Boot process for Linux

This will document the x86 boot process from a linux perspective. This document will attempt to provide a technical overview if you are not comfortable with hexidecimal, octal or binary you might want to brush up on them first.


Order of boot

  1. The BIOS completes it’s check (memory, cpu, video)
  2. The BIOS execututes the master boot code in the MBR
  3. The master boot code then has two functions identify any active partitions and any extended partitions.
  4. If the master boot code identifies a extended partition it follows the link to the extended partition and so on until it finds no additional partitions.
  5. The master boot loader moves to the active partition and turns over booting to that partition.
  6. The boot loader enters stage 1
  7. The boot loader enters stage 1.5 and displays the menu
  8. The boot loader enters stage 2 and waits for user input or default selection timout
  9. The Kernel initilizes the hardware.
  10. The boot loader loads drivers and modules out of the initrd in /boot/initrd
  11. The boot loader turns over booting to the kernel
  12. /sbin/init executes the rest of the system.
  13. int starts the run level scripts

The boot process starts with a 512 byte piece of code called the master boot record. The MBR is stored on the first 512 bytes of a drive. The BIOS accesses this section and it contains code that points to the rest of the boot process. The master boot record contains the partition table, bootloader and a section called the magic number. The bootloader takes the first 446 bytes. The partition table takes the next 64 bytes. The magic number takes the last 2 bytes.


The magic number is used as a crc check for your mbr it should always contain 0xAA55. You can dump the mbr on your system using:
dd if=/dev/hda of=/mbr.dump bs=512 count=1
This will dump the first 512 bytes of your hda drive to the file /mbr.dump. You can also rewrite this mbr to the file system using:
dd if=/mbr.dump of=/dev/hda bs=512 count=1
You can use strings to view the current boot loader:strings /mbr.dump

linuxmoney:~ # strings /mbr.dump
Hard Disk

You can view the partitions on the disk by using:file /mbr.dumpThis will produce a output listing partitions and start and stop sectors:x86 boot sector;
partition 1: ID=0x83, starthead 1, startsector 63, 417627 sectors;
partition 2: ID=0x82, starthead 0, startsector 417690, 2104515 sectors; partition 3: ID=0x83, starthead 0, startsector 2522205, 4209030 sectors;
partition 4: ID=0xf, active, starthead 0, startsector 6731235, 149565150 sectors, code offset 0x48
You can see that partition 4 is active the ID displays a the type of partition. You can find a list of partition ID codes Here. Since each sector has 512 bytes we can find the size of each partition: For example partition 1 is 417627 sectors. You can find the size using:echo $(((417627/2)/1024))You can compare this information to a df -k outputFilesystem 1K-blocks  Used      Available Use%  Mounted on
/dev/hda5 20641788    5464224   14128924  28%   /
/dev/hda6 52964408    4147160   46126764   9%   /home
/dev/hdc1 244076732   100537572 143539160 42%   /data

You can dump hex of the mbr using: od -Ad -tx1 /mbr.dump

You can also dump it using hexdump

Color Description
RED Boot Loader
GREEN 1st Partition table
YELLOW 2nd Partition table
BROWN 3rd Partition table
PINK 4th Partition table
BLUE Magic Number

