Booting Processes/Sequences of RHEL 7

A quick view of booting sequence:

• Power on
• CPU jumps to BIOS
• BIOS runs POST
• Finds first bootable device
• Load and execute MBR
• Load OS
• User prompt

This a rough idea what happens in Linux booting. Below are the detailed stages in Linux Booting process.

Stages of Booting:

1)System startup(Hardware )

2)Boot loader Stage 1 (MBR loading)

3)Boot loader Stage 2 (GRUB loader)

4)Kernel

5)INIT

6)User prompt

Step 1: System startup

This is the first stage of booting process. When you power on/Restart your machine the power is supplied to SMPS (switched-mode power supply) which converts AC to DC. The DC power is supplied to all the devices connected to that machine such as Motherboard HDD's, CD/DVD-ROM, Mouse, keyboard etc. The most intelligent device in the computer is Processor(CPU), when supplied with power will start running its sequence operations stored in its memory. The first instruction it will run is to pass control to BIOS(Basic Input/Output System) to do POST(Power On Self Test). Once the control goes to BIOS it will take care of two things

Run POST operation.

Selecting first Boot device.

POST operation: POST is a processes of checking hardware availability. BIOS will have a list of all devices which are present in previous system boot. In order to check if a hardware is available for the present booting or not it will send an electric pulse to each and every device in the list that it already have. If an electrical pulse is returned from that device it will come to a conclusion the hardware is working fine and ready for use. If it does not receive a single from a particular device it will treat that device as faulty or it was removed from the system. If any new hardware is attached to the system it will do the same operation to find if its available or not. The new list will be stored in BIOS memory for next boot.

Selecting First Boot Device: Once the POST is completed BIOS will have the list of devices available. BIOS memory will have the next steps details like what is the first boot device it has to select etc. It will select the first boot device and gives back the control to Processor(CPU). Suppose if it does not find first boot device, it will check for next boot device, if not third and so on. If BIOS do not find any boot device it will alert user stating "No boot device found".

Step 2: The Master Boot Record

In case of hard disks and many other mass storage media, the boot sector is MBR. MBR consists of 512 bytes at the first sector of the hard disk. It is important to note that MBR is not located inside any partition. MBR precedes the first partition. The layout of MBR is as follows:

• First 446 bytes contain bootable code.
• Next 64 bytes contain partition information for 4 partitions (16x4). That is why the hard disks can have only 4 primary partitions, as the MBR can store information for 4 partitions only. So if you need more than 4 partitions on the hard disk, one of the primary partition has to be made extended, and out of this extended partition, logical partitions are created.
• Last 2 bytes are for MBR signature, also called magic number. (Thus total of 446 + 64 + 2 = 512 bytes).

|=================|======|======|======|======|===|
| 446 | 16 | 16 | 16 | 16 | 2 |
|=================|======|======|======|======|===|

The first 446 bytes of MBR contain the code that locates the partition to boot from. The rest of booting process takes place from that partition. This partition contains a software program for booting the system called the 'bootloader'.

Step 3: Boot loader Stage 2 (GRUB loader)

Once the Bootloader stage 1 is completed and able to find the actual bootloader location, Stage 1 bootloader start second stage by loading Bootloader into memory. In this stage GRUB(Grand Unified Bootloader) which is located in the first 30 kilobytes of hard disk immediately following the MBR is loaded into RAM for reading its configuration and displays the GRUB boot menu (where the user can manually specify the boot parameters) to the user. GRUB loads the user-selected (or default) kernel into memory and passes control on to the kernel. If user do not select the OS, after a defined timeout GRUB will load the default kernel in the memory for starting it.

Step 4: Kernel

Once the control is given to kernel which is the central part of all your OS and act as a mediator of hardware and software components. Kernel once loaded into to RAM it always resides on RAM until the machine is shutdown. Once the Kernel starts its operations the first thing it do is executing INIT process.

Step 5: INIT

‘init’ is the first process started by kernel (initialization process). It is parent of all processes. The PID (Process ID) of init process is always 1. This process persists till the computer halts. It is responsible for the whole state of system. The settings for this process are stored in its configuration file, /etc/inittab (system initialization table).Before diving deeper into the details of this file and proceeding any further with the boot process, let’s discuss about runlevels

Step 6: Runlevels

Runlevel is the state in which a system boots. It can boot in a single user mode, multiuser mode, with networking, and with graphics etc. Following are the default runlevels defined by Linux:

0: Halt or shutdown the system

1: Single user mode

2: Multi-user mode, without networking

3: Full multi user mode, with NFS (typical for servers)

4: Officially not defined; Unused

5: Full multi user with NFS and graphics (typical for desktops)

6: Reboot

References:

http://linoxide.com/booting/boot-process-of-linux-in-detail/

http://www.linuxnix.com/linux-booting-process-explained/