Unified Extensible Firmware Interface

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Unified Extensible Firmware Interface (or UEFI for short) is a new type of firmware that was initially designed by Intel (known as EFI then) mainly for its Itanium based systems. It introduces new ways of booting an OS that is distinct from the commonly used "MBR boot code" method followed for BIOS systems. It started as Intel's EFI in versions 1.x and then a group of companies called the UEFI Forum took over its development from which it was called Unified EFI starting with version 2.0 . As of 23 May 2012, UEFI Specification 2.3.1 is the most recent version.

Note: Unless specified as EFI 1.x , EFI and UEFI terms are used interchangeably to denote UEFI 2.x firmware. Also unless stated explicitely, these instructions are general and not Mac specific. Some of them may not work or may be different in Macs. Apple's EFI implementation is neither a EFI 1.x version nor UEFI 2.x version but mixes up both. This kind of firmware does not fall under any one UEFI Specification version and therefore it is not a standard UEFI firmware.

Booting an OS using BIOS

A BIOS or Basic Input-Output System is the very first program that is executed once the system is switched on. After all the hardware has been initialized and the POST operation has completed, the BIOS executes the first boot code in the first device in the device booting list.

If the list starts with a CD/DVD drive, then the El-Torito entry in the CD/DVD is executed. This is how bootable CD/DVD works. If the list starts with a HDD, then BIOS executes the very first 440 bytes MBR boot code. The boot code then chainloads or bootstraps a much larger and complex bootloader which then loads the OS.

Basically, the BIOS does not know how to read a partition table or filesystem. All it does is initialize the hardware, then load and run the 440-byte boot code.

Multiboot on BIOS

Since very little can be achieved by a program that fits into the 440-byte boot code area, multi-booting using BIOS requires a multi-boot capable bootloader (multi-boot refers to booting multiple operating systems, not to booting a kernel in the Multiboot format specified by the GRUB developers). So usually a common bootloader like GRUB or GRUB2 or Syslinux or LILO would be loaded by the BIOS, and it would load an operating system by either chain-loading or directly loading the kernel.

Booting an OS using UEFI

UEFI firmware does not support booting through the above mentioned method which is the only way supported by BIOS. UEFI has support for reading both the partition table as well as understanding filesystems.

The commonly used UEFI firmwares support both MBR and GPT partition table. EFI in Apple-Intel Macs are known to support Apple Partition Map also apart from MBR and GPT. Most of the UEFI firmwares have support for accessing FAT12 (floppy disks) , FAT16 and FAT32 filesystems in HDD and ISO9660 (and UDF) in CD/DVDs. EFI in Apple-Intel Macs can access HFS/HFS+ filesystems also apart from the mentioned ones.

UEFI does not launch any boot code in the MBR whether it exists or not. Instead it uses a special partition in the partition table called "EFI SYSTEM PARTITION" in which files required to be launched by the firmware is stored. Each vendor can store its files under <EFI SYSTEM PARTITION>/EFI/<VENDOR NAME>/ folder and can use the firmware or its shell (UEFI shell) to launch the boot program. An EFI System Partition is usually formatted as FAT32.

Under UEFI, every program whether they are OS loaders or some utilities (like memory testing apps) or recovery tools outside the OS, should be a UEFI Application corresponding to the EFI firmware architecture. Most of the UEFI firmware in the market, including recent Apple Macs use x86_64 EFI firmware. Only some older macs use i386 EFI firmware while no non-Apple UEFI system is known to use i386 EFI firmware.

A x86_64 EFI firmware does not include support for launching 32-bit EFI apps unlike the 64-bit Linux and Windows which include such support. Therefore the bootloader must be compiled for that architecture correctly.

Multibooting on UEFI

Since each OS or vendor can maintain its own files within the EFI SYSTEM PARTITION without affecting the other, multi-booting using UEFI is just a matter of launching a different UEFI application corresponding to the particular OS's bootloader. This removes the need for relying on chainloading mechanisms of one bootloader to load another to switch OSes.

Linux Windows x86_64 UEFI-GPT Multiboot

Windows Vista (SP1+) and 7 pr 8 x86_64 versions support booting natively using UEFI firmware. But for this they need GPT partitioning of the disk used for UEFI booting. Windows x86_64 versions support either UEFI-GPT booting or BIOS-MBR booting. Windows 32-bit versions support only BIOS-MBR booting. Follow the instructions provided in the forum link given in the references sections as to how to do this. See http://support.microsoft.com/default.aspx?scid=kb;EN-US;2581408 for more info.

