Hardware Boot Sequence
Vocabulary
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BIOS = Basic Input Output System
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UEFI = Unified Extensible Firmware Interface
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POST= Power On Self Test
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BR = Boot Record (aka MBR)
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BC = Boot Code (aka MBC)
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GUID = Globally Unique Identifier
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GPT = GUID Partition Table
BIOS
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Instructions used to start the computer from a
cold start. (power off to power on).
The BIOS instructions are written on non-volatile
RAM.
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EEP-ROM is the common media choice for the
BIOS, installed on the motherboard.
The BIOS instructions are based on the chip-set
installed on the motherboard
BIOS

The BIOS primary functions are

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Issue the instructions for starting the hardware
at boot time.
Load the OS from the boot device.
Hardware Boot Sequence

Turn on the power switch

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All memory and cache is empty at startup.
A reset signal is generated by the chipset to the
CPU until the power is ready.
The CPU powers up and reads address
xFFFF0 from the ROM. This contains a jump
instruction to the start of the BIOS instructions.
POST

The POST (Power On Self Test)

Provide power to the motherboard-speaker


This allows error-code signals to be broadcast by
the mobo-speaker when errors are detected
Test if components on the motherboard are
receiving power and functioning as intended.
Hardware Boot Sequence


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Video and keyboard are activated early in the
boot sequence. (video start instruction in ROM
address is xC000)
Other devices are activated like the disk drives,
optical drives, sound-card, NIC, USB bus, etc.
BIOS setup becomes available for the user.
BIOS: User Interface

There is a curses based user interface that
allows:
a)Selection of a boot device
b)Setting the system clock
c)Configuring hardware (includes enabling or
disabling some devices)
d)Setting passwords for booting, & the UI, etc.
e)Also shows access to some system config
information: memory size, disk size, etc.
BIOS Completion

Memory count

Search for the OS Boot device

Optical drive

USB device

Network device

HDD
HDD Boot Sequence

If the HDD is the boot device then, ...
HDD Boot Sequence


The BR is read from the first cylinder, first track,
first sector.
The BR contains the BC (boot code) and a
partition table.
HDD Boot Sequence

The boot code will do one of two things when
executed:
1. Load a boot loader (GRUB, LILO, MS-BM)
1. Allows one to choose from different OSes or
different versions of the same OS.
2. Load the OS if no boot loader is available.
Partition Table

The PT contains information about 4 primary
partitions.

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Only one of the 4 primary partitions is
designated as the active partition
The active partition contains the OS for booting.
One of the 4 primary partitions can be
designated as the extended partition.
The extended partition can be divided into
additional logical partitions.
BIOS/UEFI

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In the beginning there was the BIOS
Intel creates the Extensible Firmware Interface
in 1998
UEFI now supersedes EFI
–
UEFI can run on-top-of the traditional BIOS or
in place of the BIOS.
BIOS/UEFI

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BIOS is often used to describe UEFI
Modern OSes allow backward compatibility
between the traditional BIOS boot record and
the UEFI GPT.
GPT

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Globally Unique Identifier – This is required to
allow a file system to exceed 2TB.
The 2TB limit is a result of limitations of the
original BR partition table
(Each partition has only16 bytes of storage in the
partition table.)