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Definition


WHAT IS THE UNIFIED EXTENSIBLE FIRMWARE INTERFACE (UEFI)?


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By
 * Rahul Awati
 * Garry Kranz

Unified Extensible Firmware Interface (UEFI) is a specification for a software
program that connects a computer's firmware to its operating system (OS). UEFI
is expected to eventually replace basic input/output system (BIOS) but is
compatible with it. The specification is most often pronounced by naming the
letters U-E-F-I.

UEFI is a standard specification that defines how a computer's OS and firmware
will interface. Through data tables, and boot and runtime service calls, UEFI
makes a standard environment available to boot a system's OS, control the
booting process, run pre-boot applications, and pass control of the system to
the OS.

UEFI functions via special firmware installed on a computer's motherboard.
Specifically, it stores information related to system initialization and startup
in an EFI file. The file is stored on the hard disk's EFI System Partition
(ESP), a special and independent partition that contains the UEFI bootloader (a
set of applications and drivers) and enables the computer to boot the OS. UEFI
is programmable, enabling OEM developers to add applications, drivers and UEFI
to function as a lightweight OS.


WHAT DOES UEFI DO?

UEFI defines a new method by which OSes and platform firmware communicate,
providing a lightweight BIOS alternative that uses only the information needed
to launch the OS boot process. In addition, UEFI provides enhanced computer
security features and supports most existing BIOS systems with backward
compatibility.

The interface contains platform-related data tables and boot and runtime service
calls used by the OS loader. Together, this information defines the required
interfaces and structures that must be implemented for firmware and hardware
devices to support UEFI. UEFI also checks to see which hardware components are
attached, wakes up the components and hands them over to the OS.


UEFI'S EVOLUTION FROM EFI

In the late 1990s, Intel started the Intel Boot Initiative in recognition of the
many limitations of BIOS firmware. The initiative later became the Extensible
Firmware Interface or EFI. The National Institute of Standards and Technology
(NIST) defines EFI as "a specification for the interface between the operating
system and the platform firmware."

Intel developed EFI as an outgrowth of its 64-bit Itanium server architecture,
which the company codeveloped with computer maker Hewlett-Packard (HP). The EFI
framework was developed in the C programming language. It included numerous
commands that were like common disk operating system (DOS) and Linux commands to
list a directory's contents, change the directory, move files or directories to
a different location, and display help information, for example. Also, like DOS
and Linux, EFI could run programs listed in the environmental path from the
root.

The industry perceived EFI to address the memory and processing limitations of
BIOS in X86 server architectures. Those limitations included 16-bit computing
mode, bounded system memory, no independence of in-use CPU, very basic user
interface for administrators and tedious assembly language programming. Also,
this specification was meant to reduce the dependence of the OS on firmware
implementation details and to provide an alternative to legacy BIOS,
particularly around ease of use and providing ubiquity across platforms.

A subsequent avatar of EFI, EFI 1.10 also simplified device drivers, reduced
driver footprint and enabled deterministic driver selection by the platform.
Many computing device vendors released new devices with the 32-bit EFI 1.10
implemented. One example is Apple, which released the MacBook Pro with 32-bit
EFI 1.10 in 2006. Intel later developed the EFI Development Kit v2. This version
implemented Intel's updated UEFI specification, which added 64-bit support.

EFI remains the property of Intel, and the company still licenses the
specification. However, the company ceased sole development of EFI following the
release of version 1.10. By then, Intel had also phased out its Itanium
processor line, following product delays and other hiccups. Intel contributed
EFI 1.10 to the UEFI Forum, an alliance of chipset, hardware, system, firmware
and OS vendors. Some of the companies in this industry consortium include the
following:

 * Microsoft.
 * Lenovo.
 * HP.
 * IBM.
 * AMD.
 * Apple.
 * Dell.
 * AMI.

The UEFI Forum developed UEFI, which is based on EFI 1.10, albeit with numerous
corrections and changes. In general, the UEFI Forum owns the UEFI specification
standard and manages its ongoing development. The group includes contributors
that help to build the specification, adopters that use EFI implementations and
work groups that handle technical activities, such as designing and developing
the test suite and delivering the formal specification.


