EFI/UEFI Boot Loaders
Legend |
|
Yes |
Feature known to exist |
? |
Indeterminate feature presence |
No |
Feature does not exist |
UEFI defines a new method of interfacing between PC operating systems and firmware. The UEFI interface is composed of data tables containing platform-related information, plus boot and runtime functions that are available to the operating system and the boot loader. These provide a standard environment for booting and running pre-boot applications.
The table following lists known features found in current EFI/UEFI capable boot loaders and boot managers. (Note: Boot managers present a menu of boot options, whereas boot loaders load a kernel. Many programs can do both tasks, although rEFIt can load a kernel on PC hardware only if it includes an EFI stub loader.)
Comparison table
Features |
EFI/UEFI Bootloaders |
EFI/UEFI Boot Managers |
BIOS Bootloader |
||||
Oldest/most stable Linux EFI bootloader, very limited features |
Most complex Linux EFI bootloader available, actively developed |
Linux kernels since v3.3.0 are bootable via system firmware; no bootloader needed |
EFI graphical boot manager, development stopped in 2010 |
EFI graphical boot manager with OS auto-detection, semi-active development |
EFI text-menu boot manager with modern features but very simple interface, actively developed |
No EFI/UEFI support; for comparison only |
|
Graphical interface mode |
No |
Yes |
No |
Yes |
Yes |
No |
Background imgs only |
Mouse support |
No |
No |
No |
No |
Yes |
No |
No |
Text menu interface mode |
Yes |
Yes |
No |
Yes |
Yes |
Yes |
Yes |
File-based configuration |
Yes |
Yes |
No |
Yes |
Yes |
Yes |
Yes |
select from multiple kernels |
Yes |
Yes |
No |
Yes |
Yes |
Yes |
Yes |
interactive mode |
Yes |
Yes |
No |
No |
Yes |
Yes |
Yes |
load initrd |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
kernel selector menu |
Yes |
Yes |
No |
Yes |
Yes |
Yes |
Yes |
kernel boot params |
Yes |
Yes |
Yes |
? |
Yes |
Yes |
Yes |
Boot old kernel if upgrade fails |
Yes |
Yes |
No |
No |
Yes |
Yes |
Yes |
Paramaterize kernel cmdline opts1 |
Yes |
No |
No |
No |
Yes |
No |
No |
Enable legacy BIOS |
? |
? |
No |
Yes |
Yes |
No |
BIOS ONLY |
Chain loading |
No |
Yes |
No |
Yes |
Yes |
Yes |
Yes |
Gzip compression support |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Load ELF binaries |
Yes |
Yes |
No |
Yes |
Yes |
Yes |
Yes |
Apple fat image |
? |
? |
? |
Yes |
? |
? |
? |
Boot unmodified linux kernel |
Yes |
Yes |
Yes |
? |
Yes |
Yes |
Yes |
Relocate Kernel |
No |
Yes |
No |
No |
Yes |
Yes |
Yes |
IA-64 modified kernel |
Yes |
Yes |
Yes |
? |
? |
? |
? |
Load FPSWA driver2 |
Yes (IA-64 only) |
? |
? |
? |
? |
? |
N/A |
Custom font support |
Yes |
Yes |
No |
Yes |
Yes |
No |
? |
Architecture Support: |
|||||||
x86-64 |
Yes |
Yes |
Yes |
Yes |
Yes |
? |
Yes |
x86-32 |
Yes |
Yes |
Yes |
Yes |
Yes |
? |
Yes |
IA-64 |
Yes |
Yes |
Yes |
? |
? |
? |
? |
File System Support:3 |
|||||||
Amiga FFS |
? |
Yes |
? |
? |
Yes |
? |
? |
AFS |
? |
Yes |
? |
? |
Yes |
? |
? |
Btrfs |
? |
Yes |
? |
? |
Yes |
? |
? |
CPIO |
? |
Yes |
? |
? |
Yes |
? |
? |
EXT2 |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
EXT3 |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
EXT4 |
? |
Yes |
? |
? |
Yes |
Yes |
Yes (Jaunty+) |
FAT |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
FFS |
? |
Yes |
? |
? |
No |
? |
Yes |
HFS |
? |
Yes |
? |
? |
Yes |
? |
? |
HFS+ |
Yes |
Yes |
Yes |
Yes |
Yes |
? |
? |
ISO9660 |
Yes |
Yes |
Yes |
Yes |
Yes |
? |
Yes |
JFS |
? |
Yes |
? |
? |
Yes |
? |
Yes |
MINIX |
? |
Yes |
? |
? |
Yes |
? |
Yes |
NTFS |
? |
Yes |
? |
? |
Yes |
? |
? |
NTFS Compressed |
? |
Yes |
? |
? |
? |
? |
? |
ReiserFS |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
SFS (Amiga) |
? |
Yes |
? |
