UEFI replaces BIOS
DXE – Driver Execution Environment
This stage consist of C modules and a dependency-aware dispatcher. With main memory now available, CPU, chipset, mainboard and boot devices are initialized in DXE and BDS.TSL – Transient System Load
This is the stage between boot device selection and hand-off to the OS. At this point one may enter UEFI shell, or execute an UEFI application such as the OS boot loader.