Boot Process

Following is the typical boot process. Some distros don't use the initrd mechanism. Run level details can vary depending on whether the distro follows SysV or BSD conventions.

1. BIOS runs POST and loads the boot sector from the boot device.
2. Boot loader starts the kernel and loads initrd. The kernel converts initrd into a "normal" RAM disk and frees the memory used by initrd.
3. Initrd is mounted read-write as /, /linuxrc is executed (can be a shell script or program) to load additional kernel modules and mount the the "real" root file system.
4. The kernel finishes initializing hardware and calls /sbin/init (process 1).
5. Init checks the inittab to find and run the system intialization script (si:). Then init checks inittab again for the initdefault: script to determine which runlevel to start. The runlevel script loads the daemons defined for that runlevel (and the X Window system if defined in the runlevel scripts).
6. When the runlevel script is done, init checks inittab again to determine how many getty (or mingetty) processes to start. Each getty process creates a tty console and displays a login prompt.