UEFI (Unified Extensible Firmware Interface) is a set of

specifications A specification often refers to a set of documented requirements to be satisfied by a material, design, product, or service. A specification is often a type of technical standard. There are different types of technical or engineering specificati ...

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, Del ...

. 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 building ...

of the platform firmware used for booting 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 services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also i ...

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

backwards 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 in ...

with the BIOS using CSM booting.

Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 seri ...

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 comput ...

systems in the mid-1990s. BIOS limitations (such as 16-bit real mode, 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 80 ...

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, Del ...

, 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 adver ...

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 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 technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washin ...

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 W ...

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 implem ...

. 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 Advanced Configuration and Power Interface (ACPI) is an open standard that operating systems can use to discover and configure computer hardware components, to perform power management (e.g. putting unused hardware components to sleep), auto c ...

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 ...

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 i ...

(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 o ...

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 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 mo ...

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, AR ...

) 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 In human anatomy, the arm refers to the upper limb in common usage, although academically the term specifically means the upper arm between the glenohumeral joint (shoulder joint) and the elbow joint. The distal part of the upper limb between th ...

(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, AR ...

(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 si ...

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

. 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 data bus instead of the 16-bit bus of the 8086. The 16-bit registers and ...

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 o ...

, 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, 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 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 (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, w ...

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 GNU GRUB (short for GNU GRand Unified Bootloader, commonly referred to as GRUB) is a boot loader package from the GNU Project. GRUB is the reference implementation of the Free Software Foundation's Multiboot Specification, which provides a u ...

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 The BIOS boot partition is a partition on a data storage device that GNU GRUB uses on legacy BIOS-based personal computers in order to boot an operating system, when the actual boot device contains a GUID Partition Table (GPT). Such a layo ...

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 (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, especiall ...

, 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 Windows 8.1 is a release of the Windows NT operating system developed by Microsoft. It was released to manufacturing on August 27, 2013, and broadly released for retail sale on October 17, 2013, about a year after the retail release of its pre ...

, 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 In computer science, a memory map is a structure of data (which usually resides in memory itself) that indicates how memory is laid out. The term "memory map" can have different meanings in different contexts. *It is the fastest and most flexible ...

services ; SMM services ;

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 pr ...

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 ...

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, a ...

, 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 Bavar ...

, rEFInd, Gummiboot, and Windows Boot Manager; 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 BIOS interrupt calls are a facility that operating systems and application programs use to invoke the facilities of the Basic Input/Output System firmware on IBM PC compatible computers. Traditionally, BIOS calls are mainly used by DOS programs a ...

Device drivers

In addition to standard instruction set architecture-specific device drivers, EFI provides for a ISA-independent device driver 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 typi ...

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-based

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

and Sun Microsystems

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 develope ...

computers, among others. Some architecture-specific (non-EFI Byte Code) EFI drivers for some device types can have interfaces for use by the OS. This allows the OS to rely on EFI for drivers to perform basic graphics and network functions before, and if, operating-system-specific drivers are loaded. In other cases, the EFI driver can be filesystem drivers that allow for booting from other types of disk volumes. Examples include ''efifs'' for 37 file systems (based on GRUB2 code), used by

Rufus Rufus is a masculine given name, a surname, an Ancient Roman cognomen and a nickname (from Latin '' rufus'', "red"). Notable people with the name include: Given name Politicians * Rufus Ada George (born 1940), Nigerian politician * Rufus ...

for chain-loading NTFS ESPs.

Graphics features

The EFI 1.0 specification defined a UGA (Universal Graphic Adapter) protocol as a way to support graphics features. UEFI did not include UGA and replaced it with GOP (Graphics Output Protocol). UEFI 2.1 defined a "Human Interface Infrastructure" (HII) to manage user input, localized strings, fonts, and forms (in the

HTML The HyperText Markup Language or HTML is the standard markup language for documents designed to be displayed in a web browser. It can be assisted by technologies such as Cascading Style Sheets (CSS) and scripting languages such as JavaSc ...

sense). These enable

s (OEMs) or independent BIOS vendors (IBVs) to design graphical interfaces for pre-boot configuration. Most early UEFI firmware implementations were console-based. Today many UEFI firmware implementations are GUI-based.

EFI system partition

An EFI system partition, often abbreviated to ESP, is a data storage device partition that is used in computers adhering to the UEFI specification. Accessed by the UEFI firmware when a computer is powered up, it stores UEFI applications and the files these applications need to run, including operating system

boot loader 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, a ...

s. Supported

partition table 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, ...

schemes include MBR and GPT, as well as El Torito volumes on optical discs. For use on ESPs, UEFI defines a specific version of the FAT file system, which is maintained as part of the UEFI specification and independently from the original FAT specification, encompassing the

FAT32 File Allocation Table (FAT) is a file system developed for personal computers. Originally developed in 1977 for use on floppy disks, it was adapted for use on hard disks and other devices. It is often supported for compatibility reasons by ...

FAT16 File Allocation Table (FAT) is a file system developed for personal computers. Originally developed in 1977 for use on floppy disks, it was adapted for use on hard disks and other devices. It is often supported for compatibility reasons by ...

and

FAT12 File Allocation Table (FAT) is a file system developed for personal computers. Originally developed in 1977 for use on floppy disks, it was adapted for use on hard disks and other devices. It is often supported for compatibility reasons by ...

file systems. The ESP also provides space for a boot sector as part of the backward BIOS compatibility.

