Chapter 1: Introduction to UEFI

Understanding the Unified Extensible Firmware Interface architecture.

Overview

What is UEFI?

UEFI (Unified Extensible Firmware Interface) is a specification that defines the interface between an operating system and platform firmware. It replaces the legacy BIOS (Basic Input/Output System) that has been used since the IBM PC era.

UEFI provides:

  • Modern architecture: 32-bit or 64-bit execution environment (not 16-bit real mode)
  • Modular design: Drivers and applications can be loaded dynamically
  • Standardized interfaces: Protocols for accessing hardware and services
  • Security features: Secure Boot, measured boot, and authenticated variables
  • Rich pre-boot environment: GUI support, networking, and file system access

UEFI vs Legacy BIOS

Feature Legacy BIOS UEFI
Execution Mode 16-bit real mode 32-bit or 64-bit protected mode
Addressing 1 MB limit Full memory access
Partition Table MBR (2 TB limit) GPT (9.4 ZB limit)
Boot Process INT 19h, boot sector Boot Manager, EFI applications
Driver Model Option ROMs UEFI drivers (portable)
Extensibility Limited Highly modular
User Interface Text-based GUI capable
Security None built-in Secure Boot, TPM integration
Network Boot PXE only HTTP Boot, iSCSI, etc.

Key Concepts

Concept Description
Handle An opaque reference to a collection of related protocols
Protocol A named interface (GUID) providing specific functionality
GUID 128-bit Globally Unique Identifier for protocols and variables
Boot Services Functions available before ExitBootServices()
Runtime Services Functions available to the OS after boot
Driver Code that provides services via protocols
Application Standalone executable that runs and exits

UEFI Boot Stages

The UEFI boot process consists of several distinct phases, each with specific responsibilities and available services.

flowchart TB
    subgraph SEC [SEC Phase - Security]
        S1[Power On / Reset]
        S2[CPU Microcode]
        S3[Cache-as-RAM Setup]
        S4[Trust Initialization]
    end

    subgraph PEI [PEI Phase - Pre-EFI Initialization]
        P1[PEI Core]
        P2[PEIMs Execute]
        P3[Memory Init]
        P4[Recovery Check]
        P5[S3 Resume Check]
    end

    subgraph DXE [DXE Phase - Driver Execution Environment]
        D1[DXE Core]
        D2[DXE Dispatcher]
        D3[Driver Loading]
        D4[Protocol Installation]
        D5[Boot & Runtime Services]
    end

    subgraph BDS [BDS Phase - Boot Device Selection]
        B1[BDS Core]
        B2[Console Init]
        B3[Boot Options]
        B4[OS Loader]
    end

    subgraph TSL [TSL - Transient System Load]
        T1[OS Loader Runs]
        T2[ExitBootServices]
    end

    subgraph RT [Runtime Phase]
        R1[OS Control]
        R2[Runtime Services Only]
    end

    SEC --> PEI
    PEI --> DXE
    DXE --> BDS
    BDS --> TSL
    TSL --> RT

    style SEC fill:#e74c3c,color:#fff
    style PEI fill:#e67e22,color:#fff
    style DXE fill:#2ecc71,color:#fff
    style BDS fill:#3498db,color:#fff
    style TSL fill:#9b59b6,color:#fff
    style RT fill:#34495e,color:#fff

SEC Phase (Security)

Purpose: Establish the root of trust and prepare the environment for PEI.

Characteristics:

  • Runs from reset vector (typically at 0xFFFFFFF0)
  • Executes in place from flash (no RAM yet)
  • Sets up temporary memory using Cache-as-RAM (CAR)
  • Initializes basic CPU state
  • Verifies PEI core integrity (if Secure Boot enabled)

Services Available: None (assembly code, no API)

flowchart LR
    A[Reset Vector] --> B[CPU Init]
    B --> C[CAR Setup]
    C --> D[Find PEI Core]
    D --> E[Jump to PEI]

    style A fill:#e74c3c,color:#fff
    style E fill:#e67e22,color:#fff

PEI Phase (Pre-EFI Initialization)

Purpose: Initialize permanent memory and prepare the system for DXE.

Characteristics:

  • Runs with limited memory (CAR only initially)
  • Executes PEI Modules (PEIMs)
  • Uses PPI (PEIM-to-PEIM Interface) for communication
  • Initializes main memory (DRAM)
  • Creates Hand-Off Blocks (HOBs) for DXE

Services Available: PEI Services (limited set)

PEI Service Description
InstallPpi Install a PEIM-to-PEIM Interface
LocatePpi Find a PPI by GUID
NotifyPpi Register for PPI installation notification
GetBootMode Get current boot mode (normal, recovery, S3)
CreateHob Create a Hand-Off Block for DXE 
flowchart TB
    subgraph PEI Phase
        PC[PEI Core]
        PM1[Memory Init PEIM]
        PM2[CPU PEIM]
        PM3[Platform PEIM]
        HOB[HOB List]
    end

    PC --> |Dispatch| PM1
    PC --> |Dispatch| PM2
    PC --> |Dispatch| PM3
    PM1 --> |Create| HOB
    PM2 --> |Create| HOB
    PM3 --> |Create| HOB

    style PC fill:#e67e22,color:#fff
    style HOB fill:#3498db,color:#fff

DXE Phase (Driver Execution Environment)

Purpose: Load drivers, initialize the full platform, and provide boot services.

