linker and loader ppt

8/19/2019 Linker and Loader Ppt

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Loaders and Linkers

Submitted to: Submitted by:

Ramakrishna A. Sonu Devi Asst. Prof. Ayush Upadhyay

Dipti Rani

Lincoln


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Content ntroduction

!asic Loader "unctions Desi#n of an Absolute Loader

Al#orithm for an Absolute Loader

A Simple !ootstrap Loader

$achine%Dependent Loader "eatures

Relocatin# Loaders

$odification Record

Relocation !it

Pro#ram Linkin#

Linkin# Loader

Al#orithm and Data structures of a Linkin# Loader


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ntroduction

Loadin#: brin#s the ob&ect pro#ram into memoryfor e'ecution

Relocation: modifies the ob&ect pro#ram so thatit can be loaded at an address different from thelocation ori#inally specified


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Linkin#: combines t(o or more separate

ob&ect pro#rams and supplies theinformation needed to allo( referencesbet(een them.

A loader is a system pro#ram thatperforms the loadin# function


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!asic Loader "unctions

!rin#in# an ob&ect pro#ram into memory andstartin# its e'ecution.

Absolute loader: )he *eader record is checked to verify that the

correct pro#ram has been presented for loadin#

As each )e't record is read+ the ob&ect code itcontains is moved to the indicated address in memory


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,hen the -nd record is encountered+ the loader &umps to the specified address to be#in e'ecution

!ootstrap loader: loads the first pro#ram to berun by the computerusually an operatin#

system


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Desi#n of an Absolute Loader

Absolute Pro#ram Advanta#e

Simple and efficient

Disadvanta#e the need for pro#rammer to specify the actual

address

difficult to use subroutine libraries

Pro#ram Lo#ic


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Al#orithm for an Absolute Loader


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A Simple !ootstrap Loader

!ootstrap Loader

,hen a computer is first tuned on or restarted+ a special type of

absolute loader+ called bootstrap loader is e'ecuted

)his bootstrap loads the first pro#ram to be run by the computer

%% usually an operatin# system


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-'ample /SC bootstrap loader0

)he bootstrap itself be#ins at address 1

t loads the 2S startin# address 1'31

4o header record or control information+ the ob&ect code is

consecutive bytes of memory


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$achine%Dependent Loader

"eatures Relocation

Pro#ram linkin#

Linkin# loader


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Relocatin# Loaders

$otivation

efficient sharin# of the machine (ith lar#er memory and (hen

several independent pro#rams are to be run to#ether

support the use of subroutine libraries efficiently


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)(o methods for specifyin# relocation

modification record

relocation bit

each instruction is associated (ith one relocation bit

these relocation bits in a )e't record is #athered into bit

masks


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$odification Record

"or comple' machines

Also called RLD specificationRelocation and Linka#e Directory

$odification recordcol 5: $col 6%7: relocationaddress

col 3%8: len#th /halfbyte0col 51: fla# /9%0col 55%57: se#mentname


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Relocation !it

"or simple machines

Relocation bit

1: no modification is necessary

5: modification is needed

)(elve%bit mask is used in each )e't record

since each te't record contains less than 56 (ords

unused (ords are set to 1 any value that is to be modified durin# relocation must coincide (ith one

of these ;%byte se#ments

e.#. line 651

)e't recordcol 5: )col 6%7: startin# addresscol 3%8: len#th /byte0col 51%56: relocation bitscol 5;%76: ob&ect code


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Pro#ram Linkin#


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Pro#ram Linkin# -'ample

Program A Program B Program C

Label Expression LISTA, ENDA LISTB, ENDB LISTC, ENDC

REF1 LISTA local, R, PC external external

REF LISTB!" external local, R, PC external

REF# ENDA$LISTA local, A external external

REF" ENDA$LISTA!LISTC local, A external local, R

REF% ENDC$LISTC$1& external external local, A

REF' ENDC$LISTC!LISTA$1 local, R external local, A

REF( ENDA$LISTA$)ENDB$LISTB* local, A local, A external

REF+ LISTB$LISTA local, R local, R external


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Linkin# Loader

)(o important variables are PR2


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Linkin# Loader

)he last step performed by the loader is usually thetransferrin# of control to the loaded pro#ram to be#ine'ecution

)he -nd record for each control section may contain theaddress of the first instruction in that control section to bee'ecuted

f more than one control section specifies a transferaddress+ the loader arbitrarily uses the last one

encountered f no control section contains transfer address+ the loaderuses the be#innin# of the linked pro#ram as the transferpoint


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Linkin# Loader

Assi#n a reference number to each e'ternalsymbol referred to in a control section to makethe al#orithm more efficient

)he main advanta#e of this reference%numbermechanism is that it avoids multiple searches of-S)A! for the same symbol

An e'ternal reference symbol can be looked upin -S)A! once for each control section that usesit

)he values for code modification can then beobtained by simply inde'in# into an array ofthese values


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Al#orithms and Data structures for a linkin# loader

$odification record:

$ake the linkin# and relocation to be processed similarly

.

)(o passes:

Assi#n addresses to all e'ternal symbols

Perform actual loadin#+ relocation+ and linkin#.

Data structures used by linkin# loaders

-'ternal symbol tab: -S)A!

4ame+ address+ and control session belon#in# to

Also control sessions /name+ address+ len#th0

*ash table

-S)A! (ith control sessions and e'ternal symbols

http://figures/Fig-extra-Page143.pdfhttp://figures/Fig-extra-Page143.pdf


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Al#orithms and Data structures for a linkin# loader /cont.0

Pro#ram load address : PR2


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Al#orithms for Pass 5 of a linkin# loader

n Pass 5:

>ust process *eader and Define records

PR2


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n pass 6:

$ay print a load map for all control sessions and symbols+ useful in pro#ram debu##in#

Actually loadin#+ relocation+ and linkin#

CSADDR is used as in Pass 5 "or each )e't record+ the ob&ect code is placed to the address /@the address in the

instruction 9 CSADDR0.

"or each $odification record+ from the name+ lookin# for -S)A! to #et the address /in

another control session0+ then add or subtract from the indicated location.

)ransfer the control to the startin# address indicated by -=-CADDR


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)*A4 B2U

linker and loader ppt

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