Booting

UEFI booting

Unlike the legacy PC BIOS, UEFI does not rely on boot sectors, defining instead a boot manager as part of the UEFI specification. When a computer is powered on, the boot manager checks the boot configuration and, based on its settings, then executes the specified OS

or operating system kernel (usually boot loader). The boot configuration is defined by variables stored in

, including variables that indicate the file system paths to OS loaders or OS kernels. OS boot loaders can be automatically detected by UEFI, which enables easy booting from removable devices such as

USB flash drive Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and power supply ( interfacing) between computers, peripherals and other computers. A bro ...

s. This automated detection relies on standardized file paths to the OS boot loader, with the path varying depending on the

computer architecture In computer engineering, computer architecture is a description of the structure of a computer system made from component parts. It can sometimes be a high-level description that ignores details of the implementation. At a more detailed level, the ...

. The format of the file path is defined as ; for example, the file path to the OS loader on an

system is , and on ARM64 architecture. UEFI boot process

Booting UEFI systems from GPT-partitioned disks is commonly called ''UEFI-GPT booting''. Despite the fact that the UEFI specification requires MBR partition tables to be fully supported, some UEFI firmware implementations immediately switch to the BIOS-based CSM booting depending on the type of boot disk's partition table, effectively preventing UEFI booting to be performed from EFI System Partition on MBR-partitioned disks. Such a boot scheme is commonly called ''UEFI-MBR''. It is also common for a boot manager to have a textual user interface so the user can select the desired OS (or setup utility) from a list of available boot options.

CSM booting

To ensure backward compatibility, UEFI firmware implementations on PC-class machines could support booting in legacy BIOS mode from MBR-partitioned disks through the ''Compatibility Support Module (CSM)'' that provides legacy BIOS compatibility. In this scenario, booting is performed in the same way as on legacy BIOS-based systems, by ignoring the partition table and relying on the content of a boot sector. BIOS-style booting from MBR-partitioned disks is commonly called ''BIOS-MBR'', regardless of it being performed on UEFI or legacy BIOS-based systems. Furthermore, booting legacy BIOS-based systems from GPT disks is also possible, and such a boot scheme is commonly called ''BIOS-GPT''. The ''Compatibility Support Module'' allows legacy operating systems and some legacy

option ROM An Option ROM for the PC platform (i.e. the IBM PC and derived successor computer systems) is a piece of firmware that resides in ROM on an expansion card (or stored along with the main system BIOS), which gets executed to initialize the device an ...

s that do not support UEFI to still be used. It also provides required legacy

System Management Mode System Management Mode (SMM, sometimes called ring −2 in reference to protection rings) is an operating mode of x86 central processor units (CPUs) in which all normal execution, including the operating system, is suspended. An alterna ...

(SMM) functionality, called ''CompatibilitySmm'', as an addition to features provided by the UEFI SMM. An example of such a legacy SMM functionality is providing USB legacy support for keyboard and mouse, by emulating their classic PS/2 counterparts. In November 2017, Intel announced that it planned to phase out support for CSM by 2020. In July, of 2022, Kaspersky Labs published information regarding a Rootkit designed to chain boot malicious code on machines using Intel's H81 chipset and the Compatibility Support module of affected motherboards.

Network booting

The UEFI specification includes support for booting over network via the

Preboot eXecution Environment In computing, the Preboot eXecution Environment, PXE (most often pronounced as ''pixie'', often called PXE Boot/''pixie boot''.) specification describes a standardized client–server environment that boots a software assembly, retrieved from ...

(PXE). PXE booting

network protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchroniza ...

s include

Internet Protocol The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet. ...

( IPv4 and

IPv6 Internet Protocol version 6 (IPv6) is the most recent version of the Internet Protocol (IP), the communications protocol that provides an identification and location system for computers on networks and routes traffic across the Internet. IPv ...

User Datagram Protocol In computer networking, the User Datagram Protocol (UDP) is one of the core communication protocols of the Internet protocol suite used to send messages (transported as datagrams in packets) to other hosts on an Internet Protocol (IP) network ...

(UDP), Dynamic Host Configuration Protocol (DHCP),

Trivial File Transfer Protocol Trivial File Transfer Protocol (TFTP) is a simple lockstep File Transfer Protocol which allows a client to get a file from or put a file onto a remote host. One of its primary uses is in the early stages of nodes booting from a local area networ ...

(TFTP) and

iSCSI Internet Small Computer Systems Interface or iSCSI ( ) is an Internet Protocol-based storage networking standard for linking data storage facilities. iSCSI provides block-level access to storage devices by carrying SCSI commands over a TCP/IP ...

. OS images can be remotely stored on

storage area network A storage area network (SAN) or storage network is a computer network which provides access to consolidated, block-level data storage. SANs are primarily used to access data storage devices, such as disk arrays and tape libraries from ser ...

s (SANs), with Internet Small Computer System Interface (iSCSI) and Fibre Channel over Ethernet (FCoE) as supported protocols for accessing the SANs. Version 2.5 of the UEFI specification adds support for accessing boot images over the

HTTP The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide We ...

protocol.

Secure Boot

The UEFI 2.3.1 Errata C specification (or higher) defines a protocol known as ''Secure Boot'', which can secure the boot process by preventing the loading of UEFI drivers or OS boot loaders that are not signed with an acceptable digital signature. The mechanical details of how precisely these drivers are to be signed are not specified. 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 UEFI drivers and OS boot 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. 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. Secure Boot is supported by

and

Windows Server 2012 Windows Server 2012, codenamed "Windows Server 8", is the sixth version of the Windows Server operating system by Microsoft, as part of the Windows NT family of operating systems. It is the server version of Windows based on Windows 8 and succe ...

and 2012 R2,

Windows 10 Windows 10 is a major release of Microsoft's Windows NT operating system. It is the direct successor to Windows 8.1, which was released nearly two years earlier. It was released to manufacturing on July 15, 2015, and later to retail on J ...

