MIOS is a special version of IOS that runs when the Wii enters GameCube mode. The same MIOS is used for all GameCube games and software. It has seen minimal updates in the Wii's lifetime.
It contains a full copy of the GC IPL, slightly modified for the Wii. It performs patching of some games when they load, presumably to enhance compatibility with problematic titles.
||Blocks the GameCube Action Replay.
||Overwrites memory to prevent the Tweezer Attack.