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Home > BIOS Systems > BIOS Index Pages > Unified Extensible Firmware Interface (UEFI)
These pages will provide an understanding of what the Basic Input Output System is and how it works in a personal computer.
On This Page:
Introduction To The Basic Input Output System The Power On Self Test (POST Routine) CMOS Settings What's On Board?
How Operating Systems Work BIOS Upgrades Page BIOS Beep Codes
Video Demonstration
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Unified Extensible Firmware Interface (UEFI)

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.

UEFI was initially created by Intel as just EFI, and the EFI 1.01 specification was originally released in 2000. EFI was an outgrowth of the Intel Boot Initiative (IBI) program that began in 1998. You can see the original Intel EFI specifications Intel updated the EFI specification through 2003, and in 2005 it contributed the specification to the Unified EFI (UEFI) Forum, a nonprofit organization formed to promote and manage the UEFI standard. The UEFI Forum is led by representatives from 11 major companies including AMD, AMI, Apple, Dell, HP, IBM, Insyde, Intel, Lenovo, Microsoft, and Phoenix Technologies. You can download the latest version of the UEFI specification from the Unified EFI Forum at

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UEFI and BIOS Limitations
The original PC BIOS was released in 1981 as part of the first IBM PC. The BIOS at that time consisted of only 8KiB of 16-bit code running in what is known as the “real mode” of Intel x86 (or compatible) processors, which limits memory addressability to just 1 mebibyte (MiB). Since then the BIOS has grown somewhat in size, but it still must run as 16-bit x86 code and fit into a meager 128-kibibyte (KiB) window of memory. In addition, add-on cards that need to be functional at boot time (such as video and RAID cards) must contain a ROM of limited size with 16-bit x86 code. This is why the POST, BIOS Setup, RAID Setup, and other preboot code you normally interact with usually runs in text mode and must be navigated using only the keyboard. All of this looks and feels crude in comparison to the high-res graphical mouse-driven interfaces found in modern OSs.

Another issue is that because 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 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. That started to be a problem a few years ago for drive arrays as it became possible to combine multiple drives in an array potentially larger than that. But the 2.2-terabyte (TB) limitation started to become a real 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.

Note that 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. UEFI does not specifically define POST or BIOS Setup functionality, but it does allow enhanced versions of those preboot functions to be created along with additional preboot applications. Because of this, a motherboard with UEFI firmware may look exactly like a standard BIOS Setup from the user’s perspective, with the only difference being a UEFI Boot option somewhere in the BIOS Setup. Enabling this option allows the UEFI boot loader to replace the BIOS boot loader, thus enabling the ability to boot from a drive 2.2TB or greater in capacity.

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. You may find the following features in the UEFI firmware of motherboards:

  • Diagnostics— Tests that are much more comprehensive than the standard POST, including tests for memory and hard disks.

  • Live Update— A tool that can check the motherboard manufacturer website to see if a firmware/BIOS update is available and then automatically download and install the proper update.

  • HDD Backup— A tool that backs up or clones a hard disk drive (HDD) from outside the OS. This allows for complete backups and easy HDD upgrades.

  • Overclocking— Enhanced intelligent overclocking utilities that virtually automate the process and can easily undo the settings if they fail.

  • Boot Logo— The ability to easily update or change the graphical boot logo seen when you turn the system on.

    As time goes on, I expect to see applications for web browsing, email, and other useful capabilities that will almost make a PC with no OS useful all by itself.
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UEFI Support
Even though EFI has been around since 2000, full support in the PC environment was virtually nonexistent 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. MSI shipped a few specific motherboards in 2008 that had optional/experimental UEFI firmware (called Click BIOS), but few people really used it because such support really wasn’t necessary unless you had a boot drive equal to or greater than 2.2TB in capacity, and drives bigger than that didn’t come along until late 2010.

Coinciding with the introduction of the Intel 6x Series chipsets in 2011, many motherboards began including UEFI firmware as a standard feature. This was just in time to support the new 3TB drives that were beginning to appear on the market, as well as the growing move to 64-bit OSs such as Windows 7 x64.

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), whereas most newer 32-bit and 64-bit Linux versions support UEFI as well. Neither Windows XP nor 32-bit versions of 7/Vista or later support UEFI, which means they cannot boot from 2.2TB or larger (or GPT-formatted) drives.


That although Windows XP and 32-bit versions of 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 7/Vista or later, but with 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.

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