Windows Server 2016 Windows Server 2016 is the eighth release of the Windows Server Server (computing), server operating system developed by Microsoft as part of the Windows NT family of operating systems. It was developed concurrently with Windows 10 and is the su ...

2019 File:2019 collage v1.png, From top left, clockwise: Hong Kong protests turn to widespread riots and civil disobedience; House of Representatives votes to adopt articles of impeachment against Donald Trump; CRISPR gene editing first used to experim ...

, and

2022 File:2022 collage V1.png, Clockwise, from top left: Road junction at Yamato-Saidaiji Station several hours after the assassination of Shinzo Abe; Anti-government protest in Sri Lanka in front of the Presidential Secretariat; The global monkeypo ...

, and

Windows 11 Windows 11 is the latest major release of Microsoft's Windows NT operating system, released in October 2021. It is a free upgrade to its predecessor, Windows 10 (2015), and is available for any Windows 10 devices that meet the new Windows 11 ...

, VMware vSphere 6.5 and a number of Linux distributions including Fedora (since version 18),

openSUSE openSUSE () is a free and open source RPM-based Linux distribution developed by the openSUSE project. The initial release of the community project was a beta version of SUSE Linux 10.0. Additionally the project creates a variety of tools, s ...

(since version 12.3), RHEL (since version 7), CentOS (since version 7), Debian (since version 10), and

Ubuntu Ubuntu ( ) is a Linux distribution based on Debian and composed mostly of free and open-source software. Ubuntu is officially released in three editions: '' Desktop'', ''Server'', and ''Core'' for Internet of things devices and robots. All ...

(since version 12.04.2). , FreeBSD support is in a planning stage.

UEFI shell

UEFI provides a shell environment, which can be used to execute other UEFI applications, including UEFI

s. Apart from that, commands available in the UEFI shell can be used for obtaining various other information about the system or the firmware, including getting the memory map (memmap), modifying boot manager variables (bcfg), running partitioning programs (diskpart), loading UEFI drivers, and editing text files (edit). Source code for a UEFI shell can be downloaded from the

's TianoCore UDK/EDK2 project. A pre-built ShellBinPkg is also available. 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. Methods used for launching UEFI shell depend on the manufacturer and model of the system motherboard. Some of them already provide a direct option in firmware setup for launching, e.g. compiled x86-64 version of the shell needs to be made available as /SHELLX64.EFI. Some other systems have an already embedded UEFI shell which can be launched by appropriate key press combinations. For other systems, the solution is either creating an appropriate USB flash drive or adding manually (bcfg) a boot option associated with the compiled version of shell.

Commands

The following is a list of commands supported by the EFI shell. * alias * attrib * bcfg * cd * cls *

comp Comp, COMP or Comps may refer to: Places In England: * Comp, Kent In France: * Comps, Drôme * Comps, Gard * Comps, Gironde * Comps-la-Grand-Ville * Comps-sur-Artuby Arts, entertainment, and media ;Music *Accompaniment, especially in jazz ...

* cp *

date Date or dates may refer to: *Date (fruit), the fruit of the date palm (''Phoenix dactylifera'') Social activity *Dating, a form of courtship involving social activity, with the aim of assessing a potential partner ** Group dating *Play date, a ...

* dblk * dh * dmpstore *

echo In audio signal processing and acoustics, an echo is a reflection of sound that arrives at the listener with a delay after the direct sound. The delay is directly proportional to the distance of the reflecting surface from the source and the lis ...

* Edd30 * EddDebug * edit * err * guid *

help Help is a word meaning to give aid or signal distress. Help may refer to: Arts, entertainment, and media Films * ''Help'' (2010 film), a Bollywood horror film * ''Help'' (2021 theatrical film), a British psychological thriller film * '' ...

* load * ls * map * mem * memmap * mkdir * mm * mode * mount * pause * pci * reset * rm * set * stall *

time Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, ...

* type * unload *

ver Ver or VER may refer to: * Voluntary Export Restraints, in international trade * VER, the IATA airport code for General Heriberto Jara International Airport * Volk's Electric Railway, Brighton, England * VerPublishing, of the German group VDM P ...

* vol

Extensions

Extensions to UEFI can be loaded from virtually any non-volatile storage device attached to the computer. For example, an

(OEM) can distribute systems with an EFI system partition on the hard drive, which would add additional functions to the standard UEFI firmware stored on the motherboard's

ROM Rom, or ROM may refer to: Biomechanics and medicine * Risk of mortality, a medical classification to estimate the likelihood of death for a patient * Rupture of membranes, a term used during pregnancy to describe a rupture of the amniotic sac * ...

UEFI Capsule

UEFI Capsule defines a Firmware-to-OS firmware update interface, marketed as modern and secure.

, Windows 8.1,

, and

Fwupd fwupd is an open-source daemon for managing the installation of firmware updates on Linux-based systems, developed by GNOME maintainer Richard Hughes. It is designed primarily for servicing the Unified Extensible Firmware Interface (UEFI) firm ...

for Linux each support the UEFI Capsule.

Hardware

Like BIOS, UEFI initializes and tests system hardware components (e.g. Memory training, PCIe link training, USB link training), and then loads the

from a

mass storage device In computing, mass storage refers to the storage of large amounts of data in a persisting and machine-readable fashion. In general, the term is used as large in relation to contemporaneous hard disk drives, but it has been used large in relati ...

or through a network connection. In

x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was intr ...

systems, the UEFI firmware is usually stored in the NOR flash chip of the motherboard.

