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UEFI -- Unified Extensible Firmware Interface bootstrapping procedures


The UEFI Unified Extensible Firmware Interface provides boot- and runtime services to operating systems. UEFI is a replacement for the legacy BIOS on the i386 and amd64 CPU architectures, and is also used on arm, arm64 and ia64.

The UEFI specification is the successor to the Extensible Firmware Interface (EFI) specification. The terms are often used interchangeably.

The UEFI boot process loads system bootstrap code located in an EFI System Partition (ESP). The ESP is a GPT or MBR partition with a specific identifier that contains an msdosfs(5) FAT file system with a specified file hierarchy.

Partition Scheme ESP Identifier
GPT C12A7328-F81F-11D2-BA4B-00A0C93EC93B

The UEFI boot process proceeds as follows

1. UEFI firmware runs at power up and searches for an OS loader in the EFI system partition. The path to the loader may beset by an EFI environment variable. If not set, an architecture-specific default is used.

Architecture Default Path

The default UEFI boot configuration for FreeBSD installs boot1.efi in the default path.

2. boot1.efi reads boot configuration from /boot.config or /boot/config. Unlike other first-stage boot loaders, boot1.efi passes the configuration to the next stage boot loader and does not itself act on the contents of the file.

3. boot1.efi searches partitions of type freebsd-ufs and freebsd-zfs for loader.efi. The search begins with partitions on the device from which boot1.efi was loaded, and continues with other available partitions. If both freebsd-ufs and freebsd-zfs partitions exist on the same device the freebsd-zfs partition is preferred. boot1.efi then loads and executes loader.efi.

4. loader.efi loads and boots the kernel, as described in loader(8).

The vt(4) system console is automatically selected when booting via UEFI.


File Description
/boot/boot1.efi First stage UEFI bootstrap
/boot/boot1.efifat msdosfs(5) FAT file system image containing boot1.efi for use by bsdinstall(8) and the bootcode argument to gpart(8).
/boot/loader.efi Final stage bootstrap
/boot/kernel/kernel default kernel
/boot/kernel.old/kernel typical non-default kernel (optional)

The next steps are described as Bootstrap Process.

Additional information about UEFI[edit]

FreeBSD about UEFI