Unified Extensible Firmware Interface

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Extensible Firmware Interface's position in the software stack.
The Unified Extensible Firmware Interface (UEFI) is a specification that defines a software interface between an operating system and platform firmware. UEFI is meant to replace the Basic Input/Output System (BIOS) firmware interface, present in all IBM PC-compatible personal computers.[1][2] In practice, most UEFI images provide legacy support for BIOS services. UEFI can support remote diagnostics and repair of computers, even without another operating system.[3]
The original EFI (Extensible Firmware Interface) specification was developed by Intel. Some of its practices and data formats mirror ones from Windows.[4][5] In 2005, UEFI deprecated EFI 1.10 (final release of EFI). The UEFI specification is managed by the Unified EFI Forum.



The original motivation for EFI came during early development of the first Intel–HP Itanium systems in the mid-1990s. BIOS limitations (such as 16-bit processor mode1 MB addressable space and PC AT hardware) were unacceptable for the larger server platforms Itanium was targeting.[6] The effort to address these concerns was initially called Intel Boot Initiative, which began in 1998[7] and was later renamed EFI.[8][9]
In July 2005 Intel ceased development of the EFI spec at version 1.10, and contributed it to the Unified EFI Forum, which has evolved the specification as the Unified Extensible Firmware Interface (UEFI). The original EFI spec remains owned by Intel, which exclusively provides licenses for EFI-based products, but the UEFI specification is owned by the Forum.[6][10]
Version 2.1 of the UEFI (Unified Extensible Firmware Interface) specification was released on 7 January 2007. It added cryptography, network authentication and the User Interface Architecture (Human Interface Infrastructure in UEFI). The current UEFI specification, version 2.3.1, was approved in April 2011.


Interaction between the EFI boot manager and EFI drivers
The interface defined by the EFI specification includes data tables that contain platform information, and boot and runtime services that are available to the OS loader and OS. UEFI firmware provides several technical advantages over a traditional BIOS system:[11]
  • Ability to boot from large disks (over 2 TiB) with a GUID Partition TableGPT.[12][13]
  • CPU-independent architecture[12]
  • CPU-independent drivers[12]
  • Flexible pre-OS environment, including network capability
  • Modular design

Processor compatibility

As of version 2.3, processor bindings exist for Itanium, x86, x86_64 and ARM. Only little-endian processors can be supported.[14]
The BIOS is limited to a 16-bit processor mode and 1 MB of addressable space due to the design being based on the IBM 5150, which used the 16-bit Intel 8088.[6][15] In comparison, the UEFI processor mode can be either 32-bit (x86-32, ARM) or 64-bit (x86-64 and Itanium).[6][16] 64-bit UEFI understands long mode, which allows applications in the pre-boot execution environment to have direct access to all of the memory using 64-bit addressing.[17]
UEFI requires the firmware and operating system loader to be size-matched; i.e. a 64-bit UEFI implementation can only load a 64-bit UEFI OS boot loader. After the system transitions from "Boot Services" to "Runtime Services," the operating system kernel takes over. At this point, the kernel can change processor modes if it desires, but this bars usage of runtime services[18] (unless the kernel switches back again). Presently, the only operating system that supports running a kernel that is not size-matched to the firmware is Mac OS X.

Disk device compatibility

In addition to the standard PC disk partition scheme, which uses a master boot record (MBR), EFI works with a new partitioning scheme: GUID Partition Table (GPT). GPT is free from many of the limitations of MBR. In particular, the MBR limits on the number and size of disk partitions (up to 4 primary partitions per disk, up to 2 TiB (240 bytes) per disk) are relaxed.[19] GPT allows for a maximum disk and partition size of 8 ZiB (270 bytes).[19][20] The UEFI specification explicitly requires support for FAT32 for system partitions, and FAT12/FAT16 for removable media; specific implementations may support other file systems.


Linux supports GPT. The Linux kernel should be built with option "CONFIG_EFI_PARTITION" enabled.[21]

Microsoft Windows

64-bit versions of Microsoft Windows Vista[22] and later, and the Itanium versions of Windows XP and Server 2003 can boot from disks with a partition size larger than 2 TiB.


