Unified Extensible Firmware Interface

Unified Extensible Firmware Interface
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 a more secure replacement for the older BIOS firmware interface, present in all IBM PC-compatible personal computers, which is vulnerable to bootkit malware.[1][2]

The original EFI (Extensible Firmware Interface) specification was developed by Intel. In 2005, development of the EFI specification ceased in favour of UEFI, which had evolved from EFI 1.10. The UEFI specification is being developed by the industry-wide organization Unified EFI Forum.

UEFI is not restricted to any specific processor architecture and can run on top of, or instead of, older BIOS implementations.[3]



The original motivation for EFI came during early development of the first Intel–HP Itanium systems in the mid-1990s. PC BIOS limitations (16-bit processor mode, 1 MB addressable space, PC AT hardware dependencies, etc.) were seen as unacceptable for the larger server platforms Itanium was targeting.[4] The effort to address these concerns was initially called Intel Boot Initiative, which began in 1998[5] and was later renamed EFI.[6][7]

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.[4][8]

Version 2.1 of the UEFI (Unified Extensible Firmware Interface) specification was released on January 7, 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.

An overview of late 2011 usages of UEFI by independent by hardware, BIOS, and software vendors.[9]


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:[10]

  • Ability to boot from large disks (over 2 TiB)[11]
  • Faster boot-up
  • CPU-independent architecture
  • CPU-independent drivers
  • Flexible pre-OS environment, including network capability
  • Modular design

Some existing enhancements to PC BIOS, such as the Advanced Configuration and Power Interface (ACPI) and System Management BIOS (SMBIOS), are also present in EFI, as they do not rely on a 16-bit runtime interface.

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 DOS MBR. In particular, the DOS MBR limits on the number and size of disk partitions (up to 4 partitions per disk, up to 2.2 TB (241 bytes) per disk) are relaxed.[12] GPT allows for a maximum disk and partition size of 9.4 ZB (273 bytes).[12][13] The EFI specification does not prescribe any particular file system. The only Microsoft Windows versions that can boot from disks larger than 2.2 TB are 64-bit versions of Windows Vista and later client operating systems, Windows Server 2008 and later, and the Itanium versions of Windows Server 2003 and Windows XP.

Processor compatibility

As of version 2.3, processor bindings exist for Itanium, x86, x86_64 and ARM.

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.[4][14] In comparison, the UEFI processor mode can be either 32-bit (x86-32, ARM) or 64-bit (x86-64 and Itanium).[4][15] 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.[16]

UEFI requires the firmware and operating system to be size-matched; i.e. a 64-bit UEFI implementation can only boot a 64-bit UEFI operating system.


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.


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.


UEFI does not rely on a working boot sector only, but needs a special partition table referring to a special partition containing a specially located file with a standardized name depending on the actual architecture to boot (\EFI\BOOT\boot[architecture name].efi).

Boot 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. Boot variables, stored in NVRAM, indicate the paths to the loaders. Boot loaders can also be auto-detected by firmware, for instance to enable booting on removable devices.

It is common for UEFI firmware to include a boot manager, to allow the user to select and load the operating system among the possible options.

The EFI shell

EFI provides a shell environment.[17] The shell 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 XScale, Itanium 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 the "TianoCore project." TianoCore can be used as a payload for coreboot[citation needed].

Phoenix Technologies currently sells Phoenix SecureCore Tiano and Phoenix MicroCore Bios implementing UEFI.[18]

Aptio or Aptio 4 is American Megatrend's next-generation BIOS firmware based on UEFI and the Intel Platform Innovation Framework for UEFI/EFI.[19]

InsydeH2O is Insyde Software's implementation of the Intel Platform Innovation Framework for UEFI/EFI.[20]

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 Windows, Linux, FreeBSD and HP-UX; OpenVMS 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.[21] On April 5, 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.

During 2005, more than one million Intel systems shipped with the Framework.[22] New mobile, desktop and server products, using the Framework, started shipping in 2006. For instance, boards that use the Intel 945 chipset series use the Framework.

Since 2005, EFI has also been implemented on non-PC architectures, such as embedded systems based on XScale cores.[22]

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 have adopted UEFI. While many of these systems still allow booting only the BIOS-based OSes via the Compatibility Support Module (CSM) (thus does not appear to the user that the system is 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.[23] Many Sandy Bridge PC platforms use UEFI.

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.

