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[url=http://www.wiibrew.org/wiki/IOS
IOS[/url]]It provides the services that are used by Wii code to access most of the system devices.
IOS is a somewhat unique system in that many games use (and come with) their own version of the IOS, though many games use the same IOS versions. There is no "true" version of the operating system, instead there are many of what could be called "slots" for different versions of IOS to sit in. IOS versions are not aware of each other, and are not used or invoked at all unless software specifically requests to run using them. In this way it can be considered "safe" to install custom IOS modules, or to patch any IOS module so long as it is not used by the system menu, as if it becomes corrupt it can be deleted and replaced without damaging the system as a whole.