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

When you asked "In what mode(Real,Protected,Long) does a system with UEFI start?", what do you mean? The processor starts executing in a mode that is similar to the mode of the 80386 of the past. But do you really care. Don't you really care about the mode of the processor when your OS loader code is given control? And you care about what services are provided to your OS loader code.

The environment is defined in the UEFI spec. Latest Versions of the UEFI Specifications

As for which language you use, assembly is good to start with. It's probably easier to use C or some other high level language after a bit.

Other background:

There are a lot of terms here that we are not always careful about using correctly.

The code that executes when the processor comes out of reset is system firmware that does a lot of initialization of various hardware in the system.

In the old days on a x86 PC system before the UEFI Forum existed, the system firmware was called BIOS. The BIOS in those days executed all of its initialization code, then loaded some code off of a floppy diskette or the hard drive and jumped into the code. The BIOS also provided some interfaces between the hardware and the operating system to help isolate the OS from hardware differences. Nothing was standardized though. The only standards were the OS and applications level software that used the BIOS interfaces.

If the OS and apps functioned correctly then the BIOS was considered correct. But you could only prove correctness by lack of failure. Thus new OSs or applications would work on one correct system yet fail on a different correct system.

Today we've tried to provide some actual standardization of those interfaces. They are defined by the UEFI Forum. Today I can prove that my system is correct according to meeting the UEFI specifications.

When people say things like the UEFI, they are usually referring to the actual system firmware installed on the system that is executed before the OS starts executing. But a lot of us still toss around these terms like we've spilled alphabet soup.

The boot loader is actually OS owned code that is loaded in memory by the system firmware and the system firmware gives control of the hardware to the boot loader. One could say that the end of the system firmware is the UEFI Boot Loader. Or you could just say BDS uses the system policy to find the OS. And you could still find someone that disagreed with the words.