feat: implement timers

src/bus.rs

@@ -83,13 +83,13 @@ impl Bus {
 
                 match self.cartridge.as_ref() {
                     Some(cart) => cart.read_byte(addr),
-                    None => panic!("Tried to read from a non-existant cartridge"),
+                    None => panic!("Tried to read from a non-existent cartridge"),
                 }
             }
             0x4000..=0x7FFF => match self.cartridge.as_ref() {
                 // 16KB ROM Bank 01 -> NN (switchable via MB)
                 Some(cart) => cart.read_byte(addr),
-                None => panic!("Tried to read from a non-existant cartridge"),
+                None => panic!("Tried to read from a non-existent cartridge"),
             },
             0x8000..=0x9FFF => {
                 // 8KB Video RAM
@@ -98,7 +98,7 @@ impl Bus {
             0xA000..=0xBFFF => match self.cartridge.as_ref() {
                 // 8KB External RAM
                 Some(cart) => cart.read_byte(addr),
-                None => panic!("Tried to read from the external RAM of a non-existant cartridge"),
+                None => panic!("Tried to read from the external RAM of a non-existent cartridge"),
             },
             0xC000..=0xCFFF => {
                 // 4KB Work RAM Bank 0
@@ -123,7 +123,9 @@ impl Bus {
                     0xFF00 => self.joypad.status.into(),
                     0xFF01 => self.serial.next,
                     0xFF02 => self.serial.control.into(),
+                    0xFF04 => self.timer.divider,
                     0xFF05 => self.timer.counter,
+                    0xFF06 => self.timer.modulo,
                     0xFF07 => self.timer.control.into(),
                     0xFF0F => self.interrupt_flag().into(),
                     0xFF11 => self.sound.ch1.sound_duty.into(),
@@ -210,7 +212,9 @@ impl Bus {
                     0xFF00 => self.joypad.status = byte.into(),
                     0xFF01 => self.serial.next = byte,
                     0xFF02 => self.serial.control = byte.into(),
+                    0xFF04 => self.timer.divider = 0x00,
                     0xFF05 => self.timer.counter = byte.into(),
+                    0xFF06 => self.timer.modulo = byte.into(),
                     0xFF07 => self.timer.control = byte.into(),
                     0xFF0F => self.set_interrupt_flag(byte),
                     0xFF11 => self.sound.ch1.sound_duty = byte.into(),
@@ -276,9 +280,12 @@ impl Bus {
         let vblank = self.ppu.interrupt.vblank();
         let lcd_stat = self.ppu.interrupt.lcd_stat();
 
-        // Read the currrent interrupt information from the Joypad
+        // Read the current interrupt information from the Joypad
         let joypad = self.joypad.interrupt();
 
+        // Read the current interrupt information from the Timer
+        let timer = self.timer.interrupt();
+
         // Copy the Interrupt Flag register 0xFF0F
         let mut flag = self.interrupt.flag;
 
@@ -286,6 +293,7 @@ impl Bus {
         flag.set_vblank(vblank);
         flag.set_lcd_stat(lcd_stat);
         flag.set_joypad(joypad);
+        flag.set_timer(timer);
         flag
     }
 
@@ -296,6 +304,7 @@ impl Bus {
         let vblank = self.interrupt.flag.vblank();
         let lcd_stat = self.interrupt.flag.lcd_stat();
         let joypad = self.interrupt.flag.joypad();
+        let timer = self.interrupt.flag.timer();
 
         // Update the PPU's instance of the following interrupts
         self.ppu.interrupt.set_vblank(vblank);
@@ -303,5 +312,8 @@ impl Bus {
 
         // Update the Joypad's instance of the following interrupts
         self.joypad.set_interrupt(joypad);
+
+        // Update the Timer's instance of the following interrupts
+        self.timer.set_interrupt(timer);
     }
 }

