1) BIOS
2) Master
Boot Record (MBR)
3) LILO or
GRUB
4) Kernel
5) init
6) Run
Levels
1) BIOS
Load boot sector from one
of:
Floppy
CDROM
The boot order can be
changed from within the BIOS. BIOS setup can be entered by
pressing a key during boot up. The exact key depends varies, but
is often one of Del, F1, F2, or F10.
2)(DOS)
Master Boot Record (MBR)
DOS in the context includes
MS-DOS, Win95, and Win98.
BIOS loads and execute the
first 512 bytes off the disk (/dev/hda)
Standard DOS MBR will:
look for a primary
partition (/dev/hda1-4) marked bootable
load and execute first 512
bytes of this partition
can be restored with fdisk
/mbr from DOS
3) LILO
does not understand
filesystems
code and kernel image to be
loaded is stored as raw disk offsets
uses the BIOS routines to
load
Loading sequence
load menu code, typically
/boot/boot.b
prompt for (or timeout to
default) partition or kernel
for "image=" (ie Linux)
option load kernel image
for "other=" (ie DOS)
option load first 512 bytes of the partition
Reconfiguring LILO
One minute guide to
installing a new kernel
copy kernel image (bzImage)
and modules to /boot and /lib/modules
edit /etc/lilo.conf
duplicate image= section,
eg:
image=/bzImage-2.4.14
label=14
read-only
man lilo.conf for details.Click
here
for manpage of lilo.conf
run /sbin/lilo
reboot to test
GRUB
Understands file systems
config lives in /boot/grub/menu.lst
or /boot/boot/menu.lst
4)Kernel
initialise devices
(optionally loads initrd,
see below)
mounts root filesystem
specified by lilo or loadin
with root= parameter
kernel prints: VFS: Mounted
root (ext2 filesystem) readonly.
runs /sbin/init which is
process number 1 (PID=1)
init prints: INIT: version
2.76 booting
can be changed with boot=
parameter to lilo, eg boot=/bin/sh can be useful to rescue a
system which is having trouble booting.
initrd
Allows setup to be
performed before root FS is mounted
lilo or loadlin loads ram
disk image
kernel runs /linuxrc
load modules
initialise devices
/linuxrc exits
"real" root is mounted
kernel runs /sbin/init
Details in /usr/src/linux/Documentation/initrd.txt
(part of the kernel source).
5) /sbin/init
reads /etc/inittab (see man
inittab which specifies the scripts below for manpage click
here)
Run boot scripts:
debian: run /etc/init.d/rcS
which runs:
/etc/rcS.d/S* scripts
/etc/rc.boot/*
(depreciated)
run programs specified in
/etc/inittab
6)Run
Levels
0 halt
1 single user
2 Full Multi-User mode
(default)
3-5 Same as 2
6 Reboot
Default is defined in /etc/inittab,
eg:
id:3:initdefault:
The current runlevel can be
changed by running /sbin/telinit # where # is the new runlevel,
eg typing telinit 6 will reboot.
Run Level programs
Scripts in /etc/rc*.d/* are
symlinks to /etc/init.d
Scripts prefixed with S
will be started when the runlevel is entered, eg
/etc/rc5.d/S99xdm
Scripts prefixed with K
will be killed when the runlevel is entered, eg
/etc/rc6.d/K20apache
X11 login screen is
typically started by one of S99xdm, S99kdm, or S99gdm.
Run programs for specified
run level
/etc/inittab lines:
1:2345:respawn:/sbin/getty
9600 tty1
Always running in runlevels
2, 3, 4, or 5
Displays login on console
(tty1)
2:234:respawn:/sbin/getty
9600 tty2
Always running in runlevels
2, 3, or 4
Displays login on console
(tty2)
l3:3:wait:/etc/init.d/rc 3
Run once when switching to
runlevel 3.
Uses scripts stored in
/etc/rc3.d/
ca:12345:ctrlaltdel:/sbin/shutdown
-t1 -a -r now
Run when control-alt-d
REFERENCE