CURRENT VERSION OF UEFI

The emergence of UEFI parallels the increased drive densities used for modern
application workloads. The latest version is UEFI 2.10, released in August 2022.
Some of the code and protocol names in UEFI 2.10 retain the EFI designation.

UEFI 2.10 is a pure interface specification, defining the interfaces and
structures that platform firmware must implement, and that the OS may use in
booting. Also, UEFI 2.10 provides a standardized way for the OS and platform
firmware to communicate the information necessary for the OS boot process.


UEFI 2.10 FEATURES

Users can download the UEFI specification from the UEFI Forum website. There is
no charge for using the specification.

UEFI 2.10, released in August 2022 (Errata A released in August 2024), features
several enhancements, including the following:

 * Extension of platform firmware by loading UEFI driver and UEFI application
   images.
 * Consolidation of boot menus from the OS loader and platform firmware into a
   single platform firmware menu.
 * Option to include legacy boot options, such as booting from the A: or C:
   drive in the menu.
 * Booting from media containing a UEFI OS loader or a UEFI-defined system
   partition.
 * Boot manager to load applications or UEFI drivers from any file on a
   UEFI-defined file system or by using a UEFI-defined image loading service.
 * Common boot environment abstraction for use by UEFI drivers, UEFI
   applications and UEFI OS loaders.


UEFI VS. BIOS

Turning on a computer kick-starts a chain of events that occurs before the OS is
loaded. Firmware rouses the computer's subsystem to execute a series of tests
and locates the boot loader, which, in turn, starts the OS kernel. This entire
process can be done by either BIOS or UEFI.

In general, BIOS is considered a vestige from earlier computing, whereas UEFI is
regarded as the wave of the future. For ease of understanding, some information
technology users refer to the processes collectively as UEFI BIOS, despite their
substantial differences.

BIOS and UEFI both use low-level software to manage startup functions prior to
booting an OS, albeit using different techniques. Also, like BIOS, UEFI is
installed at the time of manufacturing and is the first program that runs when
booting a computer.


BIOS LIMITATIONS

BIOS has been in use since the advent of DOS computers in the mid-1970s. BIOS
resides on a chip on the machine's motherboard and initializes the central
processing unit, random access memory, Peripheral Component Interconnect Express
(PCIe) cards and network devices. BIOS runs a power-on self-test (POST)
diagnostic sequence. POST ensures that hardware is configured properly, and all
components are functioning as intended.

To accomplish its task, BIOS consults the Master Boot Record to locate the OS
and launch the boot loader. MBR uses 32-bit values to describe the offset and
length of a partition, thus limiting BIOS systems to 2 terabyte (TB) drives and
no more than four partitions.

Also, BIOS runs only in 16-bit processor mode, which limits the number of
software commands the firmware can execute at any one time. BIOS allots 1
megabyte of memory in which tasks can be executed. Interfaces and devices thus
are initialized sequentially, which can contribute to a sluggish startup.


HOW UEFI OVERCOMES BIOS LIMITATIONS

The UEFI specification addresses several limitations of BIOS, including
restrictions on hard disk partition size and the amount of time BIOS takes to
perform its tasks.

UEFI behaves like a miniaturized OS that sits between firmware and the OS. It
performs the same diagnostics as BIOS at startup but offers more flexibility.
The OS boots directly in UEFI. This eliminates the need to repeatedly press
toggle keys, as is required to boot BIOS.

UEFI stores initialization data in an EFI file partition in non-volatile flash
memory, rather than in the firmware. UEFI also can load during boot from a drive
or a network share. UEFI also deploys a more flexible partitioning scheme than
MBR, known as a Globally Unique Identifier Partition Table, or GPT (also created
by Intel as part of EFI). GPT uses 64-bit values to enable the creation of up to
128 partitions and is required for systems launched from 2 TB drives and larger.
The EFI partition uses the file allocation table (FAT), including FAT16, FAT32
or virtual FAT.


