fix: allow for the ppu to modify interrupt flags

src/bus.rs

@@ -1,7 +1,7 @@
 use super::cartridge::Cartridge;
 use super::high_ram::HighRam;
 use super::instruction::Cycles;
-use super::interrupt::Interrupt;
+use super::interrupt::{Interrupt, InterruptFlag};
 use super::ppu::Ppu;
 use super::serial::Serial;
 use super::sound::Sound;
@@ -118,7 +118,7 @@ impl Bus {
                     0xFF01 => self.serial.next,
                     0xFF02 => self.serial.control.into(),
                     0xFF07 => self.timer.control.into(),
-                    0xFF0F => self.interrupt.flag.into(),
+                    0xFF0F => self.interrupt_flag().into(),
                     0xFF11 => self.sound.ch1.sound_duty.into(),
                     0xFF12 => self.sound.ch1.vol_envelope.into(),
                     0xFF14 => self.sound.ch1.freq_hi.into(),
@@ -203,7 +203,7 @@ impl Bus {
                     0xFF01 => self.serial.next = byte,
                     0xFF02 => self.serial.control = byte.into(),
                     0xFF07 => self.timer.control = byte.into(),
-                    0xFF0F => self.interrupt.flag = byte.into(),
+                    0xFF0F => self.set_interrupt_flag(byte),
                     0xFF11 => self.sound.ch1.sound_duty = byte.into(),
                     0xFF12 => self.sound.ch1.vol_envelope = byte.into(),
                     0xFF13 => self.sound.ch1.freq_lo = byte.into(),
@@ -251,3 +251,39 @@ impl Bus {
         self.write_byte(addr, (word & 0x00FF) as u8);
     }
 }
+
+impl Bus {
+    fn interrupt_flag(&self) -> InterruptFlag {
+        // Read the current Interrupt status from the PPU
+        let ppu_vblank = self.ppu.interrupt.vblank();
+        let ppu_lcd_stat = self.ppu.interrupt.lcd_stat();
+
+        // We actually don't care about what the InterruptFlag currently says
+        // about vblank and lcd_stat, because the PPU has the more accurate
+        // knowledge about what state these interrupt flags are in.
+
+        // In order to have the PPU be the source of truth
+        // (and accounting for the fact that we aren't able to update)
+        // the interrupt flag register 0xFF0F in the method, we can
+        // mask over those two interruptss
+
+        // Copy the Interrupt Flag register 0xFF0F
+        let mut flag = self.interrupt.flag;
+        flag.set_vblank(ppu_vblank);
+        flag.set_lcd_stat(ppu_lcd_stat);
+
+        flag
+    }
+
+    fn set_interrupt_flag(&mut self, byte: u8) {
+        // Update the Interrupt register 0xFF0F
+        self.interrupt.flag = byte.into();
+
+        let vblank = self.interrupt.flag.vblank();
+        let lcd_stat = self.interrupt.flag.lcd_stat();
+
+        // Update the PPU's internal tracking of the interrupt register 0xFF0F
+        self.ppu.interrupt.set_vblank(vblank);
+        self.ppu.interrupt.set_lcd_stat(lcd_stat);
+    }
+}

src/ppu.rs

@@ -5,6 +5,7 @@ const GB_WIDTH: usize = 160;
 const GB_HEIGHT: usize = 144;
 #[derive(Debug, Clone)]
 pub struct Ppu {
+    pub interrupt: Interrupt,
     pub lcd_control: LCDControl,
     pub monochrome: Monochrome,
     pub pos: ScreenPosition,
@@ -60,6 +61,7 @@ impl Ppu {
                     self.pos.line_y += 1;
 
                     let next_mode = if self.pos.line_y >= 144 {
+                        self.interrupt.set_vblank(true);
                         Mode::VBlank
                     } else {
                         Mode::OamScan
@@ -138,6 +140,7 @@ impl Ppu {
 impl Default for Ppu {
     fn default() -> Self {
         Self {
+            interrupt: Interrupt::default(),
             lcd_control: Default::default(),
             monochrome: Default::default(),
             pos: Default::default(),
@@ -149,6 +152,31 @@ impl Default for Ppu {
         }
     }
 }
+
+#[derive(Debug, Clone, Copy, Default)]
+pub struct Interrupt {
+    _vblank: bool,
+    _lcd_stat: bool,
+}
+
+impl Interrupt {
+    pub fn vblank(&self) -> bool {
+        self._vblank
+    }
+
+    pub fn set_vblank(&mut self, enabled: bool) {
+        self._vblank = enabled;
+    }
+
+    pub fn lcd_stat(&self) -> bool {
+        self._lcd_stat
+    }
+
+    pub fn set_lcd_stat(&mut self, enabled: bool) {
+        self._lcd_stat = enabled;
+    }
+}
+
 bitfield! {
     pub struct LCDStatus(u8);
     impl Debug;