EFI

EFI, or Extensible Firmware Interface, is a specification for the interface between a computer's firmware and its operating system. Developed by Intel in the 1990s as a replacement for the legacy BIOS, the specification evolved into Unified Extensible Firmware Interface (UEFI) and is now maintained by the UEFI Forum. EFI defines a modular pre-boot environment with firmware drivers, a boot manager, runtime services, and a set of protocols that enable communication between firmware and OS loaders.

The firmware uses a dedicated partition, the EFI System Partition (ESP), to store bootloaders and device drivers. Modern systems typically boot from a GUID Partition Table (GPT) disk, with the ESP containing one or more bootloaders or applications. EFI can load the operating system bootloader directly and can provide Secure Boot, which verifies the signatures of key firmware and software components to prevent unauthorized code from running during boot. It also offers runtime services that the operating system can call after boot, such as time and cryptographic operations.

Compatibility with legacy systems is often provided via a Compatibility Support Module (CSM), which emulates BIOS behavior to support older operating systems and hardware. Implementations include open-source efforts like TianoCore EDK II and coreboot, as well as vendor-specific firmware from companies such as AMI, Phoenix, and Insyde. EFI/UEFI-based systems have broad adoption across personal computers, servers, and embedded devices, and they form the modern standard for firmware interfaces.

EFI also abbreviates electronic fuel injection in internal combustion engines, an unrelated technology.