This limitation does not exist in Linux Kernel but rather depends on the bootloader used. For the sake of Windows UEFI booting, the Linux bootloader used should also be installed in UEFI-GPT mode if booting from the same disk.

Boot Process under UEFI

  1. System switched on - Power On Self Test, or POST process.
  2. UEFI firmware is loaded.
  3. Firmware reads its Boot Manager to determine which UEFI application to be launched and from where (ie. from which disk and partition).
  4. Firmware launches the UEFI application from the FAT32 formatted UEFISYS partition as defined in the boot entry in the firmware's boot manager.
  5. UEFI application may launch another application (in case of UEFI Shell or a boot manager like rEFInd) or the kernel and initramfs (in case of a bootloader like GRUB2) depending on how the UEFI application was configured.

Detecting UEFI Firmware Arch

If you have a non-mac UEFI system, then you have a x86_64 (aka 64-bit) UEFI 2.x firmware.

Some of the known x86_64 UEFI 2.x firmwares are Phoenix SecureCore Tiano, AMI Aptio, Insyde H2O.

Some of the known systems using these firmwares are Asus EZ Mode BIOS (in Sandy Bridge P67 and H67 motherboards), MSI ClickBIOS, HP EliteBooks, Sony Vaio Z series, many Intel Server and Desktop motherboards

Pre-2008 Macs mostly have i386-efi firmware while >=2008 Macs have mostly x86_64-efi. All macs capable of running Mac OS X Snow Leopard 64-bit Kernel have x86_64 EFI 1.x firmware.

To find out the arch of the efi firmware in a Mac, boot into Mac OS X and type the following command

ioreg -l -p IODeviceTree | grep firmware-abi

If the command returns EFI32 the it is i386 EFI 1.x firmware. If it returns EFI64 then it is x86_64 EFI 1.x firmware. Macs do not have UEFI 2.x firmware as Apple's efi implementation is not fully compliant with UEFI Specification.

UEFI Support in Linux Kernel

Linux Kernel config options for UEFI

The required Linux Kernel configuration options for UEFI systems are :


UEFI Runtime Variables/Services Support - 'efivars' kernel module . This option is important as this is required to manipulate UEFI Runtime Variables using tools like efibootmgr.

Note: This option is compiled as module in Arch core/testing kernel.
Note: For Linux to access UEFI Runtime Services, the UEFI Firmware processor architecture and the Linux kernel processor architecture must match. This is independent of the bootloader used.
Note: If the UEFI Firmware arch and Linux Kernel arch are different, then the "noefi" kernel parameter must be used to avoid the kernel panic and boot successfully. The "noefi" option instructs the kernel not to access the UEFI Runtime Services.

GUID Partition Table GPT config option - mandatory for UEFI support

Note: All of the above options are required to boot Linux via UEFI, and are enabled in Archlinux kernels in official repos.

Retrieved from http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=blob_plain;f=Documentation/x86/x86_64/uefi.txt;hb=HEAD .

UEFI Variables Support

UEFI defines variables through which an operating system can interact with the firmware. UEFI Boot Variables are used by the boot-loader and used by the OS only for early system start-up. UEFI Runtime Variables allow an OS to manage certain settings of the firmware like the UEFI Boot Manager or managing the keys for UEFI Secure Boot Protocol etc.

Note: The below steps will not work if the system has been booted in BIOS mode and will not work if the UEFI processor architecture does not match the kernel one, i.e. x86_64 UEFI + ix86 32-bit Kernel and vice-versa config will not work. This is true only for efivars kernel module and efibootmgr step. The other steps (ie. upto setting up <UEFISYS>/efi/arch/grub.{efi,cfg} ) can be done even in BIOS/Legacy boot mode.

Access to UEFI Runtime services is provided by "efivars" kernel module which is enabled through the CONFIG_EFI_VAR=m kernel config option. This module once loaded exposes the variables under the directory /sys/firnware/efi/vars. One way to check whether the system has booted in UEFI boot mode is to load the "efivars" kernel module and check for the existence of /sys/firnware/efi/vars directory with contents similar to :

Sample output (x86_64-UEFI 2.3.1 in x86_64 Kernel):

# ls -1 /sys/firmware/efi/vars/

The UEFI Runtime Variables will not be exposed to the OS if you have used "noefi" kernel parameter in the boot-loader menu. This parameter instructs the kernel to completely ignore UEFI Runtime Services.

