UEFI (Unified Extensible Firmware Interface) is a set of specifications written by the

UEFI Forum UEFI Forum, Inc. is an alliance between technology companies to coordinate the development of the UEFI specifications. The board of directors includes representatives from twelve ''promoter'' companies: AMD, American Megatrends, ARM, Apple, De ...

. They define the

architecture Architecture is the art and technique of designing and building, as distinguished from the skills associated with construction. It is both the process and the product of sketching, conceiving, planning, designing, and constructing buildings ...

of the platform

firmware In computing, firmware is a specific class of computer software that provides the low-level control for a device's specific hardware. Firmware, such as the BIOS of a personal computer, may contain basic functions of a device, and may provide ...

used for

booting In computing, booting is the process of starting a computer as initiated via Computer hardware, hardware such as a button or by a software command. After it is switched on, a computer's central processing unit (CPU) has no software in its ma ...

and its

interface Interface or interfacing may refer to: Academic journals * ''Interface'' (journal), by the Electrochemical Society * '' Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics'' * '' Int ...

for interaction with the

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...

. Examples of firmware that implement these specifications are AMI Aptio, Phoenix SecureCore, TianoCore EDK II and InsydeH2O. UEFI replaces the

BIOS In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the ...

which was present in the

boot ROM The boot ROM is a type of ROM that is used for booting a computer system. There are two types: a mask boot ROM that cannot be changed afterwards and a boot EEPROM, which can contain an UEFI implementation. Purpose Upon power up, hardware usuall ...

of all

personal computers A personal computer (PC) is a multi-purpose microcomputer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or tec ...

that are

IBM PC-compatible IBM PC compatible computers are similar to the original IBM PC, XT, and AT, all from computer giant IBM, that are able to use the same software and expansion cards. Such computers were referred to as PC clones, IBM clones or IBM PC clones. ...

, although it can provide backwards compatibility with the BIOS using CSM booting.

Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the devel ...

developed the original ''Extensible Firmware Interface'' (''EFI'') specifications. Some of the EFI's practices and data formats mirror those of Microsoft Windows. In 2005, UEFI deprecated EFI 1.10 (the final release of EFI). UEFI is independent of platform and programming language, but C is used for the reference implementation TianoCore EDKII.

History

The original motivation for EFI came during early development of the first Intel–HP

Itanium Itanium ( ) is a discontinued family of 64-bit Intel microprocessors that implement the Intel Itanium architecture (formerly called IA-64). Launched in June 2001, Intel marketed the processors for enterprise servers and high-performance comp ...

systems in the mid-1990s.

limitations (such as 16-bit

real mode Real mode, also called real address mode, is an operating mode of all x86-compatible CPUs. The mode gets its name from the fact that addresses in real mode always correspond to real locations in memory. Real mode is characterized by a 20-bit s ...

, 1MB addressable memory space, assembly language programming, and

PC AT The IBM Personal Computer/AT (model 5170, abbreviated as IBM AT or PC/AT) was released in 1984 as the fourth model in the IBM Personal Computer line, following the IBM PC/XT and its IBM Portable PC variant. It was designed around the Intel 8 ...

hardware) had become too restrictive for the larger server platforms Itanium was targeting. The effort to address these concerns began in 1998 and was initially called ''Intel Boot Initiative''. It was later renamed to ''Extensible Firmware Interface'' (EFI). In July 2005, Intel ceased its development of the EFI specification at version 1.10, and contributed it to the

Unified EFI Forum UEFI Forum, Inc. is an alliance between technology companies to coordinate the development of the UEFI specifications. The board of directors includes representatives from twelve ''promoter'' companies: AMD, American Megatrends, ARM, Apple, De ...

