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IOS is versioned in a somewhat unique way. Instead of there being a single canonical version of IOS, there are multiple branches, each typically corresponding to one or more versions of the Wii SDK. Each branch is apparently specified to have a completely frozen API, and old versions are only updated to patch bugs (often security bugs) - Nintendo at one point created an entirely new IOS branch that differed only in the default value for the TCP buffer size. A fully updated Wii contains one copy of the latest version of each branch of IOS. On a Wii, these are installed as separate titles, often called "IOS slots". Due to this design, it is generally considered safe to uninstall, reinstall, or patch an IOS or IOS module, as long as it is not the slot used by the System Menu - if anything goes wrong, the broken version can be safely uninstalled and a vanilla copy reinstalled. IOS slots have title IDs 1-3 through 1-255. Unused (high) IOS slots are often used to install patched versions of IOS or alternative Starlet software (e.g. [[BootMii]] as IOS is installed as IOS254, which when invoked will subsequently load armboot.bin from the SD card, typically [[mini]]). See [[IOS History]] for a comprehensive list of IOS slots and versions.
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−:''Not to be confused with Apple's iOS, which runs on the iPhone, which was released half a year after the Wii, or [https://wiiubrew.org/wiki/IOSU IOSU], which runs on the Wii U, sometimes referred to as IOS.''
+:''Not to be confused with [https://wiiubrew.org/wiki/IOSU IOSU], which runs on the Wii U, sometimes referred to as IOS.''
'''IOS''' (sometimes internally referred to as '''IOP''' - possibly "Input Output Proxy") is the operating system that runs on the [[Hardware/Starlet|Starlet]] (IOP) coprocessor ([https://wiiubrew.org/wiki/Hardware/Starbuck Starbuck] on the [[vWii]]) inside the [[Hollywood]] package. It provides services that are used by Wii code to access many system devices: USB, networking, security, app management, NAND flash storage, SD card, optical disc, and also WiiConnect24 features.
'''IOS''' (sometimes internally referred to as '''IOP''' - possibly "Input Output Proxy") is the operating system that runs on the [[Hardware/Starlet|Starlet]] (IOP) coprocessor ([https://wiiubrew.org/wiki/Hardware/Starbuck Starbuck] on the [[vWii]]) inside the [[Hollywood]] package. It provides services that are used by Wii code to access many system devices: USB, networking, security, app management, NAND flash storage, SD card, optical disc, and also WiiConnect24 features.
All software using the Wii [[SDK]] or [[libogc]] relies on a running IOS on the Starlet (with a few exceptions in the latter case - it is possible to shut down IOS services from libogc and work without it). Typically, the only times IOS is not in use is when running GameCube software (which uses [[MIOS]] instead - effectively a dummy IOS), or when [[BootMii]] and related software is in use (which uses [[mini]] instead).
All software using the Wii [[SDK]] or [[libogc]] relies on a running IOS on the Starlet (with a few exceptions in the latter case - it is possible to shut down IOS services from libogc and work without it). Typically, the only times IOS is not in use is when running GameCube software (which uses [[MIOS]] instead - effectively a dummy IOS), or when [[BootMii]] and related software is in use (which uses [[mini]] instead).
−IOS is not a "hypervisor", as it runs on a dedicated, separate CPU. However, IOS does isolate its memory from access by the main [[Hardware/Broadway|Broadway]] CPU, has the ability to reboot (and hence bootstrap) it, and is designed to be secure if the PowerPC side is compromised (although in practice many exploits have been found). In that sense, IOS is higher in the security hierarchy than code running on the PowerPC.
IOS is not a "hypervisor", as it runs on a dedicated, separate CPU. However, IOS does isolate its memory from access by the main [[Hardware/Broadway|Broadway]] CPU, has the ability to reboot (and hence bootstrap) it, and is designed to be secure if the PowerPC side is compromised (although in practice many exploits have been found). In that sense, IOS is higher in the security hierarchy than code running on the PowerPC.
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*[[ARM Binaries]]
*[[ARM Binaries]]
*[[/vWii IOS List|vWii IOS List]]
*[[/vWii IOS List|vWii IOS List]]
+
+== Versioning ==
+IOS is versioned in a somewhat unique way. Instead of there being a single canonical version of IOS, there are multiple branches, each typically corresponding to one or more versions of the Wii SDK. Each branch is apparently specified to have a completely frozen API, and old versions are only updated to patch bugs (often security bugs) - Nintendo at one point created an entirely new IOS branch that differed only in the default value for the TCP buffer size. A fully updated Wii contains one copy of the latest version of each branch of IOS. On a Wii, these are installed as separate titles, often called "IOS slots". Due to this design, it is generally considered safe to uninstall, reinstall, or patch an IOS or IOS module, as long as it is not the slot used by the System Menu - if anything goes wrong, the broken version can be safely uninstalled and a vanilla copy reinstalled. IOS slots have title IDs 1-3 through 1-255. Unused (high) IOS slots are often used to install patched versions of IOS or alternative Starlet software (e.g. [[BootMii]] as IOS is installed as IOS254, which when invoked will subsequently load armboot.bin from the SD card, typically [[mini]]). See [[IOS History]] for a comprehensive list of IOS slots and versions.
+
+Some IOS branches are identical outside of minor build information, such as [[IOS14]] and [[IOS15]]. These branches are referred to as "twins" on the respective pages about these branches. Most twins have identical version numbers for corresponding versions, which makes identifying possible twins simple. Twins are typically built at very similar times, and in some cases, certain modules are substituted, such as FS and FFS being switched.
+
+There are also some cases (mainly in the [[4.3]] batch IOS update) where some IOS branches have been replaced with copies of other branches, such as [[IOS33]] and [[IOS34]] being replaced with copies of [[IOS35]]. Through content sharing, this reduces the storage space required by IOSes, and it reduces the number of IOSes that need to be reverse engineered in a batch update. Such IOSes have been marked as shadow versions.
== Architecture ==
== Architecture ==