Userspace Tools

There are few tools that can access/modify the UEFI variables, namely

  1. efibootmgr - Used to create/modify boot entries in the UEFI Boot Manager - efibootmgr or efibootmgr-gitAUR
  2. uefivars - simply dumps the variables - uefivars-gitAUR - uses efibootmgr library
  3. Ubuntu's Firmware Test Suite - fwts - fwts-gitAUR - uefidump command - fwts uefidump

Non-Mac UEFI systems


Warning: Using efibootmgr in Apple Macs will brick the firmware and may need reflash of the motherboard ROM. There have been bug reports regarding this in Ubuntu/Launchpad bug tracker. Use bless command alone in case of Macs. Experimental "bless" utility for Linux by Fedora developers - mactel-bootAUR.
Note: efibootmgr command will work only if you have booted the system in UEFI mode itself, since it requires access to UEFI Runtime Variables which are available only in UEFI boot mode (with "noefi" kernel parameter NOT being used). Otherwise the message Fatal: Couldn't open either sysfs or procfs directories for accessing EFI variables is shown.

Initially the user may be required to manually launch the boot-loader from the firmware itself (using maybe the UEFI Shell) if the UEFI boot-loader was installed when the system is booted in BIOS mode. Then efibootmgr should be run to make the UEFI boot-loader entry as the default entry in the UEFI Boot Manager.

To use efibootmgr, first load the 'efivars' kernel module:

# modprobe efivars

If you get no such device found error for this command, that means you have not booted in UEFI mode or due to some reason the kernel is unable to access UEFI Runtime Variables (noefi?).

Verify whether there are files in /sys/firmware/efi/vars/ directory. This directory and its contents are created by "efivars" kernel module and it will exist only if you have booted in UEFI mode, without the "noefi" kernel parameter.

If /sys/firmware/efi/vars/ directory is empty or does not exist, then efibootmgr command will not work. If you are unable to make the ISO/CD/DVD/USB boot in UEFI mode try https://gitorious.org/tianocore_uefi_duet_builds/pages/Linux_Windows_BIOS_UEFI_boot_USB.

Note: The below commands use grub-efi-x86_64 boot-loader as example.

Assume the boot-loader file to be launched is /boot/efi/efi/arch_grub/grubx64.efi. /boot/efi/efi/arch_grub/grubx64.efi can be split up as /boot/efi and /efi/arch_grub/grubx64.efi, wherein /boot/efi is the mountpoint of the UEFI System Partition, which is assumed to be /dev/sdXY (here X and Y are just placeholders for the actual values - eg:- in /dev/sda1 , X=a Y=1).

To determine the actual device path for the UEFI System Partition, try :

# cat /proc/self/mounts | grep /boot/efi | awk '{print $1}'

Then create the boot entry using efibootmgr as follows :

# efibootmgr --create --gpt --disk /dev/sdX --part Y --write-signature --label "Arch Linux (GRUB2)" --loader '\EFI\arch_grub\grubx64.efi'

In the above command /boot/efi/efi/arch_grub/grubx64.efi translates to /boot/efi and /efi/arch_grub/grubx64.efi which in turn translate to drive /dev/sdX -> partition Y -> file /EFI/arch_grub/grubx64.efi.

UEFI uses backward slash as path separator (similar to Windows paths).

The 'label' is the name of the menu entry shown in the UEFI boot menu. This name is user's choice and does not affect the booting of the system. More info can be obtained from efibootmgr GIT README .

FAT32 filesystem is case-insensitive since it does not use UTF-8 encoding by default. In that case the firmware uses capital 'EFI' instead of small 'efi', therefore using \EFI\arch_grub\grubx64.efi or \efi\arch_grub\grubx64.efi does not matter (this will change if the filesystem encoding is UTF-8).

Linux Bootloaders for UEFI

See UEFI Bootloaders.

Create an UEFI System Partition in Linux

Note: The UEFISYS partition can be of any size supported by FAT32 filesystem. According to Microsoft Documentation, the minimum partition/volume size for FAT32 is 512 MiB. Therefore it is recommended for UEFISYS partition to be atleast 512 MiB. Higher partition sizes are fine, especially if you use multiple UEFI bootloaders, or multiple OSes booting via UEFI, so that there is enough space to hold all the related files. If you are using Linux EFISTUB booting, then you need to make sure there is adequate space available for keeping the Kernel and Initramfs files in the UEFISYS partition.

