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. Unlike BIOS, UEFI is designed to be extensible and supports larger storage capacities, faster boot times, and enhanced security features.
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.
Table: UEFI vs. Legacy BIOS Features
| 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.