# Unified Extensible Firmware Interface

Related articles

- Arch boot process
- Master Boot Record
- GUID Partition Table

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 24 July 2013, UEFI Specification 2.4 (released July 11, 2013) is the most recent version.

Note:

- This page explains What is UEFI and UEFI support in Linux kernel. It does not describe setting up UEFI Boot Loaders. For that information see Boot loaders.
- Unless specified as EFI 1.x, EFI and UEFI terms are used interchangeably to denote UEFI 2.x firmware. Also unless stated explicitly, these instructions are general and some of them may not work or may be different in Apple 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 (U)EFI specification and therefore is not a standard UEFI firmware.

## Differences between BIOS and UEFI

See Arch boot process#Firmware_types for more details.

## Boot Process under UEFI

1. System switched on - Power On Self Test, or POST process.
2. UEFI firmware is loaded. Firmware initializes the hardware required for booting.
3. Firmware then reads its Boot Manager data to determine which UEFI application to be launched and from where (i.e. from which disk and partition).
4. Firmware then launches the UEFI application as defined in the boot entry in the firmware's boot manager.
5. The launched 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 boot loader like GRUB) depending on how the UEFI application was configured.

Note: On some UEFI systems the only possible way to launch UEFI application on boot (if it does not have custom entry in UEFI boot menu) is to put it in this fixed location: `<EFI SYSTEM PARTITION>/EFI/BOOT/BOOTX64.EFI` (for 64-bit x86 system)

### Multibooting in 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 boot loader to load another to switch OSes.

#### Booting Microsoft Windows

### Detecting UEFI Firmware bitness

#### Non Macs

Check whether the dir `/sys/firmware/efi` exists, if it exists it means the kernel has booted in EFI mode. In that case the UEFI bitness is same as kernel bitness. (ie. i686 or x86_64)

Note: Intel Atom System-on-Chip systems ship with 32-bit UEFI (as on 2 November 2013). See this page for more info.

#### Apple Macs

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, type the following into the Mac OS X terminal:

    ioreg -l -p IODeviceTree | grep firmware-abi

If the command returns EFI32 then it is IA32 (32-bit) EFI firmware. If it returns EFI64 then it is x86_64 EFI firmware. Most of the Macs do not have UEFI 2.x firmware as Apple's EFI implementation is not fully compliant with UEFI 2.x Specification.

### Secure Boot

For an overview about Secure Boot in Linux see http://www.rodsbooks.com/efi-bootloaders/secureboot.html. This section focuses on how to set up Secure Boot in Arch Linux. For the time being, this section is limited to explain the procedure of booting the archiso with Secure Boot enabled. Booting the archiso with Secure Boot enabled is possible since the efi applications PreLoader.efi and HashTool.efi have been added to it. A message will show up that says Failed to Start loader... I will now execute HashTool. To use HashTool for enrolling the hash of loader.efi and vmlinuz.efi, follow these steps.

- Select `OK`
- In the HashTool main menu, select `Enroll Hash`, choose `\loader.efi` and confirm with `Yes`. Again, select `Enroll Hash` and `archiso` to enter the archiso directory, then select `vmlinuz-efi` and confirm with `Yes`. Then choose `Exit` to return to the boot device selection menu.
- In the boot device selection menu choose `Arch Linux archiso x86_64 UEFI CD`

The archiso boots, and you are presented with a shell prompt, automatically logged in as root. To check if the archiso was booted with SecureBoot, use this command:

    $ od -An -t u1 /sys/firmware/efi/vars/SecureBoot-1234abcde-5678-/data

If yes, this command returns 1. The characters denoted by 1234 differ from machine to machine. To help with this, you can use tab completion or list the efi variables.

## Linux Kernel Config options for UEFI

The required Linux Kernel configuration options for UEFI systems are :

    CONFIG_RELOCATABLE=y
    CONFIG_EFI=y
    CONFIG_EFI_STUB=y
    CONFIG_FB_EFI=y
    CONFIG_FRAMEBUFFER_CONSOLE=y

UEFI Runtime Variables Support (efivarfs filesystem - `/sys/firmware/efi/efivars`). This option is important as this is required to manipulate UEFI Runtime Variables using tools like `/usr/bin/efibootmgr`. The below config option has been added in kernel 3.10 and above.

