A modern standard specification for the software interface between an operating system (OS) and platform firmware. It replaces the legacy BIOS (Basic Input/Output System), serving as the first program that runs when a computer is powered on to initialize hardware and launch the OS.
Core Capabilities
- Drive Support: Unlike BIOS, which is limited to 2.2 TB, UEFI supports drives up to 9.4 Zettabytes by utilizing the GPT partitioning scheme.
- Architecture Independence: Operates in 32-bit or 64-bit mode, allowing the firmware to address more memory and provide a richer graphical user interface (GUI) with mouse support.
- Pre-Boot Environment: A miniature OS that can run standalone applications, such as hardware diagnostics, disk imaging tools, or web browsers, without an OS installed.
- Parallel Initialization: Speeds up boot times by initializing hardware components simultaneously rather than sequentially.
Key Terminology
- Secure Boot: A security standard that ensures a device boots using only software that is trusted by the Original Equipment Manufacturer (OEM) to prevent rootkits.
- GPT (GUID Partition Table): The modern partitioning standard required by UEFI that allows for nearly unlimited partitions and massive disk sizes.
- ESP (EFI System Partition): A dedicated physical partition on a storage device where UEFI stores bootloaders, drivers, and utility software.
- CSM (Compatibility Support Module): A component of UEFI firmware that emulates a legacy BIOS environment, allowing older operating systems (like Windows 7) to boot.
- UEFI Shell: A command-line interface used by advanced users and developers to execute UEFI applications, manage files, and flash firmware.
UEFI vs. Legacy BIOS
| Feature | UEFI | Legacy BIOS |
| Max Boot Disk Size | ~9.4 Zettabytes | 2.2 Terabytes |
| Partition Table | GPT (GUID Partition Table) | MBR (Master Boot Record) |
| Interface | Graphical (GUI) + Mouse | Text-only + Keyboard |
| Security | Secure Boot (Signed code) | Minimal (Passwords only) |
| Execution Mode | 32-bit or 64-bit | 16-bit |
Boot Process Phases
- SEC (Security): Handles power-on events and establishes a temporary memory store.
- PEI (Pre-EFI Initialization): Initializes the CPU, temporary memory, and motherboard chipset.
- DXE (Driver Execution Environment): Loads drivers for hardware like storage controllers and network cards.
- BDS (Boot Device Selection): Interprets configuration variables to choose which device (SSD, USB, Network) to boot from.
- TSL (Transient System Load): Hands off control to the operating system’s bootloader.
Technical Note: While many users still refer to UEFI as the BIOS, they are architecturally distinct. UEFI is an extensible framework, whereas BIOS is a static firmware type that has remained largely unchanged since the 1980s.