UEFI PHASING OUT BIOS

Originally developed collaboratively by HP, Intel, Microsoft, Phoenix
Technologies and Toshiba, the Advanced Configuration and Power Interface (ACPI)
is an open standard for BIOS that governs how much power is delivered to each
peripheral device. In 2013, custody of the ACPI was transferred to the UEFI
Forum.

Motherboard -- or system-specific utilities for accessing UEFI/BIOS -- differ
from vendor to vendor and how they appear, but vary little in terms of
functionality.


Most modern computer systems are equipped to support traditional BIOS, as well
as UEFI. In the future, computer manufacturers may continue to support BIOS, but
the transition to UEFI has already been underway.

Intel is phasing out BIOS support in newer PCs. Most new desktop PCs, laptops
and some tablets bundle UEFI firmware that runs in compatibility support mode
for older 32-bit Windows.

As computer makers move away from BIOS, they typically integrate UEFI firmware
that runs with Compatibility Support Module (CSM) in modern devices. Although
not intended as a long-term solution, CSM enables UEFI-based machines to launch
in legacy BIOS mode to work with older Windows versions and other OSes. However,
users may want to upgrade to the latest version of the OS to realize the value
of UEFI.


ADVANTAGES OF UEFI

UEFI provides many significant enhancements over BIOS, such as the following:

 * Boot mode. Microsoft Windows users can run 32-bit UEFI or 64-bit UEFI,
   whereas BIOS can only run in 16-bit mode. It's important to note that experts
   recommend that the OS bit mode and the firmware bit mode should be the same
   to avoid communication issues during runtime.
 * Boot speeds. UEFI enables faster booting and resume times compared to BIOS.
 * Drives. UEFI supports boot drives of 2.2 TB and higher capacities, including
   drives with theoretical capacity of 9.4 zettabytes. That far exceeds the
   maximum drive capacities available with BIOS. UEFI also supports drives with
   more than four partitions.
 * Drivers. UEFI supports discrete drivers, whereas BIOS drive support is stored
   in read-only memory, which necessitates tuning it for compatibility when
   drives are swapped out or changes are made. Simply put, it can be difficult
   to update UEFI BIOS firmware.
 * Graphical user interface (GUI). UEFI enables navigation via a mouse and GUI.
   It also enables new modules to be added more easily, including device drivers
   for motherboard hardware and attached peripheral devices. In contrast, BIOS
   navigation is harder since it can only be done via a keyboard.
 * Multiple OS support. Whereas BIOS allows a single boot loader, UEFI lets
   users install loaders for Debian-based Ubuntu and other Linux variants, along
   with Windows OS loaders, in the same EFI system partition.
 * Programming. UEFI firmware is written predominantly in C language, which
   enables users to add or remove functions with less programming than BIOS,
   which is written in an assembler language, sometimes in combination with C.
 * Security. Secure Boot is a UEFI protocol for Windows 10 or later Windows
   versions. Secure Boot makes a system's firmware the root of trust to verify
   device and system integrity, preventing hackers from installing rootkits in
   the time between bootup and handoff to the OS. Secure Boot also enables an
   authorized user to configure networks and troubleshoot issues remotely,
   something a BIOS administrator must be physically present to do.
 * Multicast deployment. Device manufacturers can broadcast a PC image to
   multiple PCs without overwhelming the network or image server.


UEFI DISADVANTAGES

Security is one of the biggest concerns with UEFI.

Software is always a target for threat actors, and UEFI is no exception. UEFI
implementation flaws may allow threat actors to gain and maintain access to a
compromised system, then take advantage of such persistence to install malware.
This malware may cause a compromised component in the motherboard or a corrupted
PCI to persist, even if the physical hard drive is replaced, creating serious
security issues that can only be eliminated by completely replacing the device.
By targeting UEFI and its various components (platform initializers, drivers,
bootloaders, etc.), cyberattackers may also be able to evade defensive actions
such as turning a device on or off or prevent a reinstalled OS from being
treated as a clean device (another standard defensive practice).