    CONFIG_EFIVAR_FS=y

UEFI Runtime Variables Support (old efivars sysfs interface - `/sys/firmware/efi/vars`). This option should be disabled to prevent any potential issues with both efivarfs and sysfs-efivars enabled.

    CONFIG_EFI_VARS=n

GUID Partition Table GPT config option - mandatory for UEFI support

    CONFIG_EFI_PARTITION=y

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

Retrieved from https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/plain/Documentation/x86/x86_64/uefi.txt .

## UEFI Variables

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. You can get the list using

    $ efivar -l

### UEFI Variables Support in Linux Kernel

Linux kernel exposes EFI variables data to userspace via 2 interfaces:

- OLD sysfs-efivars interface (CONFIG_EFI_VARS) - populated by `efivars` kernel module at `/sys/firmware/efi/vars` - 1024 byte maximum per-variable data size limitation, no UEFI Secure Boot variables support (due to the size limitation) and not recommended by kernel upstream anymore. Still supported by kernel upstream but completely disabled in Arch's official kernels.

- NEW efivarfs (EFI VARiable FileSystem) interface (CONFIG_EFIVAR_FS) - mounted using `efivarfs` kernel module at `/sys/firmware/efi/efivars` - replacement for the OLD sysfs-efivars interface, has no maximum per-variable size limitation, supports UEFI Secure Boot variables and recommended by kernel upstream. Introduced in kernel 3.8 and NEW `efivarfs` module split from OLD `efivars` kernel module in kernel 3.10 .

#### Inconsistency between efivarfs and sysfs-efivars

Enabling both OLD sysfs-efivars and NEW efivarfs can cause data inconsistency issues (see See https://lkml.org/lkml/2013/4/16/473 for more info). Due to this OLD sysfs-efivars is completely disabled in Arch's official kernels (since core/linux-3.11 and core/linux-lts-3.10) and only NEW efivarfs is enabled/supported going forward. All the UEFI Variables related tools and utilities in official repositories support efivarfs as of 01 October 2013.

Note: As a side-effect of disabling OLD sysfs-efivars, `efi_pstore` module is also disabled in the official Arch kernels as EFI pstore functionality in the kernel depends of OLD sysfs-efivars support.

If you have both interfaces enabled, you need to disable one of them, and disable and re-enable the other interface (to refresh the data, to prevent inconsistencies) before accessing the EFI VAR data using any userspace tool:

To disable sysfs-efivars and refresh efivarfs:

    # modprobe -r efivars

    # umount /sys/firmware/efi/efivars
    # modprobe -r efivarfs

    # modprobe efivarfs
    # mount -t efivarfs efivarfs /sys/firmware/efi/efivars

To disable efivarfs and refresh sysfs-efivars:

    # umount /sys/firmware/efi/efivars
    # modprobe -r efivarfs

    # modprobe -r efivars
    # modprobe efivars

### Requirements for UEFI Variables support to work properly

1. EFI Runtime Services support should be present in the kernel (`CONFIG_EFI=y`, check if present with `zgrep CONFIG_EFI /proc/config.gz`).
2. Kernel processor bitness/arch and EFI processor bitness/arch should match
3. Kernel should be booted in EFI mode (via EFISTUB or any EFI boot loader, not via BIOS/CSM or Apple's "bootcamp" which is also BIOS/CSM)
4. EFI Runtime Services in the kernel SHOULD NOT be disabled via kernel cmdline, i.e. `noefi` kernel parameter SHOULD NOT be used
5. `efivarfs` filesystem should be mounted at `/sys/firmware/efi/efivars`, otherwise follow #Mount efivarfs section below.
6. `efivar` should list (option `-l`) the EFI Variables without any error. For sample output see #Sample_List_of_UEFI_Variables.

If EFI Variables support does not work even after the above conditions are satisfied, try the below workarounds:

1. If any userspace tool is unable to modify efi variables data, check for existence of `/sys/firmware/efi/efivars/dump-*` files. If they exist, delete them, reboot and retry again.
2. If the above step does not fix the issue, try booting with `efi_no_storage_paranoia` kernel parameter to disable kernel efi variable storage space check that may prevent writing/modification of efi variables.

Note: `efi_no_storage_paranoia` should only be used when needed and should not be left as a normal boot option. The effect of this kernel command line parameter turns off a safeguard that was put in place to help avoid the bricking of machines when the NVRAM gets too full.

