How to Create Your Very Own Chip-8 Emulator

Have you ever wondered how classic video game console emulators work under the hood? Building an emulator may seem like complex, arcane programming magic reserved for genius developers. However, by starting with a relatively simple system like Chip-8, you too can learn the core concepts of emulation and create a working emulator from scratch!

In this in-depth guide, we‘ll walk through the process of developing a Chip-8 emulator step-by-step. Chip-8 is an interpreted programming language developed in the 1970s for simple video games. Its simplicity and well-documented architecture make it an ideal starting point for aspiring emulator developers.

Chip-8 Overview

Before diving into implementation, let‘s briefly review the key specifications of the Chip-8 system:

• 4KB (4096 bytes) of 8-bit memory
• 16 8-bit data registers named V0 to VF
• 16-bit address register I and 16-bit program counter PC
• 8-bit delay and sound timers
• 16 level stack for 16-bit addresses
• 16 key hex keypad for input
• 64×32 pixel monochrome display

Chip-8 draws graphics on screen through sprites, which are 8 pixels wide and 1-15 pixels tall. Sprites are drawn using an XOR operation, meaning both setting and clearing pixels is possible.

The CPU of Chip-8 contains 35 unique instructions, each 2 bytes long and identified by the first 4 bits (nibble). The instruction set includes operations like ADD, AND, JUMP, CALL, LOAD, and more.

Memory

To emulate Chip-8‘s 4KB of 8-bit memory, we can use a simple array in JavaScript:

const MEMORY_SIZE = 4096;
const memory = new Uint8Array(MEMORY_SIZE);

Chip-8 reserves the first 512 bytes of memory for the interpreter, and most programs start at memory location 0x200 (512 in decimal). The uppermost 256 bytes are reserved for display refresh. Chip-8 also stores the 16 possible 5-byte sprites representing the hex digits 0-F in the reserved interpreter memory.

To load sprites into memory:

const sprites = [
  0xF0, 0x90, 0x90, 0x90, 0xF0, // 0
  0x20, 0x60, 0x20, 0x20, 0x70, // 1
  ...
];

// Load sprites into reserved interpreter memory
for (let i = 0; i < sprites.length; i++) {
  memory[i] = sprites[i];
}

Registers and Timers

The 16 8-bit data registers and special 16-bit address/program counter registers can be implemented as:

const registers = new Uint8Array(16);
let I = 0; // 16-bit address register
let PC = 0x200; // 16-bit program counter starts at 0x200

For the stack, a simple 16×16-bit array suffices:

const stack = new Uint16Array(16);
let SP = 0; // 8-bit stack pointer

The delay and sound timers continuously decrement at 60Hz until they reach 0. We can emulate them with:

let delayTimer = 0;
let soundTimer = 0;

function emulateCycle() {
  // Execute instruction
  ...

  // Update timers
  if (delayTimer > 0) delayTimer--;
  if (soundTimer > 0) soundTimer--;
}

setInterval(emulateCycle, 1000 / 60); // 60Hz

Display

To represent the 64×32 pixel monochrome display, a 2D array works well:

const DISPLAY_WIDTH = 64;
const DISPLAY_HEIGHT = 32;
const display = new Array(DISPLAY_WIDTH * DISPLAY_HEIGHT).fill(0);

Drawing pixels involves an XOR operation that toggles values between 0 (off) and 1 (on):

function drawPixel(x, y) {
  // Wrap out-of-bounds coordinates
  x = x % DISPLAY_WIDTH;
  y = y % DISPLAY_HEIGHT;

  const i = y * DISPLAY_WIDTH + x;
  display[i] ^= 1; // Toggle pixel

  return !display[i]; // True if pixel erased
}

To render the display array to an HTML canvas:

// Set up 64x32 canvas
const scale = 10;
const canvas = document.querySelector(‘canvas‘);
canvas.width = DISPLAY_WIDTH * scale;
canvas.height = DISPLAY_HEIGHT * scale;
const ctx = canvas.getContext(‘2d‘);

function render() {
  // Clear canvas
  ctx.fillStyle = ‘black‘;
  ctx.fillRect(0, 0, canvas.width, canvas.height);

  // Render display
  ctx.fillStyle = ‘white‘;
  for (let i = 0; i < display.length; i++) { 
    const x = (i % DISPLAY_WIDTH) * scale;
    const y = Math.floor(i / DISPLAY_WIDTH) * scale;

    if (display[i] === 1) {
      ctx.fillRect(x, y, scale, scale);
    }
  }

  requestAnimationFrame(render);
}

requestAnimationFrame(render);

