Open source GBA BIOS replacement aims to match Nintendo's chip without copying it

A new project called emibios offers a freely redistributable Game Boy Advance BIOS built by observing, not copying, Nintendo's original, for emulator testing and preservation.

AI-drafted from 2 sources. This story was written with AI from the outside reporting listed under Sources, and checked by automated tools before it went up. The originals have the full detail.

A developer has published emibios, a free replacement for the Game Boy Advance's BIOS chip, on GitHub. It is not meant to make games run better so much as to give emulator authors, hardware clone makers and preservationists something legally clean to test against, instead of relying on a dump of Nintendo's own copyrighted firmware.

The project's GitHub page describes emibios as “a research Game Boy Advance BIOS replacement that aims to be as accurate as possible when running commercial games while being freely redistributable.” It builds on Cult-of-GBA BIOS, an earlier open replacement by fleroviux and DenSinH, and is currently at version 0.1.4.

Why the GBA's BIOS has always been awkward

Every Game Boy Advance contains a small startup ROM, the BIOS, that handles booting, checks a logo on the cartridge header, and provides a set of software interrupt calls (SWIs) that commercial games lean on for things like memory copies and sound mixing. That BIOS is Nintendo's copyrighted code, so it cannot legally be shipped with an emulator. For years the standard workaround has been to ask users to supply their own dump, extracted from a real console, or to fall back on approximations written from scratch by emulator teams, an approach known as high level emulation, which trades some accuracy for not needing the real thing.

emibios sits in the second camp but pushes much harder on accuracy than most stopgap implementations. According to the GitHub readme, most SWIs match the retail BIOS in both result and side effects on registers and flags, many are timed to match the original for most inputs, and some have been adjusted for timing quirks caused by bus, IRQ and DMA contention where testing on real commercial games showed it mattered. It also implements Multiboot and JoyBus link cable booting, tested, the readme says, on FPGA hardware, on GameCube link cables, and against mGBA and Dolphin.

How the developer avoided touching Nintendo's code

The project's stated method is to observe the retail BIOS's behaviour rather than reference or reuse any of its actual code. The readme is explicit on this point: emibios “is not intended to contain, or use as reference, any proprietary code directly derived from the retail BIOS.” The developer's AI disclosure adds a specific detail that is likely to raise eyebrows on the r/retrogamedev discussion thread: large language models were used to propose and run what the readme calls “thousands of experiments” needed to reverse the BIOS's behaviour, with the developer saying care was taken to “firewall” the models from ever seeing proprietary code or binaries.

What still doesn't work

The project is upfront about its limits. Its own disclaimer calls it a research project that “should not be considered production-ready just yet,” and notes that many games still desync during tool assisted speedrun (TAS) playback compared with the real BIOS, something accuracy-focused emulator projects care about a great deal since it implies subtle timing mismatches. Sound SWIs and Multiboot are flagged as needing more testing from anyone willing to try commercial carts. The developer also says emibios will “probably” run on real GBA silicon given the right setup, but admits not knowing of any way to actually do that, and asks whether anyone with debug hardware might confirm it. Until then, that claim stays unconfirmed.

For emulator developers this is another accurate reference to check work against, alongside the retail dump and Cult-of-GBA. For preservation more broadly it is a small but useful hedge: one more path to running Game Boy Advance software correctly that does not depend on Nintendo's original chip surviving in usable form.

Sources

• emibios, an accuracy-focused GBA BIOS replacement (GitHub)
• Discussion: emibios, an accuracy-focused GBA BIOS replacement (Reddit)

Image: Illustration: AI-generated for Gaming World