UEFI classes

UEFI machines can have one of the following "classes", which were used to help ease the transition to UEFI. Starting from the 10th Gen Intel Core, Intel no longer provides Legacy Video BIOS for the iGPU (

Intel Graphics Technology Intel Graphics Technology (GT) is the collective name for a series of integrated graphics processors (IGPs) produced by Intel that are manufactured on the same package or die as the central processing unit (CPU). It was first introduced in 20 ...

). Legacy boot with those CPUs requires a Legacy Video BIOS, which can still be provided by a video card. * Class 0: Legacy BIOS * Class 1: UEFI with a CSM interface and no external UEFI interface. The only UEFI interfaces are internal to the firmware. * Class 2: UEFI with CSM and external UEFI interfaces * Class 3: UEFI without a CSM interface and with an external UEFI interface * Class 3+: UEFI class 3 that has Secure Boot enabled

Boot stages

SEC – Security Phase

This is the first stage of the UEFI boot but may have platform specific binary code that precedes it. (e.g., Intel ME,

AMD PSP The AMD Platform Security Processor (PSP), officially known as AMD Secure Technology, is a trusted execution environment subsystem incorporated since about 2013 into AMD microprocessors. According to an AMD developer's guide, the subsystem is "r ...

, CPU microcode). It consists of minimal code written in assembly language for the specific architecture. It initializes a temporary memory (often CPU cache as RAM) and serves as the system's software root of trust with the option of verifying PEI before hand-off.

PEI – Pre-EFI Initialization

The second stage of UEFI boot consists of a dependency-aware dispatcher that loads and runs PEI modules (PEIMs) to handle early hardware initialization tasks such as main memory initialization and firmware recovery operations. Additionally, it is responsible for discovery of the current boot mode and handling many ACPI S0ix/ACPI S3 operations. In the case of ACPI S0ix/ACPI S3 resume, it is responsible for restoring many hardware registers to a pre-sleep state. PEI also uses CPU cache as RAM.

DXE – Driver Execution Environment

This stage consist of C modules and a dependency-aware dispatcher. With main memory now available, CPU, chipset, mainboard and boot devices are initialized in DXE and BDS.

BDS – Boot Device Select

BDS is a part of the DXE. In this stage, boot devices are initialized, UEFI drivers or

Option ROM An Option ROM for the PC platform (i.e. the IBM PC and derived successor computer systems) is a piece of firmware that resides in ROM on an expansion card (or stored along with the main system BIOS), which gets executed to initialize the device an ...

s of PCI devices are executed according to system configuration, and boot options are processed.

TSL – Transient System Load

This is the stage between boot device selection and hand-off to the OS. At this point one may enter UEFI shell, or execute an UEFI application such as the OS boot loader.

RT – Runtime

The UEFI hands off to the

(OS) after is executed. A UEFI compatible OS is now responsible for exiting boot services triggering the firmware to unload all no longer needed code and data, leaving only runtime services code/data, e.g. SMM and

. A typical modern OS will prefer to use its own programs (such as

kernel 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 ot ...

s) to control hardware devices. When a legacy OS is used, CSM will handle this call ensuring the system is compatible with legacy BIOS expectations.

Usage

Implementations

InsydeH20 UEFI security custom Secure Boot screenshot

Intel's implementation of EFI is the ''Intel Platform Innovation Framework'', codenamed ''Tiano''. Tiano runs on Intel's

XScale XScale is a microarchitecture for central processing units initially designed by Intel implementing the ARM architecture (version 5) instruction set. XScale comprises several distinct families: IXP, IXC, IOP, PXA and CE (see more below), with some ...

and

processors, and is proprietary software, although a portion of the code has been released under the

BSD license BSD licenses are a family of permissive free software licenses, imposing minimal restrictions on the use and distribution of covered software. This is in contrast to copyleft licenses, which have share-alike requirements. The original BSD lice ...

Eclipse Public License The Eclipse Public License (EPL) is a free and open source software license most notably used for the Eclipse IDE and other projects by the Eclipse Foundation. It replaces the Common Public License (CPL) and removes certain terms relating to ...

(EPL) as TianoCore EDK II. TianoCore can be used as a payload for

coreboot coreboot, formerly known as LinuxBIOS, is a software project aimed at replacing proprietary firmware (BIOS or UEFI) found in most computers with a lightweight firmware designed to perform only the minimum number of tasks necessary to load and r ...

Phoenix Technologies Phoenix Technologies Ltd is an American company that designs, develops and supports core system software for personal computers and other computing devices. The company's products commonly referred to as BIOS (Basic Input/Output System) or firm ...

' implementation of UEFI is branded as SecureCore Technology (SCT). American Megatrends offers its own UEFI firmware implementation known as Aptio, while Insyde Software offers InsydeH2O. In December 2018,

released an open source version of its TianoCore EDK2-based UEFI implementation from the

Surface A surface, as the term is most generally used, is the outermost or uppermost layer of a physical object or space. It is the portion or region of the object that can first be perceived by an observer using the senses of sight and touch, and is ...

line, Project Mu. An implementation of the UEFI API was introduced into the Universal Boot Loader (

Das U-Boot Das U-Boot (subtitled "the Universal Boot Loader" and often shortened to U-Boot; see ''History'' for more about the name) is an open-source, primary boot loader used in embedded devices to package the instructions to boot the device's operating ...

