Most embedded PC designs and some ARM designs have firmware based on the Universal Extensible Firmware Interface (UEFI) standard, see the official website at http://www.uefi.org for more information. The boot sequence is fundamentally the same as described in the preceding section:
Phase 1: The processor loads the UEFI boot manager firmware from flash memory. In some designs, it is loaded directly from NOR flash memory, in others there is ROM code on-chip which loads the boot manager from SPI flash memory. The boot manager is roughly equivalent to the SPL, but may allow user interaction through a text-based or graphical interface.
Phase 2: The boot manager loads the boot firmware from the EFI System Partition (ESP) or a hard disk or SSD, or from a network server via PXE boot. If loading from a local disk drive, the EXP is identified by a well-known GUID value of C12A7328-F81F-11D2-BA4B-00A0C93EC93B. The partition should be formatted using the FAT32 format. The third stage bootloader should be in a file named <efi_system_partition>/boot/boot<machine_type_short_name>.efi.
For example, the file path to the loader on an x86_64 system is: /efi/boot/bootx64.efi
Phase 3: The TPL in this case has to be a bootloader that is capable of loading a Linux kernel and an optional RAM disk into memory. Common choices are:
- GRUB 2: This is the GNU Grand Unified Bootloader, version 2, and it is the most commonly used Linux loader on PC platforms. However, there is one controversy in that it is licensed under GPL v3, which may make it incompatible with secure booting since the license requires the boot keys to to be supplied with the code. The website is https://www.gnu.org/software/grub/.
- gummiboot: This is a simple UEFI-compatible bootloader which has since been integrated into systemd, and is licensed under LGPL v2.1 The website is https://wiki.archlinux.org/index.php/Systemd-boot.