The Wii has several IOSes on his internal memory, organized in "slots" from 1 to 254.

Every "slot" contains a specific version of IOS.

IOS stands for "Input OS" and manages, transparently for the game, all internal Wii devices.

For example, for managing an USB keyboard, is mandatory to have an IOS support for that. So, Zelda Twilight Princess, that is hardcoded to use the IOS9, will be never be able to use an USB keyboard.

Or, the "Wii speak" microphone that is bundled with Animal Crossing, can only be managed by an IOS with version number greater than 38. Every previous game, will not be able to use that. So, for this reason, the "Boogie" microphone, is not compatible with "Wii speak"...

Hackers has done two unofficial IOSes:

• IOS254, called DVDx, based on the IOS36, coded by the Team Twiizers, that enabled the support for DVD videos and data DVDs. This IOS enables, for example, to read DVD video, or DVD with divx files with Mplayer-wii
• IOS249, called CIOS, Custom IOS, also based on the IOS36, coded by Waninkoko, that adds features like:
  • loading of copied games, without a modchip
  • loading of games from USB drives and SD cards
  • installation of "not-authorized channels"
  • SDHC support