) in 2017. On the

ARMv8 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 ...

architecture

distributions use the U-Boot UEFI implementation in conjunction with

GNU GRUB GNU GRUB (short for GNU GRand Unified Bootloader, commonly referred to as GRUB) is a boot loader package from the GNU Project. GRUB is the reference implementation of the Free Software Foundation's Multiboot Specification, which provides a u ...

for booting (e.g. SUSE Linux), the same holds true for OpenBSD. For booting from iSCSI iPXE can be used as a UEFI application loaded by U-Boot.

Platforms

's first

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 Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for ser ...

, FreeBSD and HP-UX; OpenVMS added UEFI capability in June 2003. In January 2006,

Apple Inc. Apple Inc. is an American multinational technology company headquartered in Cupertino, California, United States. Apple is the largest technology company by revenue (totaling in 2021) and, as of June 2022, is the world's biggest company ...

shipped its first Intel-based Macintosh computers. These systems used EFI instead of

, which had been used on its previous PowerPC-based systems. 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. During 2005, more than one million Intel systems shipped with Intel's implementation of UEFI. 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 firmware implementation. Since 2005, EFI has also been implemented on non-PC architectures, such as

embedded system An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is ''embedded'' ...

s based on

cores. The EDK (EFI Developer Kit) includes an NT32 target, which allows EFI firmware and EFI applications to run within a

application. But no direct hardware access is allowed by EDK NT32. This means only a subset of EFI application and drivers can be executed by 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, HP EliteBook Notebook PCs. In 2009, IBM shipped System x machines (x3550 M2, x3650 M2, iDataPlex dx360 M2) and

BladeCenter The IBM BladeCenter was IBM's blade server architecture, until it was replaced by Flex System in 2012. The x86 division was later sold to Lenovo in 2014. History Introduced in 2002, based on engineering work started in 1999, the IBM eSe ...

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. In 2011, major vendors (such as

ASRock ASRock Inc. is a Taiwanese manufacturer of motherboards, industrial PCs and home theater PCs (HTPC). Founded by Ted Hsu, it was founded in 2002 and is currently owned by Taiwanese electronics company Pegatron. History ASRock was originally sp ...

, Asus, Gigabyte, and MSI) launched several consumer-oriented motherboards using the Intel 6-series

LGA 1155 LGA 1155, also called Socket H2, is a zero insertion force flip-chip land grid array (LGA) CPU socket designed by Intel for their CPUs based on the Sandy Bridge (2nd Gen) and Ivy Bridge (3rd Gen) microarchitectures. It is the successor of L ...

chipset and AMD 9 Series AM3+ chipsets with UEFI.Asus P67 Motherboard Preview
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 (which allows devices to have power management comparable to

smartphone A smartphone is a portable computer device that combines mobile telephone and computing functions into one unit. They are distinguished from feature phones by their stronger hardware capabilities and extensive mobile operating systems, whic ...

s, with an almost instantaneous return from standby mode), 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. In October 2017, Intel announced that it would remove legacy PC BIOS support from all its products by 2020, in favor of UEFI Class 3.

Operating systems

An operating system that can be booted from a (U)EFI is called a (U)EFI-aware operating system, defined by (U)EFI specification. Here the term ''booted from a (U)EFI'' means directly booting the system using a (U)EFI operating system loader stored on any storage device. The default location for the operating system loader is /BOOT/BOOT.EFI, where short name of the machine type can be IA32, X64, IA64, ARM or AA64. Some operating systems vendors may have their own boot loaders. They may also change the default boot location. * The Linux kernel has been able to use EFI at boot time since early 2000s, using the elilo EFI boot loader or, more recently, EFI versions of

. Grub+Linux also supports booting from a GUID partition table without UEFI. The distribution

added support for UEFI Secure Boot as of version 12.10. Furthermore, the Linux kernel can be compiled with the option to run as an EFI bootloader on its own through the EFI bootstub feature. * HP-UX has used (U)EFI as its boot mechanism on

IA-64 IA-64 (Intel Itanium architecture) is the instruction set architecture (ISA) of the Itanium family of 64-bit Intel microprocessors. The basic ISA specification originated at Hewlett-Packard (HP), and was subsequently implemented by Intel in col ...

systems since 2002. * OpenVMS has used EFI on IA-64 since its initial evaluation release in December 2003, and for production releases since January 2005. The x86-64 port of OpenVMS also uses UEFI to boot the operating system. *

Apple An apple is an edible fruit produced by an apple tree (''Malus domestica''). Apple trees are cultivated worldwide and are the most widely grown species in the genus ''Malus''. The tree originated in Central Asia, where its wild ancestor, ' ...

uses EFI for its line of Intel-based Macs.

Mac OS X v10.4 Mac OS X Tiger (version 10.4) is the 5th major release of macOS, Apple's desktop and server operating system for Mac computers. Tiger was released to the public on April 29, 2005 for US$129.95 as the successor to Mac OS X 10.3 Panther. Som ...

Tiger and

Mac OS X v10.5 Mac OS X Leopard (version 10.5) is the sixth major release of macOS, Apple's desktop and server operating system for Macintosh computers. Leopard was released on October 26, 2007 as the successor of Mac OS X 10.4 Tiger, and is available in t ...

Leopard implement EFI v1.10 in 32-bit mode even on newer 64-bit CPUs, but full support arrived with OS X v10.8 Mountain Lion. * The

versions of

Windows 2000 Windows 2000 is a major release of the Windows NT operating system developed by Microsoft and oriented towards businesses. It was the direct successor to Windows NT 4.0, and was released to manufacturing on December 15, 1999, and was officiall ...

