chore: DMG compat support

src/gb.rs

@@ -127,6 +127,12 @@ pub struct GameBoyConfig {
     /// CPU frequency, PPU frequency, Boot rome size, etc.
     mode: GameBoyMode,
 
+    /// Flag that controls if the DMG compatibility mode is
+    /// enabled meaning that some of the PPU decisions will
+    /// be made differently to address this special situation
+    /// (CGB only).
+    dmg_compat: bool,
+
     /// If the PPU is enabled, it will be clocked.
     ppu_enabled: bool,
 
@@ -170,6 +176,14 @@ impl GameBoyConfig {
         self.mode = value;
     }
 
+    pub fn dmg_compat(&self) -> bool {
+        self.dmg_compat
+    }
+
+    pub fn set_dmg_compat(&mut self, value: bool) {
+        self.dmg_compat = value;
+    }
+
     pub fn ppu_enabled(&self) -> bool {
         self.ppu_enabled
     }
@@ -215,6 +229,7 @@ impl Default for GameBoyConfig {
     fn default() -> Self {
         Self {
             mode: GameBoyMode::Dmg,
+            dmg_compat: false,
             ppu_enabled: true,
             apu_enabled: true,
             timer_enabled: true,
@@ -236,6 +251,12 @@ pub struct GameBoy {
     /// kept for performance reasons.
     mode: GameBoyMode,
 
+    /// Flag that controls if the DMG compatibility mode is
+    /// enabled meaning that some of the PPU decisions will
+    /// be made differently to address this special situation
+    /// (CGB only).
+    dmg_compat: bool,
+
     /// If the PPU is enabled, it will be clocked.
     /// This is a clone of the configuration value
     /// kept for performance reasons.
@@ -312,6 +333,7 @@ impl GameBoy {
     pub fn new(mode: GameBoyMode) -> Self {
         let gbc = Rc::new(RefCell::new(GameBoyConfig {
             mode,
+            dmg_compat: false,
             ppu_enabled: true,
             apu_enabled: true,
             timer_enabled: true,
@@ -329,6 +351,7 @@ impl GameBoy {
 
         Self {
             mode,
+            dmg_compat: false,
             ppu_enabled: true,
             apu_enabled: true,
             timer_enabled: true,
@@ -574,6 +597,18 @@ impl GameBoy {
         String::from(COMPILATION_TIME)
     }
 
+    pub fn is_dmg(&self) -> bool {
+        self.mode == GameBoyMode::Dmg
+    }
+
+    pub fn is_cgb(&self) -> bool {
+        self.mode == GameBoyMode::Cgb
+    }
+
+    pub fn is_sgb(&self) -> bool {
+        self.mode == GameBoyMode::Sgb
+    }
+
     pub fn mode(&self) -> GameBoyMode {
         self.mode
     }
@@ -585,6 +620,15 @@ impl GameBoy {
         self.ppu().set_gb_mode(value);
     }
 
+    pub fn dmg_compat(&self) -> bool {
+        self.dmg_compat
+    }
+
+    pub fn set_dmg_compat(&mut self, value: bool) {
+        self.dmg_compat = value;
+        (*self.gbc).borrow_mut().set_dmg_compat(value);
+    }
+
     pub fn ppu_enabled(&self) -> bool {
         self.ppu_enabled
     }
@@ -784,6 +828,7 @@ impl GameBoy {
 
     pub fn load_rom(&mut self, data: &[u8]) -> &Cartridge {
         let rom = Cartridge::from_data(data);
+        self.set_dmg_compat(self.is_cgb() && rom.is_legacy());
         self.mmu().set_rom(rom);
         self.mmu().rom()
     }

src/rom.rs

@@ -404,6 +404,14 @@ impl Cartridge {
         }
     }
 
+    /// A cartridge is considered legacy if it does
+    /// not have a CGB flag bit (bit 7 of 0x0143) set.
+    /// These are the monochromatic only Cartridges built
+    /// for the original DMG Game Boy.
+    pub fn is_legacy(&self) -> bool {
+        self.rom_data[0x0143] & 0x80 == 0x00
+    }
+
     pub fn rom_type(&self) -> RomType {
         match self.rom_data[0x0147] {
             0x00 => RomType::RomOnly,