, which has developed the specification as the ''Unified Extensible Firmware Interface'' (UEFI). The original EFI specification remains owned by Intel, which exclusively provides licenses for EFI-based products, but the UEFI specification is owned by the UEFI Forum. Version 2.0 of the UEFI specification was released on 31 January 2006. It added

cryptography Cryptography, or cryptology (from grc, , translit=kryptós "hidden, secret"; and ''graphein'', "to write", or ''-logia'', "study", respectively), is the practice and study of techniques for secure communication in the presence of adve ...

and security. Version 2.1 of the UEFI specification was released on 7 January 2007. It added network authentication and the

user interface In the industrial design field of human–computer interaction, a user interface (UI) is the space where interactions between humans and machines occur. The goal of this interaction is to allow effective operation and control of the machine f ...

architecture ('Human Interface Infrastructure' in UEFI). The latest UEFI specification, version 2.9, was published in March 2021. The first

open source Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use the source code, design documents, or content of the product. The open-source model is a decentralized sof ...

UEFI implementation, Tiano, was released by Intel in 2004. Tiano has since then been superseded by EDK and EDK2 and is now maintained by the TianoCore community. In December 2018,

Microsoft Microsoft Corporation is an American multinational corporation, multinational technology company, technology corporation producing Software, computer software, consumer electronics, personal computers, and related services headquartered at th ...

announced Project Mu, a fork of TianoCore EDK2 used in

Microsoft Surface Microsoft Surface is a series of touchscreen-based personal computers, tablets and interactive whiteboards designed and developed by Microsoft, running the Microsoft Windows operating system, apart from the Surface Duo, which runs on Andro ...

and

Hyper-V Microsoft Hyper-V, codenamed Viridian, and briefly known before its release as Windows Server Virtualization, is a native hypervisor; it can create virtual machines on x86-64 systems running Windows. Starting with Windows 8, Hyper-V superseded Wi ...

products. The project promotes the idea of

Firmware as a Service TianoCore EDK II (formerly Tiano) is the reference implementation of UEFI by Intel. EDK is the abbreviation for EFI Development Kit and is developed by the TianoCore community. TianoCore EDK II is the de facto standard generic UEFI services impl ...

. In October 2018, Arm announce
Arm ServerReady
a compliance certification program for landing the generic off-the-shelf operating systems and

hypervisor A hypervisor (also known as a virtual machine monitor, VMM, or virtualizer) is a type of computer software, firmware or hardware that creates and runs virtual machines. A computer on which a hypervisor runs one or more virtual machines is called ...

s on Arm-based servers. The program requires the system firmware to comply with Server Base Boot Requirements (SBBR). SBBR requires UEFI, ACPI and SMBIOS compliance. In October 2020, Arm announced the extension of the program to the

edge Edge or EDGE may refer to: Technology Computing * Edge computing, a network load-balancing system * Edge device, an entry point to a computer network * Adobe Edge, a graphical development application * Microsoft Edge, a web browser developed b ...

and IoT market. The new program name i
Arm SystemReady
Arm SystemReady defined the Base Boot Requirements
BBR
specification that currently provides three recipes, two of which are related to UEFI: 1) SBBR: which requires UEFI, ACPI and SMBIOS compliance suitable for enterprise level operating environments such as Windows, Red Hat Enterprise Linux, and VMware ESXi; and 2) EBBR: which requires compliance to a set of UEFI interfaces as defined in the Embedded Base Boot Requirements
EBBR
suitable for embedded environments such as Yocto. Many Linux and BSD distros can support both recipes.

Advantages

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: * Ability to boot a disk containing large partitions (over 2 TB) with a

GUID Partition Table The GUID Partition Table (GPT) is a standard for the layout of partition tables of a physical computer storage device, such as a hard disk drive or solid-state drive, using universally unique identifiers, which are also known as globally unique ...

(GPT) * Flexible pre-OS environment, including network capability, GUI, multi language * 32-bit (for example

IA-32 IA-32 (short for "Intel Architecture, 32-bit", commonly called i386) is the 32-bit version of the x86 instruction set architecture, designed by Intel and first implemented in the 80386 microprocessor in 1985. IA-32 is the first incarnation of ...

ARM32 ARM (stylised in lowercase as arm, formerly an acronym for Advanced RISC Machines and originally Acorn RISC Machine) is a family of reduced instruction set computer (RISC) instruction set architectures for computer processors, configured ...

) or 64-bit (for example x64,

AArch64 AArch64 or ARM64 is the 64-bit extension of the ARM architecture family. It was first introduced with the Armv8-A architecture. Arm releases a new extension every year. ARMv8.x and ARMv9.x extensions and features Announced in October 2011, ...