(Advanced Server Limited Edition and Datacenter Server Limited Edition) implemented EFI 1.10 in 2002. MS

Windows Server 2003 Windows Server 2003 is the sixth version of Windows Server operating system produced by Microsoft. It is part of the Windows NT family of operating systems and was released to manufacturing on March 28, 2003 and generally available on April 24, 2 ...

for

, MS

Windows XP 64-bit Edition Windows XP, which is the next version of Windows NT after Windows 2000 and the successor to the consumer-oriented Windows Me, has been released in several editions since its original release in 2001. Windows XP is available in many languages. In ...

and

Advanced Server Limited Edition, all of which are for the Intel

family of processors, implement EFI, a requirement of the platform through the DIG64 specification. * Microsoft introduced UEFI for x64 Windows operating systems with

Windows Vista SP1 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 ...

and

Windows Server 2008 Windows Server 2008 is the fourth release of the Windows Server operating system produced by Microsoft as part of the Windows NT family of the operating systems. It was released to manufacturing on February 4, 2008, and generally to retail on F ...

however only UGA (Universal Graphic Adapter) 1.1 or Legacy BIOS INT 10h is supported; Graphics Output Protocol (GOP) is not supported. Therefore, PCs running 64-bit versions of

Windows Vista SP2 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 ...

Windows 7 Windows 7 is a major release of the Windows NT operating system developed by Microsoft. It was released to manufacturing on July 22, 2009, and became generally available on October 22, 2009. It is the successor to Windows Vista, released nearly ...

and

Windows Server 2008 R2 Windows Server 2008 R2 is the fifth version of the Windows Server operating system produced by Microsoft and released as part of the Windows NT family of operating systems. It was released to manufacturing on July 22, 2009, and became generall ...

are compatible with UEFI Class 2. 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 In computer architecture, 64-bit integers, memory addresses, or other data units are those that are 64 bits wide. Also, 64-bit CPUs and ALUs are those that are based on processor registers, address buses, or data buses of that size. A co ...

finally introduced further optimizations for UEFI systems, including Graphics Output Protocol (GOP) support, a faster startup, 32-bit UEFI support, and Secure Boot support. Microsoft began requiring UEFI to run Windows with

. * On 5 March 2013, the

FreeBSD Foundation The FreeBSD Foundation is a United States-based 501(c)(3) registered non-profit organization dedicated to supporting the FreeBSD project, its development and its community. Funding comes from individual and corporate donations, and is used to sp ...

awarded a grant to a developer seeking to add UEFI support to the FreeBSD kernel and bootloader. The changes were initially stored in a discrete branch of the FreeBSD source code, but were merged into the mainline source on 4 April 2014 (revision 264095); the changes include support in the installer as well. UEFI boot support for amd64 first appeared in FreeBSD 10.1 and for arm64 in FreeBSD 11.0. * Oracle Solaris 11.1 and later support UEFI boot for x86 systems with UEFI firmware version 2.1 or later.

2 is used as the boot loader on x86. * OpenBSD 5.9 introduced UEFI boot support for 64-bit x86 systems using its own custom loader, OpenBSD 6.0 extended that support to include ARMv7.

Use of UEFI with virtualization

HP Integrity Virtual Machines Integrity Virtual Machines is software from Hewlett-Packard that allows multiple virtual machines to run concurrently on any Itanium server running HP-UX, notably the HPE Integrity Servers line. It is part of HP's Virtual Server Environment suite ...

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. *

VMware Fusion VMware Fusion is a software hypervisor developed by VMware for Mac computers. VMware Fusion allows Macs with Intel or the Apple M series of chips to run virtual machines with guest operating systems, such as Microsoft Windows, Linux, or macOS, ...

3 software for Mac OS X can boot Mac OS X Server virtual machines using UEFI. *

VMware Workstation VMware Workstation Pro (known as VMware Workstation until release of VMware Workstation 12 in 2015) is a hosted (Type 2) hypervisor that runs on x64 versions of Windows and Linux operating systemshttps://kb.vmware.com/selfservice/microsites/sear ...

prior to version 11 unofficially supports UEFI, but is manually enabled by editing the .vmx file.

version 11 and above supports UEFI, independently of whether the physical host system is UEFI-based. VMware Workstation 14 (and accordingly, Fusion 10) adds support for the

Secure Boot UEFI (Unified Extensible Firmware Interface) is a set of specifications written by the UEFI Forum. They define the architecture of the platform firmware used for booting and its interface for interaction with the operating system. Examples of ...

feature of UEFI. * The vSphere ESXi 5.0 hypervisor officially support UEFI. Version 6.5 adds support for Secure Boot. *

VirtualBox Oracle VM VirtualBox (formerly Sun VirtualBox, Sun xVM VirtualBox and Innotek VirtualBox) is a type-2 hypervisor for x86 virtualization developed by Oracle Corporation. VirtualBox was originally created by Innotek GmbH, which was acquired by S ...

has implemented UEFI since 3.1, but limited to Unix/Linux operating systems and Windows 8 and later (does not work with Windows Vista x64 and Windows 7 x64). *

QEMU QEMU is a free and open-source emulator (Quick EMUlator). It emulates the machine's central processing unit, processor through dynamic binary translation and provides a set of different hardware and device models for the machine, enabling it t ...

/ KVM can be used with the Open Virtual Machine Firmware (OVMF) provided by TianoCore. * The VMware ESXi version 5 hypervisor, part of

VMware VMware, Inc. is an American cloud computing and virtualization technology company with headquarters in Palo Alto, California. VMware was the first commercially successful company to virtualize the x86 architecture. VMware's desktop software ru ...

vSphere, supports virtualized UEFI as an alternative to the legacy PC BIOS inside a virtual machine. * The second generation of the Microsoft

virtual machine supports virtualized UEFI. * Google Cloud Platform Shielded VMs support virtualized UEFI to enable Secure Boot.

