Not satisfied with the sprites in your NES game? This guide will show you how to hack the ROM from a Nintendo Entertainment System* (NES) game to make your favorite games even better by allowing you to change the sprites and graphics within the game! Change the main character to your own creation! Or simplify the game into a mediative floating cloud paradise. The power is in your hands!

*The NES is a popular gaming system from the 80's.

What's a ROM Anyway?

ROM stands for "Read Only Memory" and is a file that contains data from a read only memory chip such as a video game cartridge. ROM files are often run in video game emulators to play the games they are copied from. But if ROM means read only memory, how can we write over it to add our own sprites? That's where the hacking comes in :)

A ROM Hacking History

In order to hack these games, Arcangel had to dig into the game cartridge and do some soldering and other wizardy.

Fortunately we now have NES files that can be edited on a computer so no NES cartridge soldering is necessary! 

A more action-packed example comes from Kenna W, who decided to swap out Link for Zelda in the classic Legend of Zelda:

For me, I played my first Zelda game when I was pretty young, and at the time, I thought the game did star Princess Zelda. I figured I'd get to play as a magical battle princess that saved her kingdom. The game was fun, but I was bummed out that I never got to play as Zelda. But like I said, I'm an adult now. There's no one to stop me from eating candy before bed and there's nothing standing in the way of me creating the games I want to play. 

In addition to playing these cool mods on computer emulators, we can also play them remotely on a handheld gaming device like the PyGamer. We can store quite a few NES ROMs on the PyGamer (as well as other boards with Adafruit Arcada support) to have our choice of a multitude of classics and mods where ever we go!

Prerequisite Guides:

Pixel Art Fundamentals

• A great overview of pixel art, and methods of how to create your own sprites.

NES Emulator for Arcada

• If you plan to run any NES ROMs on hardware, looking through this guide before continuing would be a good idea. This guide will show you how to port and play NES ROMs on boards with Arcada support such as the PyGamer, PyBadge, and PyBadge LC

Parts

Play your new games on (in theory) any board with Arcada support - but we recommend ones that have a gamepad + buttons:

The "Impossible" Vision

In this guide we'll cover:

• How to find a ROM to hack.
• How to edit the sprites and graphics in the ROM.
• How to play the new game in an emulator or on Arcada supported hardware.

I changed BurgerTime, the 1982 classic, to Impossible BurgerTime, starring a cow that makes plant-based burgers!

Here's a sample of the original game starring a chef:

And here's a sample of the mod we'll create starring a cow:

You can mod any NES game of your choosing and use this guide as a reference. On the next page, learn how to find some great NES ROMS to hack up!

Now that you have the ROM you wanna hack, let's jump into editing the sprites in the game. First we'll need to download an editor to be able to access the ROM. Download an editor based on the operating system you use.

Download an NES sprite editor

Mac OS

Windows

Linux

There isn't a well known NES sprite editor for Linux that I could find but you can always use the Wine platform to run Windows on your OS then run Tile Layer Pro from there.

Once you have an editor downloaded and an NES file, it's time to start hacking away at those sprites. I will be using the Mac OS NES CHR editor for this lesson, however if you are on Windows and using the Tile Layer Pro editor, the Mac OS editor works similarly and a lot of the same concepts and functionality will transfer over. If you get stuck with the Tile Layer Pro editor, this is a great guide that walks through how to use the editor.

What is NES CHR data?

The majority of NES cartridges store sprite information in a part of the ROM called CHR which is usually a single chip 8K or larger. Sprites and other pattern data are stored in 8 x 8 pixel tiles. Most sprites are then made up of multiple tiles put together. Mario, below is made up of 4 CHR tiles.

Read more on how NES CHR data works here.

Open the NES file in the editor

Navigating the NES CHR Editor

Find the Desired Sprites

Draw the New Sprite

You may want to use an online sprite editor such as Piskel, to play around with the shapes until you are ready to save to the CHR editor. Here's a great guide on pixel art fundamentals if you're new to this.

Picking Colors

You have 4 options for picking colors. White is transparent, and the other 3 correspond to preset colors in each game. Unfortunately there is no current way to change the these preset colors. 

Draw the Sprite in the Editor

Save the New Sprite Tiles

Tips and Tricks

• Use the copy and paste functions in the editor to copy and paste tiles that are reused or similar to ones you are trying to make.
• You can click any of the boxes in the small 4x4 grid at the bottom right of the editor to change which quadrant the next tile you click from the CHR bank goes into.
• Tiles are not typically placed in any particular order in the CHR bank so make sure to keep track of which tiles correspond to which sprites.
• The CHR editor may unexpectedly crash, so please remember to save your files constantly!

Test out the New Game

Uploading to and Playing on Hardware

When you're ready to upload the game to any hardware such as the PyGamer or PyBadge check out this guide!

If you want to change more than just a single sprite you will have to do some more shuffling around in the CHR bank.

Find some screen shots of the creation of "Impossible BurgerTime" below.

Find a link to the Piskel sprites here!

"Impossible BurgerTime" Mods

Changing Chef to Cow

Front

Back

Side

Attack (Throw Salt)

("Dying" sprites not shown)

Ice Cream changed to Burger King Crown

Splash or Title Screen

Our new mods in action!

Can you change the enemies to something more creative? Enjoy and happy ROM hacking!