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boot2 is the Wii's third-stage bootloader. It is responsible for loading the System Menu TMD and loading the appropriate IOS under normal circumstances, although it can also load MIOS if it detects the lowered clock speed set by BC (if syscall 55 returns 162).
Like IOS, boot2 has multiple modules. In this way, it can be thought of as an IOS installed to a special part of the NAND. However, all modules are contained within a single binary, much like IOSes before IOS28. Boot2 has a kernel with IOSC functionality, an ES module, and an FS module.
boot2 is also launched by MIOS when the power button is pressed; the System Menu is initialized in this case, and 0x80003164 is set to 1 if the SRSTB_CPU bit is set in HW_RESETS; the System Menu checks 0x80003164 and shuts down if it is enabled and state.dat is set appropriately, with the message "Shutdown system from GC!".
boot2 is built from the same code as a regular IOSes, but has a kernel flavor (syscall 47) of 0 instead of 3; boot2's ES module includes all of the normal ES functionality but it goes unused due to this kernel flavor value. Regular IOSes also have the same check for a flavor less than 3, but the bootmii path goes unused.
boot2 was updated in the 4.2 update to boot2v4, the only time Nintendo has ever pushed a boot2 update, besides on prelaunch consoles. This was done by calling ES_ImportBoot to install it into the boot2 region of the NAND. Unfortunately, Nintendo did not test ES_ImportBoot very well, which led to many Wiis receiving a low-level brick, even if they were previously unmodded. The cause of this is unknown, although it may be related to the failure to write ECC data.
|v1||Present on Prelaunch consoles.|
|v2||Present on launch-day Wii's.|
|v3||Only found in Wiis released before LU64+ that were not updated to 4.2. Hardware compatibility updates.|
|v4||Packaged in the 4.2 update to erase BootMii as boot2. Also found preinstalled in LU64+ Wiis. Improves hardware compatibility, fixes the signing bug in the unused signature check code, and unintentionally breaks the HackMii Installer|
|v5||Only found on Arabian Wiis, the Wii Mini and newer RVL-101 units.|