  • Linux has been able to use EFI at boot time since early 2000, using the elilo EFI boot loader or, more recently, EFI versions of GRUB.[24]
  • 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.[25]
  • Apple uses EFI for its line of Intel-based Macs. Mac OS X v10.4 Tiger for Intel and Mac OS X v10.5 Leopard implement EFI v1.10 in 32-bit mode, even on 64-bit CPUs (newer Macs have 64-bit EFI).[26]
  • 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.[27]
  • 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. Microsoft does not implement 32-bit UEFI since vendors did not have any interest in producing native 32-bit UEFI firmware because of the mainstream status of 64-bit computing.[28] Microsoft has released a video with Andrew Ritz and Jamie Schwartz explaining Pre-OS UEFI functions on Windows Vista and Windows Server 2008.[29]
  • Microsoft will require computers with the "Designed for Windows 8" logo to use UEFI with secure boot (which will only allow signed software to run on the device) enabled by default.[30][31] Red Hat developer Matthew Garrett raised concerns over the requirement for secure booting to be enabled by default and Microsoft responded by saying that there was no mandate from Microsoft that prevents secure booting from being disabled in firmware or that keys could not be updated and managed.[30][31]

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.[32]
  • 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,[33] but limited to Unix/Linux operating systems (doesn't work with Windows Vista x64 and Windows 7 x64).[34][35]
  • Qemu can be used with an EFI bios.
  • The VMware ESXi version 5 hypervisor, part of VMware vSphere, supports virtualized EFI as an alternative to BIOS inside a virtual machine.

Consumer market

In 2011 ASRock, ASUSTeK, Gigabyte and MSI launched several consumer-based motherboards using the Intel 6-series LGA 1155 chipset and AMD 9 Series Chipset for Upcoming AM3+ AMD FX (Bulldozer) and AMD Fusion Processors with EFI.[36]

Graphics features

EFI provides graphical menus and features, such as is implemented on Aptio or Great Wall UEFI.[37]

There are two graphics output protocols defined by EFI specifications. The first one is UGA, which stands for Universal Graphic Adapters. The second one is GOP, which stands for Graphic Output Protocol. The two are similar. UGA works only at EFI 1.1 or older. EFI does not define a user interface. This depends fully on the implementation by BIOS vendors. Currently most EFI-enabled machines only have a legacy BIOS-like text mode UI.


EFI was criticized by coreboot creator Ronald G. Minnich as an attempt to preserve intellectual property.[38] 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.[39]

TianoCore[40] is an open-source project which provides the tools to make a fully free firmware based on UEFI, but it lacks the specialized drivers that initialize chipset functions, thus requiring additional features from the chipset vendor. TianoCore is a payload option for Coreboot, which includes chipset initialization code.

Because UEFI can allow for more flexible remote network booting than current BIOS technology, there are concerns over the security provisions in the standard.[41]

Red Hat developer Matthew Garrett in his article "UEFI secure booting" raised a concern that UEFI "secure boot" feature may impact Linux (machines with the Windows 8 logo with secure boot enabled that ships with only OEM and Microsoft keys will not boot a generic copy of Linux)[42][43] In response, Microsoft stated that customers may be able to disable the secure boot feature in the BIOS.[2][44] Concern remains that some OEMs might omit that capability in their computers.

Josh Gay of Free Software Foundation raised concerns on "secure boot" implementation to UEFI and FSF declared a public statement open for signing which states as such: "We, the undersigned, urge all computer makers implementing UEFI's so-called "Secure Boot" to do it in a way that allows free software operating systems to be installed. To respect user freedom and truly protect user security, manufacturers must either allow computer owners to disable the boot restrictions, or provide a sure-fire way for them to install and run a free software operating system of their choice. We commit that we will neither purchase nor recommend computers that strip users of this critical freedom, and we will actively urge people in our communities to avoid such jailed systems."[45][46]

