How does UEFI work?
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 with UEFI start? If normal boot loaders cant work with UEFI, Then what is the alternate of boot loader while dealing with UEFI? And do I need any other programming to create one, than assembly?
Answer
UEFI firmware runs in 64 bit long mode for 64 bit platforms and flat mode for 32 bit platforms; Unlike BIOS, UEFI features its own architecture, independent of the CPU, and its own device drivers. UEFI can mount partitions and read certain file systems.
When an x86 computer equipped with UEFI, the interface searches the system storage for a partition labeled with a specific globally unique identifier (GUID) that marks it as the EFI System Partition (ESP). BTW Windows doesn't mount this partition and you cannot see it in the OS. But there is a trick, you simply change the partition type (using HexWorkshop) in VBR to regular FAT32 code and it will be mounted into the OS.
This partition contains applications compiled for the EFI architecture. In general you don't have to deal with assembler to write a UEFI application/loader, it's just a regular C code. By the default it is located at "EFI/BOOT/BOOTX64.EFI". When a bootloader is selected, manually or automatically, UEFI reads it into the memory and yields control of the boot process to it.