src/instruction.rs

@@ -2146,6 +2146,34 @@ impl std::ops::RemAssign<u32> for Cycles {
     }
 }
 
+impl std::ops::Sub for Cycles {
+    type Output = Cycles;
+
+    fn sub(self, rhs: Self) -> Self::Output {
+        Self(self.0 - rhs.0)
+    }
+}
+
+impl std::ops::Sub<u32> for Cycles {
+    type Output = Cycles;
+
+    fn sub(self, rhs: u32) -> Self::Output {
+        Self(self.0 - rhs)
+    }
+}
+
+impl std::ops::SubAssign for Cycles {
+    fn sub_assign(&mut self, rhs: Self) {
+        *self = Self(self.0 - rhs.0);
+    }
+}
+
+impl std::ops::SubAssign<u32> for Cycles {
+    fn sub_assign(&mut self, rhs: u32) {
+        *self = Self(self.0 - rhs);
+    }
+}
+
 impl From<u32> for Cycles {
     fn from(num: u32) -> Self {
         Self(num)

src/timer.rs

@@ -1,16 +1,69 @@
+use crate::Cycles;
+use crate::LR35902_CLOCK_SPEED;
 use bitfield::bitfield;
 
-use crate::instruction::Cycles;
+const DIVIDER_REGISTER_HZ: u32 = 16384;
 
 #[derive(Debug, Clone, Copy)]
 pub struct Timer {
     pub control: TimerControl,
     pub counter: u8,
+    pub modulo: u8,
+    pub divider: u8,
+    divider_cycles: Cycles,
+    timer_cycles: Cycles,
+    interrupt: bool,
 }
 
 impl Timer {
-    pub fn step(&mut self, _cycles: Cycles) {
-        //
+    pub fn step(&mut self, cycles: Cycles) {
+        self.timer_cycles += cycles;
+        self.divider_cycles += cycles;
+
+        // Divider Register
+        let divider_wait = Cycles::new(LR35902_CLOCK_SPEED / DIVIDER_REGISTER_HZ);
+        let timer_wait = self.timer_cycles();
+
+        if self.divider_cycles >= divider_wait {
+            // The Divider Timer has ticked
+            self.divider_cycles %= divider_wait;
+
+            self.divider = self.divider.wrapping_add(1);
+        }
+
+        if self.control.enabled() {
+            if self.timer_cycles >= timer_wait {
+                // The Timer has ticked
+                self.timer_cycles %= timer_wait;
+
+                let (result, did_overflow) = self.counter.overflowing_add(1);
+                self.counter = if did_overflow {
+                    self.interrupt = true;
+                    self.modulo
+                } else {
+                    result
+                };
+            }
+        }
+    }
+
+    fn timer_cycles(&self) -> Cycles {
+        let difference = match self.control.speed() {
+            TimerSpeed::Hz4096 => LR35902_CLOCK_SPEED / 4096,
+            TimerSpeed::Hz262144 => LR35902_CLOCK_SPEED / 262144,
+            TimerSpeed::Hz65536 => LR35902_CLOCK_SPEED / 65536,
+            TimerSpeed::Hz16384 => LR35902_CLOCK_SPEED / 16384,
+        };
+
+        Cycles::new(difference)
+    }
+
+    pub fn interrupt(&self) -> bool {
+        self.interrupt
+    }
+
+    pub fn set_interrupt(&mut self, value: bool) {
+        self.interrupt = value;
     }
 }
 
@@ -18,26 +71,31 @@ impl Default for Timer {
     fn default() -> Self {
         Self {
             control: Default::default(),
+            timer_cycles: Default::default(),
+            divider_cycles: Default::default(),
             counter: 0,
+            modulo: 0,
+            divider: 0,
+            interrupt: false,
         }
     }
 }
 
 #[derive(Debug, Clone, Copy)]
 pub enum TimerSpeed {
-    Freq4096Hz = 0,
-    Freq262144Hz = 1,
-    Freq65536Hz = 2,
-    Freq16384Hz = 3,
+    Hz4096 = 0,
+    Hz262144 = 1,
+    Hz65536 = 2,
+    Hz16384 = 3,
 }
 
 impl From<u8> for TimerSpeed {
     fn from(byte: u8) -> Self {
         match byte {
-            0b00 => Self::Freq4096Hz,
-            0b01 => Self::Freq262144Hz,
-            0b10 => Self::Freq65536Hz,
-            0b11 => Self::Freq16384Hz,
+            0b00 => Self::Hz4096,
+            0b01 => Self::Hz262144,
+            0b10 => Self::Hz65536,
+            0b11 => Self::Hz16384,
             _ => unreachable!("{:#04X} is not a valid value for TimerSpeed", byte),
         }
     }