Getting to know how your Computer System starts up when you press the Power Button!!
You start by powering ON your computer, at this moment, the first software to run is the BIOS/UEFI, which is a low-level software( one which interacts directly with hardware). The BIOS initializes the computer hardware to make sure everything is good to go.
The BIOS then runs a special process called POST i.e Power On Self Test whose Single job is to run various diagnostic tests to ensure that the system is in proper working order and everything is OK.
Next, depending on the configuration, a boot device is selected i.e this boot device may be a hard drive, a USB device, or a CD device. This boot device has something called as Bootloader, which is a small program responsible for loading up the OS. As soon as BIOS finds the Bootloader in one of the boot devices, it executes it. This bootloader then in turn loads up the OS.
Now when we say OS loads up, it basically means that the Kernel has been loaded up as OS is nothing but KERNEL + some Application software like Browser, MS-office etc. What we interact with most of the time are Application software or Apps, who in turn go to kernal asking to perform different things user has asked for as Kernal is responsible for managing our Computer’s resources like device drivers,memory etc.
After that essential system process like user login, a desktop environment, different user spaces are launched and at this point the control is transferred from BIOS to OS.
Image Credit : Google Search, youtube video titled “Operating System — The Boot Process” by ‘intrigano’.