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What happens when we turn on computer?

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  • Difficulty Level : Medium
  • Last Updated : 28 Sep, 2022
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A computer without a program running is just an inert hunk of electronics. The first thing a computer has to do when it is turned on is to start up a special program called an operating system. The operating system’s job is to help other computer programs work by handling the messy details of controlling the computer’s hardware. 

An overview of the boot process  

The boot process is something that happens every time you turn your computer on. You don’t really see it, because it happens so fast. You press the power button and come back a few sec (or minutes if on slow storage like HDD) later and Windows 10, or Windows 11, or whatever Operating System you use is all loaded. 

The BIOS chip tells it to look in a fixed place, usually on the lowest-numbered hard disk (the boot disk) for a special program called a boot loader (under Linux the boot loader is called Grub or LILO). The boot loader is pulled into memory and started. The bootloader’s job is to start the real operating system. 

Functions of BIOS

1. POST (Power On Self Test): The Power On Self Test happens each time you turn your computer on. It sounds complicated and that’s because it kind of is. Your computer does so much when it’s turned on and this is just part of that. 

  • It initializes the various hardware devices. 
  • It is an important process to ensure that all the devices operate smoothly without any conflicts. BIOSes following ACPI create tables describing the devices in the computer. 
  • The POST first checks the bios and then tests the CMOS RAM. 
  • If there is no problem with this then POST continues to check the CPU, hardware devices such as the Video Card, and the secondary storage devices such as the Hard Drive, Floppy Drives, Zip Drive, or CD/DVD Drives. 
  • If some errors are found then an error message is displayed on the screen or a number of beeps are heard. 
  • These beeps are known as POST beep codes. 

2. Master Boot Record: The Master Boot Record (MBR) is a special boot sector at the beginning of the disk. The MBR contains the code that loads the rest of OS, known as bootloader. This complicated process (called the Boot Process) starts with the POST (Power On Self Test) and ends when the Bios searches for the MBR on the Hard Drive, which is generally located in the first sector, first head, first cylinder (cylinder 0, head 0, sector 1). 
A typical structure looks like this: 


The bootstrap loader is stored in the computer’s EPROM, ROM, or another non-volatile memory. When the computer is turned on or restarted, it first performs the power-on-self-test, also known as POST. If the POST is successful and no issues are found, the bootstrap loader will load the operating system for the computer into memory. The computer will then be able to quickly access, load, and run the operating system. 

3. init: init is the last step of the kernel boot sequence. It looks for the file /etc/inittab to see if there is an entry for initdefault. It is used to determine the initial run level of the system. A run-level is used to decide the initial state of the operating system. 
Some of the run levels are: 

  • Level 0: System Halt.
  • Level 1: Single user mode.
  • Level 2: Full multiuser mode without network.
  • Level 3: Full multiuser mode with network.
  • Level 4: user definable.
  • Level 5: Full multiuser mode with network and X display manager.
  • Level 6: Reboot.

The above design of init is called SysV- pronounced as System five. Several other implementations of init have been written now. Some of the popular implementations are systemd and upstart. Upstart is being used by ubuntu since 2006. More details of the upstart can be found here

The next step of init is to start up various daemons that support networking and other services. X server daemon is one of the most important daemons. It manages the display, keyboard, and mouse. When X server daemon is started you see a Graphical Interface and a login screen is displayed. 

This article is contributed by Saket Kumar. If you like GeeksforGeeks and would like to contribute, you can also write an article using or mail your article to See your article appearing on the GeeksforGeeks main page and help other Geeks. 

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above. 

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