In which UEFI boot process phase is the UEFI program cleared from memory and control is transferred to the OS?

The UEFI boot process involves several distinct phases, and understanding each phase helps clarify the overall workflow. The correct answer pertains to the Runtime phase, during which the UEFI program completes its operations, and control is handed over to the operating system.

During the Runtime phase, the firmware has done its job in initializing hardware and preparing the environment for the operating system to take over. This is the last phase of the UEFI boot process where any remaining UEFI services can be used by the OS before the firmware is cleared from memory. Once the OS has fully taken control, the UEFI firmware is no longer needed and is therefore cleared from memory. This handover ensures optimal memory management, allowing the operating system to utilize resources without the overhead of the firmware residing in memory.

In contrast, the Pre-EFI Initialization phase involves setting up the system before the UEFI firmware runs its main routines, which does not involve transferring control to the OS. The Driver Execution Environment phase allows for the initialization and loading of drivers but does not represent the final transition to the operating system. Finally, the Boot Device Selection phase concerns the identification and selection of the boot device but does not encompass the act of transferring control to the OS either.

Understanding the sequence and roles of