#### Mount efivarfs

If `efivarfs` is not automatically mounted at `/sys/firmware/efi/efivars` by systemd during boot, then you need to manually mount it to expose UEFI Variable support to the userspace tools like `efibootmgr` etc.:

    # mount -t efivarfs efivarfs /sys/firmware/efi/efivars

Note: The above command should be run both OUTSIDE (BEFORE) and INSIDE chroot, if any.

It is also a good idea to auto-mount `efivarfs` during boot via `/etc/fstab` as follows:

    /etc/fstab

    efivarfs    /sys/firmware/efi/efivars    efivarfs    defaults    0    0

### Userspace Tools

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

1. efivar - Library and Tool to manipulate UEFI Variables (used by efibootmgr) - https://github.com/vathpela/efivar - efivar or efivar-git
2. efibootmgr - Tool to manipulate UEFI Firmware Boot Manager Settings - https://github.com/vathpela/efibootmgr - efibootmgr or efibootmgr-git
3. uefivars - Dumps list of EFI variables with some additional PCI related info (uses efibootmgr code internally) - https://github.com/fpmurphy/Various/tree/master/uefivars-2.0 supports only efivarfs and https://github.com/fpmurphy/Various/tree/master/uefivars-1.0 supports only sysfs-efivars . AUR package uefivars-git
4. efitools - Tools to Create and Setup own UEFI Secure Boot Certificates, Keys and Signed Binaries (requires efivarfs) - efitools-git
5. Ubuntu's Firmware Test Suite - https://wiki.ubuntu.com/FirmwareTestSuite/ - fwts (along with fwts-efi-runtime-dkms) or fwts-git

#### efibootmgr

Warning:

- Using `efibootmgr` in Apple Macs may 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-boot.

Note:

- If `efibootmgr` completely fails to work in your system, you can reboot into UEFI Shell v2 and use `bcfg` command to create a boot entry for the bootloader.
- If you are unable to use `efibootmgr`, some UEFI BIOSes allow users to directly manage uefi boot options from within the BIOS. For example, some ASUS BIOSes have a "Add New Boot Option" choice which enables you to select a local EFI System Partition and manually enter the EFI stub location. (for example `\EFI\refind\refind_x64.efi`).
- The below commands use refind-efi boot-loader as example.

Assuming the boot-loader file to be launched is `/boot/efi/EFI/refind/refind_x64.efi`, `/boot/efi/EFI/refind/refind_x64.efi` can be split up as `/boot/efi` and `/EFI/refind/refind_x64.efi`, wherein `/boot/efi` is the mountpoint of the EFI 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 EFI System Partition (assuming mountpoint `/boot/efi` for example) (should be in the form `/dev/sdXY`), try :

    # findmnt /boot/efi
    TARGET SOURCE  FSTYPE OPTIONS
    /boot/efi  /dev/sdXY  vfat         rw,flush,tz=UTC

Verify that uefi variables support in kernel is working properly by running:

    # efivar -l

If efivar lists the uefi variables without any error, then you can proceed. If not, check whether all the conditions in #Requirements for UEFI Variables support to work properly are met.

Then create the boot entry using efibootmgr as follows:

    # efibootmgr -c -d /dev/sdX -p Y -l /EFI/refind/refind_x64.efi -L "rEFInd"

Note: UEFI uses backward slash `\` as path separator (similar to Windows paths), but the official efibootmgr pkg support passing unix-style paths with forward-slash `/` as path-separator for the `-l` option. Efibootmgr internally converts `/` to `\` before encoding the loader path. The relevant git commit that incorporated this feature in efibootmgr is http://linux.dell.com/cgi-bin/cgit.cgi/efibootmgr.git/commit/?id=f38f4aaad1dfa677918e417c9faa6e3286411378 .

In the above command `/boot/efi/EFI/refind/refind_x64.efi` translates to `/boot/efi` and `/EFI/refind/refind_x64.efi` which in turn translate to drive `/dev/sdX` -> partition `Y` -> file `/EFI/refind/refind_x64.efi`.

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\refind\refindx64.efi` or `\efi\refind\refind_x64.efi` does not matter (this will change if the filesystem encoding is UTF-8).

