow_freq_shifted = self.shadow_freq >> self.sweep.shift_count();
         let new_freq = match self.sweep.direction() {
-            Increase => self.shadow_freq + shifted_shadow_freq,
+            Increase => self.shadow_freq + shadow_freq_shifted,
-            Decrease => self.shadow_freq - shifted_shadow_freq,
+            Decrease => self.shadow_freq - shadow_freq_shifted,
         };
 
         // Overflow check
@@ -587,11 +588,9 @@ impl Channel2 {
 
     /// 0xFF16 | NR21 - Channel 2 Sound length / Wave Pattern Duty
     pub(crate) fn set_duty(&mut self, byte: u8) {
-        if self.enabled {
         self.duty = byte.into();
         self.length_timer = 64 - self.duty.sound_length() as u16;
     }
-    }
 
     /// 0xFF17 | NR22 - Channel 2 Volume ENvelope
     pub(crate) fn envelope(&self) -> u8 {
@@ -600,17 +599,13 @@ impl Channel2 {
 
     /// 0xFF17 | NR22 - Channel 2 Volume ENvelope
     pub(crate) fn set_envelope(&mut self, byte: u8) {
-        if self.enabled {
         self.envelope = byte.into()
     }
-    }
 
     /// 0xFF18 | NR23 - Channel 2 Frequency low (lower 8 bits only)
     pub(crate) fn set_freq_lo(&mut self, byte: u8) {
-        if self.enabled {
         self.freq_lo = byte;
     }
-    }
 
     /// 0xFF19 | NR24 - Channel 2 Frequency high
     pub(crate) fn freq_hi(&self) -> u8 {
@@ -619,7 +614,6 @@ impl Channel2 {
 
     /// 0xFF19 | NR24 - Channel 2 Frequency high
     pub(crate) fn set_freq_hi(&mut self, byte: u8) {
-        if self.enabled {
         self.freq_hi = byte.into();
 
         if self.freq_hi.initial() {
@@ -631,7 +625,8 @@ impl Channel2 {
             if self.length_timer == 0 {
                 self.length_timer = 64;
             }
-            }
+
+            self.enabled = true;
         }
     }
 
@@ -664,13 +659,21 @@ pub(crate) struct Channel3 {
 
 impl BusIo for Channel3 {
     fn read_byte(&self, addr: u16) -> u8 {
+        if self.enabled {
+            self.wave_ram[self.offset as usize]
+        } else {
             self.wave_ram[(addr - Self::WAVE_RAM_START_ADDR) as usize]
         }
+    }
 
     fn write_byte(&mut self, addr: u16, byte: u8) {
+        if self.enabled {
+            self.wave_ram[self.offset as usize] = byte;
+        } else {
             self.wave_ram[(addr - Self::WAVE_RAM_START_ADDR) as usize] = byte;
         }
     }
+}
 
 impl Channel3 {
     const WAVE_RAM_START_ADDR: u16 = 0xFF30;
@@ -692,11 +695,9 @@ impl Channel3 {
 
     /// 0xFF1B | NR31 - Sound Length
     pub(crate) fn set_len(&mut self, byte: u8) {
-        if self.enabled {
         self.len = byte;
         self.length_timer = 256 - self.len as u16;
     }
-    }
 
     /// 0xFF1C | NR32 - Channel 3 Volume
     pub(crate) fn volume(&self) -> u8 {
@@ -707,7 +708,6 @@ impl Channel3 {
     pub(crate) fn set_volume(&mut self, byte: u8) {
         use Ch3Volume::*;
 
-        if self.enabled {
         self.volume = match (byte >> 5) & 0x03 {
             0b00 => Mute,
             0b01 => Full,
@@ -716,14 +716,11 @@ impl Channel3 {
             _ => unreachable!("{:#04X} is not a valid value for Channel3Volume", byte),
         };
     }
-    }
 
     /// 0xFF1D | NR33 - Channel 3 Frequency low (lower 8 bits)
     pub(crate) fn set_freq_lo(&mut self, byte: u8) {
-        if self.enabled {
         self.freq_lo = byte;
     }
-    }
 
     /// 0xFF1E | NR34 - Channel 3 Frequency high
     pub(crate) fn freq_hi(&self) -> u8 {
@@ -732,7 +729,6 @@ impl Channel3 {
 