See also


  1. ^ Michael Kinney (2000-09-01). "Solving BIOS Boot Issues with EFI" (PDF). pp. 47–50. http://systems.cs.colorado.edu/Documentation/IntelDataSheets/xscalemagazine.pdf. Retrieved 2010-09-14. 
  2. ^ a b . The Register. 23 September 2011. http://www.theregister.co.uk/2011/09/23/ms_denies_uefi_lock_in/. Retrieved 24 September 2011. 
  3. ^ About, UEFI Forum, http://www.uefi.org/about/ .
  4. ^ a b c d "Emulex UEFI Implementation Delivers Industry-leading Features for IBM Systems" (PDF). Emulex. http://www.emulex.com/artifacts/757d23e7-8acb-41a7-872a-afb733ab0688/elx_tb_all_uefi_ibm.pdf. Retrieved 2010-09-14. 
  5. ^ Extensible Firmware Interface (EFI) and Unified EFI (UEFI), Intel, http://web.archive.org/web/20100105051711/http://www.intel.com/technology/efi/ .
  6. ^ Wei, Dong (2006), "foreword", Beyond BIOS, Intel Press, ISBN 978-0-9743649-0-2 .
  7. ^ "1.10 Specification overview", Extensible Firmware Interface, Intel, http://www.intel.com/technology/efi/main_specification.htm .
  8. ^ About, Unified EFI Forum, http://www.uefi.org/about/, "Q: What is the relationship between EFI and UEFI? A: The UEFI specification will be based on the EFI 1.10 specification published by Intel with corrections and changes managed by the Unified EFI Forum. Intel still holds the copyright on the EFI 1.10 specification, but has contributed it to the Forum so that the Forum can evolve it. There will not be any future versions of the EFI specification, but customers who license it can still use it under the terms of their license from Intel. The license to the Unified EFI Specification will come from the Forum, not from Intel" 
  9. ^ UEFI Today: Bootstrapping the Continuum, Intel Press, http://www.intel.com/technology/itj/2011/v15i1/index.htm .
  10. ^ "UEFI and Windows". Microsoft. 2009-09-15. http://www.microsoft.com/whdc/system/platform/firmware/UEFI_Windows.mspx. Retrieved 2010-09-14. 
  11. ^ Only when using GUID Partition Table as its partition mapping method and an x64 operating system.
  12. ^ a b "FAQ: Drive Partition Limits" (PDF). UEFI Forum. http://www.uefi.org/learning_center/UEFI_MBR_Limits_v2.pdf. Retrieved 2010-06-09. 
  13. ^ Bill Boswell (2002-07-01). "FAQ: Drive Partition Limits". Redmond Mag. http://redmondmag.com/Articles/2002/07/01/The-64Bit-Question.aspx. Retrieved 2010-06-09. "GPT disks also support very large partitions thanks to a 64-bit Logical Block Address scheme. A logical block corresponds to one sector, or 512 bytes, yielding a maximum theoretical capacity of eight zettabytes," 
  14. ^ Ben Hardwidge (2010-06-01). "LBA explained — Solving the 3TB Problem?". bit-tech. http://www.bit-tech.net/hardware/storage/2010/06/01/are-we-ready-for-3tb-hard-disks/2. Retrieved 2010-06-18. 
  15. ^ Brian Richardson (2010-05-10). "Ask a BIOS Guy: "Why UEFI"". Intel Architecture Blog. http://community.edc.intel.com/t5/New-to-Intel-Architecture-Blog/Ask-a-BIOS-Guy-quot-Why-UEFI-quot/ba-p/2781. Retrieved 2010-06-18. 
  16. ^ Gary Simpson. "UEFI Momentum — The AMD perspective" (PPTX). AMD. http://download.microsoft.com/download/5/e/6/5e66b27b-988b-4f50-af3a-c2ff1e62180f/cor-t605_wh08.pptx. Retrieved 2010-06-18. 
  17. ^ Efi-shell.tianocore.org for EFI shell information
  18. ^ "SecureCore Tiano™". Phoenix Technologies, LTD. http://www.phoenix.com/pages/phoenix-securecore-tiano-tm. Retrieved 2010-09-14. 
  19. ^ "Aptio®: The Complete UEFI Product Solution" (PDF). American Megatrends, Inc. http://www.ami.com/support/doc/AMI_Aptio_4.x_Datasheet_PUB_2009-09-14.pdf. Retrieved 2011-01-08. 
  20. ^ "InsydeH2O UEFI Framework". Insyde Software Corp. http://www.insydesw.com/products/products-efi-h2o.htm. Retrieved 2011-01-08. 
  21. ^ Apple Computer. "Universal Binary Programming Guidelines, Second Edition: Extensible Firmware Interface (EFI)"
