Overview

This chapter describes the fundamental services that are made available in an EFI-compliant system. The services are defined by interface functions that may be used by code running in the EFI environment. Such code may include protocols that manage device access or extend platform capabilities. In this chapter, the Runtime services will be the focus of discussion. These runtime services are functions that are available both during EFI operation and when the OS has been launched and running.

During boot, system resources are owned by the firmware and are controlled through a variety of system services that expose callable APIs. In EFI there are two primary types of services:

• Boot Services - Functions that are available prior to the launching of the boot target (such as the OS), and prior to the calling of the ExitBootServices() function.

• Runtime Services - Functions that are available both during the boot phase prior to the launching of the boot target and after the boot target is executing. :

Figure 5.1 illustrates the phases of boot operation that a platform evolves through.

Figure 5.1: Phases of Boot Operation

In Figure 5.1, it is clearly evident that the two previously mentioned forms of services (Boot Services and Runtime Services) are available during the early launch of the EFI infrastructure and only the runtime services are available after the remainder of the firmware stack has relinquished control to an OS loader.