The primary objective of Stage III is to enable a minimal boot path that successfully loads the UEFI Shell. A secondary objective for Stage III is to be silicon and board agnostic. All silicon and board specific details should be leveraged from Stages I and II. Demonstrating the capability to load the UEFI shell does not imply that the UEFI shell is a required component in the end product firmware. It does ensure that the platforms with a terminal boot stage target greater than Stage II can load the UEFI shell so the system can be analyzed and configured in the UEFI boot services environment with well-defined behavior in a consistent manner with other Minimum Platform specification-compliant systems.
The minimal UI capability that is required is serial console. UEFI variables must be supported with at least emulated variable behavior. UEFI variable storage to a non-volatile media such as SPI NOR flash is acceptable if the platform requirements mandate such support. Additional capabilities are optional and must not be assumed. These include USB input devices, graphics devices, and other storage devices.
5.1.1 Major Execution Activities
DXE Initial Program Load (IPL)
DXE Core initialization and dispatcher execution
Initialize the generic infrastructure required for the DXE environment o Installation of DXE architectural protocols o Initialization of architecturally required hardware such as timers
Tests for Memory Map, Cache Map, architectural hardware
5.1.2 Main Control Flow
Stage III extends Stage II control flow by executing Driver Execution Environment (DXE), executing Boot Device Selection (BDS) and invoking the UEFI Shell.
Stage III Main Control Flow
Figure 8 Stage III Main Control Flow
After memory is installed during Stage II, the remaining silicon and platform initialization must take place in the PEI phase only. All silicon initialization tasks should have been completed in Stage II, and there should be no silicon-specific initialization required in the DXE phase. The default console information should be transferred via a HOB and initialized and used in Stage III.