Emulator: Add state machine to PPU

src/emu.cpp

@@ -64,6 +64,16 @@ void Emu::removeMemorySpace(std::string_view name)
 
 void Emu::writeMemory(uint32_t address, uint32_t value)
 {
+	// Bail if the CPU tries to write to a read-only address
+	switch (address) {
+	case 0xff44:
+		ruc::error("writing to read-only address: {:#06x}", address);
+		VERIFY_NOT_REACHED();
+		break;
+	default:
+		break;
+	}
+
 	for (auto& memory_space : m_memory_spaces) {
 		auto& memory = memory_space.second;
 		if (address >= memory.start_address && address <= memory.end_address) {
@@ -96,6 +106,13 @@ void Emu::writeMemory(uint32_t address, uint32_t value)
 
 uint32_t Emu::readMemory(uint32_t address) const
 {
+	switch (address) {
+	case 0xff44:
+		return *m_processing_units.at("PPU")->sharedRegister("LY");
+	default:
+		break;
+	};
+
 	for (const auto& memory_space : m_memory_spaces) {
 		const auto& memory = memory_space.second;
 		if (address >= memory.start_address && address <= memory.end_address) {

src/emu.h

@@ -9,7 +9,6 @@
 
 #include <cstdint> // uint32_t
 #include <memory>  // std::shared_ptr
-#include <string>
 #include <string_view>
 #include <unordered_map>
 #include <vector>

src/ppu.cpp

@@ -18,10 +18,15 @@
 
 #include "emu.h"
 #include "ppu.h"
+#include "ruc/meta/assert.h"
 
 PPU ::PPU(uint32_t frequency)
 	: ProcessingUnit(frequency)
 {
+	m_shared_registers.emplace("LY", &m_lcd_y_coordinate);
+
+	// Setup screen
+
 	auto& scene = Inferno::Application::the().scene();
 	m_entity = scene.createEntity("Screen");
 
@@ -46,12 +51,54 @@ void PPU::update()
 
 	// print("PPU update\n");
 
-	// Increment LY (LCD Y Coordinate)
-	uint32_t ly_register = Emu::the().readMemory(0xff44) + 1;
-	if (ly_register >= 154) {
-		ly_register = 0;
-	}
-	Emu::the().writeMemory(0xff44, ly_register);
+	m_clocks_into_frame++;
+
+	switch (m_state) {
+	case State::OAMSearch:
+		// OAM logic goes here..
+
+		if (m_clocks_into_frame % 20 == 0) {
+			m_state = State::PixelTransfer;
+		}
+		break;
+	case State::PixelTransfer:
+		// PixelTransfer logic goes here..
+		m_lcd_x_coordinate++;
+
+		if (m_lcd_x_coordinate == 160) {
+			m_lcd_x_coordinate = 0;
+			m_state = State::HBlank;
+		}
+		break;
+	case State::HBlank:
+		// H-Blank logic goes here..
+
+		if (m_clocks_into_frame % (20 + 43 + 51) == 0) {
+			if (m_lcd_y_coordinate == 144) {
+				m_state = State::VBlank;
+			}
+			else {
+				m_state = State::OAMSearch;
+			}
+			m_lcd_y_coordinate++;
+		}
+		break;
+	case State::VBlank:
+		// V-Blank logic goes here..
+
+		if (m_clocks_into_frame % (20 + 43 + 51) == 0) {
+			if (m_lcd_y_coordinate == 154) {
+				m_lcd_y_coordinate = 0;
+				m_clocks_into_frame = 0;
+				m_state = State::OAMSearch;
+			}
+			m_lcd_y_coordinate++;
+		}
+
+		break;
+	default:
+		VERIFY_NOT_REACHED();
+	};
 }
 
 void PPU::render()
@@ -141,3 +188,11 @@ void PPU::drawTile(uint32_t x, uint32_t y, uint32_t tile_address, uint8_t bg_pal
 		screen_index += SCREEN_WIDTH * FORMAT_SIZE;
 	}
 }
+
+void PPU::resetFrame()
+{
+	m_state = State::OAMSearch;
+	m_clocks_into_frame = 0;
+	m_lcd_x_coordinate = 0;
+	m_lcd_y_coordinate = 0;
+}

src/ppu.h

@@ -38,12 +38,25 @@ public:
 	PPU(uint32_t frequency);
 	~PPU();
 
+	enum State : uint8_t {
+		OAMSearch,
+		PixelTransfer,
+		HBlank,
+		VBlank,
+	};
+
 	void update() override;
 	void render();
 
 	void drawTile(uint32_t screen_x, uint32_t screen_y, uint32_t tile_address, uint8_t bg_palette);
+	void resetFrame();
 
 private:
+	State m_state { State::OAMSearch };
+	uint32_t m_clocks_into_frame { 0 };
+	uint32_t m_lcd_x_coordinate { 0 };
+	uint32_t m_lcd_y_coordinate { 0 }; // Note: includes V-Blank
+
 	uint32_t m_entity;
 	std::array<uint8_t, SCREEN_WIDTH * FORMAT_SIZE * SCREEN_HEIGHT> m_screen;
 };