What exactly happens when we switch on the computer?

I'll try and explain the method by which a linux system boots.

Functions of BIOS

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

It initializes the various hardware devices. It is an important process so as 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 problems with this then POST continues to check the CPU, hardware devices such as the Video Card, the secondary storage devices such as the Hard Drive, Floppy Drives, Zip Drive or CD/DVD Drives.

If some errors found then an error message is displayed on screen or a number of beeps are heard. These beeps are known as POST beep codes.

Master Boot Record

The Master Boot Record (MBR) is a small program that starts when the computer is booting, in order to find the operating system (eg. Windows XP). 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).

The MBR is 512 bytes long.

1. Primary bootloader info in 1st 446 bytes.
2. Partition table info in next 64 bytes.
3. MBR validation check in last 2 bytes.

A typical structure looks like:

Image courtsey : Microsoft

Grand Unified Bootloader (GRUB) waits for you to select the kernel image you want to load. Grub has the information about kernel image and initrd images. It loads and executes them both.

Next, the kernel is decompressed from its image. It uses a temporary root file system (Initrd - Initial RAM Disk) . The filesystem are loaded. Trap, IRQ (Interrupt request) and scheduling are initialized. move_to_user_mode() is executed and init process is forked

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 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

1 –> Single user mode

3 –> Full multiuser mode with network

5 –> Full multiuser mode with network and X display manager

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 implementatios 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 daemon. It manages display, keyboard, and mouse. When X server daemon is started you see a Graphical Interface and a login screen is displayed.