Boot Stages

A UEFI system goes through several distinct phases during the boot process.

1. Platform Initialization. This early-boot phase is mostly outside the scope of the uefi crate. It is described by the UEFI Platform Initialization Specification, which is separate from the main UEFI Specification.
2. Boot Services. This is when UEFI drivers and applications are loaded. Functions in both the boot module and runtime module can be used. This stage typically culminates in running a bootloader that loads an operating system. The stage ends when boot::exit_boot_services is called, putting the system in Runtime mode.
3. Runtime. This stage is typically active when running an operating system such as Linux or Windows. UEFI functionality is much more limited in the Runtime mode. Functions in the boot module can no longer be used, but functions in the runtime module are still available. Once the system is in Runtime mode, it cannot return to the Boot Services stage until after a system reset.