How operating system of a personal computer comes into working order?

Answer 1

A PC going through its booting sequence upon staring, a personal computer's CPU runs the instruction located at the memory location of the Basic Input/ Output System (BIOS). It contains a jump instruction that transfer execution to the location of the BIOS startup program. This program runs a Power On Self Test to check that devices the computer rely on are functioning; it also initializes these devices.

Then the BIOS go through a preconfigured list of devices until it finds one that is bootable. If the BIOS find a bootable device, it loads and executes its boot sector. In the case of a hard drive, this is referred to as the master boot record (MBR) and is often not operating system specific. Usually MBR code checks the partition table for an active portion. If one is found, the MBR code loads that partition's boot sector and executes it. The boot sector is often operating system specific, however in most operating systems its main function is to load and execute the kernel, which continues startup and in the case of multi-user operating systems prompt the login screen.