Boot Process

All computer systems begin the boot process through executing the code in ROM (specially, the BIOS) to load the sector from sector 0 of the boot drive. The boot drive is commonly the first hard disk. BIOS then try to execute this sector. On all hard disks, sector 0, holds the beginning of an operating system kernel, like as Unix.

Once the Kernel is fully loaded, it goes by a few basic device initializations. Then the kernel has to look for the root file system; it halts if it does not find a loadable image there.

At this point the system searches the init program on the root file system (in /bin or /sbin) and executes it. An init program reads its configuration file /etc/inittab, such as for a line-designated sysinit and executes the named script. The sysinit script is commonly something such as /etc/rc.d/rc.sysinit. This given script is a set of shell commands in which set up the basic system services like as given below

1. Loading necessary Kernel modules
2. Starts swapping,
3. Running fsck on all disks,
4. Initializing the network
5. Mounting the disks mentioned in /etc/fstab

This script frequently invokes several other scripts to do modular initialization. For instance, in the general SysV init structure, the directory /etc/rc.d holds a complex structure of subdirectories whose files specify how to shutdown and startup most system services. Through, on a boot disk the sysinit script is frequently extremely easy.