Extensible Firmware Interface

Extensible Firmware Interface (EFI) is a set of specifications developed by Intel to define the interface between an operating system (OS) and platform firmware, aiming to reduce OS dependence on details of the firmware implementation.

EFI 1.x (32-bit)
Apple phased out its previous use of Open Firmware in favor of EFI during its transition from PowerPC to Intel processors. The first Intel-based Mac was the MacBook Pro, introduced in January 2006 with a 32-bit implementation of the EFI 1.10 standard. In 2007, Apple unilaterally implemented a "fat" binary of EFI 1.10 to support booting on ""-based models in either 32 or 64-bit mode before Intel was able to come up with its own official specification.

UEFI 2.x (64-bit)
The Extensible Firmware Interface Development Kit (EDK) version 2 implemented Intel's updated Unified Extensible Firmware Interface (UEFI) specification, which formally adds 64-bit support. Apple began implementing UEFI 2.0 features along with EFI 1.10 in late 2008. To protect against UEFI firmware s, Apple rearchitected the booting process of Intel-based Macs to place its root of trust in the Apple T2 security chip.

Deprecation
Starting in November 2020, Apple began phasing out its use of UEFI in favor iBoot, which uses the Apple M1's built-in Secure Enclave. Mac boot volumes may still retain an EFI partition for backwards compatibility.