Boot Process of SUSE

Following is the boot process of the suse linux

1. BIOS

After the computer has been turned on, the BIOS initializes the  screen and keyboard and tests the main memory. Up to this stage, the machine does not access any mass storage media. Subsequently, the information about the current date, time, and the most important peripherals are loaded from the CMOS values (CMOS Setup). When the first hard disk and its geometry are recognized, the system control passes from the BIOS to the bootloader.

2. Boot Loader

The first physical 512-byte data sector of the first hard disk is loaded into the main memory and the boot loader that resides at the beginning of this sector takes over. The commands executed by the boot loader determine the remaining part of the boot process. Therefore, the first 512 bytes on the first hard disk are referred to as the Master Boot Record (MBR). The boat loader then passes control to the actual operating system, in this case, the Linux kernel.

3. Kernel and initrd

To pass system control, the boot loader loads both the kernel and an initial RAM disk (initrd) into memory.The Linux kernel contains an option of having small file systems loaded to a RAM disk and running programs before the actual root file system is mounted. The kernel then decompresses the initrd and mounts it as a temporary root file system. The contents of initrd is a minimal Linux system that contains an executable called linuxrc. This executable is executed before the real root file system is mounted. If possible, the kernel frees the memory occupied by initrd and starts init after linuxrc terminates successfully.

4. linuxrc

This program performs all actions needed to mount the proper root file system, like providing kernel functionality for the needed file system and device drivers for mass storage controllers. As soon as
the actual root file system has been successfully mounted, linuxrc stops and the kernel starts the init program.

5. init

Init phase handles the actual booting of the system through several different levels providing different functionality.