For GPT partitioned disks

Two choices:

  • Using GNU Parted/GParted: Create a FAT32 partition. Set "boot" flag on for that partition.
  • Using GPT fdisk (aka gdisk): Create a partition with gdisk type code "EF00". Then format that partition as FAT32 using mkfs.vfat -F32 /dev/<THAT_PARTITION>
Note: Setting "boot" flag in parted in a MBR partition marks that partition as active, while the same "boot" flag in a GPT partition marks that partition as "UEFI System Partition".
Warning: Do not use util-linux fdisk, cfdisk or sfdisk to change the type codes in a GPT disk. Similarly do not use gptfdisk gdisk, cgdisk or sgdisk on a MBR disk, it will be automatically converted to GPT (no data loss will occur, but the system will fail to boot).

For MBR partitioned disks

Two choices:

  • Using GNU Parted/GParted: Create FAT32 partition. Change the type code of that partition to 0xEF using fdisk, cfdisk or sfdisk.
  • Using fdisk: Create a partition with partition type 0xEF and format it as FAT32 using mkfs.vfat -F32 /dev/<THAT_PARTITION>
Note: It is recommended to use always GPT for UEFI boot as some UEFI firmwares do not allow UEFI-MBR boot.

UEFI Shell

The UEFI Shell is a shell/terminal for the firmware which allows launching uefi applications which include uefi bootloaders. Apart from that, the shell can also be used to obtain various other information about the system or the firmware like memory map (memmap), modifying boot manager variables (bcfg), running partitioning programs (diskpart), loading uefi drivers, editing text files (edit), hexedit etc.

UEFI Shell download links

You can download a BSD licensed UEFI Shell from Intel's Tianocore UDK/EDK2 Sourceforge.net project.

Shell 2.0 works only in UEFI 2.3+ systems and is recommended over Shell 1.0 in those systems. Shell 1.0 should work in all UEFI systems irrespective of the spec. version the firmware follows. More info at ShellPkg and this mail

Launching UEFI Shell

Few Asus and other AMI Aptio x86_64 UEFI firmware based motherboards (from Sandy Bridge onwards) provide an option called "Launch EFI Shell from filesystem device" . For those motherboards, download the x86_64 UEFI Shell and copy it to your UEFI SYSTEM PARTITION as <UEFI_SYSTEM_PARTITION>/shellx64.efi (mostly /boot/efi/shellx64.efi) .

Systems with Phoenix SecureCore Tiano UEFI firmware are known to have embedded UEFI Shell which can be launched using either F6, F11 or F12 key.

Note: If you are unable to launch UEFI Shell from the firmware directly using any of the above mentioned methods, create a FAT32 USB pen drive with Shell.efi copied as (USB)/efi/boot/bootx64.efi . This USB should come up in the firmware boot menu. Launching this option will launch the UEFI Shell for you.

Important UEFI Shell Commands

More info at http://software.intel.com/en-us/articles/efi-shells-and-scripting/


BCFG command is used to modify the UEFI NVRAM entries, which allow the user to change the boot entries or driver options. This command is described in detail in page 83 (Section 5.3) of "UEFI Shell Specification 2.0" pdf document.

Note: Users are recommended to try bcfg only if efibootmgr fails to create working boot entries in their system.
Note: UEFI Shell 1.0 does not support bcfg command.

To dump a list of current boot entries -

Shell> bcfg boot dump -v

To add a boot menu entry for grub2's grubx64.efi (for example) as 4th (numbeering starts from zero) option in the boot menu

Shell> bcfg boot add 3 fs0:\EFI\arch\grubx64.efi "Arch Linux (GRUB2)"

where fs0: is the mapping corresponding to the UEFI System Partition and \EFI\arch\grubx64.efi is the file to be launched.

To remove the 4th boot option

Shell> bcfg boot rm 3

To move the boot option #3 to #0 (i.e. 1st or the default entry in the UEFI Boot menu)

Shell> bcfg boot mv 3 0

For bcfg help text

Shell> help bcfg -v -b


Shell> bcfg -? -v -b


EDIT command provides a basic text editor with an interface similar to nano text editor, but slightly less functional. It handles UTF-8 encoding and takes care or LF vs CRLF line endings.

To edit, for example grub2's grub.cfg in the UEFI System Partition (fs0: in the firmware)

Shell> fs0:
FS0:\> cd \efi\grub
FS0:\efi\grub\> edit grub.cfg

Hardware Compatibility

Main page HCL/Firmwares/UEFI

Create UEFI bootable USB from ISO

Note: dd'ing the ISO (isohybrid method) to the USB drive will not work for UEFI boot.


