Download The Boot Process

IT2204:
Systems Administration I
8b).Booting
(Supplementary
Notes)
Derived From Ubuntu
Unleased, Chapter 14: The
Boot Process
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A sneak pick into the booting
process
• After you turn on the power switch, the boot
process begins with the computer executing code
stored in a chip called the BIOS (Basic Input
Output System); this process occurs no matter what
operating system you have installed.
• The Linux boot process begins when the code
known as the boot loader starts loading the Linux
kernel and ends only when the login prompt
appears.
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• Although most people consider a computer to be either
on or off, in Ubuntu and Linux in general, there are a
number of states in between. Known as runlevels, they
define what system services are started upon boot.
• These services are simply applications running in the
background that provide some needed function to your
system, such as getting information from your mouse
and sending it to the display; or a service could monitor
the partitions to see whether they have enough free
space left on them. Services are typically loaded and run
(also referred to as being started) during the boot
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process
Beginning the Boot Loading
Process
• The BIOS (basic input/output system) is an application
stored in a chip on the motherboard that initializes the
hardware on the motherboard (and often the
hardware that’s attached to the motherboard). The
BIOS gets the system ready to load and run the
software that we recognize as the operating system.
• As a last step, the BIOS code looks for a special
program known as the boot loader or boot code. The
instructions in this little bit of code tell the BIOS
where the Linux kernel is located, how it should be
loaded into memory, and how it should be started.
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• If all goes well, the BIOS looks for a bootable volume
such as a floppy disk, CD-ROM, hard drive, RAM disk,
USB drive, or other media. The bootable volume
contains a special hexadecimal value written to the
volume by the boot loader application.
• The BIOS searches volumes in the order established by
the BIOS settings (for example, USB first, followed by a
DVD-ROM, and then a hard drive) and then boots
from the first bootable volume it finds. Modern BIOSs
allow considerable flexibility in choosing the device
used for booting the system.
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Note:
• If the BIOS detects a hardware problem, the
boot process fails, and the BIOS generates a
few beeps from the system speaker. These
“beep codes” indicate the nature of the
problem the BIOS has encountered.
• The codes vary among manufacturers. If you
encounter a problem, troubleshoot, consult the
motherboard manual or contact the
manufacturer of the motherboard.
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• Next, the BIOS looks on the bootable volume for boot
code in the partition boot sector also known as the Master
Boot Record (MBR) of the first hard disk. The MBR contains the
boot loader code and the partition table—think of it like an
index for a book, plus a few comments on how to start
reading the book.
• If the BIOS finds a boot loader, it loads the boot loader
code into memory. At that point, the BIOSs job is
completed, and it passes control of the system to the boot
loader.
• The boot loader locates the Linux kernel on the disk and
loads it into memory. After that task is completed, the boot
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loader passes control of the system to the Linux kernel.
Loading the Linux Kernel
• In a general sense, the kernel manages the system
resources. As the user, you do not often interact
with the kernel, but instead you interact with the
applications that you are using.
• Linux refers to each application as a process, and
the kernel assigns each process a number called a
process ID (PID). Traditionally, the Linux kernel loads
and runs a process named init, which is also known
as the “ancestor of all processes” because it starts
every subsequent process. Ubuntu has replaced
init with Upstart, which is being written by
Ubuntu developers and made available for any
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distribution to use.
• This next step of the boot process traditionally
begins with a message that the Linux kernel is
loading, and a series of messages that are printed
to the screen, giving you the status of each
command. A failure should display an error
message.
• The quiet option may be passed to the kernel at
boot time to suppress many of these messages.
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• Ubuntu does not display these messages by default, but
instead uses a boot process created by the Fedora/Red
Hat developers called Plymouth that is fast and
incorporates a beautiful boot screen.
• If the boot process were halted at this point, the
system would just sit idle and the screen would be
blank. To make the system useful for users, we need to
start the system services. Those services are some of
the applications that allow us to interact with the
system.
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System Services and
Runlevels
• The init command traditionally boots a Linux system to a
specific system state, commonly referred to as its runlevel.
• Runlevels determine which of the many available system
services are started, as well as in which order they start.
A special runlevel is used to stop the system, and a special
runlevel is used for system maintenance. There are other
runlevels for special purposes.
• You traditionally use runlevels to manage the system
services running on a Linux computer. All these special
files and scripts are set up during installation but you can
change and control them manually.
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Runlevel Definitions
• The runlevels are defined in a traditional Linux system in
/etc/init.d.
• Each runlevel tells the init command what services to start or
stop. Although runlevels might all have custom definitions,
Ubuntu has adopted some standards for runlevels:
• Runlevel 0—Known as “halt,” this runlevel is used to shut
down the system.
• Runlevel 1—This is a special runlevel, defined as “single,”
which boots Ubuntu to a root access shell prompt where only
the root user may log in. It has networking and multi-user access
turned off. This is the maintenance or rescue mode. It allows the
system administrator to perform work on the system, make
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backups, or repair configuration or other files.