You can also do a hex dump using xxd /mbr.dump


linuxmoney:~ # xxd mbr.dump
0000000: eb48 90d0 66bc 007c 0000 8ec0 8ed8 89e6  .H..f..|........
0000010: 66bf 0006 0000 66b9 0001 0000 f3a5 ea23  f.....f........#
0000020: 0600 0080 fa80 7c05 80fa 877e 02b2 8088  ......|....~....
0000030: 1649 0766 bfbe 0700 0031 f666 b904 0302  .I.f.....1.f....
0000040: ff00 0020 0100 0000 0002 fa90 90f6 c280  ... ............
0000050: 7502 b280 ea59 7c00 0031 c08e d88e d0bc  u....Y|..1......
0000060: 0020 fba0 407c 3cff 7402 88c2 52be 817d  . ..@|<.t...R..}
0000070: e836 01f6 c280 7456 b441 bbaa 55cd 135a  .6....tV.A..U..Z
0000080: 5272 4b81 fb55 aa75 45a0 417c 84c0 783e  RrK..U.uE.A|..x>
0000090: 7505 83e1 0174 3766 8b4c 10be 057c c644  u....t7f.L...|.D
00000a0: ff01 668b 1e44 7cc7 0410 00c7 4402 0100  ..f..D|.....D...
00000b0: 6689 5c08 c744 0600 7066 31c0 8944 0466  f.\..D..pf1..D.f
00000c0: 8944 0cb4 42cd 1372 05bb 0070 eb7d b408  .D..B..r...p.}..
00000d0: cd13 730a f6c2 800f 84e8 00e9 8d00 be05  ..s.............
00000e0: 7cc6 44ff 0066 31c0 88f0 4066 8944 0431  |
00000f0: d288 cac1 e202 88e8 88f4 4089 4408 31c0  
0000100: 88d0 c0e8 0266 8904 66a1 447c 6631 d266  .....f..f.D|f1.f
0000110: f734 8854 0a66 31d2 66f7 7404 8854 0b89  .4.T.f1.f.t..T..
0000120: 440c 3b44 087d 3c8a 540d c0e2 068a 4c0a  D.;D.}<.T.....L.
0000130: fec1 08d1 8a6c 0c5a 8a74 0bbb 0070 8ec3  .....l.Z.t...p..
0000140: 31db b801 02cd 1372 2a8c c38e 0648 7c60  1......r*....H|`
0000150: 1eb9 0001 8edb 31f6 31ff fcf3 a51f 61ff  ......1.1.....a.
0000160: 2642 7cbe 877d e840 00eb 0ebe 8c7d e838  &B|..}
0000170: 00eb 06be 967d e830 00be 9b7d e82a 00eb  .....}.0...}.*..
0000180: fe47 5255 4220 0047 656f 6d00 4861 7264  .GRUB .Geom.Hard
0000190: 2044 6973 6b00 5265 6164 0020 4572 726f   Disk.Read. Erro
00001a0: 7200 bb01 00b4 0ecd 10ac 3c00 75f4 c300  r.........<.u...
00001b0: 0000 0000 0000 0000 5147 0a00 0000 0001  ........QG......
00001c0: 0100 83fe 3f19 3f00 0000 5b5f 0600 0000  ....?.?...[_....
00001d0: 011a 82fe 3f9c 9a5f 0600 c31c 2000 0000  ....?.._.... ...
00001e0: 019d 83fe 7fa2 5d7c 2600 8639 4000 8000  ......]|&
00001f0: 41a3 0ffe ffff e3b5 6600 de2e ea08 55aa  A.......f.....U.

linuxmoney:~ # hexdump mbr.dump
0000000 48eb d090 bc66 7c00 0000 c08e d88e e689
0000010 bf66 0600 0000 b966 0100 0000 a5f3 23ea
0000020 0006 8000 80fa 057c fa80 7e87 b202 8880
0000030 4916 6607 bebf 0007 3100 66f6 04b9 0203
0000040 00ff 2000 0001 0000 0200 90fa f690 80c2
0000050 0275 80b2 59ea 007c 3100 8ec0 8ed8 bcd0
0000060 2000 a0fb 7c40 ff3c 0274 c288 be52 7d81
0000070 36e8 f601 80c2 5674 41b4 aabb cd55 5a13
0000080 7252 814b 55fb 75aa a045 7c41 c084 3e78
0000090 0575 e183 7401 6637 4c8b be10 7c05 44c6
00000a0 01ff 8b66 441e c77c 1004 c700 0244 0001
00000b0 8966 085c 44c7 0006 6670 c031 4489 6604
00000c0 4489 b40c cd42 7213 bb05 7000 7deb 08b4
00000d0 13cd 0a73 c2f6 0f80 e884 e900 008d 05be
00000e0 c67c ff44 6600 c031 f088 6640 4489 3104
00000f0 88d2 c1ca 02e2 e888 f488 8940 0844 c031
0000100 d088 e8c0 6602 0489 a166 7c44 3166 66d2
0000110 34f7 5488 660a d231 f766 0474 5488 890b
0000120 0c44 443b 7d08 8a3c 0d54 e2c0 8a06 0a4c
0000130 c1fe d108 6c8a 5a0c 748a bb0b 7000 c38e
0000140 db31 01b8 cd02 7213 8c2a 8ec3 4806 607c
0000150 b91e 0100 db8e f631 ff31 f3fc 1fa5 ff61
0000160 4226 be7c 7d87 40e8 eb00 be0e 7d8c 38e8
0000170 eb00 be06 7d96 30e8 be00 7d9b 2ae8 eb00
0000180 47fe 5552 2042 4700 6f65 006d 6148 6472
0000190 4420 7369 006b 6552 6461 2000 7245 6f72
00001a0 0072 01bb b400 cd0e ac10 003c f475 00c3
00001b0 0000 0000 0000 0000 4751 000a 0000 0100
00001c0 0001 fe83 193f 003f 0000 5f5b 0006 0000
00001d0 1a01 fe82 9c3f 5f9a 0006 1cc3 0020 0000
00001e0 9d01 fe83 a27f 7c5d 0026 3986 0040 0080
00001f0 a341 fe0f ffff b5e3 0066 2ede 08ea aa55

