Building a CHIP-8 Emulator (C++)

Building a CHIP-8 emulator is a great way to understand emulation concepts, low-level programming, and virtual machines. I want to share a basic outline to help you get started if you’re a newcomer in C++ and I will include all the key components you’ll need to implement.

What is CHIP-8?

CHIP-8 is a simple interpreted programming language that was designed for early microcomputers. It has a straightforward architecture with the following key components:

Memory: 4 KB of RAM.
Registers: 16 8-bit data registers (V0 to VF).
Stack: A stack for managing function calls (up to 16 levels).
Display: A 64x32 pixel display.
Input: A hexadecimal keypad with 16 keys.

These features make CHIP-8 an excellent candidate for emulation, providing a manageable complexity for developers.

Structure

Before coding, it’s important to set up a clean project structure.

Here’s my recommended layout:

/chip8-emulator
├── /src
│   └── main.cpp
│   └── chip8.cpp
│   └── chip8.h
├── /assets
│   └── roms
├── /build
└── CMakeLists.txt (if using CMake)

Defining the CHIP-8 structure

Defining the constants and structure that will represent our CHIP-8 system.

// chip8.h
#ifndef CHIP8_H
#define CHIP8_H

const int MEMORY_SIZE = 4096;
const int DISPLAY_WIDTH = 64;
const int DISPLAY_HEIGHT = 32;
const int STACK_SIZE = 16;
const int NUM_REGISTERS = 16;

struct Chip8 {
    unsigned char memory[MEMORY_SIZE]; // 4KB memory
    unsigned char V[NUM_REGISTERS]; // 16 registers
    unsigned short I; // Index register
    unsigned short PC; // Program counter
    unsigned short stack[STACK_SIZE]; // Stack
    unsigned short SP; // Stack pointer
    unsigned char display[DISPLAY_WIDTH * DISPLAY_HEIGHT]; // Display
    bool key[16]; // Keypad state
};

void initialize(Chip8 &chip8);
void loadRom(Chip8 &chip8, const char* filename);
void emulateCycle(Chip8 &chip8);

#endif // CHIP8_H

Initializing the Emulator

The initialization function sets up the registers, memory, and loads the CHIP-8 font set into memory.

// chip8.cpp
#include "chip8.h"
#include <cstring>
#include <fstream>

void initialize(Chip8 &chip8) {
    chip8.PC = 0x200; // Programs start at 0x200
    chip8.I = 0;
    chip8.SP = 0;

    // Clear display
    std::memset(chip8.display, 0, sizeof(chip8.display));

    // Load font set
    unsigned char fontset[80] = {
        // 0-F
        0xF0, 0x90, 0x90, 0x90, 0xF0, // 0
        0x20, 0x60, 0x20, 0x20, 0x70, // 1
        // (Add the rest of the font set here)
    };
    std::memcpy(chip8.memory, fontset, sizeof(fontset));
}

Loading the ROM files

Let’s create a function to load a CHIP-8 ROM into memory.

void loadRom(Chip8 &chip8, const char* filename) {
    std::ifstream file(filename, std::ios::binary);
    if (file) {
        file.read(reinterpret_cast<char*>(&chip8.memory[0x200]), MEMORY_SIZE - 0x200);
    }
}

The emulation cycle is where the magic happens. This loop continuously fetches instructions, decodes them, and executes them.

void emulateCycle(Chip8 &chip8) {
    // Fetch
    unsigned short opcode = chip8.memory[chip8.PC] << 8 | chip8.memory[chip8.PC + 1];
    
    // Decode and Execute
    switch (opcode & 0xF000) {
        case 0x0000:
            if (opcode == 0x00E0) {
                // Clear the display
                std::memset(chip8.display, 0, sizeof(chip8.display));
                chip8.PC += 2;
            } else if (opcode == 0x00EE) {
                // Return from subroutine
                chip8.SP--;
                chip8.PC = chip8.stack[chip8.SP];
                chip8.PC += 2;
            }
            break;
        // (Add additional opcodes here)
        default:
            // Unknown opcode
            chip8.PC += 2;
            break;
    }
}

In our main function, initialize the emulator, load a ROM, and run the emulation loop.

#include "chip8.h"
#include <iostream>

int main(int argc, char* argv[]) {
    Chip8 chip8;
    initialize(chip8);

    if (argc < 2) {
        std::cerr << "Please provide a ROM file." << std::endl;
        return 1;
    }

    loadRom(chip8, argv[1]);

    while (true) {
        emulateCycle(chip8);
        // Handle timing and rendering here
    }

    return 0;
}

Additional Considerations

CHIP-8 typically runs at about 60 Hz, so you’ll need to implement a timer to control the speed of the emulator.
To capture key presses, consider using libraries like SDL or SFML, which provide convenient ways to handle windowing and input.
You’ll need to create a window to display the 64x32 pixel output. Again, libraries like SDL can help you achieve this with minimal hassle.

That’s it basically. Feel free to explore existing open-source CHIP-8 emulators for inspiration, and don’t hesitate to reach out if you have any questions or need guidance.

laudz : weblog

Embedded systems, C/C++, GNU/Linux, and Infosec