Unified Extensible Firmware Interface
The second stage of UEFI contains of a dependency-aware dispatcher that loads and runs PEI modules (PEIMs) to handle hardware tasks such as main memory initialization and firmware recovery operations. It's also responsible for the discovery of the boot mode and handling many ACPI S0ix/ACPI S3 operations. In the case of ACPI S0ix/ACPI S3 resume, it is responsible for restoring many hardware registers to a pre-sleep state. PEI also uses CPU cache as RAM.