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. 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 the Windows XP 64-Bit Edition, 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 the Windows XP 64-Bit Edition. 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 creates a subdirectory called \Microsoft\Winnt50. Other subdirectories created by operating system vendors, 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 and the contents of the EFI System partition can be accessed using the EFI firmware's Boot Manager. The contents of the \Microsoft\Winnt50 subdirectory should be accessed only by authorized support technicians.

For more information about GPT, see GUID partition table For more information about EFI, see the manufacturer's documentation that came with your computer, or the Intel Web site.

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