System boot and linux boot

System boot
BIOS - checks memory and runs MBR
MBR - checks partition table and loads BootLoader(grub)
BootLoader - on OS selection, loads kernel
kernel starts the init process
// now booting is done

RUN LEVEL
0 - system halt(shutdown)
1 - single user
2-5 - multi-user
6 - restart
S - transitional run level

$ runlevel

Typical sysvinit startup
$ /etc/init.d/rc RL
runs the srcitsp in /etc/rc$RL.d
this is how sysvinit init worked

How upstart works
upstart jobs are defined in /etc/init/*.conf
hence, /etc/init/rc.conf loads /etc/init.d/rc $RL this way sysvinit functionality served.
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