Input

To map the 16 Chip-8 keys to keyboard keys, we can use an object:

const keymap = {
  ‘1‘: 0x1,
  ‘2‘: 0x2,
  ...
  ‘a‘: 0xA,
  ‘b‘: 0xB,
  ...
  ‘f‘: 0xF
};

const keys = {};
window.addEventListener(‘keydown‘, event => keys[keymap[event.key]] = 1);
window.addEventListener(‘keyup‘, event => keys[keymap[event.key]] = 0);

Then checking if a key is pressed becomes:

function isKeyPressed(keyCode) {
  return keys[keyCode];
}

Sound

While Chip-8 doesn‘t specify what sound to produce, a simple beep suffices. The Web Audio API works well for this:

const audioCtx = new AudioContext();

function playSound() {
  const oscillator = audioCtx.createOscillator();
  oscillator.type = ‘square‘;
  oscillator.frequency.value = 440; // 440Hz beep  
  oscillator.connect(audioCtx.destination);
  oscillator.start();
  oscillator.stop(audioCtx.currentTime + 0.1); // 100ms beep
}

CPU and Instructions

The CPU emulation consists of an infinite loop that fetches, decodes, and executes instructions. A switch statement handles decoding by examining instruction nibbles:

function emulateCycle() {
  // Fetch opcode
  const opcode = (memory[PC] << 8) | memory[PC + 1];

  // Decode and execute opcode
  switch (opcode & 0xF000) {
    case 0x0000:
      switch (opcode) {
        case 0x00E0: // Clear display
          display.fill(0);
          break;
        case 0x00EE: // Return from subroutine
          PC = stack[--SP]; 
          break;
      }
      break;

    case 0x1000: // Jump to address NNN
      PC = opcode & 0x0FFF;
      break;

    case 0x2000: // Call subroutine at NNN
      stack[SP++] = PC;
      PC = opcode & 0x0FFF;
      break;

    ...
  }

  // Increment program counter
  PC += 2;

  // Update timers  
  if (delayTimer > 0) delayTimer--;
  if (soundTimer > 0) {
    playSound();
    soundTimer--;
  }
}

Some instructions require bitwise operations to extract nibbles:

case 0x3000: // Skip if VX == NN
  if (registers[(opcode & 0x0F00) >> 8] === (opcode & 0x00FF)) {
    PC += 2;
  }
  break;

And others operations like sprites drawing are more complex:

case 0xD000: // Draw sprite at (VX, VY) with N bytes
  const x = registers[(opcode & 0x0F00) >> 8];
  const y = registers[(opcode & 0x00F0) >> 4];
  const height = opcode & 0x000F;

  registers[0xF] = 0; // Reset collision flag

  for (let i = 0; i < height; i++) {
    const line = memory[I + i];
    for (let j = 0; j < 8; j++) {
      const pixel = line & (0x80 >> j);
      if (pixel !== 0 && drawPixel(x + j, y + i)) {
        registers[0xF] = 1; // Set collision flag
      }
    }
  }
  break;

But most instructions are straightforward register operations.

ROM Loading and Execution

To load a Chip-8 ROM into memory and start execution:

const rom = new Uint8Array(await fetch(‘ibm.ch8‘).then(res => res.arrayBuffer()));

// Load ROM into memory starting at 0x200
memory.set(rom, 0x200);

// Set program counter to ROM entry point 
PC = 0x200;

// Begin emulation
setInterval(emulateCycle, 1000 / 500); // 500 instructions per second

With all the pieces in place, you should see the Chip-8 ROM come to life:

Next Steps

Congratulations, you now have a fully functional Chip-8 emulator! But why stop here? There are plenty of additional features and improvements to try:

• Selectable color themes and scaling options
• Pause, step, and reset controls
• Savestate support
• Debugger with breakpoints, memory inspection, and disassembly
• ROM selector and library
• Sound and rendering options and enhancements

You can find the complete source code for this Chip-8 emulator on GitHub. Feel free to fork the project and experiment with changes and additions.

I hope this guide has demystified the basics of emulator development and showed you that even seemingly complicated programs can be broken down into understandable steps. Armed with this foundation, you‘re ready to dive deeper into the world of emulation.

While more advanced systems like the Nintendo Game Boy or NES are significantly more complex than Chip-8, the same core concepts apply. Mastering Chip-8 is a great first step to developing more ambitious emulation projects.

Thanks for following along, and happy emulating!