     /// 0xFF1E | NR34 - Channel 3 Frequency high
     pub(crate) fn set_freq_hi(&mut self, byte: u8) {
-        if self.enabled {
         self.freq_hi = byte.into();
 
         if self.freq_hi.initial() {
@@ -740,7 +736,8 @@ impl Channel3 {
             if self.length_timer == 0 {
                 self.length_timer = 256;
             }
-            }
+
+            self.enabled = true;
         }
     }
 
@@ -756,13 +753,13 @@ impl Channel3 {
         }
 
         if self.freq_timer == 0 {
-            self.freq_timer = (2048 - self.frequency()) * 4;
+            self.freq_timer = (2048 - self.frequency()) * 2;
-            self.offset = (self.offset + 1) % 8;
+            self.offset = (self.offset + 1) % (WAVE_PATTERN_RAM_LEN * 2) as u8;
         }
     }
 
     fn read_sample(&self, index: u8) -> u8 {
-        let i = index as usize / 2;
+        let i = (index / 2) as usize;
 
         if index % 2 == 0 {
             // We grab the high nibble on even indexes
@@ -812,11 +809,9 @@ impl Channel4 {
 
     /// 0xFF20 | NR41 - Channel 4 Sound Length
     pub(crate) fn set_len(&mut self, byte: u8) {
-        if self.enabled {
         self.len = byte & 0x3F;
         self.length_timer = 256 - self.len as u16;
     }
-    }
 
     /// 0xFF21 | NR42 - Channel 4 Volume Envelope
     pub(crate) fn envelope(&self) -> u8 {
@@ -825,10 +820,8 @@ impl Channel4 {
 
     /// 0xFF21 | NR42 - Channel 4 Volume Envelope
     pub(crate) fn set_envelope(&mut self, byte: u8) {
-        if self.enabled {
         self.envelope = byte.into()
     }
-    }
 
     /// 0xFF22 | NR43 - Chanel 4 Polynomial Counter
     pub(crate) fn poly(&self) -> u8 {
@@ -837,10 +830,8 @@ impl Channel4 {
 
     /// 0xFF22 | NR43 - Chanel 4 Polynomial Counter
     pub(crate) fn set_poly(&mut self, byte: u8) {
-        if self.enabled {
         self.poly = byte.into();
     }
-    }
 
     /// 0xFF23 | NR44 - Channel 4 Counter / Consecutive Selector and Restart
     pub(crate) fn frequency(&self) -> u8 {
@@ -848,8 +839,7 @@ impl Channel4 {
     }
 
     /// 0xFF23 | NR44 - Channel 4 Counter / Consecutive Selector and Restart
-    pub(crate) fn set_freq_data(&mut self, byte: u8) {
+    pub(crate) fn set_frequency(&mut self, byte: u8) {
-        if self.enabled {
         self.freq = byte.into();
 
         if self.freq.initial() {
@@ -864,7 +854,8 @@ impl Channel4 {
 
             // LFSR behaviour during trigger event
             self.lf_shift = 0x7FFF;
-            }
+
+            self.enabled = true;
         }
     }
 

src/apu/gen.rs

@@ -43,10 +43,17 @@ impl<T> SampleProducer<T> {
         self.inner.push(value)
     }
 
-    #[inline]
+    pub(crate) fn available(&self) -> bool {
-    pub(crate) fn two_available(&self) -> bool {
         self.inner.slots() > 2
     }
+
+    pub(crate) fn available_block(&self) -> bool {
+        loop {
+            if self.inner.slots() > 2 {
+                break true;
+            }
+        }
+    }
 }
 
 impl<T> std::fmt::Debug for SampleProducer<T> {

src/apu/types.rs

@@ -178,12 +178,12 @@ pub(crate) mod ch4 {
         pub struct Frequency(u8);
         impl Debug;
         _initial, _: 7;
-        _idk, _: 6; // TODO: same as FrequencyHigh, figure out what this is
+        _length_disable, _: 6; // TODO: same as FrequencyHigh, figure out what this is
     }
 
     impl Frequency {
-        pub(crate) fn idk(&self) -> bool {
+        pub(crate) fn length_disable(&self) -> bool {
-            self._idk()
+            self._length_disable()
         }
 
         pub(crate) fn initial(&self) -> bool {
@@ -224,7 +224,7 @@ pub(crate) mod common {
         pub struct FrequencyHigh(u8);
         impl Debug;
         _initial, _: 7;
-        _idk, _: 6; // TODO: Figure out what the hell this is
+        _length_disable, _: 6; // TODO: Figure out what the hell this is
         pub freq_bits, set_freq_bits: 2, 0;
     }
 