? |
Yes |
? |
? |
UDF |
? |
Yes |
? |
? |
Yes |
? |
? |
UFS |
? |
Yes |
? |
? |
Yes |
? |
Yes |
XFS |
? |
Yes |
? |
? |
Yes |
? |
Yes |
ZFS |
? |
? |
? |
? |
Yes |
? |
? |
Disk Volume/Partition Support: |
|||||||
RAID |
? |
Yes |
? |
? |
? |
? |
Yes |
LVM |
? |
Yes |
? |
? |
Yes |
? |
Yes |
ADFS (Acorn partition maps) |
? |
Yes |
? |
? |
? |
? |
? |
Amiga partition tables |
? |
Yes |
? |
? |
? |
? |
? |
Macintosh partition tables |
? |
Yes |
? |
? |
? |
? |
? |
Master Boot Record (MBR) tables |
? |
Yes |
Yes |
Yes |
Yes |
? |
Yes |
Sun partition tables |
? |
Yes |
? |
? |
? |
? |
? |
GPT (GUID partition tables) |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
No |
Disk loopback |
? |
Yes |
? |
? |
? |
? |
? |
Filesystems by UUID/Label |
? |
Yes |
? |
? |
Yes |
? |
Yes (Intrepid+) |
Image File Format Support: |
|||||||
JPG images |
No |
Yes |
No |
? |
Yes |
No |
No |
BMP images |
No |
No |
No |
Yes |
Yes |
No |
No |
PNG images |
No |
Yes |
No |
No |
Yes |
No |
No |
XPM images |
No |
? |
No |
No |
No |
No |
Yes |
ICNS images |
No |
? |
No |
Yes |
Yes |
No |
? |
Command Line Operations |
|||||||
background |
? |
? |
? |
? |
? |
? |
Yes |
beep (PC speaker) |
? |
Yes |
? |
? |
? |
? |
? |
print file block list |
? |
Yes |
? |
? |
? |
? |
Yes |
set border color |
? |
? |
? |
? |
? |
? |
Yes |
read text file contents |
? |
Yes |
? |
? |
Yes |
? |
Yes |
clear screen |
? |
? |
? |
? |
? |
? |
Yes |
compare files |
? |
Yes |
? |
? |
? |
? |
Yes |
identify CPU |
? |
Yes |
? |
? |
? |
? |
? |
debug mode |
Yes |
? |
? |
? |
? |
? |
Yes |
display APM BIOS info |
N/A |
N/A |
? |
N/A |
N/A |
? |
Yes |
display RAM info |
? |
? |
? |
? |
? |
? |
Yes |
dump file (copy) |
? |
? |
? |
? |
? |
? |
Yes |
echo / print |
? |
Yes |
? |
? |
? |
? |
Yes |
fallback to known good kernel |
? |
Yes |
? |
? |
? |
Yes |
Yes |
find filename on partition |
? |
? |
? |
? |
? |
? |
Yes |
filesystem test mode |
? |
? |
? |
? |
Yes |
Yes |
Yes |
help |
? |
Yes |
? |
? |
Yes |
Yes |
Yes |
display disk geometry |
? |
? |
? |
? |
? |
? |
Yes |
manually load config. file |
? |
Yes |
? |
? |
? |
? |
? |
halt machine |
? |
Yes |
? |
Yes |
Yes |
? |
Yes |
hide partition |
? |
? |
? |
? |
Yes |
? |
Yes |
file hexdump |
? |
Yes |
? |
? |
? |
? |
? |
intel multiprocessor probe |
? |
? |
? |
? |
? |
? |
Yes |
disk I/O port probe |
? |
? |
? |
? |
? |
? |
Yes |
load/save env. variables |
? |
Yes |
? |
? |
Yes |
Yes |
? |
show directory contents |
? |
Yes |
? |
? |
Yes |
Yes |
? |
probe PCI bus |
? |
Yes |
? |
? |
? |
? |
? |
set partition boot flag |
? |
? |
? |
? |
? |
? |
Yes |
map one drive to another |
? |
? |
? |
? |
? |
? |
Yes |
screen pager |
? |
? |
? |
? |
? |
? |
Yes |
partition manipulation |
? |
? |
? |
? |
? |
? |
Yes |
pause/sleep |
? |
Yes |
? |
? |
? |
? |
Yes |
read variables from user input |
? |
Yes |
? |
? |
? |
? |
? |
read memory address |
? |
? |
? |
? |
? |
? |
Yes |
reboot computer |
? |
Yes |
? |
Yes |
Yes |
? |
? |
select terminal |
? |
Yes |
? |
? |
? |
? |
? |
serial console support |
? |
? |
? |
? |
? |
? |
Yes |
set keyboard mappings |
? |
? |
? |
? |
? |
? |
Yes |
expression handling |
? |
Yes |
? |
? |
? |
? |
? |
video testing |
? |
Yes |
? |
? |
? |
? |
? |
list vbe modes |
? |
Yes |
? |
? |
? |
? |
Yes |
vbe test |
? |
Yes |
? |
? |
? |
? |
? |
load font files |
? |
Yes |
? |
? |
? |
? |
? |
block users from using EFI shell |
? |
? |
? |
Yes |
Yes |
? |
? |
disable booting from optical drive |
? |
? |
? |
Yes |
Yes |
? |
? |
disable booting from internal drives |
? |
? |
? |
Yes |
Yes |
? |
? |
block apple HW testing |
? |
? |
? |
Yes |
Yes |
? |
? |
Toolchain Contraints |
|||||||
IA-64: gcc version |
3.1, 3.2 |
? |
? |
? |
? |
? |
? |
IA-64: binutils |
2.13.90 |
? |
? |
? |
? |
? |
? |
x86-32: gcc-version |
3.2 |
? |
? |
