Linux Boot Process

POST :- Power on self-test.

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BIOS :- Basic input output system

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MBR & GRUB will load

 

In MBR

Boot loader program 446 bytes

Partition table 64 bytes

Active sign 2 bytes

 

In GRUB three stages are there

 

1). Stage 1 :- The duty of stage 1 is to identify where is the stage 2 is located.

 

2). Stage 1.5 :- The duty of  stage 1.5 is to synchronize with deferent file systems.

                        LBA= Logical Block Addressing

At the tome of booting the BIOS can't identify more then 1024 cylinders.

Means (1Cylinder=7.8MB). Old BIOS is not identify if boot partition is more then 1024 cylinders.

 

3). Stage 2 :- It will list out the available operating systems and it will boot with the default operating system.

 

Then the kernel will load the kernel will findout the devices and it will load the drivers for the device.

·  initialize devices

·  (optionally loads initrd, see below)

·  mounts root file system

• 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 boot

Then init (parent) /etc/inittab file it will check for which run level it want load.
INIT (initialize the RAM Disk) :-
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

Run LEVEL

·  0 halt

·  1 single user

·  2 multiple user without network in CLI

• 3 multiple users with network in CLI. (The Consoles are run on "mingetty" programe).
• 4  undefined runlevel.

·  5 X11 only (0 or 1 text console)

·  6 Reboot

The Linux server is started.