Applications development

''EDK2 Application Development Kit'' (EADK) makes it possible to use

standard C library The C standard library or libc is the standard library for the C (programming language), C programming language, as specified in the ISO C standard.International Organization for Standardization, ISO/International Electrotechnical Commission, IEC ...

functions in UEFI applications. EADK can be freely downloaded from the

's TianoCore UDK / EDK2

SourceForge SourceForge is a web service that offers software consumers a centralized online location to control and manage open-source software projects and research business software. It provides source code repository hosting, bug tracking, mirroring ...

project. As an example, a port of the

Python Python may refer to: Snakes * Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia ** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia * Python (mythology), a mythical serpent Computing * Python (pro ...

interpreter is made available as a UEFI application by using the EADK. The development has moved to GitHub since UDK2015. A minimalistic " hello, world" C program written using EADK looks similar to its usual C counterpart: #include #include #include EFI_STATUS EFIAPI ShellAppMain(IN UINTN Argc, IN CHAR16 **Argv)

Criticism

Numerous digital rights activists have protested against UEFI. Ronald G. Minnich, a co-author of

, and

Cory Doctorow Cory Efram Doctorow (; born July 17, 1971) is a Canadian-British blogger, journalist, and science fiction author who served as co-editor of the blog '' Boing Boing''. He is an activist in favour of liberalising copyright laws and a proponent of ...

, a digital rights activist, have criticized UEFI as an attempt to remove the ability of the user to truly control the computer. It does not solve the BIOS's long-standing problems of requiring two different drivers—one for the firmware and one for the operating system—for most hardware. Open-source project TianoCore also provides UEFI interfaces. TianoCore lacks the specialized drivers that initialize chipset functions, which are instead provided by

, of which TianoCore is one of many payload options. The development of coreboot requires cooperation from chipset manufacturers to provide the specifications needed to develop initialization drivers.

Secure Boot

In 2011, Microsoft announced that computers certified to run its

operating system had to ship with Microsoft's public key enrolled and Secure Boot enabled. Following the announcement, the company was accused by critics and free software/open source advocates (including the

Free Software Foundation The Free Software Foundation (FSF) is a 501(c)(3) non-profit organization founded by Richard Stallman on October 4, 1985, to support the free software movement, with the organization's preference for software being distributed under copyleft (" ...

) of trying to use the Secure Boot functionality of UEFI to hinder or outright prevent the installation of alternative operating systems such as

. Microsoft denied that the Secure Boot requirement was intended to serve as a form of lock-in, and clarified its requirements by stating that x86-based systems certified for Windows 8 must allow Secure Boot to enter custom mode or be disabled, but not on systems using the ARM architecture.

allows

OEM 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 to decide whether or not Secure Boot can be managed by users of their x86 systems. 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 The GNU General Public License (GNU GPL or simply GPL) is a series of widely used free software licenses that guarantee end users the four freedoms to run, study, share, and modify the software. The license was the first copyleft for general u ...

may prevent the use of the GNU GRand Unified Bootloader without a distribution's developer disclosing the private key (however, the

has since clarified its position, assuring that the responsibility to make keys available was held by the hardware manufacturer), and that it would also be difficult for advanced users to build custom

kernel Kernel may refer to: Computing * Kernel (operating system), the central component of most operating systems * Kernel (image processing), a matrix used for image convolution * Compute kernel, in GPGPU programming * Kernel method, in machine learn ...

s that could function with Secure Boot enabled without self-signing them. 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. Several major Linux distributions have developed different implementations for Secure Boot. Garrett himself developed a minimal bootloader known as a shim, which is a precompiled, signed bootloader that allows the user to individually trust keys provided by Linux distributions. Ubuntu 12.10 uses an older version of shim pre-configured for use with

Canonical The adjective canonical is applied in many contexts to mean "according to the canon" the standard, rule or primary source that is accepted as authoritative for the body of knowledge or literature in that context. In mathematics, "canonical examp ...

's own key that verifies only the bootloader and allows unsigned kernels to be loaded; developers believed that the practice of signing only the bootloader is more feasible, since a trusted kernel is effective at securing only the

user space A modern computer operating system usually segregates virtual memory into user space and kernel space. Primarily, this separation serves to provide memory protection and hardware protection from malicious or errant software behaviour. Kernel ...

, and not the pre-boot state for which Secure Boot is designed to add protection. That also allows users to build their own kernels and use custom

kernel module In computing, a loadable kernel module (LKM) is an object file that contains code to extend the running kernel, or so-called ''base kernel'', of an operating system. LKMs are typically used to add support for new hardware (as device drivers) and/o ...

s as well, without the need to reconfigure the system. 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 wellrequiring 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. It has been disputed whether the operating system kernel and its modules must be signed as well; while the UEFI specifications do not require it, Microsoft has asserted that their contractual requirements do, and that it reserves the right to revoke any certificates used to sign code that can be used to compromise the security of the system. In Windows, only WHQL kernel driver is allowed if Secure Boot enabled. 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 In cryptography, X.509 is an International Telecommunication Union (ITU) standard defining the format of public key certificates. X.509 certificates are used in many Internet protocols, including TLS/SSL, which is the basis for HTTPS, the secu ...

key embedded in PE files signed by Microsoft. However, the proposal was criticized by Linux creator

Linus Torvalds Linus Benedict Torvalds ( , ; born 28 December 1969) is a Finnish software engineer who is the creator and, historically, the lead developer of the Linux kernel, used by Linux distributions and other operating systems such as Android. He also ...

