Have you ever wondered what actually happens in the span of you turning the PC on to get the Desktop? Some crucial operations has to be initialized from the moment of rebooting.
When the PC is powered up it needs to have an initial program to run, which is initial program or bootstrap program. Actually, it is stored in volatile memory well known as read-only memory (ROM) or Electronically erasable memory programmable read-only memory (EPROM) , which is known by general term Firmware within the computer hardware. It initializes all aspects of the system.
The bootstrap program must know how to load the operating system and execution process of that system. To do that, bootstrap program must fetch into memory the operating system Kernal (The OS is the one program running at all the times on the computer usually called ‘kernal’) . The operating system starts to execute the first process such as “init”, and waits for some event to occur.
The occurrence is usually signaled by an interrupt from either software or hardware side. Hardware trigger in interrupt by sending signal to CPU where as software trigger it by executing operation called system call or monitor call. When CPU is interrupted it stop amid of running process and immediately transfer execution to another process which need to be executed.