Boot Process of Linux

7/23/2019 Boot Process of Linux

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Welcome

Boot Process inLINUX



System Boot-up

• USER

PressesSwitch

Switch SMPS

Power Passesto

SMPS( Switch MoePower Supply! is a "in

o# RE$UL%&'R &hrou)h SMPS* Power

is Re)ulate to MainBoar an 'ther %reas

Mothe

rBoar

'ther%reas

P o w e r

P a s s

e s

t o

P o w e r P a s s e s

t o

+ ,PU %N. /%R0 0IS1 2 ,0 R'M



IN M'&/ER B'%R0 (%1% M%IN B'%R0!

BI'

S

BI'S(Basic Input3OutputSystem! is 4uilt usin) %ssem4ly

Lan)ua)e Its main #unction is toLoa 1ernel into Memory BI'S isa Pro)ram * locate at a R'M,hip in Mother4oar It is also"nown as R'M Pro)ram It1eeps the in#ormation o# all the/arware an is responsi4le #orma5or tas"s in Boot ProcessBI'S $ets Loae in MEM'R6 (Boot Strappin)!

R%M(MEM'R6! + BI'S

.,M'S

P'S&

P'S&(power-on sel# test ! is a Pro)ram thatpro7ies Status o# all the peripherals connecte tothe System an chec"s i# e7ery e7ice is#unctionin) properly or

&he BI'S per#orms the power-on sel# test (P'S&! I# there are any #atal errors* the 4oot processstopsP'S& 4eep coes can 4e #oun in this area o# the &rou4leshootin) E8pert

,M'

S

$et s

Loae

,M'S is a Pro)ram * itchec"s the 4oot e7icespriority an also ienti9es

the +st sector (: ,yliner * : &rac"s! o# /30 *;+. 4ytes in

$ e t s L o

a

e t h r u

B I ' S

,M'S

B%&&ER6

,mos(ComplementaryMetal-O8ie Semiconuctor!4attery )i7es power to

,mos pro)ram * so thatit retains all its,ontains (settin)s!

,PUPIN RESE&S

P r o 7 o " e s

,PU-E8ecute a =ump at :8: ( +st instruction a#ter

power on ! >5ump> instruction tellin) the processor where to)o to 9n the real BI'S startup pro)ram

http://www.pcguide.com/ref/mbsys/bios/bootPOST-c.html


http://www.pcguide.com/ts/x/sys/beep/index.htm







At the First Sector of HD (0 Cylinder = 0

Tracks = st Sector!

MBR

HA"DDIS#

st

Sector

R%M(MEM'R6!

+BI'S.,M'S2MBR

$e t s

Loae ( t h r u

B I ' S !Sta)e + 4oot loaer

&he primary 4oot loaer that resies in the MBR is a ;+.-4yte ima)e containin) 4oth pro)ram coe an a smallpartition ta4le (see i)ure .! &he 9rst ??@ 4ytes are theprimary 4oot loaer* which contains 4oth e8ecuta4lecoe an error messa)e te8t &he ne8t si8ty-#our 4ytesare the partition ta4le* which contains a recor #or eacho# #our partitions (si8teen 4ytes each! &he MBR enswith two 4ytes that are e9ne as the ma)ic num4er(:8%%;;! &he ma)ic num4er ser7es as a 7aliationchec" o# the MBR

&he 5o4 o# the primary 4oot loaer is to 9n an loa the

seconary 4oot loaer (sta)e .! It oes this 4y loo"in)throu)h the partition ta4le #or an acti7e partition When it9ns an acti7e partition* it scans the remainin) partitionsin the ta4le to ensure that theyAre all inacti7e When this is7eri9e* the acti7e partitionAs 4oot recor is rea #rom thee7ice into R%M an e8ecuteSta)e . 4oot loaer

&he seconary* or secon-sta)e* 4oot loaer coul 4emore aptly calle the "ernel loaer &he tas" at this sta)eis to loa the Linu8 "ernel an optional initial R%M is"



Let us Unerstan MBR in0etail

Master Boot Recor (MBR! is a ,ommon Pro)ram in 'S * whene7er you4oot your System with the Boota4le ,0 * &he Installer (%nacona inLinu8! writes the MBR at the irst Sector o# your /30 %s shown in a4o7eia)ram* MBR is i7ie into 2 main parts

$ Boot Sector (%%& Bytes! ' Boot Sector is a %rea in MBR * whichcontains the in#ormation o# Boot LoaerC li"e LIL' D $RUB (o#

Linu8 (PO)"F*+!! an N&L0R (o# Winows (+ess Po,erf-lthan +I+O . /"*B!!

