UEFI Boot Services

Overview

Boot Services are a collection of UEFI functions provided by the firmware while the system is still under firmware control.

They allow UEFI applications and boot loaders to perform tasks such as memory allocation, protocol discovery, event handling and loading additional EFI images.

Boot Services are available until the operating system loader calls ExitBootServices().

Role in the Boot Process

During early system startup, the firmware provides an execution environment for EFI applications.

The simplified flow is:

    Firmware starts
          |
          v
    UEFI Boot Services available
          |
          v
    EFI Application / Boot Loader runs
          |
          v
    ExitBootServices()
          |
          v
    Operating system takes control

After ExitBootServices() returns successfully, the firmware releases control over hardware resources managed through Boot Services.

Accessing Boot Services

Boot Services are accessed through the UEFI System Table:

    EFI_SYSTEM_TABLE
            |
            v
    EFI_BOOT_SERVICES

The system table is provided by the firmware when the application starts.

Example:

SystemTable->BootServices

This structure contains pointers to the available Boot Service functions.

Boot Services Used in This Project

Memory Management

The Memory Map Viewer uses:

GetMemoryMap()
AllocatePool()
FreePool()

These functions allow the application to inspect and manage memory before the operating system starts.

Protocol Discovery

Several applications use:

LocateProtocol()

This allows applications to find firmware interfaces dynamically.

Examples:

Simple File System Protocol
Graphics Output Protocol

File and Graphics Access

The Filesystem Explorer and Graphics Demo use Boot Services to locate the required UEFI protocols before interacting with hardware.

Boot Services Lifetime

Boot Services are temporary.

They exist only during the firmware phase.

A boot loader typically uses them to:

Load the operating system kernel
Prepare memory
Retrieve the final memory map
Configure hardware

Before transferring control to the operating system, the boot loader calls:

ExitBootServices()

After that point:

Boot Services are no longer available
The operating system manages the hardware directly

Project Usage

This project uses Boot Services extensively to explore the environment before an operating system exists.

Understanding Boot Services is essential for the later goal of implementing a Linux EFI loader.