@@ -233,8 +233,8 @@ pub(crate) mod common {
             self._initial()
         }
 
-        pub(crate) fn idk(&self) -> bool {
+        pub(crate) fn length_disable(&self) -> bool {
-            self._idk()
+            self._length_disable()
         }
     }
 

src/bus.rs

@@ -66,6 +66,7 @@ impl Bus {
         self.cartridge.as_ref()?.title()
     }
 
+    #[allow(dead_code)]
     pub(crate) fn boot_mapped(&self) -> bool {
         self.boot.is_some()
     }

src/emu.rs

@@ -33,18 +33,11 @@ pub fn run(
     game_boy: &mut SM83,
     gamepad: &mut Gilrs,
     input: &WinitInputHelper,
-    pending: Cycle,
+    target: Cycle,
 ) -> Cycle {
     let mut elapsed = Cycle::new(0);
 
-    while elapsed < pending {
+    while elapsed < target {
-        elapsed += run_unsynced(game_boy, gamepad, input);
-    }
-
-    elapsed
-}
-
-pub fn run_unsynced(game_boy: &mut SM83, gamepad: &mut Gilrs, input: &WinitInputHelper) -> Cycle {
         if GAMEPAD_ENABLED {
             if let Some(event) = gamepad.next_event() {
                 joypad::handle_gamepad_input(game_boy.joypad_mut(), event);
@@ -52,7 +45,27 @@ pub fn run_unsynced(game_boy: &mut SM83, gamepad: &mut Gilrs, input: &WinitInput
         }
 
         joypad::handle_keyboard_input(game_boy.joypad_mut(), input);
-    game_boy.step()
+        elapsed += game_boy.step();
+    }
+
+    elapsed
+}
+
+pub fn run_frame(game_boy: &mut SM83, gamepad: &mut Gilrs, input: &WinitInputHelper) -> Cycle {
+    let mut elapsed = Cycle::new(0);
+
+    while elapsed < CYCLES_IN_FRAME {
+        if GAMEPAD_ENABLED {
+            if let Some(event) = gamepad.next_event() {
+                joypad::handle_gamepad_input(game_boy.joypad_mut(), event);
+            }
+        }
+
+        joypad::handle_keyboard_input(game_boy.joypad_mut(), input);
+        elapsed += game_boy.step();
+    }
+
+    elapsed
 }
 
 pub fn draw(ppu: &Ppu, frame: &mut [u8]) {

src/instruction.rs

@@ -1456,7 +1456,6 @@ impl Instruction {
         }
     }
 
-    #[inline]
     fn unprefixed(byte: u8) -> Self {
         use Instruction::*;
 
@@ -1573,7 +1572,6 @@ impl Instruction {
         }
     }
 
-    #[inline]
     fn prefixed(byte: u8) -> Self {
         use Instruction::*;
 
@@ -1934,7 +1932,6 @@ mod table {
         }
     }
 
-    #[inline]
     pub(crate) fn group1(code: u8) -> Group1RegisterPair {
         use Group1RegisterPair::*;
 
@@ -1947,7 +1944,6 @@ mod table {
         }
     }
 
-    #[inline]
     pub(crate) fn group2(code: u8) -> Group2RegisterPair {
         use Group2RegisterPair::*;
 
@@ -1960,7 +1956,6 @@ mod table {
         }
     }
 
-    #[inline]
     pub(crate) fn group3(code: u8) -> Group3RegisterPair {
         use Group3RegisterPair::*;
 
@@ -1973,7 +1968,6 @@ mod table {
         }
     }
 
-    #[inline]
     pub(crate) fn register(code: u8) -> Register {
         use Register::*;
 
@@ -1990,7 +1984,6 @@ mod table {
         }
     }
 
-    #[inline]
     pub(crate) fn jump_cond(code: u8) -> JumpCondition {
         use JumpCondition::*;
 
@@ -2003,7 +1996,6 @@ mod table {
         }
     }
 
-    #[inline]
     pub(crate) fn flag_instr(code: u8) -> Instruction {
         use Instruction::*;
 
@@ -2020,7 +2012,6 @@ mod table {
         }
     }
 
-    #[inline]
     pub(crate) fn alu_reg_instr(alu_code: u8, reg_code: u8) -> Instruction {
         use Instruction::*;
 
@@ -2037,7 +2028,6 @@ mod table {
         }
     }
 
-    #[inline]
     pub(crate) fn alu_imm_instr(code: u8) -> Instruction {
         use Instruction::*;
 