) pre-OS environment *

C language C (''pronounced like the letter c'') is a general-purpose computer programming language. It was created in the 1970s by Dennis Ritchie, and remains very widely used and influential. By design, C's features cleanly reflect the capabilities o ...

programming * Modular design * Backward and forward compatibility

Compatibility

Processor compatibility

As of version 2.5, processor bindings exist for Itanium, x86, x86-64, ARM (AArch32) and

ARM64 AArch64 or ARM64 is the 64-bit extension of the ARM architecture family. It was first introduced with the Armv8-A architecture. Arm releases a new extension every year. ARMv8.x and ARMv9.x extensions and features Announced in October 2011, ...

(AArch64). Only

little-endian In computing, endianness, also known as byte sex, is the order or sequence of bytes of a word of digital data in computer memory. Endianness is primarily expressed as big-endian (BE) or little-endian (LE). A big-endian system stores the most ...

processors can be supported. Unofficial UEFI support is under development for POWERPC64 by implementing

TianoCore TianoCore EDK II (formerly Tiano) is the reference implementation of UEFI by Intel. EDK is the abbreviation for EFI Development Kit and is developed by the TianoCore community. TianoCore EDK II is the de facto standard generic UEFI services implem ...

on top of OPAL, the OpenPOWER abstraction layer, running in little-endian mode. Similar projects exist for MIPS and

RISC-V RISC-V (pronounced "risk-five" where five refers to the number of generations of RISC architecture that were developed at the University of California, Berkeley since 1981) is an open standard instruction set architecture (ISA) based on establi ...

. As of UEFI 2.7, RISC-V processor bindings have been officially established for 32-, 64- and 128-bit modes. Standard PC BIOS is limited to a 16-bit processor mode and 1 MB of addressable memory space, resulting from the design based on the IBM 5150 that used a 16-bit

Intel 8088 The Intel 8088 ("''eighty-eighty-eight''", also called iAPX 88) microprocessor is a variant of the Intel 8086. Introduced on June 1, 1979, the 8088 has an eight-bit external Bus (computing), data bus instead of the 16-bit computing, 16-bit bus of ...

processor. In comparison, the processor mode in a UEFI environment can be either 32-bit (

x86-32 IA-32 (short for "Intel Architecture, 32-bit", commonly called i386) is the 32-bit version of the x86 instruction set architecture, designed by Intel and first implemented in the 80386 microprocessor in 1985. IA-32 is the first incarnation of ...

, AArch32) or 64-bit (

x86-64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging ...

, Itanium, and AArch64). 64-bit UEFI firmware implementations support

long mode In the x86-64 computer architecture, long mode is the mode where a 64-bit operating system can access 64-bit instructions and registers. 64-bit programs are run in a sub-mode called 64-bit mode, while 32-bit programs and 16-bit protected mode p ...

, which allows applications in the preboot environment to use 64-bit addressing to get direct access to all of the machine's memory. UEFI requires the firmware and operating system loader (or kernel) to be size-matched; that is, a 64-bit UEFI firmware implementation can load only a 64-bit operating system (OS) boot loader or kernel (unless the CSM-based Legacy boot is used) and the same applies to 32-bit. 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 the runtime services (unless the kernel switches back again). As of version 3.15, the

Linux kernel The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU ...

supports 64-bit kernels to be booted on 32-bit UEFI firmware implementations running on

CPUs, with ''UEFI handover'' support from a UEFI boot loader as the requirement. UEFI handover protocol deduplicates the UEFI initialization code between the kernel and UEFI boot loaders, leaving the initialization to be performed only by the Linux kernel's ''UEFI boot stub''.

Disk device compatibility

In addition to the standard PC disk partition scheme that uses a

master boot record A master boot record (MBR) is a special type of boot sector at the very beginning of partitioned computer mass storage devices like fixed disks or removable drives intended for use with IBM PC-compatible systems and beyond. The concept of MB ...

