UEFI fallback boot mode
The UEFI specification defines a so called "fallback boot mode" which allows booting an efi file from disks or removable disks (e.g. USB sticks)
without a corresponding boot variable existing in NVRAM.
When the UEFI firmware is directed to boot from a disk it first searches for a EFI System partition (on GPT disks)
or a FAT32 partition (on MBR disks).
Subsequently, if such a partition is found the firmware searches for a file
\efi
\boot
\bootx64.efi
(on x86-64 machines), and if found, starts this efi file.
Example: Installation of boot manager Boot-US to USB stick
- Get an USB stick formatted with FAT32 in MBR style
- The USB stick may already contain directories and files
- Create a directory \efi\boot on the USB stick
- Install the boot manager Boot-US to a file, this creates the file bmgrus.efi
- Copy the file bmgrus.efi to the directory \efi\boot on the USB stick
- If you have specified a background image copy it to the same directory
- Rename the file bmgrus.efi on the USB stick to bootx64.efi
- Restart the computer and select the USB stick as boot medium
- The UEFI firmware starts the file bootx64.efi with the boot manager Boot-US
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