  22. ^ a b "Intel® Platform Innovation Framework for UEFI Overview". Intel. http://www.intel.com/technology/framework/overview1.htm. Retrieved 2010-09-14. 
  23. ^ (PDF) Evaluating UEFI using Commercially Available Platforms and Solutions, UEFI, 2011‐5, http://www.uefi.org/news/uefi_industry/UEFIEvaluationPlatforms_2011_05.pdf .
  24. ^ EFI version of Grub, Debian GNU/Linux, http://packages.debian.org/sid/grub-efi, retrieved 1 May 2008 .
  25. ^ OpenVMS Release History, HP, http://h71000.www7.hp.com/openvms/os/openvms-release-history.html, retrieved 16 September 2008 .
  26. ^ rEFIt — Windows Vista and EFI, SourceForge, http://refit.sourceforge.net/info/vista.html .
  27. ^ "Extensible Firmware Interface", Windows Server TechCenter, Microsoft, http://technet2.microsoft.com/WindowsServer/en/Library/6b03fad7-665e-49f3-8e7d-e1a6a5be9cf01033.mspx .
  28. ^ "Unified Extended Firmware Interface support in Windows Vista". Microsoft. 2006-10-26. http://support.microsoft.com/kb/930061. Retrieved 2010-06-12. "Microsoft determined that vendors would not have any interest in producing native UEFI 32-bit firmware because of the current status of mainstream 64-bit computing and platform costs. Therefore, Microsoft has chosen not to ship support for 32-bit UEFI implementations." 
  29. ^ "Pre-OS Video", MS Developers Network Channel 9, Microsoft, http://channel9.msdn.com/showpost.aspx?postid=292774 .
  30. ^ a b Jon Brodkin (2011-09-21). "Windows 8 secure boot could complicate Linux installs". Arstechnica. http://arstechnica.com/business/news/2011/09/windows-8-secure-boot-will-complicate-linux-installs.ars. Retrieved 2011-09-23. 
  31. ^ a b Denis Wong (2011-09-22). "Microsoft clarifies Windows 8 UEFI concerns". Neowin.net. http://www.neowin.net/news/microsoft-clarifies-windows-8-uefi-concerns. Retrieved 2011-09-23. 
  32. ^ Open Virtual Machine Firmware, SourceForge, http://sourceforge.net/apps/mediawiki/tianocore/index.php?title=OVMF .
  33. ^ 3.1 Changelog, VirtualBox, http://www.virtualbox.org/wiki/Changelog-3.1 .
  34. ^ Ticket 7702, VirtualBox, http://www.virtualbox.org/ticket/7702 .
  35. ^ "Statement by sr. software engineer at Oracle", Forum, VirtualBox, http://forums.virtualbox.org/viewtopic.php?f=1&p=183022#p114765 .
  36. ^ Asus P67 Motherboard Preview, .
  37. ^ Intel shows PC booting Windows with UEFI firmware
  38. ^ "Interview: Ronald G Minnich". Fosdem. 2007-02-06. http://archive.fosdem.org/2007/interview/ronald+g+minnich. Retrieved 2010-09-14. 
  39. ^ "coreboot (aka LinuxBIOS): The Free/Open-Source x86 Firmware". YouTube. 2008-10-31. http://www.youtube.com/watch?v=X72LgcMpM9k. Retrieved 2010-09-14. 
  40. ^ "Welcome", TianoCore, SourceForge, http://sourceforge.net/apps/mediawiki/tianocore/index.php?title=Welcome .
  41. ^ Risks, UK: NCL, http://catless.ncl.ac.uk/Risks/26.18.html#subj13 .
  42. ^ Garrett, Matthew. "UEFI secure booting". http://mjg59.dreamwidth.org/5552.html. Retrieved 20 September 2011. 
  43. ^ Garrett, Matthew. "UEFI secure booting". http://mjg59.dreamwidth.org/5850.html. Retrieved 23 September 2011. 
  44. ^ "Protecting the pre-OS Environment with UEFI". Microsoft. 22 September 2011. http://blogs.msdn.com/b/b8/archive/2011/09/22/protecting-the-pre-os-environment-with-uefi.aspx. Retrieved 24 September 2011. 
  45. ^ Gay, Josh. "Will your computer's "Secure Boot" turn out to be "Restricted Boot"?". www.fsf.org. Free Software Foundation. http://www.fsf.org/campaigns/secure-boot-vs-restricted-boot. Retrieved 25 October 2011. 
  46. ^ "Stand up for your freedom to install free software". www.fsf.org. Free Software Foundation. http://www.fsf.org/campaigns/secure-boot-vs-restricted-boot/statement. Retrieved 25 October 2011. 

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