You can manually decode the partition table using the following
information. Remember to flip the bytes to get the correct order e.g
0080 becomes 80 00.

Offset Size Description
0x00 1 byte Active flag 0x80 active otherwise 0x00
0x01 3 bytes Cylinder-head-sector address of the first sector in the partition
0x04 1 byte Partition type
0x05 3 bytes Cylinder-head-sector address of the last sector in the partition
0x08 4 bytes Logical block address of the first sector in the partition
0x0C 4 bytes Length of Parition in sectors

+--- Active partition flag 80H for active partition
|      +--- Cylinder-head-sector address of the first sector in the partition
|      |
|      |    +--- Partition Type List here.
|      |    |
|      |    |     +--- Cylinder-head-sector address of the last sector.
|      |    |     |
|      |    |     |        +--- Logical block address of the first sector.
|      |    |     |        |
|      |    |     |        |	    +--- Size of Parition in sectors.
|      |    |     |        |        |
-- -------- -- -------- -------- --------
00 01 01 00 83 fe 3f 19 3f000000 5b5f0600	1st Partition
00 00 01 1a 82 fe 3f 9c 9a5f0600 c31c2000	2nd Partition
00 00 01 9d 83 fe 7f a2 5d7c2600 86394000	3rd Partition
80 00 41 a3 0f fe ff ff e3b56600 de2eea08	4th Partition

Decoding CHS

The CHS is used to decode the location of the first of the
partition if that location exists within the first 1024 cylinders of
the hard drive. When the location goes beyond that location the CHS
value is normaly set to the max values of 1024,254,63 or FE FF FF.
Decoding the values can be a challenge without switching to the binary
value. They are stored in the order of head, sector, and cylinder, the
cylinder value requires more than 8 bits (1 byte) the sector value uses
less than 8 bits, so you have to convert the values to binary to decode them:

If the ending value for cylinder is 1023 or above then you have to figure out the ending location by adding the size to the starting location.

Remember that we can only have four partition tables per disk. This
is why extended paritions were created an extended partition uses a
link table to create unlimited partitions. The partition entries on the
table are top down. The first partition on the physical disk is the
last entry in the MBR partition table.

Extended Partitions 
      Extended Partitions are a way of getting around the four
partition limit on file systems. Extended partitions cannot be marked
as active or used as a boot device. The extended partition section in
the MBR can describe up to at least 23 (Old DOS) additional partitions
under linux the amount of partitions possible is much higher. Extended
partitions have the partition type of 05h or 0Fh depending on size of
the disk. Extended partition boot records duplicate the MBR. Normally
the first 446 bytes of the extended section is empty (LILO and GRUB
both use it for internal code) The partition table is then full of
partition entries followed by the aa55 code. In extended partitions the
total size (LBA) is the size of all extended partitions.

