The boot process - systemd and upstart

System

Power On

BIOS or EFI loads

BIOS scans and hands off to the primary disk's boot sector

Boot sector provides the MBR within the first 512 bytes of the active drive

Boot loader is executed:

LILO/GRUB/GRUB2

USER INPUT (choose boot types or kernel versions or allow default to proceed)

Linux kernel is read and executed

Root file system is mounted

systemd runs

default run target is executed (along with dependencies)

Root fylesystem is checked

Remaining local filesystems are mounted

Network devices are started

Remote filesystems are mounted (if configured)

Device initialization, module loading, and initial RAM disk (initrd) is loaded

Modules indicated within runlevel target (and its dependencies) are executed

systemd sets the runlevel as indicated in the default target

System login prompt is displayed on console, system is ready for login