## EFI System Partition

The EFI System Partition (also called ESP or EFISYS) is a FAT32 formatted physical partition (in the main partition table of the disk, not LVM or software raid etc.) from where the UEFI firmware launches the UEFI bootloader and application. It is a OS independent partition that acts as the storage place for the EFI bootloaders and applications which the firmware launches them. It is mandatory for UEFI boot. It should be marked as EF00 or ef00 type code in gdisk, or boot flag in case of GNU Parted (only for GPT disk). It is recommended to keep ESP size at 512 MiB although smaller/larger sizes are fine (smaller sizes provided it is higher than the minimum FAT32 FS partition size limit (as mandated by FAT32 specification from Microsoft). For more info visit link.

Note:

- It is recommended to use always GPT for UEFI boot as some UEFI firmwares do not allow UEFI-MBR boot.
- In GNU Parted, `boot` flag (not to be confused with `legacy_boot` flag) has different effect in MBR and GPT disk. In MBR disk, it marks the partition as active. In GPT disk, it changes the type code of the partition to `EFI System Partition` type. Parted has no flag to mark a partition as ESP in MBR disk (this can be done using fdisk though).
- Microsoft documentation noted the ESP size: For Advanced Format 4K Native drives (4-KB-per-sector) drives, the minimum size is 260 MB, due to a limitation of the FAT32 file format. The minimum partition size of FAT32 drives is calculated as sector size (4KB) x 65527 = 256 MB. Advanced Format 512e drives are not affected by this limitation, because their emulated sector size is 512 bytes. 512 bytes x 65527 = 32 MB, which is less than the 100 MB minimum size for this partition. From: http://technet.microsoft.com/en-us/library/hh824839.aspx#DiskPartitionRules
- In case of EFISTUB, the kernels and initramfs files should be stored in the EFI System Partition. For sake of simplicity, you can also use the ESP as the `/boot` partition itself instead of a separate `/boot` partition, for EFISTUB booting.

### GPT partitioned disks

- Create a partition with partition type `ef00` or `EF00` using gdisk (from gptfdisk pkg). Then format that partition as FAT32 using `mkfs.fat -F32 /dev/<THAT_PARTITION>`

(or)

- Create a FAT32 partition and in GNU Parted set/activate the `boot` flag (not `legacy_boot` flag) on that partition

Note: If you get the message `WARNING: Not enough clusters for a 32 bit FAT!`, reduce cluster size with `mkfs.fat -s2 -F32 ...` or `-s1`, otherwise the partition may be unreadable by UEFI.

### MBR partitioned disks

Create a partition with partition type `0xEF` using fdisk (from util-linux pkg). Then format that partition as FAT32 using `mkfs.fat -F32 /dev/<THAT_PARTITION>`

## 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.

### Obtaining UEFI Shell

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

- AUR uefi-shell-svn pkg (recommended) - provides x86_64 Shell in x86_64 system and IA32 Shell in i686 system - compiled directly from latest Tianocore EDK2 SVN source
- Precompiled x86_64 UEFI Shell v2 binary (may not be up-to-date)
- Precompiled x86_64 UEFI Shell v1 binary (not updated anymore upstream)
- Precompiled IA32 UEFI Shell v2 binary (may not be up-to-date)
- Precompiled IA32 UEFI Shell v1 binary (not updated anymore upstream)

Shell v2 works best in UEFI 2.3+ systems and is recommended over Shell v1 in those systems. Shell v1 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 EFI System Partition as `<EFI_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

UEFI Shell commands usually support `-b` option which makes output pause after each page. `map` lists recognized filesystems (`fs0`, ...) and data storage devices (`blk0`, ...). Run `help -b` to list available commands.

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

#### bcfg

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.
- UEFI Shell v1 official binary does not support `bcfg` command. You can download a modified UEFI Shell v2 binary which may work in UEFI pre-2.3 firmwares.

To dump a list of current boot entries:

    Shell> bcfg boot dump -v

To add a boot menu entry for rEFInd (for example) as 4th (numbering starts from zero) option in the boot menu:

    Shell> bcfg boot add 3 fs0:\EFI\refind\refind_x64.efi "rEFInd"

where `fs0:` is the mapping corresponding to the EFI System Partition and `fs0:\EFI\refind\refind_x64.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

or:

    Shell> bcfg -? -v -b

#### edit

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 rEFInd's `refind.conf` in the EFI System Partition (`fs0:` in the firmware)

    Shell> fs0:
    FS0:\> cd \EFI\arch\refind
    FS0:\EFI\arch\refind\> edit refind.conf

Type `Ctrl-E` for help.