(MBR), UEFI also works with the

(GPT) partitioning scheme, which is free from many of the limitations of MBR. In particular, the MBR limits on the number and size of disk partitions (up to four

primary partition Disk partitioning or disk slicing is the creation of one or more regions on secondary storage, so that each region can be managed separately. These regions are called partitions. It is typically the first step of preparing a newly installed disk ...

s per disk, and up to 2 TB per disk) are relaxed. More specifically, GPT allows for a maximum disk and partition size of 8 ZB .

Linux

Support for GPT in

Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which i ...

is enabled by turning on the option CONFIG_EFI_PARTITION (EFI GUID Partition Support) during kernel configuration. This option allows Linux to recognize and use GPT disks after the system firmware passes control over the system to Linux. For reverse compatibility, Linux can use GPT disks in BIOS-based systems for both data storage and booting, as both GRUB 2 and Linux are GPT-aware. Such a setup is usually referred to as ''BIOS-GPT''. As GPT incorporates the protective MBR, a BIOS-based computer can boot from a GPT disk using a GPT-aware boot loader stored in the protective MBR's bootstrap code area. In the case of GRUB, such a configuration requires a BIOS boot partition for GRUB to embed its second-stage code due to absence of the post-MBR gap in GPT partitioned disks (which is taken over by the GPT's ''Primary Header'' and ''Primary Partition Table''). Commonly 1 MB in size, this partition's

Globally Unique Identifier A universally unique identifier (UUID) is a 128-bit label used for information in computer systems. The term globally unique identifier (GUID) is also used. When generated according to the standard methods, UUIDs are, for practical purposes, un ...

(GUID) in GPT scheme is and is used by GRUB only in BIOS-GPT setups. From GRUB's perspective, no such partition type exists in case of MBR partitioning. This partition is not required if the system is UEFI-based because no embedding of the second-stage code is needed in that case. UEFI systems can access GPT disks and boot directly from them, which allows Linux to use UEFI boot methods. Booting Linux from GPT disks on UEFI systems involves creation of an EFI system partition (ESP), which contains UEFI applications such as bootloaders, operating system kernels, and utility software. Such a setup is usually referred to as ''UEFI-GPT'', while ESP is recommended to be at least 512 MB in size and formatted with a FAT32 filesystem for maximum compatibility. For

backward compatibility Backward compatibility (sometimes known as backwards compatibility) is a property of an operating system, product, or technology that allows for interoperability with an older legacy system, or with input designed for such a system, especially ...

, some UEFI implementations also support booting from MBR-partitioned disks through the Compatibility Support Module (CSM) that provides legacy BIOS compatibility. In that case, booting Linux on UEFI systems is the same as on legacy BIOS-based systems.

Microsoft Windows

The 64-bit versions of

Windows Vista Windows Vista is a major release of the Windows NT operating system developed by Microsoft. It was the direct successor to Windows XP, which was released five years before, at the time being the longest time span between successive releases of ...

SP1 and later and 32-bit versions of

Windows 8 Windows 8 is a major release of the Windows NT operating system developed by Microsoft. It was released to manufacturing on August 1, 2012; it was subsequently made available for download via MSDN and TechNet on August 15, 2012, and later to ...

, 8.1, and 10 can boot from a GPT disk that is larger than 2 TB.

Features

Services

EFI defines two types of services: ''boot services'' and ''runtime services''. Boot services are available only while the firmware owns the platform (i.e., before the ExitBootServices() call), and they 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 Non-volatile random-access memory (NVRAM) is random-access memory that retains data without applied power. This is in contrast to dynamic random-access memory (DRAM) and static random-access memory (SRAM), which both maintain data only for as lon ...

access. ; Graphics Output Protocol (GOP) services : The ''Graphics Output Protocol'' (GOP) provides runtime services; see also Graphics features section below. The operating system is permitted to directly write to the framebuffer provided by GOP during runtime mode. ; UEFI Memory map services ; SMM services ; ACPI services ; SMBIOS services ; Device tree services (for RISC processors) ; Variable services : UEFI variables provide a way to store data, in particular non-volatile data. Some UEFI variables are shared between platform firmware and operating systems. Variable namespaces are identified by GUIDs, and variables are key/value pairs. For example, UEFI variables can be used to keep crash messages in

after a crash for the operating system to retrieve after a reboot. ; Time services : UEFI provides time services. Time services include support for time zone and daylight saving fields, which allow the hardware

real-time clock A real-time clock (RTC) is an electronic device (most often in the form of an integrated circuit) that measures the passage of time. Although the term often refers to the devices in personal computers, servers and embedded systems, RTCs are ...