Grub stands for GRand Unified Bootloader. It is the most common boot loader for linux today. The boot process with GRUB is as follows:

  1. Starts executing bootloader code (GRUB stage 1) (boot.img).
  2. Bootloader jumps to the sector number of next stage. The stage 1.5 located in the “DOS compat space” immediately after the MBR.
  3. Stage 1.5 loads the file system and make full drive size available for loading. (diskboot.img+kernel.img+pc.mod+ext2.mod)
  4. Stage 2 takes over and loads the boot menu. (normal.mod+_chain.mod)
  5. After your selection the operating system is loaded.
Grub files are located in /boot/grub here you can find the stage1 stage2 and the menu.1st or grub.conf files. The configuration is done in the menu.1st or grub.conf file.
linuxmoney:/ # ls -al /boot/grub/
total 228
drwxr-xr-x 2 root root   4096 Sep 27 18:12 .
drwxr-xr-x 3 root root   4096 Jun 30 17:43 ..
-rw------- 1 root root     30 Jun 30 17:43
-rw------- 1 root root     30 Jun 30 17:37
-rw-r--r-- 1 root root   7552 Nov 25  2006 e2fs_stage1_5
-rw-r--r-- 1 root root   7424 Nov 25  2006 fat_stage1_5
-rw-r--r-- 1 root root   6688 Nov 25  2006 ffs_stage1_5
-rw-r--r-- 1 root root   6688 Nov 25  2006 iso9660_stage1_5
-rw-r--r-- 1 root root   8160 Nov 25  2006 jfs_stage1_5
-rw------- 1 root root   1385 Jun 30 17:43 menu.lst
-rw------- 1 root root   1188 Jun 30 17:36 menu.lst.joe
-rw------- 1 root root   1385 Jun 30 17:37 menu.lst.old
-rw-r--r-- 1 root root   6848 Nov 25  2006 minix_stage1_5
-rw-r--r-- 1 root root   9216 Nov 25  2006 reiserfs_stage1_5
-rw-r--r-- 1 root root    512 Nov 25  2006 stage1
-rw-r--r-- 1 root root 104042 May 19 11:13 stage2
-rw-r--r-- 1 root root   7040 Nov 25  2006 ufs2_stage1_5
-rw-r--r-- 1 root root   6240 Nov 25  2006 vstafs_stage1_5
-rw-r--r-- 1 root root   8904 Nov 25  2006 xfs_stage1_5

To reinstall grub in your mbr type:grub-install /dev/hda      Configuration for grub is done inside grub.lst (normally in /boot/grub/grub.lst) this file has the following settings:# Comments inside grub.lst ae done with a hash mark (#)


# default defines the default choice to boot without user interaction
default 0
# Time out sets how long the boot menu will display before it loads default
timeout 30
# fallback provides a another choice in case default fails.
fallback 1
# hiddenmenu allows you to choose not to display the boot menu instead boot the default
# hiddenmenu
# OS definitions begin with a title title is what is displayed on the screen to the user
title openSUSE 10.2 –
# After the title description everything that follows is part of the same boot loader until the title tag appears again.
# Common entries in linux are root, kernel, and initrd
# root defines the root partition and tries to get the size of the partition hd0 partition 4
root (hd0,4)
# kernel attempts to load the kernel image off the root device
kernel /boot/vmlinuz- root=/dev/hda5 vga=0x31a resume=/dev/hda2 splash=silent showopts
# initrd Load an initial ramdisk (allows you to modify the kernel without a recompile
initrd /boot/initrd-
Command Line Options
While at the boot menu you can also pass grub command line variables like what runlevel to boot into or additional options. To choose the run level to boot the kernel into:

  • On the graphical menu highlight the kernel you wish to boot
  • Press the e button to edit the kernel selection
  • At the prompt type the number of the run level you wish to boot into (1 to 5) single or emergecy
  • Once returned to the grub menu press b too boot the kernel and runlevl selection
      You can read more about grub options at The GNU grub menu.
      LILO (LInux LOader) is a generic boot loader for Linux. Lilo is an older boot loader it follows the same process as GRUB. Unfortunatly, it does not contain a command line interface like grud making MBR changes required each time you want to change boot parameters. Also changes to LILO can cause the system to fail to boot. It is for this reason alone that GRUB has become the standard boot loader of linux. Lilo keeps some files in /boot but it's configuration is done in /etc/lilo.conf. To reinstall lilo as the boot loader:/sbin/lilo
Command Line Options 
      While at the boot menu you can choose what runlevel you want to boot by pressing:
  • Ctrl-X to get boot:
  • Type linux runlevel


