From RGDWiki

EXI (Expansion Interface) is the proprietary (developed by Nintendo/ArtX/ATI) interface used for most peripherals on the GameCube and Wii. It is available through the memory card slots and (on GameCube) serial ports, as well as additional channels accessible only on development hardware. The Gekko and Broadway also boot from EXI, so it is used for the boot code (loaded from mask ROM, Barnacle or by IOS).