, who attacked Red Hat for supporting Microsoft's control over the Secure Boot infrastructure. On 26 March 2013, the

Spanish Spanish might refer to: * Items from or related to Spain: **Spaniards are a nation and ethnic group indigenous to Spain **Spanish language, spoken in Spain and many Latin American countries **Spanish cuisine Other places * Spanish, Ontario, Can ...

free software development group Hispalinux filed a formal complaint with the

European Commission The European Commission (EC) is the executive of the European Union (EU). It operates as a cabinet government, with 27 members of the Commission (informally known as "Commissioners") headed by a President. It includes an administrative body ...

, contending that Microsoft's Secure Boot requirements on OEM systems were "obstructive" and

anti-competitive Anti-competitive practices are business or government practices that prevent or reduce competition in a market. Antitrust laws differ among state and federal laws to ensure businesses do not engage in competitive practices that harm other, usuall ...

. At the Black Hat conference in August 2013, a group of security researchers presented a series of exploits in specific vendor implementations of UEFI that could be used to exploit Secure Boot. In August 2016 it was reported that two security researchers had found the "golden key" security key Microsoft uses in signing operating systems. Technically, no key was exposed, however, an exploitable binary signed by the key was. This allows any software to run as though it was genuinely signed by Microsoft and exposes the possibility of

rootkit A rootkit is a collection of computer software, typically malicious, designed to enable access to a computer or an area of its software that is not otherwise allowed (for example, to an unauthorized user) and often masks its existence or the exis ...

and bootkit attacks. This also makes patching the fault impossible, since any patch can be replaced (downgraded) by the (signed) exploitable binary. Microsoft responded in a statement that the vulnerability only exists in ARM architecture and

Windows RT Windows RT is a mobile operating system developed by Microsoft. It is a version of Windows 8 or Windows 8.1 built for the 32-bit ARM architecture (ARMv7). First unveiled in January 2011 at Consumer Electronics Show, the Windows RT 8 operat ...

devices, and has released two patches; however, the patches do not (and cannot) remove the vulnerability, which would require key replacements in end user firmware to fix. Many

Linux distributions A Linux distribution (often abbreviated as distro) is an operating system made from a software collection that includes the Linux kernel and, often, a package management system. Linux users usually obtain their operating system by downloading one ...

support UEFI Secure Boot now, such as

RHEL Red Hat Enterprise Linux (RHEL) is a commercial open-source Linux distribution developed by Red Hat for the commercial market. Red Hat Enterprise Linux is released in server versions for x86-64, Power ISA, ARM64, and IBM Z and a desktop versio ...

(RHEL 7 and later), CentOS (CentOS 7 and later),

, Fedora, Debian (Debian 10 and later),

OpenSUSE openSUSE () is a free and open source RPM-based Linux distribution developed by the openSUSE project. The initial release of the community project was a beta version of SUSE Linux 10.0. Additionally the project creates a variety of tools, s ...

, SUSE Linux.

Firmware problems

The increased prominence of UEFI firmware in devices has also led to a number of technical problems blamed on their respective implementations. Following the release of Windows 8 in late 2012, it was discovered that certain Lenovo computer models with Secure Boot had firmware that was hardcoded to allow only executables named " Windows Boot Manager" or "

Red Hat Enterprise Linux Red Hat Enterprise Linux (RHEL) is a commercial open-source Linux distribution developed by Red Hat for the commercial market. Red Hat Enterprise Linux is released in server versions for x86-64, Power ISA, ARM64, and IBM Z and a desktop ...

" to load, regardless of any other setting. Other problems were encountered by several

Toshiba , commonly known as Toshiba and stylized as TOSHIBA, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan. Its diversified products and services include power, industrial and social infrastructure systems, ...

laptop models with Secure Boot that were missing certain certificates required for its proper operation. In January 2013, a bug surrounding the UEFI implementation on some

Samsung The Samsung Group (or simply Samsung) ( ko, 삼성 ) is a South Korean multinational manufacturing conglomerate headquartered in Samsung Town, Seoul, South Korea. It comprises numerous affiliated businesses, most of them united under the ...

laptops was publicized, which caused them to be

bricked A brick (or bricked device) is a mobile device, game console, router or other consumer electronic device that is no longer functional due to corrupted firmware, a hardware problem, or other damage. The term analogizes the device to a brick's ...

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), Matthew Garrett discovered that the bug was actually triggered by storing too many UEFI variables to memory, and that the bug could also be triggered under Windows under certain conditions. In conclusion, he determined that the offending kernel module had caused kernel message dumps to be written to the firmware, thus triggering the bug.

Notes

References

External links

*
UEFI Specifications

Intel-sponsored open-source EFI Framework initiative

Microsoft UEFI Support and Requirements for Windows Operating Systems

How Windows 8 Hybrid Shutdown / Fast Boot feature works

Securing the Windows 10 Boot Process

LoJax: First UEFI rootkit found in the wild, courtesy of the Sednit group
{{Firmware and booting Articles with example C code

History

Advantages

Compatibility

Processor compatibility

Disk device compatibility

Linux

Microsoft Windows

Features

Services

Applications

Protocols

Device drivers

Graphics features

EFI system partition

Booting

UEFI booting

CSM booting

Network booting

Secure Boot

UEFI shell

Commands

Extensions

UEFI Capsule

Hardware

UEFI classes

Boot stages

SEC – Security Phase

PEI – Pre-EFI Initialization

DXE – Driver Execution Environment

BDS – Boot Device Select

TSL – Transient System Load

RT – Runtime

Usage

Implementations

Platforms

Operating systems

Use of UEFI with virtualization

Applications development

Criticism

Secure Boot

Firmware problems

See also

Notes

References

Further reading

External links