• (Runlevel 1 (also known as single-user mode or
maintenance mode) is most commonly used to repair file
systems and change the root password on a system when
the password has been forgotten. Trespassers with
physical access to the machine can also use runlevel 1 to
access your system.)
• Runlevel 2—This is the default runlevel for Ubuntu.
• Runlevels 3–5—These runlevels aren’t used in
Ubuntu but are often used in other Linux
distributions.
• Runlevel 6—This runlevel is used to reboot the
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system.
Booting into the Default
Runlevel
• Ubuntu boots into runlevel 2 by default, which means it
starts the system as normal and leaves you inside the X
Window System looking at the graphical login prompt. It
knows what runlevel 2 needs to load by looking in the
rc*.d directories in /etc. Ubuntu contains directories for
rc0.d through to rc5.d and rcS.d.
• Assuming that the value is 1, the rc script then executes
all the scripts under the /etc/rc.1 directory and then
launches the graphical login.
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Booting into the Default
Runlevel
• If Ubuntu is booted to runlevel 1, for example, scripts
beginning with the letter K followed by scripts beginning
with the letter S under the /etc/rc1.d directory are then
executed:
• matthew@seymour:~$ ls /etc/rc1.d/
• K10jackd K20rsync K20vboxdrv K80cups S70pppd-dns
• System service scripts that reside in the /etc/init.d
directory.
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• The rc1.d links are prefaced with a letter and number,
such as K15 or S10. The K or S in these prefixes indicate
whether a particular service should be killed (K) or
started (S) and pass a value of stop or start to the
appropriate /etc/init.d script.
• The number in the prefix executes the specific
/etc/init.d script in a particular order. The symlinks have
numbers to delineate the order in which they are
started.
• Nothing is sacred about a specific number, but some
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services need to be running before others are started.
Final Stage of Initialization
• Each /etc/init.d script, or init script, contains logic
that determines what to do when receiving a start
or stop value. The logic might be a simple switch
statement for execution.
• After all the system scripts have been run, your
system is configured and all the necessary system
services have been started. If you are using a
runlevel other than 5, the final act of the init process
is to launch the user shell—bash, tcsh, zsh, or any of
the many command shells available. The shell
launches, and you see a login prompt on the screen.
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• Other notes that may be
helpful
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Why is Booting Required ?
• Hardware doesn’t know where the operating system
resides and how to load it.
• Need a special program to do this job – Bootstrap
loader.
– E.g. BIOS – Boot Input Output System.
• Bootstrap loader locates the kernel, loads it into
main memory and starts its execution.
• In some systems, a simple bootstrap loader fetches a
more complex boot program from disk, which in
turn loads the kernel.
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How Boot process occurs ?
• Reset event on CPU (power up, reboot)
causes instruction register to be loaded with a
predefined memory location. It contains a
jump instruction that transfers execution to
the location of Bootstrap program.
• This program is in form of ROM, since RAM is
in unknown state at system startup. ROM is
convenient as it needs no initialization and
can’t be affected by virus.
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BIOS Interaction
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Tasks performed at boot up
• Run diagnostics to determine the state of machine. If
diagnostics pass, booting continues.
• Runs a Power-On Self Test (POST) to check the
devices that the computer will rely on, are
functioning.
• BIOS goes through a preconfigured list of devices
until it finds one that is bootable. If it finds no such
device, an error is given and the boot process stops.
• Initializes CPU registers, device controllers and
contents of the main memory. After this, it loads the
OS.
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BIOS Setup
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Boot Procedure
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Tasks performed at boot up
(Contd)
• On finding a bootable device, the BIOS loads and
executes its boot sector. In the case of a hard drive,
this is referred to as the master boot record (MBR)
and is often not OS specific.
• The MBR code checks the partition table for an
active partition. If one is found, the MBR code loads
that partition's boot sector and executes it.
• The boot sector is often operating system specific,
however in most operating systems its main function
is to load and execute a kernel, which continues
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startup.
Secondary Boot Loaders
• If there is no active partition or the active partition's
boot sector is invalid, the MBR may load a secondary
boot loader and pass control to it and this secondary
boot loader will select a partition (often via user
input) and load its boot sector.
• Examples of secondary boot loaders
– GRUB – GRand Unified Bootloader
– LILO – LInux LOader
– NTLDR – NT Loader
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GRUB Loader
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Booting and ROM
• System such as cellular phones, PDAs and
game consoles stores entire OS on ROM.
Done only for small OS, simple supporting
hardware, and rugged operation.
• Changing bootstrap code would require
changing ROM chips.
– EPROM – Erasable Programmable ROM.
• Code execution in ROM is slower. Copied to
RAM for faster execution.
