The EFI Boot Process

An EFI-compliant system has firmware that runs boot loader code that’s been programmed into the system’s nonvolatile RAM (NVRAM) by Windows Setup. The boot code reads the BCD’s contents, which are also stored in NVRAM. The Bcdedit.exe tool mentioned earlier also has the ability to abstract the firmware’s NVRAM variables in the BCD, allowing for full transparency of this mechanism.

The EFI standard defines the ability to prompt the user with an EFI Boot Manager that can be used to select an operating system or additional applications to load. However, to provide a consistent user interface between BIOS systems and EFI systems, Windows sets a 2- second timeout for selecting the EFI Boot Manager, after which the EFI-version of Bootmgr (Bootmgfw.efi) loads instead.

Hardware detection occurs next, where the boot loader uses EFI interfaces to determine the number and type of the following devices:
• Network adapters
• Video adapters
• Keyboards
• Disk controllers
• Storage devices

On EFI systems, all operations and programs execute in the native CPU mode with paging enabled and no part of the Windows boot process executes in 16-bit mode. Note that although EFI is supported on both 32-bit and 64-bit systems, Windows provides support for EFI only on 64-bit platforms.

Just as Bootmgr does on x86 and x64 systems, the EFI Boot Manager presents a menu of boot selections with an optional timeout. Once a boot selection is made, the loader navigates to the subdirectory on the EFI System partition corresponding to the selection and loads the EFI version of the Windows boot loader (Winload.efi).

The EFI specification requires that the system have a partition designated as the EFI System partition that is formatted with the FAT file system and is between 100 MB and 1 GB in size or up to one percent of the size of the disk, and each Windows installation has a subdirectory on the EFI System partition under EFI\Microsoft.

Note that through the unified boot process and model in Windows Vista, the components apply almost identically to EFI systems, except that those ending in .exe end in .efi, and they use EFI APIs and services instead of BIOS interrupts. Another difference is that to avoid limitations of the MBR partition format (including a maximum of four partitions per disk), EFI systems use the GPT (GUID Partition Table) format, which uses GUIDs to identify different partitions and their roles on the system.

Source of Information : Microsoft Press Windows Internals 5th Edition