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IOS is the operating system that runs on the Starlet coprocessor inside the Hollywood package. 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.

It should be noted that the IOS does not act as a hypervisor, the "Home" menu you see when you press the Home button is part of the game, it is not provided by the running IOS or the System Menu. To put it simply, the games does not run "on top of" the system menu. When a game is launched the system menu is closed and the game is started, and when the game calls to return to the system menu, the game closes and the system menu is restarted. There can only be one IOS running at a time.

The only times an IOS is not running is during GameCube mode (during which time MIOS is running instead), or when BootMii is running (during which time mini is running instead).

See Also


Communication with IOS from PPC code is done using an IPC mechanism. There are 7 calls that can be made using this system:

  1. open
  2. close
  3. read
  4. write
  5. seek
  6. ioctl
  7. ioctlv
ipc struct size = 0x40, aligned to 0x20

00:     cmd     // 1=open 2=close 3=read 4=write 5=seek 6=ioctl 7=ioctlv
04:     ret
08:     fd
0c:     arg[5]
20:     async1
24:     async2
28:     0
3F:     relaunch, used for ioctlvreboot

open:   fd = 0
        arg0, arg1: name, mode (1=read 2=write)

close:  fd

read:   fd
        arg0, arg1: addr, len

write:  fd
        arg0, arg1: addr, len

seek:   fd
        arg0, arg1: where, whence

ioctl:  fd
        arg0: ioctl #
        arg1, arg2: addr, len
        arg3, arg4: addr, len

ioctlv: fd
        arg0: ioctl #
        arg1: # in
        arg2: # out (or in-out)
        arg3: pointer to # in plus # out pairs of (addr, len)

fd is a handle you get back from ios on "open", and that you should pass back to all other calls --segher

Most non-trivial operations are performed by opening one of the below resources, then calling ioctl or ioctlv on it.

The Starlet kernel hands these calls over to the individual drivers / processes within the Starlet. The processes register themselves to handle requests by creating one or more queues and assigning them to handle requests from a particular /dev device.

IOS Modules

IOS is a Nintendo-proprietary, embedded operating system. It uses a microkernel architecture, with several independent modules that talk to each other via "resources", which are organized as entries in the /dev tree.


The Kernel is the piece of code that is actually booted; it contains a small ELF-loader header at the beginning, and then is an ELF file. In addition to the core microkernel stuff, it contains the bare minimum drivers necessary to read the rest of the drivers from the NAND filesystem -- specifically, FFS (Flash Filesystem), ES (E-Ticket Services), and IOSP (Responsible for booting and managing Broadway?).