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I tried to find the UEFI boot process yet found much more information about the BIOS boot process. I would like to understand the different stages that are involved in the UEFI boot.

Tamar
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  • There was a similar question a while ago: [link](https://stackoverflow.com/questions/58002542/how-does-uefi-boot-mode-boot-flows/58005664#58005664). – MiSimon Apr 22 '21 at 08:16

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So I found the answer using these websites:

https://edk2-docs.gitbook.io/edk-ii-build-specification/2_design_discussion/23_boot_sequence

https://blog.hansenpartnership.com/anatomy-of-the-uefi-boot-sequence-on-the-intel-galileo/

These are the steps:

UEFI boot officially has three phases (SEC, PEI and DXE). However, the DXE phase is divided into DXEBoot and DXERuntime (the former is eliminated after the call to ExitBootSerivices()). The jobs of each phase are

  1. SEC (SECurity phase). This contains all the CPU initialisation code from the cold boot entry point on. It’s job is to set the system up far enough to find, validate, install and run the PEI.

Responsible for the following:

  • Handling all platform restart events
  • Creating a temporary memory store
  • Serving as the root of trust in the system
  • Passing handoff information to the PEI Foundation
  1. PEI (Pre-Efi Initialization phase). This configures the entire platform and then loads and boots the DXE.

Responsible for the following:

  • Initializing some permanent memory complement
  • Describing the memory in Hand-Off Blocks (HOBs)
  • Describing the firmware volume locations in HOBs
  • Passing control into the Driver Execution Environment (DXE) phase
  1. DXE (Driver eXecution Environment). This is where the UEFI system loads drivers for configured devices, if necessary; mounts drives and finds and executes the boot code. After control is transferred to the boot OS, the DXERuntime stays resident to handle any OS to UEFI calls.
Tamar
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