## UEFI Linux Hardware Compatibility

See HCL/Firmwares/UEFI for the main article.

## UEFI Bootable Media

### Create UEFI bootable USB from ISO

Follow USB Flash Installation Media#BIOS and UEFI Bootable USB

### Remove UEFI boot support from Optical Media

Note: This section mentions removing UEFI boot support from a CD/DVD only (Optical Media), not from a USB flash drive.

Most of the 32-bit EFI Macs and some 64-bit EFI Macs refuse to boot from a UEFI(X64)+BIOS bootable CD/DVD. If one wishes to proceed with the installation using optical media, it might be necessary to remove UEFI support first.

- Mount the official installation media and obtain the `archisolabel` as shown in the previous section.

    # mount -o loop input.iso /mnt/iso

- Then rebuild the ISO, excluding the UEFI Optical Media booting support, using `xorriso` from libisoburn

    $ xorriso -as mkisofs -iso-level 3 \
        -full-iso9660-filenames\
        -volid "archisolabel" \
        -appid "Arch Linux CD" \
        -publisher "Arch Linux <https://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 "/mnt/iso/isolinux/isohdpfx.bin" \
        -output output.iso /mnt/iso/

- Burn `output.iso` to optical media and proceed with installation normally.

## Testing UEFI in systems without native support

### OVMF for Virtual Machines

OVMF [1] is a tianocore project to enable UEFI support for Virtual Machines. OVMF contains a sample UEFI firmware for QEMU.

You can build OVMF (with Secure Boot support) from AUR ovmf-svn and run it as follows:

    $ qemu-system-x86_64 -enable-kvm -net none -m 1024 -pflash /usr/share/ovmf/x86_64/bios.bin

### DUET for BIOS only systems

DUET is a tianocore project that enables chainloading a full UEFI environment from a BIOS system, in a way similar to BIOS OS booting. This method is being discussed extensively in http://www.insanelymac.com/forum/topic/186440-linux-and-windows-uefi-boot-using-tianocore-duet-firmware/. Pre-build DUET images can be downloaded from one of the repos at https://gitorious.org/tianocore_uefi_duet_builds. Specific instructions for setting up DUET is available at https://gitorious.org/tianocore_uefi_duet_builds/tianocore_uefi_duet_installer/blobs/raw/master/Migle_BootDuet_INSTALL.txt.

You can also try http://sourceforge.net/projects/cloverefiboot/ which provides modified DUET images that may contain some system specific fixes and is more frequently updated compared to the gitorious repos.

## Troubleshooting

### Windows 7 will not boot in UEFI Mode

If you have installed Windows to a different harddisk with GPT partitioning and still have a MBR partitioned harddisk in your computer, then it is possible that the UEFI BIOS is starting it's CSM support (for booting MBR partitions) and therefor Windows will not boot. To solve this merge your MBR harddisk to GPT partitioning or disable the SATA port where the MBR harddisk is plugged in or unplug the SATA connector from this harddisk.

Mainboards with this kind of problem:

Gigabyte Z77X-UD3H rev. 1.1 (UEFI BIOS version F19e)

- UEFI BIOS option for booting UEFI Only does not pretend the UEFI BIOS from starting CSM

### Windows changes boot order

In some motherboards (confirmed in ASRock Z77 Extreme4) Windows 8 changes the boot order in the NVRAM everytime is booted. This can be fixed making the Windows Boot Manager to load another loader instead of booting Windows. Run this command in a Administrator mode console in Windows:

    bcdedit /set {bootmgr} path \EFI\boot_app_dir\boot_app.efi

### USB media gets struck with black screen

- This issue can occur either due to KMS issue. Try Disabling KMS while booting the USB.

- If the issue is not due to KMS, then it may be due to bug in EFISTUB booting (see [2] and [3] for more information.). Both Official ISO (Archiso) and Archboot iso use EFISTUB (via Gummiboot Boot Manager for menu) for booting the kernel in UEFI mode. In such a case you have to use GRUB as the USB's UEFI bootloader by following the below section.

#### Using GRUB

- Create USB Flash Installation drive as mentioned in link. After that follow the below steps to use GRUB instead of Gummiboot.

