FWIW:
"Driver eXecution Environment (DXE) Phase
The Driver Execution Environment (DXE) phase is where most of the system initialization is performed. Pre-EFI Initialization (PEI), the phase prior to DXE, is responsible for initializing permanent memory in the platform so that the DXE phase can be loaded and executed. The state of the system at the end of the PEI phase is passed to the DXE phase through a list of position-independent data structures called Hand-Off Blocks (HOBs). HOBs are described in detail in the Platform Initialization Hand-Off Block Specification.
There are several components in the DXE phase:
“DXE Foundation”
“DXE Dispatcher”
A set of “DXE Drivers”
The Dxe Core produces a set of Boot Services, Runtime Services, and DXE Services. The DXE Dispatcher is responsible for discovering and executing DXE drivers in the correct order. The DXE drivers are responsible for initializing the processor, chipset, and platform components as well as providing software abstractions for system services, console devices, and boot devices. These components work together to initialize the platform and provide the services required to boot an operating system. The DXE phase and Boot Device Selection (BDS) phases work together to establish consoles and attempt the booting of operating systems. The DXE phase is terminated when an operating system is successfully booted. The Dxe Core is composed of boot services code, so no code from the Dxe Core itself is allowed to persist into the OS runtime environment. Only the runtime data structures allocated by the Dxe Core and services and data structured produced by runtime DXE drivers are allowed to persist into the OS runtime environment."
http://sourceforge.net/apps/mediawiki/tianocore/index.php?title=PI_Boot_Flow
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