UEFI supports booting from media that contain an UEFI OS loader or an UEFI-defined System Partition.
UEFI does not require any change to the first sector of a partition, so it is possible to build media that will boot on both legacy architectures and UEFI platforms.
UEFI uses a subset of the PE32+ image format with a modified header signature. The modification to the signature value in the PE32+ image is done to distinguish UEFI images from normal PE32 executables. The “+” addition to PE32 provides the 64-bit relocation fix-up extensions to standard PE32 format.
A UEFI OS loader is a special type of UEFI application that normally takes over control of the system from firmware conforming to this specification. When loaded, the UEFI OS loader behaves like any other UEFI application in that it must only use memory it has allocated from the firmware and can only use UEFI services and protocols to access the devices that the firmware exposes.
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