- Backup `<USB>/EFI/boot/loader.efi` to `<USB>/EFI/boot/gummiboot.efi`

- Create a GRUB standalone image and copy it to `<USB>/EFI/boot/loader.efi`

- Create `<USB>/EFI/boot/grub.cfg` with the following contents (replace `ARCH_YYYYMM` with the label of the USB disk e.g. `ARCH_201404`):

    grub.cfg for Official ISO

    insmod part_gpt
    insmod part_msdos
    insmod fat

    insmod efi_gop
    insmod efi_uga
    insmod video_bochs
    insmod video_cirrus

    insmod font

    if loadfont "${prefix}/fonts/unicode.pf2" ; then
        insmod gfxterm
        set gfxmode="1024x768x32;auto"
        terminal_input console
        terminal_output gfxterm
    fi

    menuentry "Arch Linux archiso x86_64" {
        set gfxpayload=keep
        search --no-floppy --set=root --label ARCH_YYYYMM
        linux /arch/boot/x86_64/vmlinuz archisobasedir=arch archisolabel=ARCH_YYYYMM add_efi_memmap
        initrd /arch/boot/x86_64/archiso.img
    }

    menuentry "UEFI Shell x86_64 v2" {
        search --no-floppy --set=root --label ARCH_YYYYMM
        chainloader /EFI/shellx64_v2.efi
    }

    menuentry "UEFI Shell x86_64 v1" {
        search --no-floppy --set=root --label ARCH_YYYYMM
        chainloader /EFI/shellx64_v1.efi
    }

    grub.cfg for Archboot ISO

    insmod part_gpt
    insmod part_msdos
    insmod fat

    insmod efi_gop
    insmod efi_uga
    insmod video_bochs
    insmod video_cirrus

    insmod font

    if loadfont "${prefix}/fonts/unicode.pf2" ; then
        insmod gfxterm
        set gfxmode="1024x768x32;auto"
        terminal_input console
        terminal_output gfxterm
    fi

    menuentry "Arch Linux x86_64 Archboot" {
        set gfxpayload=keep
        search --no-floppy --set=root --file /boot/vmlinuz_x86_64
        linux /boot/vmlinuz_x86_64 cgroup_disable=memory loglevel=7 add_efi_memmap
        initrd /boot/initramfs_x86_64.img
    }

    menuentry "UEFI Shell x86_64 v2" {
        search --no-floppy --set=root --file /boot/vmlinuz_x86_64
        chainloader /EFI/tools/shellx64_v2.efi
    }

    menuentry "UEFI Shell x86_64 v1" {
        search --no-floppy --set=root --file /boot/vmlinuz_x86_64
        chainloader /EFI/tools/shellx64_v1.efi
    }

## See also

- Wikipedia:UEFI
- UEFI Forum - contains the official UEFI Specifications - GUID Partition Table is part of UEFI Specification
- UEFI boot: how does that actually work, then? - A blog post by AdamW
- Wikipedia:EFI System partition
- The EFI System Partition and the Default Boot Behavior
- Linux Kernel x86_64 UEFI Documentation
- Intel's page on EFI
- Intel UEFI Community Resource Center
- Matt Fleming - The Linux EFI Boot Stub
- Matt Fleming - Accessing UEFI Variables from Linux
- Rod Smith - Linux on UEFI: A Quick Installation Guide
- UEFI Boot problems on some newer machines (LKML)
- LPC 2012 Plumbing UEFI into Linux
- LPC 2012 UEFI Tutorial : part 1
- LPC 2012 UEFI Tutorial : part 2
- Intel's Tianocore Project for Open-Source UEFI firmware which includes DuetPkg for direct BIOS based booting and OvmfPkg used in QEMU and Oracle VirtualBox
- FGA: The EFI boot process
- Microsoft's Windows and GPT FAQ
- Convert Windows x64 from BIOS-MBR mode to UEFI-GPT mode without Reinstall
- Create a Linux BIOS+UEFI and Windows x64 BIOS+UEFI bootable USB drive
- Rod Smith - A BIOS to UEFI Transformation
- EFI Shells and Scripting - Intel Documentation
- UEFI Shell - Intel Documentation
- UEFI Shell - bcfg command info
- UEFI Shell v2 binary with bcfg modified to work with UEFI pre-2.3 firmware - from Clover efiboot

Category:

- Boot process