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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 a flag set by BC, and it can simply shut down if it detects a flag set by MIOS.
boot2 was updated in the 4.2 update to boot2v4, the only time Nintendo has ever pushed a boot2 update. This was done by installing boot2 as title 1-1[check], then 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.
|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. Fixes the signing bug in the unused signature check code, and supports the next Hollywood revision.|
|v4||Packaged in the 4.2 update to erase BootMii as boot2. Also found preinstalled in LU64+ Wiis. Contains a different layout to break the HackMii Installer.|
|v5||Only found on Arabian Wiis, certain RVL-101 Wiis and the Wii Mini.|