Wednesday, October 20, 2010
Anu
Hi Guys, How r u? Today i'm posting a very important note for all guys n Teachers, bcz those who operate the Computers should know about the boot process of it.
In this post i will gv u the deep information about tht
Boot sequence summary
- BIOS
- Master Boot Record (MBR)
- Kernel
- init
BIOS
Load boot sector from one of:
- Floppy
- CDROM
- SCSI drive
- IDE drive
Master Boot Record
- MBR (loaded from /dev/hda or /dev/sda) contains:
- lilo
- load kernel (image=), or
- load partition boot sector (other=)
- DOS
- load "bootable" partition boot sector (set with fdisk)
- partition boot sector (eg /dev/hda2) contains:
- DOS
- loadlin
- lilo
- kernel
LILO
One minute guide to installing a newkernel
- edit /etc/lilo.conf
- duplicate image= section, eg:
o image=/bzImage-2.2.12
o label=12
o read-only
- man lilo.conf for details
- run /sbin/lilo
- (copy modules)
- reboot to test
Kernel
- initialise devices
- (optionally loads initrd, see below)
- mount root FS
- specified by lilo or loadin
- kernel prints:
- VFS: Mounted root (ext2 filesystem) readonly.
- run /sbin/init, PID 1
- can be changed with boot=
- init prints:
- INIT: version 2.76 booting
initrd
Allows setup to be performed beforeroot 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
/sbin/init
- reads /etc/inittab
- runs script defined by this line:
- si::sysinit:/etc/init.d/rcS
- switches to runlevel defined by
- id:3:initdefault:
sysinit
- debian: /etc/init.d/rcS which runs
- /etc/rcS.d/S* scripts
- symlinks to /etc/init.d/*
- /etc/rc.boot/* (depreciated)
- redhat: /etc/rc.d/rc.sysinit script which
- load modules
- check root FS and mount RW
- mount local FS
- setup network
- mount remote FS
Example Debian /etc/rcS.d/ directory
README
S05keymaps-lct.sh-> ../init.d/keymaps-lct.sh
S10checkroot.sh-> ../init.d/checkroot.sh
S20modutils-> ../init.d/modutils
S30checkfs.sh-> ../init.d/checkfs.sh
S35devpts.sh-> ../init.d/devpts.sh
S35mountall.sh-> ../init.d/mountall.sh
S35umsdos-> ../init.d/umsdos
S40hostname.sh-> ../init.d/hostname.sh
S40network-> ../init.d/network
S41ipmasq-> ../init.d/ipmasq
S45mountnfs.sh-> ../init.d/mountnfs.sh
S48console-screen.sh-> ../init.d/console-screen.sh
S50hwclock.sh-> ../init.d/hwclock.sh
S55bootmisc.sh-> ../init.d/bootmisc.sh
S55urandom-> ../init.d/urandom
Run Levels
- 0 halt
- 1 single user
- 2-4 user defined
- 5 X11
- 6 Reboot
- Default in /etc/inittab, eg
- id:3:initdefault:
- Change using /sbin/telinit
Run Level programs
- 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-delete is pressed
Typical /etc/rc3.d/ directory
When changing runlevels /etc/init.d/rc 3:
- Kills K##scripts
- Starts S##scripts
K25nfs-server-< ../init.d/nfs-server
K99xdm-< ../init.d/xdm
S10sysklogd-< ../init.d/sysklogd
S12kerneld-< ../init.d/kerneld
S15netstd_init-< ../init.d/netstd_init
S18netbase-< ../init.d/netbase
S20acct-< ../init.d/acct
S20anacron-< ../init.d/anacron
S20gpm-< ../init.d/gpm
S20postfix-< ../init.d/postfix
S20ppp-< ../init.d/ppp
S20ssh-< ../init.d/ssh
S20xfs-< ../init.d/xfs
S20xfstt-< ../init.d/xfstt
S20xntp3-< ../init.d/xntp3
S89atd-< ../init.d/atd
S89cron-< ../init.d/cron
S99rmnologin-< ../init.d/rmnologin
Boot Summary
- lilo
- /etc/lilo.conf
- debian runs
- /etc/rcS.d/S* and /etc/rc.boot/
- /etc/rc3.d/S* scripts
- redhat runs
- /etc/rc.d/rc.sysinit
- /etc/rc.d/rc3.d/S* scripts
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