to be set to local time or UTC. On machines using a PC-AT real-time clock, by default the hardware clock still has to be set to local time for compatibility with BIOS-based Windows, unless using recent versions and an entry in the

Windows registry The Windows Registry is a hierarchical database that stores low-level settings for the Microsoft Windows operating system and for applications that opt to use the registry. The kernel, device drivers, services, Security Accounts Manager, and u ...

is set to indicate the use of UTC.

Applications

Beyond loading an OS, UEFI can run ''UEFI applications'', which reside as files on the EFI System Partition. They can be executed from the UEFI Shell, by the firmware's

boot manager A bootloader, also spelled as boot loader or called boot manager and bootstrap loader, is a computer program that is responsible for booting a computer. When a computer is turned off, its softwareincluding operating systems, application code, an ...

, or by other UEFI applications. ''UEFI applications'' can be developed and installed independently of the

original equipment manufacturer An original equipment manufacturer (OEM) is generally perceived as a company that produces non-aftermarket parts and equipment that may be marketed by another manufacturer. It is a common industry term recognized and used by many professional or ...

s (OEMs). A type of UEFI application is an OS boot loader such as

GRUB Grub can refer to Grub (larva), of the beetle superfamily Scarabaeoidea, or as a slang term for food. It can also refer to: Places * Grub, Appenzell Ausserrhoden, Switzerland * Grub, St. Gallen, Switzerland * Grub (Amerang), a hamlet in Bavari ...

, rEFInd, Gummiboot, and

Windows Boot Manager The booting process of Windows Vista and later versions differ from the startup process part of previous versions of Windows. In this article, unless otherwise specified, what is said about Windows Vista also applies to all later NT operatin ...

; which loads some OS files into memory and executes them. Also, an OS boot loader can provide a user interface to allow the selection of another UEFI application to run. Utilities like the UEFI Shell are also UEFI applications.

Protocols

EFI defines protocols as a set of software interfaces used for communication between two binary modules. All EFI drivers must provide services to others via protocols. The EFI Protocols are similar to the BIOS interrupt calls.

Device drivers

In addition to standard

instruction set architecture In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, such as a central processing unit (CPU), is called an ...

-specific device drivers, EFI provides for a ISA-independent

device driver In computing, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabling operating systems and o ...

stored in

non-volatile memory Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retain stored information even after power is removed. In contrast, volatile memory needs constant power in order to retain data. Non-volatile memory typ ...

as ''EFI byte code'' or ''EBC''. System firmware has an interpreter for EBC images. In that sense, EBC is analogous to

Open Firmware Open Firmware is a standard defining the interfaces of a computer firmware system, formerly endorsed by the Institute of Electrical and Electronics Engineers (IEEE). It originated at Sun Microsystems, where it was known as OpenBoot, and has bee ...

, the ISA-independent firmware used in

PowerPC PowerPC (with the backronym Performance Optimization With Enhanced RISC – Performance Computing, sometimes abbreviated as PPC) is a reduced instruction set computer (RISC) instruction set architecture (ISA) created by the 1991 Apple– IBM ...

-based

Apple Macintosh The Mac (known as Macintosh until 1999) is a family of personal computers designed and marketed by Apple Inc. Macs are known for their ease of use and minimalist designs, and are popular among students, creative professionals, and software ...

and

Sun Microsystems Sun Microsystems, Inc. (Sun for short) was an American technology company that sold computers, computer components, software, and information technology services and created the Java programming language, the Solaris operating system, ZFS, ...

SPARC SPARC (Scalable Processor Architecture) is a reduced instruction set computer (RISC) instruction set architecture originally developed by Sun Microsystems. Its design was strongly influenced by the experimental Berkeley RISC system developed ...

computers, among