A three layer approach is taken. The bottom layer is made up of coreboot in an unmodified version. Its task is to initialize and gather information about the hardware in the system. When coreboot has completed execution, it hands off control to a payload, the middle layer of the architecture.
The payload is efiload, a loader for the upper layer. The loader detects and parses the coreboot tables and converts the information encoded in the tables into a form understood by the UEFI components. Then, it loads and starts the UEFI components.
The third layer is composed of the UEFI components. The design and architecture of the UEFI components is dictated by the UEFI specification and the PI architecture.
Figure 1 above illustrates the architecture on a high level. It should be noted that the DXE Core presents the UEFI interfaces to any upper layers, e.g. an operating system, and completely hides the bottom layers. In fact, when the DXE Core has been started, the bottom layers do not exist anymore.
The corebooot layer
Coreboot performs platform initialization and starts a so-called “payload” after the initialization phase. The payload needs to be a standard ELF file, which is loaded at its linked address and then executed. Coreboot passes information about the system, like e.g. memory size, to the payload in the coreboot tables.
The efiload layer
The UEFI components expect that some basic information about the system is passed in a list of Hand-off Blocks (HOBs). The HOB list must be created by the efiload component from the information found in the coreboot tables. Most imporantly, information about the memory layout and the location of the UEFI Firmware Volume must be encoded in the HOBs.
The UEFI layer
The UEFI components implement the DXE phase of the PI architecture. In the DXE phase, the so-called DXE core starts several independent DXE drivers which implement the UEFI Boot Services and the UEFI Runtime Services. When all DXE drivers in the UEFI Firmware Volume have been executed, UEFI drivers and applications are started. Those drivers and applications make use of the UEFI services to fully initialize the system. As drivers execute, new hardware may be discovered. Information about hardware is passed to the DXE core via the UEFI Boot Services. The DXE core takes that information and builds a device tree representation of the system hardware. When a boot loader or an operating system is started, it may query said information through the UEFI system table.