More than you wanted to know about how Game Boy cartridges work

More than you wanted to know about how Game Boy cartridges work

By Allison Parrish

I set out a while ago to make a Game Boy cartridge from scratch. This is not a novel goal; bootleg Game Boy cartridges have existed almost as long as the Game Boy itself has, and there are many third-party cartridges now available for purchase, or that have copyleft designs.

But I wanted to know how Game Boy cartridges work. I was also excited to use the PIO functionality of the RP2040 microcontroller. Now, after a few years (!) of research and design, I have made my design for a bootleg Game Boy cartridge available for everyone to use. In this post, I’m going to take you through everything I learned along the way.

I will be quick to note that this document doesn’t contain any new research! Instead, it’s my attempt to gather together information relevant to making custom Game Boy cartridges in one place, and present that information in a way that is easy to digest for people who start off with about the same level of knowledge I started with when I first dipped my toes into the Game Boy cartridge pool.

Please let me know if you find any errors or have questions!

Prerequisites

I’m going to assume that you have some knowledge of how digital memory works, e.g., that you know what a “byte” is and broadly how processors address memory. (If you don’t have this knowledge but you’re curious and want to get up to speed, I recommend Sebastian Lague’s “Exploring How Computers Work” series, or this chapter on Digital Memory Terms and Concepts from All About Circuits.) I’m also going to assume that you have enough knowledge of computer internals to know what a microprocessor is, and how a microprocessor might communicate with other electronic components. You should have some understanding of hexadecimal and binary numbers. However, you don’t need to know anything in particular about electronics or electronic components.

Rodrigo Copetti’s Game Boy/Game Boy Color Architecture: A Practical Analysis is a good general overview of the Game Boy hardware, and is also worth a read, to familiarize yourself with Game Boy components and terminology.

Conventions, notations and caveats

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