EFI defines two types of services: boot services and runtime services. Boot services are only available while the firmware owns the platform (before the "ExitBootServices" call). Boot services include text and graphical consoles on various devices, and bus, block and file services. Runtime services are still accessible while the operating system is running; they include services such as date, time and NVRAM access. In addition, the Graphics Output Protocol provides limited runtime services support. The operating system is permitted to directly write to the framebuffer provided by GOP during runtime mode. However, the ability to change video modes is lost after transitioning to runtime services mode until the OS graphics driver is loaded.

Variable services

UEFI variables provide a way to store data, in particular non-volatile data, that is shared between platform firmware and operating systems or UEFI applications. Variable namespaces are identified by GUIDs, and variables are key/value pairs. For example, variables can be used to keep a core dump in memory after a crash for the operating system to retrieve after a reboot.[23]

Time services

UEFI provides device-independent time services. Time services include support for timezone and daylight saving fields, which allow the hardware real-time clock to be set to local time or UTC.[24] On machines using a PC-AT real-time clock, the clock still has to be set to local time for compatibility with BIOS-based Windows.[5]


EFI defines protocols as set of software interfaces used for communication between two binary modules. All EFI drivers must provide services to others via protocols.

Device drivers

In addition to standard architecture-specific device drivers, the EFI specification provides for a processor-independent device driver environment, called EFI Byte Code or EBC. System firmware is required by the UEFI specification to carry an interpreter for any EBC images that reside in or are loaded into the environment. In that sense, EBC is similar to Open Firmware, the hardware-independent firmware used in PowerPC-based Apple Macintosh and Sun Microsystems SPARC computers, among others.
Some architecture-specific (non-EBC) EFI device driver types can have interfaces for use from the operating system. This allows the OS to rely on EFI for basic graphics and network functions until OS specific drivers are loaded.

Graphics features

The EFI specification defined a UGA (Universal Graphic Adapter) protocol as a way to support device-independent graphics. UEFI did not include UGA and replaced it with GOP (Graphics Output Protocol), with the explicit goal of removing VGA hardware dependencies. The two are similar.
UEFI 2.1 defined a "Human Interface Infrastructure" (HII) to manage user input, localized strings, fonts, and forms (in the HTML sense). These enable OEMs or IBVs to design graphical interfaces for pre-boot configuration. UEFI itself does not define a user interface.
Most early UEFI implementations were console-based, but as early as 2007 some implementations featured a graphical user interface.[25]


The UEFI specification defines a "boot manager", a firmware policy engine that is in charge of loading the OS loader and all necessary drivers. The boot configuration is controlled by a set of global NVRAM variables, including boot variables that indicate the paths to the loaders.
OS loaders are a class of UEFI applications. As such, they are stored as files on a file system that can be accessed by the firmware. Supported file systems include FAT32FAT16 and FAT12. Supported partition table schemes include MBR and GPT. UEFI does not rely on aboot sector.
Boot loaders can also be auto-detected by firmware, to enable booting on removable devices. Auto-detection relies on a standardized file path to the OS loader, depending on the actual architecture to boot (\EFI\BOOT\BOOT[architecture name].EFI, e.g.\EFI\BOOT\BOOTx64.EFI).
It is common for UEFI firmware to include a user interface to the boot manager, to allow the user to select and load the operating system among the possible options.

Secure boot

The UEFI 2.2 specification adds a protocol known as Secure boot, which can secure the boot process by preventing the loading of drivers or OS loaders that are not signed with an acceptable digital signature. When secure boot is enabled, it is initially placed in "setup" mode, which allows a public key known as the "Platform key" (PK) to be written to the firmware. Once the key is written, secure boot enters "User" mode, where only drivers and loaders signed with the platform key can be loaded by the firmware. Additional "Key Exchange Keys" (KEK) can be added to a database stored in memory to allow other certificates to be used, but they must still have a connection to the private portion of the Platform key.[26] Secure boot can also be placed in "Custom" mode, where additional public keys can be added to the system that do not match the private key.[27]
Secure boot is supported by Windows 8Windows Server 2012, and selected Linux distributions.

