Unified Extensible Firmware Interface
Unified Extensible Firmware Interface (UEFI) defines an interface between an OS and computer hardware and firmware.
The interface consists of tables containing system specific information as well as boot and runtime services that are available to the OS loader and to the OS after it is loaded. These tables and services provide an industry standard environment for running preboot applications, booting an OS, and providing drivers for devices that need to be active during boot.
Because UEFI is based on the EFI specification originated by Intel, it is common to use the terms EFI and UEFI interchangeably.
Starting in 2011, virtually all Intel and AMD based motherboards included a BIOS with UEFI support.
Operating systems with UEFI support include the 64-bit versions of Windows Vista SP1 and later, including Windows 7 and 8.1, Windows 10, and most recent Linux distributions.
The BIOS is so limited in size and functionality (it exists mainly to test/initialize the hardware, and then find and load an OS), a PC without an OS installed (or with one that has been corrupted) has little or no functionality besides entering the BIOS Setup. In other words, you cannot run extensive diagnostics, go online with a web browser, send or receive email, or read or write files.
But perhaps the most important limitation with the PC BIOS is that it was only designed to boot from Master Boot Record (MBR) formatted disks. Because the MBR can only define up to four primary partitions on a disk of just under 2.2TB in maximum size, BIOS based systems cannot boot from a drive that is 2.2TB or greater in capacity.
The 2.2 terabyte (TB) limitation started to become an bigger problem in 2010 when the first 3TB drives were released to the market. The solution to the limitations caused by the MBR format is a more flexible partitioning scheme called the GUID (globally unique identifier) Partition Table (GPT). The GPT is a newer boot sector and format design that supports up to 128 partitions on a disk of up to 9.4 zettabytes (ZB, where 1 zettabyte = 1 billion terabytes or 1 sextillion bytes).
While the GPT breaks the 2.2TB barrier, GPT formatted drives can only be used as data drives because the PC BIOS supports only MBR and cannot boot from a GPT drive. The solution to the GPT boot problem is UEFI, which replaces the BIOS boot loader with a UEFI loader that supports both MBR and GPT formatted drives, thus breaking the 2.2TB limitation.
UEFI does not completely replace the BIOS, at least not yet. At a minimum, it runs on top of the BIOS as a replacement for the boot loader and runtime services.
Although some UEFI firmware implementations will look like a standard BIOS, most will be notable by their enhanced mouse-driven graphical interfaces combined with additional features not found in older PC BIOS. UEFI firmware may also include applications for web browsing, email, and other useful capabilities that can almost make a PC with no OS useful all by itself.
Even though EFI has been around since 2000, full support in the PC environment was virtually non existent until 2008, and it wasn’t really widespread until 2011. Intel began shipping some of the first motherboards with UEFI boot capability in 2006; however, OS support for UEFI wasn’t available until Vista SP1 x64 was released in 2008.
If you want to boot from a GPT formatted disk (2.2TB and larger disks must be formatted as GPT), you need two things: a motherboard with UEFI firmware (at a minimum a UEFI Boot option that is enabled) and an OS with UEFI support. Windows UEFI support is provided only in the 64-bit versions of Vista SP1 and later (including Windows 7/8.1/10), whereas most newer 32-bit and 64-bit Linux versions support UEFI as well. Neither Windows XP nor 32-bit versions of Vista or later support UEFI, which means they cannot boot from 2.2TB or larger (or GPT formatted) drives. Although Windows XP and 32-bit versions of Windows 10/8.1/7/Vista cannot boot from GPT disks, they can use GPT formatted drives as data disks. This support is provided natively in XP x64 and 10/8.1/7/Vista x64. For Windows XP x86 (32-bit), you need a third party utility such as the Paragon GPT Loader or the ASUS Disk Unlocker to enable GPT support.
Compatibility Support Module (CSM)
To ensure backward compatibility, most UEFI firmware implementations on PC class machines also 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.
The UEFI 2.2 specification adds a protocol known as Secure Boot, which can secure the boot process by preventing the loading of drivers or OS loaders that are not signed with an acceptable digital signature. Secure boot is supported by Windows 8 and 8.1, Windows 10, Windows Server 2012 and 2012 R2, and a number of Linux distributions including Fedora (since version 18), openSUSE (since version 12.3), and Ubuntu (since version 12.04.2). As of June 2015, FreeBSD support is in a planning stage.
Secure Boot must be disable to install 64-bit versions of Vista SP1 and 64-bit Windows 7 OS.
Click the Secure Boot link to watch video clip from Microsoft explanation about UEFI firmware Secure Boot feature .