? |
? |
? |
? |
x86-32: binutils |
2.13.90.0.2 |
? |
? |
? |
? |
? |
? |
x86-64: gcc-version |
4.1.1+ |
? |
? |
? |
? |
? |
? |
x86-64: bintil |
2.17.50.0.14+ |
? |
? |
? |
? |
? |
? |
Netboot Methods: |
|||||||
PXE |
Yes |
? |
? |
? |
Yes |
? |
? |
DCHP |
Yes |
? |
? |
? |
? |
? |
Yes |
TFTP |
Yes |
? |
? |
? |
? |
? |
Yes |
BOOTP |
? |
? |
? |
? |
? |
? |
Yes |
IFCONFIG |
? |
? |
? |
? |
? |
? |
Yes |
RARP |
? |
? |
? |
? |
? |
? |
Yes |
Footnotes:
e.g. root=/dev/nfs ip=%I:%z:%G:%N, where the utility can generate the needed values based on information it can probe for (1)
Floating Point Software Assist (2)
Loading of kernel+initrd occurs using EFI firmware, hence only supports FAT16/32 (but if given 3rd-party EFI drivers can then read: ext2, ext3, ext4, ReiserFS, Btrfs, HFS+, or ISO-9660) (3)
Booting from EFI
All EFI compliant systems have an EFI System Partition (ESP), which is a volume formatted with either a FAT16 or FAT32 filesystem containing EFI firmware modules and boot images. Note that there can be multiple EFI system partitions present on a system, with their order of precedence configured in the firmware setup menu. The ESP is effectively the boot partition, and contains the necessary images to boot a system. This boot partition should be mounted on a Linux system as /efi (formerly /boot/efi) for the updating of new kernels and initrd images.
Each vendor "must" use a dedicated directory within the \EFI\ directory (i.e. \EFI\vendor) for its EFI executable and other related files.
Shell.efi is the name of the UEFI Shell binary generated by the TianoCore EFI Development Kit (EDK) II. If present, it will execute a boot script with the filename startup.nsh in the root directory of an ESP. startup.nsh can be used to load and execute the preferred boot loader, or perform other tasks related to the ESP contents. More commonly, a list of boot loaders is maintained in NVRAM variables with names of the format Boot####, where #### is a hexadecimal value in a series beginning with 0000. The boot order is stored in the BootOrder variable and determines the order in which the system will attempt to execute the boot loaders that are configured. These variables can be manipulated in Linux with the efibootmgr utility. If the NVRAM contains no such entries or if they're all invalid, recent EFI implementations fallback to using the boot loader located at \EFI\BOOT\boot{arch}.efi, where {arch} is an architecture code, such as "x64" for x86-64 or "ia32" for x86.
All EFI executable images contain a PE/COFF header defining the format of the executable code. The code may be one of:
- IA-32
- x86-64 (aka X64)
- Itaninum
- ARM
- Processor agnostic generic EFI byte code
ELILO, the kernel's EFI stub loader, Grub 2 (with EFI support), rEFIt, and rEFInd are all processor native executable code.
KERNEL configuration
Build the kernel with the following configuration.
CONFIG_FB_EFI=y CONFIG_FRAMEBUFFER_CONSOLE=y
If EFI runtime services are expected, the following configuration should be selected.
CONFIG_EFI=y CONFIG_EFI_VARS=y or m # optional
To use the EFI stub loader, you must set the following option:
CONFIG_EFI_STUB=y
The kernel can then be loaded from an EFI shell or by entering it into the EFI's boot list in NVRAM. Getting the options right can be tricky, though. Using rEFInd to launch a kernel with EFI stub support can simplify matters.
External Links
Implementations
Documentation
Intel UEFI resources (September 6, 2019)
UEFI Shell Specification (v2.2 - January 26, 2016, a UEFI scripting primer)
The Extensible Firmware Interface (September 1, 2005) and EFI Architecture (May 10, 2007) from Dr. Dobb's Journal
Mac OS X Internals: More Power To Firmware by Amit Singh (June 2004, archived by The Wayback Machine on January 3, 2020)