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Example : DOS
• After identifying the location of boot files, BIOS looks
at the first sector (512 bytes) and copies information
to specific location in RAM (7C00H) - Boot Record.
• Control passes from BIOS to a program residing in
the boot record.
• Boot record loads the initial system file into RAM. For
DOS, it is IO.SYS .
• The initial file, IO.SYS includes a file called SYSINIT
which loads the remaining OS into the RAM.
• SYSINIT loads a system file MSDOS.SYS that knows
how to work with BIOS.
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Example : DOS
• One of the first OS files that is loaded is the
system configuration file, CONFIG.SYS in case of
DOS. Information in the configuration file tells
loading program which OS files need to be
loaded (e.g. drivers)
• Another special file that is loaded is one which
tells what specific applications or commands user
wants to be performed as part of booting
process. In DOS, it is AUTOEXEC.BAT. In
Windows, it’s WIN.INI .
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References
• Operating System Principles – Silberchatz,
Galvin and Gagne.
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Questions
• What is the effect on boot sector and boot
loader when you install two OSs, for e.g.
Windows and Linux in two separate partitions ?
• Suppose, you install Windows first. The default
boot loader installed in MBR is NTLDR and
contains information regarding the active
partition of Windows. When you install Linux on
this system, the installation prompts to overwrite
a new secondary boot loader which identifies
both Windows and Linux active partitions and
therefore we get a choice of booting the desired
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OS when the system is started.
Questions Cont’d
In contrast, if Linux is installed first and then
Windows, the Windows Installer overwrites
the MBR with its own boot loader which
doesn’t recognize the Linux active partition.
This creates a problem.
The problem can be corrected by using a
LiveCD or any bootable disc which can be
used to reinstall a secondary boot loader
which identifies both the OSs and gives true
choice.
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Questions
• If I have only a single OS, is there a secondary boot
loader present on the system ?
• Older machines might not have this feature. But nowa-days, even Windows is installed with a default
secondary boot loader (NTLDR). Linux is also
installed usually with LILO or GRUB as the default
boot loader.
You might have also encountered that your system
(Windows) is not able to boot after flashing the BIOS
and is displaying the message “NTLDR missing”. This
is because the primary boot loader transfer to
NTLDR which might have become corrupt or deleted
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by mistake.
Questions
• Difference between Boot Loader and Boot
Manager ?
• Basically, these two are different but are
sometimes combined into a single program.
• IBM's Boot Manager, PowerQuest's BootMagic and
V Communications' System Commander are some
examples of boot managers.
• Dual booting is the act of installing multiple
operating systems on a computer, and being able
to choose which one to boot when switching on
the computer. The program, which makes dual
booting possible is called a boot loader.
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Information
• Dual Boot
• In the OS/2 world, the term dual boot has a
more specific meaning.
• In a dual boot installation, two (or more)
operating systems are installed in a single
partition. Selection of which operating system to
boot is performed by running a dual boot utility
program, which switches around the necessary
boot loaders programs (by renaming files and
copying boot sectors) to ensure that the chosen
operating system is loaded at the next boot. 37
Information
• In a boot manager installation, by contrast, the two
(or more) operating systems are installed in their
own, separate, individual, partitions. Rather than
booting directly into an operating system, the
machine boots into a specialised, operating system
neutral, boot loader program (such as IBM's
eponymous Boot Manager) installed on a floppy disk
or in its own partition on a hard disk.
• This boot loader program presents a list of the
available bootable partitions from which the user
can choose, and then loads and invokes the boot
loader in the boot sector of the chosen partition,
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to boot the chosen operating system.
Information
• When installing an OS on a computer from scratch,
here is how the partition table is created.
• The hard disk is denoted as “hda” where hd=hard disk,
and the third letter could mean the hard-disk on the
system. For e.g. the first hard disk is “hda”, the second
is “hdb”.
• When the partitioning is done, “hda0” is the place of
MBR. “hda1” is the primary partition. Then a
secondary partition may be created which is further
subdivided into logical drives. Another OS could be
installed on any of these logical drives.
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hda0 – MBR
hda1 – Primary Partition e.g. Windows XP
hda2 – Secondary Partition
hda3 – Logical Drive 1 (FAT32 or NTFS partition)
hda4 – Logical Drive 2 (FAT32 or NTFS partition)
hda5 – Logical Drive 3 (Swap for Linux Partition)
hda6 – Logical Drive 4 (Root for Linux Partition)
The above example is a simple example. Specific cases
can be different.
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Information
• When the kernel is being loaded, the control is in the
privileged mode. If the user is allowed to login in the
same mode, any user will be “root” or
“administrator” (super-user).
• When the booting is almost complete, which is with
the privileged right. But this login program, after
verifying your password, gives you a shell by creating
another process which intentionally drops the superuser privileges and assume the privileges of this user.
Login program is trusted by the kernel. If that is
hacked or replaced, you can get a root shell from any
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login.
Q&A