Game Boy Advance BIOS

Game Boy Advance BIOS
Developer: Nintendo Publisher: Nintendo Platform: Game Boy Advance Released in JP: March 21, 2001 Released in US: June 11, 2001 Released in EU: June 22, 2001 This game has unused code. This game has hidden developer credits. This game has a prototype article This game has a prerelease article

To do: Stuff from the source code in the 2020-07-25 Nintendo leak

The Game Boy Advance (GBA) was essentially the true successor to the original Game Boy line launched in 2001. It added an ARM7-based 32-bit CPU on the top of the Z80-based 8-bit CPU used by previous Game Boy models, allowing for full backwards compatibility with the entire Game Boy (GB) and Game Boy Color (GBC) software library (aside from a few games that used GB/GBC exclusive features, mind you) in addition to its own games. Due to hardware simplification, this feature would be removed in the later Game Boy Micro models though, limiting its support only to GBA cartridges (similarly, the original Nintendo DS and the Nintendo DS Lite are also limited to only GBA cartridges for their backwards compatibility).

Its BIOS was more sophisticated than previous models' bootstrap ROMs, containing a variety of routines that games could use to speed up or simplify operations (or dump the normally read-protected BIOS data, thanks to programming oversights).

Developer Credit

// Coded by Kawasedo       

The five keys (normal, multi, joybus, [unknown], and constant) are used by the GBA to decrypt multiboot images sent over the link cable. When combined, they form this message from the developer.

Kawasedo

The last two keys also appear later in the BIOS. "Kawasedo" is a nickname used by "Tomohiro Kawasae", a Nintendo employee most notable for his work on official emulation projects such as the NES emulators included in Animal Crossing and Metroid Prime.

Unused Fade to GBC Mode

The BIOS contains code to detect when a Game Boy or Game Boy Color cartridge is inserted into the slot (by reading bit 15 of REG_WAITCNT 4000204h). If it detects a GBC cartridge, it will fade to an image of a screen border, then perform a software switch to GBC mode (by setting bit 3 of REG_DISPCNT 4000000h).

However, Nintendo couldn't get their hardware to work this way, possibly due to cartridge voltage differences, so they used a physical switch in the cartridge slot to immediately switch to GBC mode instead. The Game Boy Micro model, as well as the DS and DS Lite, lacked this switch.