A well-known malware targeting UEFI is known as BlackLotus. In April 2023,
Microsoft released a guidance document to help organizations assess whether they
have been compromised by the exploitation of CVE-2022-21894 (BatonDrop) via the
BlackLotus UEFI bootkit.

This bootkit writes malicious bootloader files to the ESP and can run at
computer startup (before the OS even loads). Consequently, it can interfere with
and even deactivate built-in OS security mechanisms. Successful exploitation of
the CVE via BlackLotus could allow attackers to take control of an affected
system and to maintain persistence in the system.

Real-world attacks on UEFI have been detected recently. One such attack, dubbed
TrickBot, surfaced in December 2020. TrickBot malware works by attempting to spy
on device firmware, which could permit malicious actors to subvert the boot
process and gain access to the OS.

The TrickBot episode came on the heels of 2018 findings by ESET Research, a
Slovak outlet for the information security community, which claimed to have
discovered a rootkit in the wild that potentially enabled hackers to surveil
UEFI firmware and install malicious code.



Aside from security issues, organizations switching to UEFI may incur a cost
related to booting from flash. While flash booting is faster than booting from
hard disk drives, older systems may require a retrofit, namely a larger flash
die on the motherboard to switch to UEFI booting.

Another potential drawback of UEFI is its reliance on the FAT file format, which
is maintained by the OS. Larger drive partitions can add too much system
overhead, thus defeating some of the performance advantages of UEFI. In this
scenario, BIOS can be a more useful option, especially for a computer running an
older OS version and smaller boot disks.


HOW TO DETERMINE AND ACCESS UEFI/BIOS SETTINGS

To determine whether a computer boots from BIOS or UEFI, press the Windows and R
keys on the keyboard to launch the Run configuration box. Type MSInfo32 in the
dialog box and hit the Enter key. A system summary screen appears. Look for the
entry entitled BIOS Mode, and make note of the corresponding value. If the value
says Legacy, the system has BIOS. Otherwise, UEFI will appear in the value
field.

Windows users can access UEFI via the PC Settings option in the search bar. The
path is PC Settings > Update & Security > Recovery > Advanced Startup, and
select the Restart Now option. From the menu, select Troubleshoot > Advanced
Options > UEFI Firmware Settings, and restart again.

Linux machines with UEFI installed will show it in the sys/firmware/efi
directory. This will also be reflected in the Linux Grand Unified Bootloader
boot manager as grub-efi, rather than grub-pc for BIOS.


COREBOOT AND UEFI

Like UEFI, open source coreboot is another option vying to replace legacy BIOS.
Formerly known as LinuxBIOS, coreboot is a flexible firmware for modern
computers and embedded systems. The firmware performs a minimal amount of
hardware initialization before executing a payload (Linux kernel, FILO, SeaBIOS,
etc.). Coreboot can be faster than BIOS and UEFI, with additional benefits of
high performance, stability, enhanced security and easier maintenance. Since
coreboot is open source, any improvements are shared with all users.

The coreboot project kicked off in 1999. Since then, many people have
contributed to its code and it remains a community-based development project,
with numerous supporters. Some of its key supporters include Libreboot
(blob-free coreboot distribution), Skulls (simple coreboot images for IBM
ThinkPad laptops) and MrChromebox (custom coreboot firmware and utilities for
Chromebook devices).

Windows 11, now 2 years old, recently added Microsoft Copilot.

Explore the differences between Windows 11 vs. Windows 10 and how to extend
Windows 10 support as it approaches end of life. Find out if it's worthwhile to
upgrade to Windows 11 right now and how to plan a Windows 11 upgrade project.

This was last updated in November 2024

CONTINUE READING ABOUT WHAT IS THE UNIFIED EXTENSIBLE FIRMWARE INTERFACE (UEFI)?

 * How to fix Windows 11 when Secure Boot and TPM don't work

 * Steps to identify and fix Windows 11 performance issues

 * Common Linux vulnerabilities admins need to detect and fix

 * Major server hardware components you should know

 * Is Windows 11 finally ready for take-off?



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