Compatibility Support Module

The Compatibility Support Module (CSM) is a component of the firmware that emulates a BIOS environment; allowing legacy operating systems and some option ROMs that do not support UEFI to still be used.

The EFI shell

EFI provides a shell environment, which can be used to execute other EFI applications.


Extensions to EFI can be loaded from virtually any non-volatile storage device attached to the computer. For example, an original equipment manufacturer (OEM) can distribute systems with an EFI partition on the hard drive, which would add additional functions to the standard EFI firmware stored on the motherboard's ROM.

Implementation and adoption

Intel EFI

Intel's implementation of EFI is the Intel Platform Innovation Framework, codenamed "Tiano." Tiano runs on Intel's XScaleItanium and IA-32 processors, and is proprietary software, although a portion of the code has been released under the BSD license or Eclipse Public License (EPL) as TianoCore. TianoCore can be used as a payload for coreboot.[28]
Phoenix Technologies' implementations of UEFI include its SecureCore and SecureCore Tiano products.[29] American Megatrends offers its own UEFI implementation known as Aptio,[30] while Insyde Software offers InsydeH2O, its own implementation of Tiano.[31]

Platforms using EFI/UEFI

Intel's first Itanium workstations and servers, released in 2000, implemented EFI 1.02.
Hewlett-Packard's first Itanium 2 systems, released in 2002, implemented EFI 1.10; they were able to boot WindowsLinuxFreeBSD and HP-UXOpenVMS added UEFI capability in June, 2003.
In January 2006, Apple Inc. shipped its first Intel-based Macintosh computers. These systems used EFI instead of Open Firmware, which had been used on its previous PowerPC-based systems.[32] On 5 April 2006, Apple first released Boot Camp, which produces a Windows drivers disk and a non-destructive partitioning tool to allow the installation of Windows XP or Vista without requiring a reinstallation of Mac OS X. A firmware update was also released that added BIOS compatibility to its EFI implementation. Subsequent Macintosh models shipped with the newer firmware.[33]
During 2005, more than one million Intel systems shipped with Intel's implementation of UEFI.[34] New mobile, desktop and server products, using Intel's implementation of UEFI, started shipping in 2006. For instance, boards that use the Intel 945 chipset series use Intel's UEFI implementation.
Since 2005, EFI has also been implemented on non-PC architectures, such as embedded systems based on XScale cores.[34]
The EDK (EFI Developer Kit) includes an NT32 target, which allows EFI firmware and EFI applications to run within a Windows application. But no direct hardware access is allowed by EDK NT32. This means only a subset of EFI application and drivers can be executed at the EDK NT32 target.
In 2008, more x86-64 systems adopted UEFI. While many of these systems still allow booting only the BIOS-based OSes via the Compatibility Support Module (CSM) (thus not appearing to the user to be UEFI-based), other systems started to allow booting UEFI-based OSes. For example, IBM x3450 server, MSI motherboards with ClickBIOS, all HP EliteBook Notebook and Tablet PCs, newer HP Compaq Notebook PCs (e.g., 6730b, 6735b, etc.).
In 2009, IBM shipped System x machines (x3550 M2, x3650 M2, iDataPlex dx360 M2) and BladeCenter HS22 with UEFI capability. Dell shipped PowerEdge T610, R610, R710, M610 and M710 servers with UEFI capability. More commercially available systems are mentioned in a UEFI whitepaper.[35] Many Sandy Bridge PC platforms use UEFI.
In 2011 ASRockASUSTeKGigabyte Technology, and MSI launched several consumer-based motherboards using the Intel 6-series LGA 1155 chipset and AMD 9 Series chipset for AM3+ AMD FX (Bulldozer) and AMD Fusion processors with EFI.[36]
With the release of Windows 8 in October 2012, Microsoft's certification requirements now require that computers include firmware that implements the UEFI specification. Furthermore, if the computer supports the "Connected Standby" feature of Windows 8, then the firmware is not permitted to contain a Compatibility Support Module (CSM). As such, systems that support Connected Standby are incapable of booting Legacy BIOS operating systems.[37]

