

Either /bios.bin, /gba/bios.bin or /_gba/bios.bin will work. Place your GBA bios on your SD card (recommended for better compatibility). You can either replace the GBARunner2_arm7dldi_3ds.nds file on the \_nds folder, or place it anywhere on your SD card (except for in \_gba). If you are using TWiLight Menu++, GBARunner2 will come bundled with it and you should find it already on the \_nds folder.

A GBA bios is thus optional, but it can greatly improve compatibility with some games. Note: GBARunner2 has the open source bios of Normmatt integrated, with some small modifications. GBARunner2 is a hypervisor that runs GBA games on DS/DSi/3DS in DS mode, basically like Nintendont does for running Gamecube games on Wii.
