feat: instant boot supprot

CHANGELOG.md

@@ -9,7 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
-*
+* Instant boot support using the `GameBoy.boot()` method
 
 ### Changed
 

examples/sdl/src/main.rs

@@ -158,7 +158,7 @@ fn main() {
     game_boy.load_boot_default();
 
     let mut emulator = Emulator::new(game_boy, SCREEN_SCALE);
-    emulator.load_rom("../../res/roms.prop/dr_mario.gb");
+    emulator.load_rom("../../res/roms.prop/super_mario.gb");
     emulator.run();
 }
 

src/cpu.rs

@@ -76,6 +76,31 @@ impl Cpu {
         self.cycles = 0;
     }
 
+    /// Sets the CPU registers and some of the memory space to the
+    /// state expected after the Game Boy boot ROM executes, using
+    /// these values its possible to skip the boot loading process.
+    pub fn boot(&mut self) {
+        self.pc = 0x0100;
+        self.sp = 0xfffe;
+        self.a = 0x01;
+        self.b = 0xff;
+        self.c = 0x13;
+        self.d = 0x00;
+        self.e = 0xc1;
+        self.h = 0x84;
+        self.l = 0x03;
+        self.zero = false;
+        self.sub = false;
+        self.half_carry = false;
+        self.carry = false;
+
+        // updates part of the MMU state, disabling the
+        // boot memory overlap and setting the LCD control
+        // register to enabled (required by some ROMs)
+        self.mmu.set_boot_active(false);
+        self.mmu.write(0xff40, 0x91);
+    }
+
     pub fn clock(&mut self) -> u8 {
         // gathers the PC (program counter) reference that
         // is going to be used in the fetching phase

src/gb.rs

@@ -69,6 +69,10 @@ impl GameBoy {
         self.timer().clock(cycles)
     }
 
+    pub fn boot(&mut self) {
+        self.cpu.boot();
+    }
+
     pub fn load_boot(&mut self, data: &[u8]) {
         self.cpu.mmu().write_boot(0x0000, data);
     }

src/mmu.rs

@@ -17,13 +17,19 @@ pub struct Mmu {
     /// the I/O access to this device.
     pad: Pad,
 
+    /// The timer controller to be used as part of the I/O access
+    /// that is memory mapped.
     timer: Timer,
 
     /// The cartridge ROM that is currently loaded into the system,
     /// going to be used to access ROM and external RAM banks.
     rom: Cartridge,
 
+    /// Flag that control the access to the boot section in the
+    /// 0x0000-0x00fe memory area, this flag should be unset after
+    /// the bool sequence has been finished.
     boot_active: bool,
+
     boot: [u8; BOOT_SIZE],
     ram: [u8; RAM_SIZE],
 }
@@ -66,6 +72,10 @@ impl Mmu {
         self.boot_active
     }
 
+    pub fn set_boot_active(&mut self, value: bool) {
+        self.boot_active = value;
+    }
+
     pub fn read(&mut self, addr: u16) -> u8 {
         match addr & 0xf000 {
             // BOOT (256 B) + ROM0 (4 KB/16 KB)

src/ppu.rs

@@ -303,7 +303,7 @@ impl Ppu {
             switch_window: false,
             window_map: false,
             switch_lcd: false,
-            first_frame: true,
+            first_frame: false,
             stat_hblank: false,
             stat_vblank: false,
             stat_oam: false,
@@ -337,6 +337,7 @@ impl Ppu {
         self.switch_window = false;
         self.window_map = false;
         self.switch_lcd = false;
+        self.first_frame = false;
         self.stat_hblank = false;
         self.stat_vblank = false;
         self.stat_oam = false;