Operating systems

An operating system that can be booted from a (U)EFI is called a (U)EFI-aware OS, defined by (U)EFI specification. Here the term booted from a (U)EFI means directly booting the system using a (U)EFI OS loader stored on any storage device. The default location for the OS loader is \EFI\BOOT\boot[architecture name].efi, where the architecture name can be e.g. IA32, X64 or IA64. Some OS vendors may have their own OS loader. They may also change the default boot location.
  • The Linux kernel has been able to use EFI at boot time since early 2000,[38] using the elilo EFI boot loader or, more recently, EFI versions of GRUB.[39] Grub+Linux also supports booting from a GUID partition table without UEFI.[13] The distribution Ubuntu added support for UEFI secure boot as of version 12.10.[40]
  • HP-UX has used (U)EFI as its boot mechanism on IA-64 systems since 2002.
  • HP OpenVMS has used (U)EFI on IA-64 since its initial evaluation release in December 2003, and for production releases since January 2005.[41]
  • Apple uses EFI for its line of Intel-based MacsMac OS X v10.4 Tiger and Mac OS X v10.5 Leopard implement EFI v1.10 in 32-bit mode even on newer 64-bit CPUs, but full support arrived with Mac OS X v10.8 Mountain Lion.[42]
  • The Itanium versions of Windows 2000 (Advanced Server Limited Edition and Datacenter Server Limited Edition) implemented EFI 1.10 in 2002. MS Windows Server 2003 for IA-64, MS Windows XP 64-bit Edition and Windows 2000 Advanced Server Limited Edition, all of which are for the Intel Itanium family of processors, implement EFI, a requirement of the platform through the DIG64 specification.[43]
  • Microsoft introduced UEFI for x86-64 Windows operating systems with Windows Server 2008 and Windows Vista Service Pack 1 so the 64-bit versions of Windows 7 are compatible with EFI. 32-bit UEFI was originally not supported since vendors did not have any interest in producing native 32-bit UEFI firmware because of the mainstream status of 64-bit computing.[44] Windows 8 includes further optimizations for UEFI systems, including a faster startup, 32-bit support, and secure boot support.[45][46]
  • On March 5, 2013, the FreeBSD Foundation awarded a grant to a developer seeking to add UEFI support to the FreeBSD kernel and bootloader.[47]

Use of UEFI with virtualization

  • HP Integrity Virtual Machines provides UEFI boot on HP Integrity Servers. It also provides a virtualized UEFI environment for the guest UEFI-aware OSes.
  • Intel hosts an Open Virtual Machine Firmware project on SourceForge.[48]
  • VMware Fusion 3 software for Mac OS X can boot Mac OS X Server virtual machines using EFI.
  • VirtualBox has implemented UEFI since 3.1,[49] but limited to Unix/Linux operating systems (doesn't work with Windows Vista x64 and Windows 7 x64).[50][51]
  • QEMU can be used with an EFI system firmware.
  • The VMware ESXi version 5 hypervisor, part of VMware vSphere, supports virtualized EFI as an alternative to BIOS inside a virtual machine.


Numerous digital rights activitists have protested against UEFI. Ronald G. Minnich, a co-author of coreboot, and Cory Doctorow, a digital rights activist, have criticized EFI as an attempt to remove the ability of the user to truly control the computer.[52][53] It does not solve any of the BIOS's long-standing problems of requiring two different drivers—one for the firmware and one for the operating system—for most hardware.[54]
The open-source project, TianoCore,[55] provides the UEFI interfaces. TianoCore lacks the specialized drivers that initialize chipset functions, which are instead provided by Coreboot, of which TianoCore is one of many payload options. The development of Coreboot requires the cooperation from chipset manufactures to provide specifications needed to develop initialization drivers.

