An Epic Quest to Build a Game Boy From Scratch

The Nintendo Game Boy is one of the video game industry’s more surprising success stories. Its direct competitors, including the Sega Game Gear, Atari Lynx, and TurboExpress, were all more powerful and had much better screens. But the Game Boy outsold all of them combined many times over. There is a lot of debate about how Nintendo managed that, but battery life was likely a major factor. Despite that success, that first Game Boy was notorious for having an awful screen. That screen gave Raphael Stäbler the motivation to embark on an epic quest to build his own Game Boy from scratch.

There are, of course, much easier solutions if you just want a better screen. You can upgrade an original Game Boy with a backlight and a better filter to enhance the contrast, use a Game Boy Color or another model, or use some kind of modern device with a game emulator. But Stäbler's motivation wasn’t purely practical. He also wanted to learn more about electronics hardware, and this seemed like the kind of project that would force him to learn a lot.

The original Game Boy ran on a Zilog Z80 processor clocked at 4.9MHz. The Z80 was an extremely popular CPU in the 8-bit era, and was used in a number of computers, video game consoles, and even calculators like the TI-83. Stäbler considered using “real hardware,” but decided to go the emulation route. That might sound like cheating, but he isn’t just installing RetroPie on a Raspberry Pi. Instead he decided to develop his own emulator that runs on “bare metal.”

In this case, that bare metal is a Teensy 4.0 with an Arm Cortex-M7 processor that runs at 600MHz. Stäbler emulated the Game Boy’s CPU, including the Instruction Set and Register access, for the full fetch-decode-execute instruction cycle. When combined with memory access, which is also how the CPU communicates with peripherals, that is enough to start running a game with a cobbled-together video controller.

Stäbler was able to test his new emulator by running Tetris, and it seemed to run exactly as it should. Right now, this is at the stage that he considers a “proof of concept.” The plan is to continue forward and develop hardware that will take advantage of the bare metal emulator that he created. Be sure to follow Stäbler so you can see how the rest of the project progresses.