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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), and it can simply shut down if it detects a flag[check] set by MIOS. There is also a path that loads BC, although it appears unused.
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.|