The document discusses the boot process of a computer system. It begins with the BIOS initializing hardware and checking for a boot sector. If found, the boot sector loads the operating system kernel which takes control. Common boot devices include local hard drives, optical drives, USB drives, and network interfaces. The boot process completes when the operating system is loaded into memory and the computer is ready for use.

1. Presented by:
Anusha BABOOA
10/7/2012 1

2. 1. Introduction
2. The bootloader
3. Device drivers
4. Boot sequence on a standard PC
5. Outputting the results of an addition
6. Summary
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3. Introduction
In computing, booting (also known as "booting up") is
a bootstrapping process that starts operating systems
when the user turns on a computer system. A boot
sequence is the initial set of operations that the
computer performs when power is switched on. The
bootloader typically loads the main operating system
for the computer.
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4. The boot loader
 Modern operating systems and application program
code and data are stored on nonvolatile data storage
devices, such as hard disk drives, CD, DVD, flash
memory cards, USB flash drives disks.
 When switched on, the computer runs a relatively
small program (known as bootstrap loader or boot
loader) stored in ROM along with the least amount of
data needed to access the non-volatile devices from
which operating system programs and data are loaded
into RAM.
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5.  The main task of the bootloader is to load other data
and programs which are then executed from RAM.
 Multiple- stage boot loaders are usually used which
loads several complex programs one after the other in a
process of chain-loading.
 Computers often use automatic bootload mechanisms
to ensure that it starts quickly and with a
predetermined software configuration e.g., in desktop
computers bootstrapping starts with the CPU
executing software(e.g., BIOS )found le tin ROM at a
predefined address.
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6. Common primary boot loaders:
 BIOS (Basic Input/Output System)
 EFI (Extensible Firmware Interface)
 Open BIOS
 SLOF (Slimline Open Firmware)
These software contain essential functionality to
search for devices eligible to participate in booting.
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7. Second stage bootloader
 Small program is most often not itself an operating
system but only a second stage boot loader such as
GRUB, BOOTMGR, Syslinux, NTLDR, LILs
 The OS initializes itself and may load device driver
needed for the normal operations of the OS. After
that it starts loading normal system programs.
 Bootloaders can be configured to give the user
multiple booting choices including different
versions, loading options of the operating system.
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8.  Usually a default choice is preselected with a time
delay which a user can press a key to change the
choice, after which the default choice is automatically
run, so normal booting can occur without interaction.
 Boot process is complete when the computer is ready
to interact with the user or the OS is capable of
running system programs or application program like
spreadsheet, word processor.
 Typical personal computers boot in about 1 minute
among which 15 seconds are taken by the power-on-
self-test (POST) and a preliminary boot loader and the
rest by loading the OS and other software
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9. Network booting
o Most computers are also capable of booting over
computer network.
o In this scenario, the operating system is stored on the
disk of a server, and certain parts of it are transferred
to the client using a simple protocol such as the Trivial
File Transfer protocol.
o After these parts have been transferred, the operating
system then takes over control of the booting process.
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10. Boot devices
 The boot device is the device from which the operating
system is loaded.
 A modern PC BIOS supports booting from various
devices, typically a local hard disk drive (or one of
several partitions on such a disk), an optical disk drive,
a USB device (flash drive, hard disk drive, optical disc
drive, etc.), a flash memory card such as an SD card in
a multi-media card slot, or a network interface card
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11. Boot sequence on standard PC
 Power supply sends signal to components in system unit
 The CPU of the personal computer executes instruction
located at the memory location of the BIOS, stored in
ROM, which is located at the 0xFFFF0 linear address to
start the computer.
 BIOS performs a POWER ON SELF TEST (POST test), to
check components such as mouse, keyboard connectors,
and expansion cards, in essence, the BIOS is taking
inventory
 The BIOS goes through a pre-configured list of boot device
sequence until it finds one that can be read (bootable)from
the boot signature.
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12.  Results of the POST are compared to data in the CMOS
chip, the CMOS knows what is supposed to be detected. If
something is missing, the computer will alert you with a
tone, or an on-screen message.
 BIOS looks for boot files in a CD drive, or another drive
such as a USB flash drive, if present. If it finds no boot files,
it then looks to drive C
 Once BIOS has found a bootable device, it loads the boot
sector and transfers execution to the master boot record
(MBR).
 The MBR code checks the MBR's partition table for a
partition set as bootable.
 If an active partition is found, the MBR code loads the boot
sector code from that partition and executes it.
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13.  The main function of the boot sector is to load and
execute the operating system kernel, which continues
startup.
 If there is no active partition, or the active partition's
boot sector is invalid, the MBR may load a secondary
boot loader which will select a partition (often via user
input) and load its boot sector, which usually loads the
corresponding operating system kernel into RAM from
boot drive.
 Once the operating system is loaded in memory, it
takes control of computer.
 Operating system loads configuration information and
displays desktop on screen. OS executes any programs
In Start Up folder and the computer is ready to use.
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14. Power on self test (POST)
 The computer power-on self-test tests the computer to
make sure it meets the necessary system requirements
and that all hardware is working properly before
starting the remainder of the boot process.
 If the computer passes the POST, the computer may
have a single beep (with some computer BIOS
suppliers it may beep twice) as the computer starts and
the computer will continue to start normally.
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15. Booting Digital Signal Processors
 When, a DSP and a CPU or a microcontroller are co-
existing, the boot sequence is as following. The DSP is
booted by another processor which is called Host processor
or also Master because it usually boots first from its own
memories and then controls the overall system behaviors
(that means booting of the DSP and further controlling the
DSP’s behavior). What is interesting here is that often the
DSP lacks its own boot memories and relies on the boot
processor to supply the required code instead. Examples of
systems with such a design are cell phones, modems, audio
and video player.
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16. Outputting the results of an
addition
The processor is composed by a control unit (which
controls the operations of a computer) and a
Arithmetic Logic Unit (which performs mathematical
calculations).
 When a computer performs an addition, two signal
ALUs operate in series based on signals received from
the control unit.
 The pixel input streams and two constant values are
input to a multiplexer and delay circuit
 The multiplexer and delay circuit chooses among these
inputs to produce two inputs to the system.
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17.  The two system inputs are coupled to the first ALU
wherein the ALU performs its operation on the two
inputs based on instructions from the control unit.
 The output from this ALU is input to another multiplexer
and delay circuit wherein the inputs to the second ALU
may be selected.
 The multiplexer and delay circuit selects from among the
two original ALU inputs, the output from the first ALU
and one of the constants originally input to the first
multiplexer and delay circuit to produce two input values
for the second ALU.
 After the control unit evaluates the output from the first
ALU, the second ALU performs its operation on these two
values based on instructions from the control unit.
 The result of the second operation is communicated to an
output latch which forms a one-bit output stream and a
16-bit output stream.
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18. Summary
Outline of the boot sequence
Phase
Post
BIOS is started. Some basic
hardware checks are
performed. Hardware is Boot sector takes over the
Power on identified and initialized. BIOS computer and begins to kernel
looks for a boot sector in the start an operating system.
order of the devices specified
by the BIOS setiing.
GUI and
System startup init
beyond
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