Extensible Firmware Interface

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Extensible Firmware Interface

The Extensible Firmware Interface (EFI) is a new type of interface between a computer's firmware, hardware, and the operating system. Computers with the Intel Itanium family of processors use EFI to start and load Windows XP 64-bit Edition (Itanium) and the Itanium-based versions of Windows Server 2003. EFI serves the same purpose for Itanium-based computers as the BIOS found in x86-based computers, but has expanded capabilities that provide:

  • A consistent way to start any compatible operating system.

  • An easy way to add EFI drivers for new bootable devices without the need to update the computer's firmware.

With EFI, an Itanium-based computer can have hard disks with the GUID partition table (GPT) partitioning style and the master boot record (MBR) partitioning style. However, in order for EFI to start and load Windows XP 64-bit Edition (Itanium) or the Itanium-based versions of Windows Server 2003, the operating system must reside on a GPT disk. In a mixed GPT and MBR disk environment, MBR disks are used solely for data storage.

Every Itanium-based computer must have one GPT disk with an EFI System partition. Like the system volume on x86-based computers, the EFI System partition contains the operating system loader, drivers, and other files required to start Windows XP 64-bit Edition (Itanium) or the Itanium-based versions of Windows Server 2003. In computers that contain only one GPT disk, the EFI System partition is the first partition on the disk.

The EFI System partition is formatted as FAT and contains an EFI directory at the root directory. Windows XP 64-bit Edition (Itanium) and the Itanium-based versions of Windows Server 2003 create a subdirectory called \EFI\Microsoft\Winnt50. Other subdirectories created by operating system vendors, original equipment manufacturers (OEMs), BIOS vendors, and other tools vendors might also be present. The \Msutil subdirectory contains utilities that are used by the EFI firmware and operating system. These utilities as well as the contents of the EFI System partition, can be accessed using the EFI firmware's Boot Manager. The contents of the \EFI\Microsoft\Winnt50 subdirectory should be accessed only by authorized support technicians.


  • When you update the software in the EFI System partition, apply only authentic updates provided by your OEM or other trusted source. Applying other updates might introduce a virus, Trojan horse, or other malicious code that might make your computer inoperable.

For more information about GPT, see GUID partition table, or Disk Management at the Microsoft Windows Resource Kits Web site. For more information about EFI, see the manufacturer's documentation that came with your computer, or the Intel Web site.

Web addresses can change, so you might be unable to connect to the Web site or sites mentioned here.