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Unified Extensible Firmware Interface

aliases:

  • UEFI

The modern firmware developed to replace BIOS. Also responsible for managing the PC's startup process, locating and loading an installed OS into memory.

Features:

  • provides support for storage sizes greater than 2.2 TB
  • provides support for 64-bit memory addresses
  • provides better security against rootkit attacks (malware that targets the bootloader)
  • provides faster startup times over BIOS
  • backwards compatible with BIOS

UEFI Boot Order

another long ahh list 😭

Stage Description
Security | SEC - Power to CPU
- x16 Real Mode to x64 Protected Mode
- Special instruction set executed to switch to PEI mode
Pre-EFI Initialization | PEI - CPU and chipset are verified and initialized
- System Memory is verified and initialized
- PEI modules are loaded and allocate system resources
- Switch to DXE
Driver Execution Environment | DXE - Devices drivers are loaded into memory
- Hardware devices and other peripherals are all loaded
- System drives are mounted and loaded
- EFI boot manager starts
Boot Device Selection | BDS - EFI boot manager identifies system config settings and decides how to boot the system

If an ESP partition is found, it searches for \EFI\Microsoft\bootmgfw.efi. That EFI file locates for OS installations stored in the BCD of the system. If there's only one, that one's automatically loaded. If there's multiple, a list of bootable OS's is shown to the end user.

If not, the process continues using Legacy BIOS Mode, where the MBR looks for the bootmgr.exe file and runs it. bootmgr.exe searches for the BCD store in the C:\ drive or in the System Reserved partition.
Transient System Load | TSL - OS is located and loaded.

BCD store identifies the location of the OS (e.g. for Windows, it's found at C:\Windows\system32\winload.efi) and starts it.
Runtime | RT Control of the system is handed off to the OS and the OS's splash screen is loaded, and soon awaits user login to finish booting.

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