Secure boot

In 2011, Microsoft was accused by critics and free software/open source advocates (including the Free Software Foundation) of trying to use the secure boot functionality of UEFI to hinder or outright prevent the installation of alternative operating systems such as Linux, by requiring that new computers certified to run its Windows 8 operating system ship with secure boot enabled using a Microsoft private key. Following the criticism, Microsoft denied that the secure boot requirement was intended to serve as a form of lock-in, and clarified its requirements by stating that systems certified for Windows 8 must allow secure boot to enter custom mode or be disabled, but not on systems using the ARM architecture.[27][56]
Other developers raised concerns about the legal and practical issues of implementing support for secure boot on Linux systems in general. Former Red Hat developer Matthew Garrett noted that conditions in the GNU General Public License version 3 may prevent the use of the GRUB bootloader without a distribution's developer disclosing the private key (however, the Free Software Foundation has since clarified its position, assuring that the responsibility to make keys available was held by the hardware manufacturer),[40] and that it would also be difficult for advanced users to build custom kernels that could function with secure boot enabled without self-signing them.[56] Other developers suggested that signed builds of Linux with another key could be provided, but noted that it would be difficult to persuade OEMs to ship their computers with the required key alongside the Microsoft key.[2]
Several major Linux distributions have developed different implementations for secure boot. Matthew Garrett himself developed a minimal bootloader known as shim; a pre-compiled, signed bootloader that allows the user to individually trust keys provided by distributors.[57]Ubuntu 12.10 uses an older version of shim pre-configured for use with Canonical's own key that only verifies the bootloader and allows unsigned kernels to be loaded: developers believed this practice of only signing the bootloader is more feasible, since a trusted kernel is only effective at securing user space and not the pre-boot state (which secure boot is designed to protect). This also allows users to build their own kernels and use custom kernel modules as well, without needing to re-configure the system.[58][40][59] Canonical also maintains its own private key to sign installations of Ubuntu pre-loaded on certified OEM computers that run the operating system, and also plans to enforce a secure boot requirement as well—requiring both a Canonical key and a Microsoft key (for compatibility reasons) to be included in their firmware. Fedora also uses shim, but requires that both the kernel and its modules be signed as well.[58]
It has been disputed whether the kernel and its modules must be signed as well; while the UEFI specifications do not require it, Microsoft has asserted that that their certification requirements do, and that it will revoke any certificates used to sign code that can be used to compromise the security of the system.[59] In February 2013, another Red Hat developer attempted to submit a patch to the Linux kernel that would allow it to parse Microsoft's authenticode signing using a master X.509 key embedded in a UEFI executable signed by Microsoft. However, Linus Torvalds, the creator of Linux, attacked the proposal for supporting Red Hat's desire to “deep-throat” Microsoft and participate in their “dick-sucking” for supporting their regime.[60]
On March 26 2013, the Spanish free software development group Hispalinux filed a formal complaint with the European Commission, contesting that Microsoft's secure boot requirements on OEM systems were "obstructive" and anti-competitive.[61]

Firmware issues

The increased prominence of UEFI firmware in devices have also led to a number of technical issues blamed on their respective implementations.[62]
In 2012, it was discovered that certain Lenovo computer models with secure boot had firmware that was hard-coded to only allow "Windows Boot Manager" or "Red Hat Enterprise Linux" to load, regardless of their secure boot settings, preventing even a signed Linux distribution from loading.[63] Other issues were encountered by several Toshiba laptop models with secure boot that were missing certain certificates required for its proper operation.[62]
In January 2013, a bug surrounding the UEFI implementation on some Samsung laptops was publicized, which caused them to be bricked after installing a Linux distribution in UEFI mode. While potential conflicts with a kernel module designed to access system features on Samsung laptops were initially blamed (also prompting kernel maintainers to disable the module on UEFI systems as a safety measure), experiments by Linux developer Matthew Garret uncovered that the bug was actually triggered by storing too many UEFI variables to memory, and that the bug could also be triggered under Windows using a custom-made program intentionally built to write a large number of variables to memory. In conclusion, he noted that conflicts caused by the kernel module had caused core dumps to be written to the firmware, thus triggering the bug.[64][65][23]

See also


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Further reading

External links