How does UEFI work?

Question

I was studying about bootloaders when exactly came upon the term UEFI. I can understand some things about UEFI. But still, In what mode(Real,Protected,Long) does a system wi

Answer 1

Here's a good answer to this question:

Other modern 64-bit machines have new EFI firmwares. These don't load a bootstrap program from sector #0 of a disc at all. They bootstrap by the EFI Boot Manager loading and running an EFI boot loader application. Such programs are run in protected mode. This is the EFI bootstrap process.

EFI firmwares in general switch to protected mode within a few instructions of exiting processor reset. Switching to protected mode is done early on in the so-called "SEC Phase" of EFI firmware initialization. Technically, 32-bit and greater x86 processors don't even start in real mode proper, but in what is colloquially known as unreal mode. (The initial segment descriptor for the CS register does not describe the conventional real mode mapping and is what makes this "unreal".)

As such, it could be said that those EFI systems never enter real mode proper at all, when bootstrapping natively to an EFI bootloader (i.e. when they don't employ a compatibility support module), since they switch from unreal mode directly to protected mode and stay in protected mode from then on.