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Booting

booting (booting up) is a bootstrapping process that starts

operating systems when the user turns on a computer system.

Boot sequence: The initial set of operations that the computer

performs when power is switched on .

Bootloader: typically loads the main operating system for the

computer.

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Boot Sequence

Power is turned ON (It takes some time until it generates reliable

power after that it sends Power Good signal to processor) Processor always look to BIOS at the same place in the system

BIOS ROM for the start of the BIOS boot program. This isnormally location FFFF0h, right at the end of the systemmemory.

BIOS performs POST BIOS looks for vide card etc for their built in BIOS program and

runs it to initialize devices

BIOS displays its start up screen

BIOS performs some tests for RAM count and to check what kind

of hardware is in the system. BIOS will also check logicaldevices such as LPT & COM

If the BIOS supports the Plug and Play standard, it will detectand configure Plug and Play devices at this time and display amessage on the screen for each one it finds.

contd.

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The BIOS will display a summary screen about your system'sconfiguration.

The BIOS begins the search for a drive to boot from.

If the first device that the system tries (floppy, hard disk, etc.) is notfound, the BIOS will then try the next device in the boot sequence, andcontinue until it finds a bootable device.

If it finds what it is looking for, the BIOS looks for boot information tostart the operating system boot process. If it is searching a hard disk, itlooks for a master boot record at cylinder 0, head 0, sector 1 (the firstsector on the disk).

The BIOS starts the process of booting the operating system, using theinformation in the boot sector.

If no boot device at all can be found, the system will normally display

an error message and then freeze up the system.This process is called a "cold boot" (since the machine was off, or cold,when it started). A "warm boot" is the same thing except it occurs whenthe machine is rebooted using {Ctrl}+{Alt}+{Delete} or similar. In thiscase the POST is skipped

Rest of the process in O.S. dependent

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Bootloader:

CPU can only execute program code found in Read-OnlyMemory (ROM) and Random Access Memory (RAM).

Modern operating systems and application program codeand data are stored on nonvolatile data storage devices,such as hard disk drives.

When a computer is first powered on, it does not have anoperating system in ROM or RAM.

The computer must initially execute a small program storedin ROM along with the bare minimum of data needed toaccess the nonvolatile devices from which the operatingsystem programs and data are loaded into RAM .

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Bootloader:

The small program that starts this sequence of loading into RAM,

is known as a bootstrap loader, bootstrap or boot loader

Boot loader program's only job is to load other data and programswhich are then executed from RAM .

The small program is most often not itself an operating system, butonly a second-stage boot loader, such as GRUB, BOOTMGR,

LILO or NTLDR.

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Bootloader:

It will then be able to load the operating system properly, and

finally transfer execution to it.

The Operating system will initialize itself, and may load devicedrivers and other programs that are needed for the normal operation

of the OS.

Many boot loaders (like GRUB, BOOTMGR, LILO, and NTLDR)can be configured to give the user multiple booting choices.

These choices can include different operating systems (for dual ormulti-booting from different partitions or drives).

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Salient features of GRUB LILO does not understand file systems whereas GRUB

understands. The currently supported filesystem types are BSD

FFS, DOS FAT16 and FAT32, Minix fs, Linux ext2fs, ReiserFS,JFS, XFSetc.

GRUB supports both direct loading and chain loading, allowingyou to use almost any operating system, most popular filesystems, and almost any hard disk your BIOS recognize.

GRUBs configuration file is read from the disk every time thesystem boots preventing you from having to write over the MBRevery time you change the boot options.

GRUB provides a true command based pre-OS environment onx86 m/c to provide maximum flexiblity

GRUB supports logical block addressing mode, needed toaccess many IDE and all SCSI hard disks.

Support automatic decompression . Can decompress files whichwere compressed by gzip.

Support n/w booting, logical block address mode, remoteterminals.

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Anatomy of MBR (Master Boot Record)

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Top view of Booting Linux

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Booting sequence of linux

The kernel when it is loaded, finds init in sbin and executes it.

When init starts it becomes the parent or grand parent of all theprocesses that startup automatically on your linux system.

Init reads an initialization configuration script for the environmentwhich sets the path, start swapping, checks the file system,

setting the clock, initializing serial port and so on and so on. After deciding the default run level (run level 1 is single user

mode, run level 2 is multi user run level and so on) for yoursystem init starts all of the background processes necessary forthe system to run by looking in the appropriate rc directory for

that run level. After that it runs all kill scripts and then start scriptsso that all

services and applications are started correctly

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Booting XP using NTLDR

NTLDR is located at the Boot Sector (The first sector of theactive partition). That is where NTLDR is located

NTLDR will allow memory addressing, initiate the file system,read boot.ini and load the boot menu.

NTLDR has to be in root of the active partition as doNTDETECT.COM, BOOT.INI, BOOTSECT.DOS (for multi OSbooting) and NTBOOTD.SYS (if u have SCSI adopter)

Once XP is selected from the boot menu, NTLDR will runNTDETECT.COM, BOOT.INI and BOOTSECT.DOS to getproper OS selected and loaded

The system starts in 16 bit real mode and then moves to 32-bitprotected mode.

