Legacy Boot Mode / Alternative Bootloader (AltFw)

ChromeOS’ Legacy Boot Mode (old older devices) or Alternative Bootloader (aka AltFw, on newer devices) provides a method for booting an alternative OS by running a firmware payload from the RW_LEGACY firmware region. On older devices using Legacy Boot Mode, RW_LEGACY contains a single payload; on devices with AltFw, there can be multiple (though this feature is not used on most devices)

  • Unsupported (by Google) method for booting alternate OSes using the RW_LEGACY firmware region
    • On older devices, the SeaBIOS payload is used, providing a Legacy BIOS bootloader
    • On newer devices, the edk2 (aka Tianocore) payload is used, providing a UEFI bootloader.
  • Accessed via [CTRL+L] on the developer mode boot screen (older devices) or Alternative Bootloader menu option (newer devices)
  • Requires explicit enabling in Developer Mode via command line: sudo crossystem dev_boot_legacy=1 (or dev_boot_altfw=1) (installing RW_LEGACY firmware via the Firmware Utility Script will set these flags for you)
  • Most ChromeOS devices are not capable out of the box, and require a RW_LEGACY firmware update first

Note: If you hear two beeps after pressing [CTRL+L], then either your device doesn’t have a valid Legacy Boot Mode / RW_LEGACY firmware installed, or legacy boot/AltFw capability has not been been enabled via the crossystem flag(s) above.

Using the ChromeOS Firmware Utility Script to update your device’s RW_LEGACY firmware region will address both of these issues, as it not only provides an updated, fully functional RW_LEGACY firmware, but also sets the required boot flag.

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