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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).

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.

There are two copies of boot2 housed in NAND blocks 1-7 in case of failure, loaded by boot1. Additionally, a copy of boot2 may exist as title 1-1 in the NAND filesystem.

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.

Version history

Version Description
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, the Wii Mini and newer RVL-101 units.