@@ -2054,7 +2044,6 @@ mod table {
         }
     }
 
-    #[inline]
     pub(crate) fn prefix_alu(alu_code: u8, reg_code: u8) -> Instruction {
         use Instruction::*;
 

src/main.rs

@@ -4,14 +4,14 @@ use gb::{AudioSPSC, Cycle, GB_HEIGHT, GB_WIDTH};
 use gilrs::Gilrs;
 use pixels::{PixelsBuilder, SurfaceTexture};
 use rodio::{OutputStream, Sink};
-use std::time::Instant;
+use std::time::{Duration, Instant};
 use winit::dpi::LogicalSize;
 use winit::event::{Event, VirtualKeyCode};
 use winit::event_loop::{ControlFlow, EventLoop};
 use winit::window::{Window, WindowBuilder};
 use winit_input_helper::WinitInputHelper;
 
-const SCALE: f64 = 2.0;
+const WINDOW_SCALE: f64 = 2.0;
 
 fn main() -> Result<()> {
     let app = App::new(crate_name!())
@@ -38,23 +38,20 @@ fn main() -> Result<()> {
         )
         .get_matches();
 
-    // `rom` is a required value in every situation so this will
-    // always exist.
     let rom_path = m
         .value_of("rom")
         .expect("Required value 'rom' was provided");
 
     let mut game_boy =
-        gb::emu::init(m.value_of("boot"), rom_path).expect("Initialized DMG-01 Emulator");
+        gb::emu::init(m.value_of("boot"), rom_path).expect("Initialize DMG-01 Emulator");
-    let cartridge_title = gb::emu::rom_title(&game_boy);
+    let rom_title = gb::emu::rom_title(&game_boy);
 
-    // Initialize Gamepad Support
+    let mut gamepad = Gilrs::new().expect("Initialize Controller Support");
-    let mut gamepad = Gilrs::new().expect("Initialized Gilrs for Controller Input");
 
     // Initialize GUI
     let event_loop = EventLoop::new();
     let mut input = WinitInputHelper::new();
-    let window = create_window(&event_loop, cartridge_title)?;
+    let window = create_window(&event_loop, rom_title)?;
 
     let mut pixels = {
         let size = window.inner_size();
@@ -65,21 +62,20 @@ fn main() -> Result<()> {
             .build()?
     };
 
+    // Initialize Audio
     let spsc: AudioSPSC<f32> = Default::default();
     let (prod, cons) = spsc.init();
-
-    game_boy.apu_mut().set_producer(prod);
-
-    // Initialize Audio
     let (_stream, stream_handle) = OutputStream::try_default().expect("Initialized Audio");
     let sink = Sink::try_new(&stream_handle)?;
     sink.append(cons);
+    game_boy.apu_mut().set_producer(prod);
 
     std::thread::spawn(move || {
         sink.sleep_until_end();
     });
 
-    let mut now = Instant::now();
+    let mut start = Instant::now();
+    let frame_time = Duration::from_secs_f64(1.0 / 60.0); // 60 Hz on Host
     let mut cycle_count: Cycle = Default::default();
 
     event_loop.run(move |event, _, control_flow| {
@@ -104,14 +100,17 @@ fn main() -> Result<()> {
                 pixels.resize_surface(size.width, size.height);
             }
 
-            let delta = now.elapsed().subsec_nanos();
+            let mut diff = Instant::now() - start;
-            now = Instant::now();
+            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);
+                }
 
-            let pending = Cycle::new(delta / gb::emu::SM83_CYCLE_TIME.subsec_nanos());
+                diff = Instant::now() - start;
-            cycle_count += gb::emu::run(&mut game_boy, &mut gamepad, &input, pending);
+            }
+            start = Instant::now();
 
             if cycle_count >= gb::emu::CYCLES_IN_FRAME {
-                // Draw Frame
                 cycle_count %= gb::emu::CYCLES_IN_FRAME;
 
                 gb::emu::draw(game_boy.ppu(), pixels.get_frame());
@@ -138,7 +137,10 @@ fn create_window(event_loop: &EventLoop<()>, title: &str) -> Result<Window> {
 fn create_window(event_loop: &EventLoop<()>, title: &str) -> Result<Window> {
     use winit::platform::windows::WindowBuilderExtWindows;
 
-    let size = LogicalSize::new((GB_WIDTH as f64) * SCALE, (GB_HEIGHT as f64) * SCALE);
+    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)