NTLDR will then load NTOSKRNL.EXE and HAL.DLL (effectivelythese 2 files are windows XP)

NTLDR reads the registry, chooses a h/w profile and authorizesdevice drivers.

At this point NTOSKRNL.EXE takes over. It starts

WINLOGON.EXE that in turn starts LSASS.EXE.

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A Modern Computer System

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Computer-System Operation

I/O devices and the CPU can execute concurrently

Each device controller is in charge of a particular device

type

Each device controller has a local buffer

CPU moves data from/to main memory to/from localbuffers

I/O is from the device to local buffer of controller

Device controller informs CPU that it has finished its

operation by causing an interrupt

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Traps and Interrupts

Interrupts

An interrupt is a signal sent to the CPU, either from

Hardware or software.

Hardware may trigger an interrupt at any time by sending a

signal to CPU and S/W may trigger an interrupt by executing

a special operations called System calls.

An Interrupt Service routine is provided for dealing with the

interrupt.

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Common Functions of Interrupts

Interrupt transfers control to the interrupt service routine

generally, through the interrupt vector, which contains theaddresses of all the service routines

Interrupt architecture must save the address of the interrupted

instruction

Incoming interrupts are disabled while another interrupt is

being processed to prevent a lost interrupt

An operating system is interruptdriven

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Traps and Interrupts

Interrupts

When the CPU is interrupted it stops what it is doing andimmediately transfers execution to fixed location.

The fixed location usually contains the starting address

where the service routine for interrupt is located.

The interrupt service routine executes; on completion, CPU

resumes the interrupted computation.

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Traps and Interrupts

Traps

Abnormal Condition detected by the CPU that usually is

indicative of an error. For example

Dividing by zero

Accessing memory that does not exist or for which

the program has not access to.

Executing an instruction with an undefined opcode or

trying to access a non existent I/O device.

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Direct Memory Access Controller

Used for high-speed I/O devices able to transmit

information at close to memory speeds

Device controller transfers blocks of data from bufferstorage directly to main memory without CPUintervention

While the DMA controller is busy in data transfer the CPUis busy to perform other task.

Only one interrupt is generated per block, rather than the

one interrupt per byte

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Storage Structure

Main memoryonly large storage media that the CPU can

access directly

Secondary storage extension of main memory that

provides large nonvolatile storage capacity e.g. magnetic

disks.

Magnetic disks rigid metal or glass platters covered with

magnetic recording material

Disk surface is logically divided into tracks, which are

subdivided into sectors

The disk controller determines the logical interaction

between the device and the computer

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Hardware Protection

Dual-Mode Operation

I/O Protection

Memory Protection

CPU Protection

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Dual-Mode Operation

Sharing system resources requires operating system to ensure

that an incorrect program or poorly behaving human cannot

cause other programs to execute incorrectly

OS must provide hardware support to differentiate betweenat least two modes of operations

1.User modeexecution done on behalf of a user

2. Monitor mode (also kernel mode or system mode)

execution done on behalf of operating system

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Dual-Mode Operation (Cont.)

Mode bitadded to computer hardware to indicate the current

mode: monitor (0) or user (1)

When an interrupt or fault occurs hardware switches to

monitor mode

monitor user

Interrupt/fault

set user mode

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I/O Protection

To prevent users from performing illegal I/O, all I/O

instructions are defined to be privileged instructions.

Thus users can not issue I/O instruction directly, they must

do it through the OS.

Must ensure that a user program could never gain control of

the computer in monitor mode (I.e., a user program that, as

part of its execution, stores a new address in the interrupt

vector)

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Use of A System Call to Perform I/O

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Memory Protection

Must provide memory protection at least for the interrupt

vector and the interrupt service routines

In order to have memory protection, at a minimum add two

registers that determine the range of legal addresses a

program may access:

Base register holds the smallest legal physical

memory address

Limit register

contains the size of the range

Memory outside the defined range is protected

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Use of A Base and Limit Register

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Hardware Address Protection

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Hardware Protection

When executing in monitor mode, the operating

system has unrestricted access to both monitor andusers memory

The load instructions for the base and limit registers

are privileged instructions

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CPU Protection

Timer interrupts computer after specified period to ensure

operating system maintains control

Timer is decremented every clock tick

When timer reaches the value 0, an interrupt occurs

Timer commonly used to implement time sharing

Timer also used to compute the current time

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Some GRUB commands

Geometry

Root

Boot

Kernel

Initrd

Rootnoverify

Chainloader

Find

Setup

lock

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Naming convention

(hd0,1) Here, `hd' means it is a hard disk drive. The first integer

`0' indicates the drive number, that is, the first hard disk, whilethe second integer, `1', indicates the partition number .

fd (floo[pydisk) cd (cd rom) fd0-> first floppy disk

(hd0,0)/vmlinuzThis specifies the file named `vmlinuz', found onthe first partition of the first hard disk