EFI Boot

The EFI Boot Process

For All Windows Versions Running On EFI Firmware Systems

At boot time, the EFI firmware in your PC loads the customized GNU Grub EFI Boot Manager module.

For 64 bit systems, the module is /EFI/grub2win/g2bootmgr/gnugrub.kernel64.efi

For 32 bit systems, the module is /EFI/grub2win/g2bootmgr/gnugrub.kernel32.efi

The GNU Grub EFI Boot Manager then takes control and performs the following actions:

Searches all disk drives for file /grub2/g2bootmgr/gnugrub.functions.cfg to find the partition where the Grub2Win base directory is installed. 

This is usually the Windows boot partition C: drive.

Sets variable $grub2part to the address of the partition where Grub2Win is installed. 

Sets variable $prefix to the Grub2Win partition address followed by /grub2.

The $prefix variable is required by grub2 to locate its base directory and files.

If $prefix is not set properly, grub2 initializes in rescue mode (limited command line).

Loads several other GNU Grub modules to support various partition and filesystem types as well as USB drives and keyboards.

Reads file $prefix/grub.cfg and builds the grub2 boot menu display. 

This is usually file C:\grub2\grub.cfg