Boot Loaer will 4e responsi4le #or Loain) the 1ernelC (mlinu<in our case! * a#ter BI'S assi)ns it the &%S1 to o so

$ Partition Ta1le (&% Bytes! ' Partition &a4le is a)ain a su4-i7iepart o# MBR It

has ? pro)rams o# +@ Bytes Each (? X +@ 4 F @? Bytes! EachPro)ram is Responsi4le #or each Partition in /30 /ence you cannotcreate more than ? partitions in a /30 'ut o# this ? partition * youcan create one partition as E8teneC an create Su4 partitions orLo)ical Partitions in it2$ Ma3ic 4-51er ( Bytes! ' Ma)ic Num4er 4asically shows thestatus o# other two

i7isons o# MBR I# Boot Sector an Partition &a4le are writtenSucess#ully * Ma)ic Num4er will 4e 6esC * 'therwise it will 4e NoC



BI'S

irst

,/E,1S

Ma)icNum4er

I#6esC

I#

NoC

ReportsError

Partition &a4le

,hec"s #or %cti7e Partition inP&

Boot Sector

ins the Boot Loaerin BS

BI'S now Loas the BootLoaer (LIL' or $RUB in our,ase! in Memory * a"a(irst sta)e o# LIL'! an

/ans o7er the 1ernelLoain C tas" to it

R%M(MEM'R6!

+BI'S

.,M'S2MBR?LIL' G$RUB

L o a e

LIL' G$RUB



+I+O I4FO

34oot34oot4 is a4inary 9le

,/S Num4ers is

,onsiere to 4e theM'&/ER &'N$UE o#BI'S ( Int +2 n

H!

(Int +2 n .!

( Int +2 n

.!

BIOS

unctions to ,%LL BI'S+ Int +2 #n H -- >$et ri7eparameters>. Int +2 #n . -- >Rea sectors#rom ri7e>

34oot34oot4

6 61oot6Ma7 6

&o

loa

&o

, / S

N '

(

, / S

N '

(

,/S N'

BIOS

(Int +2 n .!

BIOS

, / S N ' (

+I+O /"*B

+I+O has no interacti8eco55and interface

$RUB has interacti7e commaninter#ace

+I+O does not s-77ort

1ootin3 fro5 a net,ork

$RUB oes support 4ootin) #rom a

networ"If yo- chan3e yo-r +I+O

con:3 :le; yo- ha8e to

re,rite the +I+O sta3e

one 1oot loader to the

MB"

$RUB automatically etects any

chan)e in con9) 9le an auto

loas the 'S

+I+O s-77orts only lin-<

o7eratin3 syste5

$RUB supports lar)e num4er o#

'S

Below are )i7en some #ew important iKerences a4outLIL' an $RUB



Installer (%nacona!* pro7ies the ,ylinrical /ea Sector (,/S! num4er o#34oot34oot4 * 34oot3Messa)e D 34oot3Map 9le to LIL' LIL' can Loa allthese 9le with the help o# ,/S Num4er * howe7er it oesnCt unerstan,/S num4er * So it ,alls BI'S to help it Loa all these 9les LIL' uses a#unction (Int +2 n H! #or 34oot34oot4 D #unction (Int +2 n .! #or other9les * to ,all BI'S

%#ter unction (Int +2 n H! is e8ecute *BI'S Loas 4oot4 9le intoMemory * it is "nown as Secon sta)e o# LIL' %n When unction (Int +2

n .! is e8ecute one 4y one *BI'S Loas 4oth Messa)e an Map 9le intoMemory R%M(MEM'R6!

+ BI'S. ,M'S2 MBR? LIL' GG $RUB

; Boot4@ Messa)e Map

Important 9les in 34ootirectory

+ Boot4. Messa)e2 Map

? mlinu<; Initrim)



$ 6Messa3e has CHS 4-51er of S7lash Screen (also kno,n as#ernel +istin3s!> and it calls BIOS to +oad this ; for the sa5ereason ;as it doesn>t -nderstand CHS 4-51er$

$ 6Ma7 has CHS 4-51er of 85lin-9> and it calls BIOS to +oad this ;as it doesn>t -nderstand CHS 4-51er$6Messa3e

6Ma7

( Int +2 n.!

BIOS

( Int +2 n

.!

BIOS

/ope this is Sel#E8planatory

mlinu<

(1ernel!