Characteristics:

  • Full memory available
  • Executes DXE drivers and UEFI drivers
  • Uses Protocol model for communication
  • Provides Boot Services and Runtime Services
  • Most UEFI applications run here

Services Available: Full Boot Services and Runtime Services

flowchart TB
    subgraph DXE Phase
        DC[DXE Core]
        DD[DXE Dispatcher]

        subgraph Drivers
            DRV1[CPU Driver]
            DRV2[PCI Driver]
            DRV3[USB Driver]
            DRV4[Graphics Driver]
            DRV5[Filesystem Driver]
        end

        subgraph Services
            BS[Boot Services]
            RS[Runtime Services]
        end

        subgraph Protocols
            P1[Protocols]
        end
    end

    DC --> DD
    DD --> DRV1
    DD --> DRV2
    DD --> DRV3
    DD --> DRV4
    DD --> DRV5
    DRV1 --> |Install| P1
    DRV2 --> |Install| P1
    DRV3 --> |Install| P1
    P1 --> |Use| BS
    P1 --> |Use| RS

    style DC fill:#2ecc71,color:#fff
    style BS fill:#3498db,color:#fff
    style RS fill:#9b59b6,color:#fff

BDS Phase (Boot Device Selection)

Purpose: Initialize consoles, process boot options, and launch OS loader.

Characteristics:

  • Console devices initialized (keyboard, display)
  • Boot options processed (BootOrder variable)
  • User interface available
  • OS loader selected and executed

Services Available: Full Boot Services and Runtime Services

TSL (Transient System Load)

Purpose: OS loader prepares to take control.

Characteristics:

  • OS loader runs as UEFI application
  • Gets memory map
  • Loads kernel and initrd
  • Calls ExitBootServices()

Runtime Phase

Purpose: OS has control; limited firmware services remain.

Characteristics:

  • Boot Services no longer available
  • Only Runtime Services accessible
  • Virtual addressing may be used

Services Activation by Phase

This table shows which services are available during each boot phase:

Service Category SEC PEI DXE BDS TSL Runtime
PEI Services - ✓ - - - -
Boot Services - - ✓ ✓ ✓ -
Runtime Services - - ✓ ✓ ✓ ✓
Protocols - PPI ✓ ✓ ✓ -
Memory Allocation - Limited ✓ ✓ ✓ -
Console I/O - - ✓ ✓ ✓ -
File System - - ✓ ✓ ✓ -
Network - - ✓ ✓ ✓ -
Variables - HOB ✓ ✓ ✓ ✓

Boot Services (DXE and later)

Boot Services are available from DXE phase until ExitBootServices() is called:

flowchart LR
    subgraph Boot Services
        direction TB
        MEM[Memory Services]
        EVT[Event Services]
        PROT[Protocol Services]
        IMG[Image Services]
        DRV[Driver Services]
    end

    subgraph Memory
        AP[AllocatePool]
        FP[FreePool]
        APAG[AllocatePages]
        GMM[GetMemoryMap]
    end

    subgraph Events
        CE[CreateEvent]
        SE[SetTimer]
        WE[WaitForEvent]
    end

    subgraph Protocols
        HP[HandleProtocol]
        LP[LocateProtocol]
        IP[InstallProtocol]
    end

    MEM --> AP
    MEM --> FP
    MEM --> APAG
    MEM --> GMM
    EVT --> CE
    EVT --> SE
    EVT --> WE
    PROT --> HP
    PROT --> LP
    PROT --> IP

    style MEM fill:#3498db,color:#fff
    style EVT fill:#2ecc71,color:#fff
    style PROT fill:#e74c3c,color:#fff

Runtime Services (Always Available After DXE)

Runtime Services survive ExitBootServices() and are available to the OS:

Service Description
GetTime / SetTime RTC access
GetVariable / SetVariable UEFI variable access
GetNextVariableName Enumerate variables
ResetSystem System reset/shutdown
GetNextHighMonotonicCount Monotonic counter
UpdateCapsule Firmware update
QueryVariableInfo Variable storage info

UEFI Specification Reference

Summary

  1. UEFI replaces legacy BIOS with a modern, extensible firmware interface
  2. Boot progresses through phases: SEC → PEI → DXE → BDS → Runtime
  3. Each phase has specific services: PEI Services, Boot Services, Runtime Services
  4. Protocols provide functionality: Hardware access through standardized interfaces
  5. ExitBootServices is the transition point: From firmware control to OS control

Glossary

Term Definition
UEFI Unified Extensible Firmware Interface
PI Platform Initialization (Intel specification)
SEC Security phase - first code after reset
PEI Pre-EFI Initialization phase
PEIM PEI Module
PPI PEIM-to-PEIM Interface
DXE Driver Execution Environment
BDS Boot Device Selection
HOB Hand-Off Block (PEI to DXE data)
GUID Globally Unique Identifier (128-bit)
Handle Reference to protocol collection
Protocol Named interface providing functionality
CAR Cache-as-RAM (temporary memory before DRAM)
NVRAM Non-Volatile RAM for variables

Next Steps

UEFI Spec Version: This guide references UEFI Specification 2.10 and PI Specification 1.8. Always check the latest specification for updates.

Back to top

UEFI Development Guide is not affiliated with the UEFI Forum. Content is provided for educational purposes.

This site uses Just the Docs, a documentation theme for Jekyll.