Below are the basic stages of the boot process for an x86 system:
- The system BIOS checks the system and launches the first stage boot loader on the MBR of the primary hard disk.
- The first stage boot loader loads itself into memory and launches the second stage boot loader from the /boot/ partition.
- The second stage boot loader loads the kernel into memory, which in turn loads any necessary modules and mounts the root partition read-only.
- The kernel transfers control of the boot process to the /sbin/init program.
- The /sbin/init program loads all services and user-space tools, and mounts all partitions listed in /etc/fstab.
- The user is presented with a login screen for the freshly booted Linux system.
Detailed Process:
BIOS :
The BIOS controls not only the first step of the boot process, but also provides the lowest level interface to peripheral devices. For this reason it is written into read-only, permanent memory and is always available for use.
The BIOS then loads into memory a program is residing in the first sector of this device, called the Master Boot Record or MBR. The MBR is only 512 bytes in size and contains machine code instructions for booting the machine, called a boot loader, along with the partition table. Once the BIOS finds and loads the boot loader program into memory, it yields control of the boot process to it.
Boot loader :
The default boot loader for the x86 platform, GRUB
GRUB has the advantage of being able to read ext2 and ext3 partitions and load its configuration file — /boot/grub/grub.conf — at boot time
A boot loader for the x86platform is broken into at least two stages. The first stage is a small machine code binary on the MBR. Its sole job is to locate the second stage boot loader and load the first part of it into memory.
Once the second stage boot loader is in memory, it presents the user with a graphical screen showing the different operating systems or kernels it has been configured to boot. On this screen a user can use the arrow keys to choose which operating system or kernel they wish to boot and press Enter. If no key is pressed, the boot loader loads the default selection after a configurable period of time has passed
Boot Loaders:
Once the kernel loads and hands off the boot process to the init command, The boot loader then places one or more appropriate initramfs images into memory. Next, the kernel decompresses these images from memory to /boot/, a RAM-based virtual file system, via cpio. The initramfs is used by the kernel to load drivers and modules necessary to boot the system. This is particularly important if SCSI hard drives are present or if the systems use the ext3 file system.
Once the kernel and the initramfs image(s) are loaded into memory, the boot loader hands control of the boot process to the kernel.
The Kernel:
When the kernel is loaded, it immediately initializes and configures the computer’s memory and configures. The various hardware attached to the system, including all processors, I/O subsystems, and storage devices. It then looks for the compressed initramfs image(s) in a predetermined location in memory, decompresses it directly to /sysroot/, and loads all necessary drivers. Next, it initializes virtual devices related to the file system, such as LVM or software RAID, before completing the initramfs processes and freeing up all the memory the disk image once occupied.
At this point, the kernel is loaded into memory and operational. However, since there are no user applications that allow meaningful input to the system, not much can be done with the system.
To set up the user environment, the kernel executes the /sbin/init program.
The /sbin/init Program
The /sbin/init program (also called init) coordinates the rest of the boot process and configures the environment for the user.
When the init command starts, it becomes the parent or grandparent of all of the processes that start up automatically on the system. First, it runs the /etc/rc.d/rc.sysinit script, which sets the environment path, starts swap, checks the file systems, and executes all other steps required for system initialization
The init command then runs the /etc/inittab script, which describes how the system should be set up in each SysV init runlevel. Runlevels are a state, or mode, defined by the services listed in the SysV /etc/rc.d/rc<x>.d/ directory, where <x> is the number of the runlevel.
the init command sets the source function library, /etc/rc.d/init.d/functions, for the system, which configures how to start, kill, and determine the PID of a program.
The init program starts all of the background processes by looking in the appropriate rc directory for the runlevel specified as the default in /etc/inittab. The rc directories are numbered to correspond to the runlevel they represent. For instance, /etc/rc.d/rc5.d/ is the directory for runlevel 5.
When booting to runlevel 5, the init program looks in the /etc/rc.d/rc5.d/ directory to determine which processes to start and stop.
The name of each symbolic link begins with either a K or an S. The K links are processes that are killed on that runlevel, while those beginning with an S are started
Init process PID is 1
Running Additional Programs at Boot Time :
The /etc/rc.d/rc.local script is executed by the init command at boot time or when changing runlevels. Adding commands to the bottom of this script is an easy way to perform necessary tasks like starting special services or initialize devices without writing complex initialization scripts in the /etc/rc.d/init.d/ directory and creating symbolic links.
SysV Init Runlevels :
The SysV init runlevel system provides a standard process for controlling which programs init launches or halts when initializing a runlevel. SysV init was chosen because it is easier to use and more flexible than the traditional BSD-style init process.