UEFI’s architecture is divided into several phases and components, each handling specific tasks during the boot process and runtime.
The key phases are:
1. Security Phase (SEC):
• The initial phase after power-on, where the CPU is initialized, and basic security checks are performed. This phase sets up a minimal environment for the next stages.
2. Pre-EFI Initialization (PEI):
• Initializes core hardware components like memory and the CPU. This phase prepares the system for more complex operations.
3. Driver Execution Environment (DXE):
• Loads and executes UEFI drivers for hardware components (e.g., storage, network, and graphics). This phase is highly modular, allowing drivers to be added or updated as needed.
4. Boot Device Selection (BDS):
• Manages the selection of the boot device (e.g., hard drive, USB, or network) and loads the bootloader or operating system. The user can interact with the UEFI interface during this phase to configure settings or select boot options.
5. Transient System Load (TSL):
• Transfers control to the operating system’s bootloader, which then loads the operating system kernel.
6. Runtime (RT):
• Provides services to the operating system after it has loaded, such as accessing firmware variables or managing power states.
UEFI uses a standardized firmware interface, with components written in C or assembly, stored in a firmware volume (a structured file system on the motherboard’s flash chip). It supports a variety of file systems, including FAT32, for reading bootloaders and configuration files.
Advantages of UEFI Over BIOS
• Performance: Faster boot times due to parallel hardware initialization and optimized drivers.
• Scalability: Supports modern hardware, including large storage devices and complex systems.
• Security: Secure Boot protects against unauthorized software during the boot process.
• User Experience: Graphical interfaces and mouse support make firmware configuration more accessible.
• Flexibility: Modular design allows for easy updates and customization by manufacturers.
• Backward Compatibility: The Compatibility Support Module (CSM) allows UEFI to emulate BIOS for older operating systems.
Challenges and Limitations
While UEFI is a significant improvement over BIOS, it has some challenges:
• Complexity: UEFI’s advanced features make it more complex to implement and configure, requiring expertise from manufacturers and developers.
• Secure Boot Criticism: Some users and open-source communities argue that Secure Boot can restrict the ability to install alternative operating systems (e.g., certain Linux distributions) unless properly signed or if Secure Boot is disabled.
• Compatibility Issues: Older operating systems or hardware may not fully support UEFI, requiring the use of CSM or legacy boot modes.
• Firmware Vulnerabilities: As UEFI is more complex, it can be a target for sophisticated attacks if not properly secured or updated.