fix: properly implement LY==LYC behaviour

src/bus.rs

@@ -245,7 +245,7 @@ impl Bus {
                     0xFF00 => self.joypad.status.update(byte),
                     0xFF01 => self.serial.next = byte,
                     0xFF02 => self.serial.control = byte.into(),
-                    0xFF04 => self.timer.divider = 0x00,
+                    0xFF04 => self.timer.divider = 0x0000,
                     0xFF05 => self.timer.counter = byte,
                     0xFF06 => self.timer.modulo = byte,
                     0xFF07 => self.timer.control = byte.into(),
@@ -258,7 +258,7 @@ impl Bus {
                     0xFF25 => self.sound.control.output = byte.into(),
                     0xFF26 => self.sound.control.status = byte.into(), // FIXME: Should we control which bytes are written to here?
                     0xFF40 => self.ppu.control = byte.into(),
-                    0xFF41 => self.ppu.stat = byte.into(),
+                    0xFF41 => self.ppu.stat.update(byte),
                     0xFF42 => self.ppu.pos.scroll_y = byte,
                     0xFF43 => self.ppu.pos.scroll_x = byte,
                     0xFF44 => self.ppu.pos.line_y = byte,
@@ -267,9 +267,12 @@ impl Bus {
                         self.ppu.pos.ly_compare = byte;
 
                         // Update Coincidence Flag
-                        if self.ppu.stat.coincidence_int() {
+                        let are_equal = self.ppu.pos.line_y == byte;
-                            let are_equal = self.ppu.pos.line_y == byte;
+                        self.ppu.stat.set_coincidence(are_equal);
-                            self.ppu.stat.set_coincidence(are_equal);
+
+                        // If enabled, request a LCD STAT interrupt
+                        if self.ppu.stat.coincidence_int() && are_equal {
+                            self.ppu.int.set_lcd_stat(true);
                         }
                     }
                     0xFF47 => self.ppu.monochrome.bg_palette = byte.into(),

src/ppu.rs

@@ -103,10 +103,13 @@ impl Ppu {
                         self.cycles %= 456;
                         self.pos.line_y += 1;
 
-                        // New Scanline is next, check for LYC=LY
+                        // Update LY==LYC bit
-                        if self.stat.coincidence_int() {
+                        let are_equal = self.pos.line_y == self.pos.ly_compare;
-                            let are_equal = self.pos.line_y == self.pos.ly_compare;
+                        self.stat.set_coincidence(are_equal);
-                            self.stat.set_coincidence(are_equal);
+
+                        // Request LCD STAT interrupt if conditions met
+                        if self.stat.coincidence_int() && are_equal {
+                            self.int.set_lcd_stat(true);
                         }
 
                         let next_mode = if self.pos.line_y >= 144 {
@@ -140,10 +143,13 @@ impl Ppu {
                         self.cycles %= 456;
                         self.pos.line_y += 1;
 
-                        // New Scanline is next, check for LYC=LY
+                        // Update LY==LYC bit
-                        if self.stat.coincidence_int() {
+                        let are_equal = self.pos.line_y == self.pos.ly_compare;
-                            let are_equal = self.pos.line_y == self.pos.ly_compare;
+                        self.stat.set_coincidence(are_equal);
-                            self.stat.set_coincidence(are_equal);
+
+                        // Request LCD STAT interrupt if conditions met
+                        if self.stat.coincidence_int() && are_equal {
+                            self.int.set_lcd_stat(true);
                         }
 
                         if self.pos.line_y == 154 {
@@ -687,8 +693,8 @@ impl PixelFetcher {
         let id = self.bg.tile.id.expect("Tile Number unexpectedly missing");
 
         let tile_data_addr = match control.tile_data_addr() {
-            TileDataAddress::X8800 => 0x9000u16.wrapping_add((id as i8 * 16) as u16),
+            TileDataAddress::X8800 => 0x9000u16.wrapping_add((id as i8).wrapping_mul(16) as u16),
-            TileDataAddress::X8000 => 0x8000 + (id as u16 * 16),
+            TileDataAddress::X8000 => 0x8000 + (id as u16).wrapping_mul(16),
         };
 
         let offset = 2 * if window {
@@ -737,7 +743,7 @@ impl PixelFetcher {
             (line_y + scroll_y) % 8
         };
 
-        0x8000 + (attr.tile_index as u16 * 16) + offset as u16
+        0x8000 + (tile_number as u16 * 16) + offset as u16
     }
 }
 

src/ppu/registers.rs

@@ -13,6 +13,16 @@ bitfield! {
     pub from into PpuMode, mode, set_mode: 1, 0;
 }
 
+impl LCDStatus {
+    pub fn update(&mut self, byte: u8) {
+        // Bytes 2 -> 0 are read only
+        let mask = 0b00000111;
+
+        let read_only = self.0 & mask;
+        self.0 = (byte & !mask) | read_only;
+    }
+}
+
 impl Copy for LCDStatus {}
 impl Clone for LCDStatus {
     fn clone(&self) -> Self {
@@ -26,12 +36,6 @@ impl Default for LCDStatus {
     }
 }
 
-impl From<u8> for LCDStatus {
-    fn from(byte: u8) -> Self {
-        Self(byte)
-    }
-}
-
 impl From<LCDStatus> for u8 {
     fn from(status: LCDStatus) -> Self {
         status.0