Boot Sequence

• BIOS
• Master Boot Record (MBR)
• LILO or GRUB
• Kernel
• Init
• Run Level’s

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BIOS

Load boot sector from one of:

• Floppy
• CDROM
• Hard Drive

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.

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(DOS) Master Boot Record (MBR)

• BIOS loads and execute the first 512 bytes off the disk (/dev/sda)
• Standard DOS MBR will:
• look for a primary partition (/dev/sda1-4) marked bootable
• load and execute first 512 bytes of this partition

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LILO

• does not understand filesystems
• code and kernel image to be loaded is stored as raw disk offsets

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

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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

• run /sbin/lilo

• reboot to test

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GRUB

• Understands file systems
• config lives in /boot/grub/menu.lst or /boot/boot/menu.lst

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Kernel

• initialize devices
• (optionally loads initrd)
• 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)

o      init prints: INIT: version 2.76 booting

o

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initrd

Allows setup to be performed before root FS is mounted

• lilo or loadlin loads ram disk image
• kernel runs /linuxrc
• load modules
• initialize devices
• /linuxrc exits
• "real" root is mounted
• kernel runs /sbin/init

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/sbin/init

• reads /etc/inittab (see man inittab which specifies the scripts below
• Run boot scripts:
• redhat: /etc/rc.d/rc.sysinit script which: loads modules, check root FS and mount RW, mount local FS, setup network, and mount remote FS
• switches to default runlevel eg 3.
• run programs specified in /etc/inittab

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Run Levels

• 0 halt
• 1 single user
• 2-4 user defined
• 5 X11 only
• 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. 

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