Uefi

UEFI, the Unified Extensible Firmware Interface, is the specification for the firmware that starts a modern computer and hands control to an operating system, replacing the legacy BIOS on nearly all current PCs and, in a different form, on Macs and many servers and ARM devices. Like the BIOS it is firmware stored on the motherboard, but it is built as a small modular operating environment with its own drivers, a standard programming interface, a graphical setup screen with mouse support and the ability to boot from large disks and verify what it boots. The specification is maintained by the UEFI Forum, an industry group, and many firmware images are built from open-source reference code such as TianoCore's EDK II.

UEFI began as Intel's Extensible Firmware Interface, designed around 1998-2000 for the Itanium platform, and was handed to an industry group in 2005 as the Unified EFI Forum. The first UEFI version, 2.0, appeared in 2006, and Apple adopted EFI in its Intel-based Macs that same year, giving the design its first mass-market use. Later versions added features such as Secure Boot, introduced in version 2.3.1 in 2011. Microsoft required UEFI with Secure Boot capability for Windows 8 logo machines from 2012, and Windows 11 later required UEFI and Secure Boot capability, along with a TPM 2.0 module, which finally made legacy BIOS boot unusual on new Windows PCs. Linux distributions adapted by signing small intermediate loaders so that they could boot with Secure Boot enabled.

UEFI organizes startup into well-defined phases, with the standard platform-initialization sequence running from security through to boot selection. The Security (SEC) phase takes the first instructions after reset and sets up a temporary memory area; Pre-EFI Initialization (PEI) brings up the memory controller and basic hardware; the Driver Execution Environment (DXE) loads drivers and services for devices; and Boot Device Selection (BDS) reads the boot variables stored in non-volatile memory and runs the chosen entry. Unlike a BIOS that reads a fixed sector, UEFI understands file systems: it reads files from a special partition, the EFI System Partition (ESP), formatted with FAT, and runs programs in a standard executable format with the extension .efi, such as the Windows Boot Manager, GRUB or systemd-boot. These boot entries live in firmware variables, which is why tools can add or reorder them and why the firmware can show a menu of installed operating systems directly.

Two features account for much of UEFI's practical importance: the GUID Partition Table and Secure Boot. The GUID Partition Table (GPT), part of the UEFI specification, uses 64-bit sector addresses, so disks far larger than the 2 tebibytes that the older MBR scheme reaches become usable, and it supports a large number of partitions, with a backup copy of the table at the end of the disk. Secure Boot has the firmware check cryptographic signatures on boot loaders, drivers and option ROMs against keys it holds, refusing to run anything unsigned or revoked; this blocks bootkit malware that hides before the operating system loads. The cost is complexity: users who install custom kernels or unusual operating systems may need to enroll their own keys or disable the feature, and the trust model depends on who controls the keys.

Misunderstandings about UEFI are common because the name BIOS is still used for its setup screen. UEFI is not an operating system, although it includes a small shell and drivers; it is not the same as firmware in general, which is the broader category covering SSD controllers and routers; and it is not the same as the boot process, which spans from power-on through the kernel's start. Many boards offer a compatibility support module, a CSM, to emulate a legacy BIOS, but it is being removed from new designs. UEFI is also software with bugs, and firmware vulnerabilities in it are valuable to attackers since they persist across reinstalls, so vendors ship UEFI updates through tools such as the Linux Vendor Firmware Service. The same standard underlies cloud virtual machines, where the open-source OVMF firmware lets guests boot with Secure Boot.

Read UEFI as the modern replacement for the BIOS: a modular firmware environment that understands partitions and files, loads signed boot programs and can verify their origin, which is why large GPT disks, graphical setup menus and Secure Boot are all properties of the same specification rather than separate inventions.