,/SNo

,/SNo+ BI'S

. ,M'S

2 MBR? LIL' GG $RUB; Boot4@ Messa)e MapH mlinu<

R%M(MEM'R6!

Splash

Screen



&he "ernel is the central part o# an o7eratin3 syste5* that irectlycontrols the co57-ter hard,are Usually* the "ernel is the 9rst o# theuser-installe soft,are on a computer* 4ootin) irectly a#ter the BIOS1ernel is in 4<ip #ormat1ernel has a ,/S Num4er o# initrim) (INI&I%L R%M 0IS1! * it ,alls BI'S 4y

(Int +2 #n .! #unction an it loas initrim) into Memory Initrim) islocate at 34oot an is in )<ip #ormat Initrim) has ? main 9les

+ nash - Nash is a Minimal shell * it ta"es low memory It is useto run linu8rc script . e8t2 : - "ernel nees a ri7er 9le calle e8t2 :C #or 9lesystems

without which * youyou cannot mount any partition

2 =B0 : O ItCs a /30 is" ri7er 9le ? Linu8rc O ItCs a Script 9le * ha7in) #unctions o# e8t 2 : an =B0 :an also #unction to

mount 3C (slash! partition

Initrim) is e8tracte 3 ecompresse an its ,ontents (a4o7e ? 9les! areLoae into R%M 0IS1C

W/%& IS R%M 0IS1

% R%M is" is a portion o# R%M which is 4ein) use as i# it were a is" ri7eR%M is"s ha7e 98e si<es an act li"e re ular is" artitions %ccess time



(int +2 #n.!

BIOS

initrim)

$ e t s

E 8 t r a

c t e i n t

o R 0

a n

$ e t s

L o a

e i n t o m

e m o r y

R%M(MEM'R6!

+ BI'S

. ,M'S2 MBR? LIL'; Boot4@ Messa)e Map

H imlinu<Q e8t2:+:54:

R%M

0IS1

+ Nash. E8t2 :2 =B0 :? Linu8rc

%t R%M 0IS1 * 9rst NashC will 4eacti7ate to run the script 9le *linu8rc

&his Script 9le will now run #unctions o#e8t2 : * =B0 : an mountin) #unction o#

3C (slash! * this mountin) is also calleSysrootC

In linu8rc script Mountin) o# 3C(slash! *loo"s somethin) li"e thismount 3e73root 3 ro ------ (rea

only!

, / S N o (

XXXXXXXXXXXXXX XX EN0 ' 1ERNEL L%N0

61oot68i5lin-9



N%S/ ItCs a minimal shell Uses 7ery lowmemory Nash will 4e acti7ate9rst

LINUXR,

RUNS

ItCs a Script 9le * ha7in)

instructions to loa e8t2: an 54: an also #unction tomount 3C (slash! partition inro (rea only !

Mounts3C

partition

XXXXXXXXXXXXXX XX EN0 ' 1ERNEL L%N0

mount3e73root 3 ro



S&%R& ' USERL%N0

%s the 3C (slash! Partition is now mounte * 1ernel now opens the irst 9lewith the Inoe Num4erC &he ile it opens is locate at 3s4in3initC Init is the

7ery 9rst process to start an hence it has the pi (process I! Num4er as +Init 9le is a Binary 9le (e8ecuta4le! an it has 4een )i7en an Instructions toLoa a 9le calle3etc3initta41ernel

3s4in3init

I n o e n u m 4 e r

3etc3initta4

I n s t r u

c t i o n s t o L o a



3s4in3init 3etc3initta4

/IER%R,/6

,hec"s 0e#ault RUNLEELS

System Initialisation (System !( 3etc3rc3rcSysinit!

'pens a Su4-Shell

$ 6etc6syscon:364et,ork' &his 9le is relate to Networ"

6ou can Ena4le or 0isa4leNetwor" #rom this 9le

$ Hostna5e'

6ou can also set /'S&N%ME inthis 9le e)Ser7erE8amplecom &he0e#ault /ostname islocalhostC

2$ Mo-nt ?n ?t 7roc 6 7roc



%$ 6etc6init$d6f-nctions'@3lo1al

-5ask 3lo1al PATH; de:nes shell f-nctions @ s-ccess;fail-re;7assed;,arnin3echos-ccess echofail-re echo7assed; echo,arnin3 ;kill7roc;7idof7roc;7id:leof7roc action;check7id;con:r5;stat-s;strstr;dae5on

$ 6etc6redhatErelease' 6ou can chan)e the Release

name withthis 9le

&$ Press i> to enter interacti8eset-7'