Once the boot loader has reached second stage it reads it’s configuration ahd displays a menu of available kernels to boot. Once the user or boot loader determines what kernel to load stage two boots the kernel file off the /boot partition. Once the kernel is loaded the first step is to initialize the hardware. Then the kernel loading is reading the initrd image this file contains drivers required by the kernel to load scsi devices and ext3 file systems. Once initrd image is completely loaded the boot loader turns the booting process over to the kernel file. The kernel creates a read-only root device and mounts it. At this point the kernel is loaded but since no user space files are loaded you cannot interact with it. This is where /sbin/init takes over.


init is what process the rest of the boot and provides the user environment. init becomes the parent or grandparent process for all processes on a system it has a pid of 1 always. It first runs the /etc/rc.d/rc.sysinit script that starts swap, system clock, check file systems and many other processes. It the runs /etc/inittab which sets up the run levels.



A runlevel is a collection of scripts used to start applications and services used by a system.  Linux supports multiple runlevels.  You can change between runlevels very quickly on a Linux system dismounting file systems as you go.  The configuration for the runlevels is done inside the /etc/inittab file.  You can find the default runlevel inside inittab:


The default run level on this system is 5 which is multiuser with graphical X windows interface.  inittab also possibly defines:

  • First script to be executed before runlevels /etc/init.d/boot
  • Defines the RC scripts to be executed with each run level
  • It also defines special keyboard commands
  • The getty-programs for each run level

    The /etc/init.d/boot defines the following settings:

  • Sets the terminal size and dimentions for the terminal
  • Starts the initial boot messages and coloring
  • Sets up /proc /sys /dev /sys/kernel/debug
  • Starts user defined scripts boot.local

The default runlevels for Linux are:

Runlevel  State 
 0  Shutdown
 1  Single User Mode
 2  Multiuser without network
 3  Multiuser text based
 4  Unused
 5  Multiuser with Graphical X
6  Reboot

You can quickly change the runlevels using:

init runlevel

Each runlevel executes the scripts contained inside /etc/init.d/rc_runlevel.  The scripts inside here are normaly symbolic links to scripts inside /etc/init.d/ these scripts should take at least two variables stop and start.  The links inside /etc/init.d/rc_runlevel are of two types kill (K) scripts and start (S) scripts.  The type is followed by a two digit number used to denote the order inside this runlevel for the script to be executed.  For example:

# ls -al
total 8
drwxr-xr-x  2 root root 4096 Sep  2 21:52 .
drwxr-xr-x 11 root root 4096 Nov 16 20:25 ..
lrwxrwxrwx  1 root root    9 Sep  2 21:52 K02single -> ../single
lrwxrwxrwx  1 root root   12 Sep  2 21:52 K13microcode -> ../microcode
lrwxrwxrwx  1 root root    9 Sep  2 21:52 K13splash -> ../splash
lrwxrwxrwx  1 root root    8 Sep  2 21:52 K21fbset -> ../fbset
lrwxrwxrwx  1 root root   15 Sep  2 21:52 K21irq_balancer -> ../irq_balancer
lrwxrwxrwx  1 root root    8 May 19 10:46 S01fbset -> ../fbset
lrwxrwxrwx  1 root root   15 May 19 10:45 S01irq_balancer -> ../irq_balancer
lrwxrwxrwx  1 root root    6 May 19 10:47 S09kbd -> ../kbd
lrwxrwxrwx  1 root root   12 May 19 10:51 S09microcode -> ../microcode
lrwxrwxrwx  1 root root    9 May 19 10:47 S09splash -> ../splash
lrwxrwxrwx  1 root root    9 May 19 10:47 S20single -> ../single
You can see that I have many files that start as part of runlevel 1 for example S09splash starts before S20single.  It is very easy to automatically add an item to a run level using chkconfig in linux.  For example if I wanted to see if a script in /etc/init.d is started at runtime use the following command:

# chkconfig -l apache2
apache2  0:off  1:off  2:off  3:on   4:off  5:on   6:off

chkconfig can also be used to turn on specific run levels using

#chkconfig service_name runlevel/runlevels 

For example:

#chkconfig apache2 235

Will start the apache2 script in /etc/init.d on runlevel 2, 3 and 5.  You can manually add the links using ln.  Also running chkconfig alone will display all scripts and they status at the current run level or chkconfig -l will display all runlevels.




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