PreEFI

PreEFI is an informal term used to describe the earliest boot-time environment that runs before the main UEFI firmware initialization. It refers to the initial hardware bring-up and basic system setup that occurs in firmware ROM prior to the formal UEFI boot phases. Because PreEFI is not defined in the official UEFI specification, its exact scope and implementation vary by platform, vendor, and device type.

In practice, PreEFI encompasses tasks that prepare the system for the UEFI boot process. Typical responsibilities may include low-level hardware checks, initial clock and memory controller configuration, basic PCI/PCIe enumeration, routing of early debug output, and the loading of very early microcode or ROM modules required to bring the system to a state where UEFI can take over. These activities often occur before or in parallel with the earliest PEI (Pre-EFI Initialization) actions, and they are sometimes implemented in separate ROM regions or in firmware cores that initialize hardware up to the point where the UEFI environment begins.

Relation to UEFI phases and security: The UEFI boot process is formally divided into phases such as PEI, DXE, and BDS. PreEFI-related work is typically considered to occur before or during the transition into PEI, depending on the platform. Because PreEFI runs with extensive hardware access and before the integrity checks of later firmware stages, it represents a critical trust boundary. Modern firmware security practices emphasize secure boot and signed code to minimize the risk of compromise during these early stages.

See also: UEFI, PEI, DXE, BIOS, Secure Boot.