Building a Custom Gameboy Color Emulator in C++

Introduction

Emulating classic game consoles is not only a fun exercise in nostalgia but also an insightful journey into low-level programming and systems architecture. In this post, we’ll construct a simple Gameboy Color emulator using C++. This project will help us understand the workings of CPU emulation, memory management, and I/O operations within the Gameboy system.

System Overview

The Gameboy Color (GBC) is powered by an 8-bit Sharp LR35902 processor, which is similar to the Z80 but with fewer instructions and some unique hardware features. It operates at 4.19 MHz, features 32 KB of ROM, 8 KB of internal RAM, and 8 KB of video RAM. The display is a 2.6-inch diagonal TFT color screen.

Emulator Components

Our emulator will focus on the following components:

1. CPU: Emulating the Sharp LR35902 instruction set.
2. Memory: Handling ROM, RAM, and VRAM.
3. Graphics: Simulating the GPU and rendering frames.
4. Input: Emulating button presses.
5. Timing: Ensuring everything runs at correct speeds.

Setting Up the Environment

First, ensure you have a C++ development environment set up. We will use GCC for compilation. You’ll also need the SDL2 library for graphics and input handling.

sudo apt-get install g++ libsdl2-dev

CPU Emulation

We’ll start by emulating the CPU. The CPU emulation involves decoding and executing instructions based on the binary data from the game ROM.

// Basic structure for our CPU
struct CPU {
    uint8_t registers[8]; // 8 general purpose registers
    uint16_t pc;          // Program counter
    uint16_t sp;          // Stack pointer

    // Function to execute one instruction
    void executeInstruction(uint8_t opcode) {
        switch(opcode) {
            // Add cases for each opcode
        }
    }
};

Memory Management

The Gameboy memory map is crucial for the emulator. Here’s how you can set it up:

struct Memory {
    uint8_t rom[0x8000];    // 32KB of ROM
    uint8_t ram[0x2000];    // 8KB of Internal RAM
    uint8_t vram[0x2000];   // 8KB of Video RAM

    uint8_t read(uint16_t address) {
        // Implement memory read logic based on address
        // Return the byte at the specified address
    }

    void write(uint16_t address, uint8_t data) {
        // Implement memory write logic based on address
    }
};

Graphics and Rendering

The GBC has a simple 2D GPU that scans out graphics to the display. We will simulate this using SDL2.

#include <SDL2/SDL.h>

class Display {
public:
    SDL_Window* window;
    SDL_Renderer* renderer;

    Display() {
        SDL_Init(SDL_INIT_VIDEO);
        window = SDL_CreateWindow("Gameboy Color Emulator", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, 160, 144, 0);
        renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED);
    }

    ~Display() {
        SDL_DestroyRenderer(renderer);
        SDL_DestroyWindow(window);
        SDL_Quit();
    }

    void renderFrame(uint8_t* frameBuffer) {
        // Convert frame buffer to textures and render
    }
};

Running the Emulator

Combining all parts, we initialize the components and enter the emulation loop.

int main() {
    CPU cpu;
    Memory mem;
    Display display;

    // Main emulation loop
    while (true) {
        // Fetch, decode, execute
        uint8_t opcode = mem.read(cpu.pc++);
        cpu.executeInstruction(opcode);

        // Handle graphics and input here
    }

    return 0;
}

Debugging and Challenges

Throughout the development, debugging involves a lot of logging and comparing the emulator’s behavior with that of an actual GBC. Challenges might include timing issues and accurately emulating hardware quirks.

Conclusion

Building an emulator is a complex but rewarding project that sharpens your programming and problem-solving skills while deepening your understanding of computer architecture. I encourage you to expand on this basic framework by implementing more detailed graphics emulation, sound, and more precise timing mechanisms.

For those interested in diving deeper, refer to official Gameboy programming manuals and various online resources dedicated to Gameboy emulation. Happy coding!