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EFI (an acronym for “Extensible Firmware Interface”) is the interface between a computer's firmware, hardware, and the operating system. The Extensible Firmware Interface defines a new partition style called GUID partition table (GPT). EFI serves the same purpose for Itanium-based computers as the BIOS found in x86-based computers. However, it has expanded capabilities that provide a consistent way to start any compatible operating system and 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 partitioning style and the master boot record (MBR) partitioning style. In a mixed GPT and MBR disk environment, MBR disks are used solely for data storage. In computers that contain only one GPT disk, the EFI System partition is the first partition on the disk.

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 operating system.