6ou can enter intoInteracti7eC moe tocustomise your 4ootin)

$ 6etc6syscon:36clock' &his 9le contains U&, &ime

Tone&his9le upates the 9le at



G$ 6etc6syscon:36init ' 6ou can the set this 9leCs

$raphicalC

7aria4le to 6esC or NoC &his4asically

)i7es 4ootin) in#ormation in$raphics or console

0$ 6etc6sysctl$conf'

&his 9le is #or 1ERNEL tunin)

$ 6etc6syscon:36key1oard' 6ou can Unerstan ,ontrol

"eysC with this 9le

$6fast1oot' &his 9le * i# create in 3C

(slash! partition * ensures thatthe #sc"C (ile system chec"!operation is s"ippe &his 9lewill e7en s"ip the #sc"C

operation e7en i# its mentione C



2$ 6forcefsck ' &his 9le will ma"e sure * i#

create at 3C Slash Partition*

the #sc"C operation isper#orme E7en i# its notmentione in 3etc3#sta4C 9le

%$ 6etc6syscon:36readonlyEroot'

6ou can set the Entireilesystems to Reaonly F 6esCor NoC

$ 6etc6r,ta1' &his can 4e consiere as an

e8ception to3etc3syscon9)3reaonly-rootCwhich means i# you ma"e theentire 9lesystem to rea onlyan wante to )i7e e8ceptionsto certain 9les3ir then you can

ma"e use o# this 9le to )i7e - C



$ 6etc65ta1' mta4 stans #or Mount &a4C

When you run mountC

comman * it re#ers to this 9leIt "eeps the in#ormation o# allmounte partition only

$ 6s1in6-otaon'

G$)na1lin3 6etc6fsta1 s,a7s'

0$ 68ar6lo36d5es3'

3etc3rc3rc &his 9le is responsi4le #orstartin) 3 stoppin)ser7ices when runle7elchan)es

(R, F RUNLEEL,/%N$E!



3etc3rc3rc

+ ,hec"s RUNLEEL . ,hec"s RUNLEEL ,'MM%N0

#rom3etc3initta4 9le

2 3etc3rc31

3etc3rc3S

inicates RUNLEEL Num4erwhereas 1C shows that ser7ice

name with 9rst letter 1C will 4e1ILLE0 an ser7ice namestartin) with SC will 4eS&%R&E0C %ll 9les at thislocation are sym4olic lin" o# itsparent 9le at

3etc3init3ser7icename



.:

Runle7els

• % runle7el is a so#tware con9)urationo# the system which allows only aselecte )roup o# processes to e8ist

• &he processes spawne 4y init #oreach o# these runle7els are e9ne inthe 3etc3initta4 9le

•

Init can 4e in one o# se7en runle7els:-@



6s1in65in3etty ( "-nle8elE;;2! tty+ *tty.*tty2*tty?*tty;*tty@(,an 4e increase upto tty+.!

'R

6etc6J67refd5 ("-nle8el E!

0epensUpon theRunle7elset in3etc3initta

4

6etc6iss-e

&he 9le 6etc6iss-e is a te8t 9lewhich contains a messa)e orsystem ienti9cation to 4e printe4e#ore the lo)in prompt It maycontain7arious Kchar an Lchar seuenc

es



61in6lo3in+o3in co55and is use when si)nin) onto asystem It can also 4e use to switch #rom oneuser to another at any time (most moern shells

ha7e support #or this #eature 4uilt into them*howe7er!

61in67ass,d ' Will Prompt you #orPasswor

PAM

6etc67ass,d

6etc6shado,

6etc63ro-7

6etc63shado,

6$h-shlo3in

lastlo3



6etc65otd NMessa3e of theday

68ar6s7ool65ail

61in61ash

6etc67ro:le

De:nition of PATH>

Hostna5e aria1le

HISTSIQ) (C5d historyli5it!

6etc6in7-trc

6etc67ro:le$d6R$shAfter +o3in ; yo- canr-n any scri7ts fro5this locationa-to5atically$



6etc61ashrc

-5ask

PS aria4le #orPrompt

$lo4al User settin)s can 4e one at3etc3pro9le an 3etc34ashrc

3root3$ 1ashrc

3root3 1ashrc7ro:le

3root3 1ashlo3o-t

&his 9le is #orrespecti7euser 6ou canset umas" atini7iual

le7el

In this caserootC isconsiere to4e the user



EN0 ' B''& PR',ESS

&/%N1 6'U

Boot Process of Linux

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