1. Create a directory /tmp/archiso and extract the archiso file contents to it.

2. Create a directory /tmp/archiso_efiboot and extract /tmp/archiso/EFI/archiso/efiboot.img to it using 7z command from p7zip package.

3. Run the below commands:

# mkdir -p /tmp/archiso/EFI/{archiso,boot}
# cp /tmp/archiso/arch/boot/x86_64/vmlinuz /tmp/archiso/EFI/archiso/vmlinuz.efi
# cp /tmp/archiso/arch/boot/x86_64/archiso.img /tmp/archiso/EFI/archiso/archiso.img
# cp /tmp/archiso_efiboot/EFI/boot/bootx64.efi /tmp/archiso/EFI/boot/bootx64.efi
# cp /tmp/archiso_efiboot/EFI/boot/startup.nsh /tmp/archiso/EFI/boot/startup.nsh

4. Find out the filesystem label to be used for the USB by reading "archisolabel=" part in /tmp/archiso/EFI/boot/startup.nsh. For example if /tmp/archiso/EFI/boot/startup.nsh has archisolabel=ARCH_201208 then the filesystem label to be used is ARCH_201208 .

5. Create a directory /tmp/archisousb . Format the USB drive as FAT32 (or FAT16) (no other filesystem is supported) and set the filesystem label same as the one obtained in step 4, and mount it to /tmp/archisousb .

6. Copy the contents of /tmp/archiso to /tmp/archisousb and then umount /tmp/archisousb .


1. Create a directory /tmp/archboot and extract the archiso file contents to it.

Note: Follow steps 2 and 3 only if /tmp/archboot/EFI/boot/bootx64.efi does not exist, even after extracting archboot iso to /tmp/archboot .

2. Create a directory /tmp/archboot_efiboot and extract /tmp/archboot/boot/grub/grub_uefi_x86_64.bin to it using 7z command from p7zip package.

3. Run the below commands:

# mkdir -p /tmp/archboot/EFI/boot
# cp /tmp/archboot_efiboot/EFI/boot/bootx64.efi /tmp/archboot/EFI/boot/bootx64.efi

4. Create a directory /tmp/archbootusb . Format the USB drive as FAT32 (or FAT16) (no other filesystem is supported) and mount it to /tmp/archbootusb .

5. Copy the contents of /tmp/archboot to /tmp/archbootusb and then umount /tmp/archbootusb .

Remove UEFI boot support from ISO

Most of the 32-bit EFI Macs and some 64-bit EFI Macs refuse to boot from a UEFI(X64)+BIOS bootable CD/DVD. In these cases the iso should be rebuilt without UEFI boot support, retaining only BIOS boot.


1. Obtain the ISO label from the output of file <path_to_iso>. Let it be ARCH_201208 for example.

2. Create a directory /tmp/archiso and extract the archiso file contents to it.

3. Run xorriso (part of libisoburn package) as shown below:

$ xorriso -as mkisofs -iso-level 3 \
          -full-iso9660-filenames \
          -volid "ARCH_201208" \
          -appid "Arch Linux Live/Rescue CD" \
          -publisher "Arch Linux <http://www.archlinux.org>" \
          -preparer "prepared by user" \
          -eltorito-boot isolinux/isolinux.bin \
          -eltorito-catalog isolinux/boot.cat \
          -no-emul-boot -boot-load-size 4 -boot-info-table \
          -isohybrid-mbr "/tmp/archiso/isolinux/isohdpfx.bin" \
          -output "/tmp/archiso.iso" "/tmp/archiso/"

4. Burn /tmp/archiso.iso to a CD and boot into your Mac using that CD.


1. Create a directory /tmp/archboot and extract the archboot iso file contents to it.

2. Run xorriso (part of libisoburn package) as shown below:

$ xorriso -as mkisofs -iso-level 3 -rock -joliet \
          -max-iso9660-filenames -omit-period \
          -omit-version-number -allow-leading-dots \
          -relaxed-filenames -allow-lowercase -allow-multidot \
          -volid "ARCHBOOT" -preparer "prepared by user" \
          -eltorito-boot boot/syslinux/isolinux.bin \
          -eltorito-catalog boot/syslinux/boot.cat \
          -no-emul-boot -boot-load-size 4 -boot-info-table \
          -isohybrid-mbr /tmp/archboot/boot/syslinux/isohdpfx.bin \
          -output "/tmp/archboot.iso" "/tmp/archboot/"

3. Burn /tmp/archboot.iso to a CD and boot into your Mac using that CD.

See also