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• Dynamic linking

• Overlays

Storage Management

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Recap

In the last class, you have learnt

• Address binding

• Concepts of Dynamic loading

• Advantages of Dynamic loading

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Objectives

On completion of this class, you will be able to

know

• Dynamic linking

• Overlays

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Dynamic Linking

• Concept of dynamic linking is similar to dynamic

loading

• Instead of loading, linking is postponed till

execution

• This feature is used with system libraries

• A stub is included in the image for each library

routine reference

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Dynamic Linking

• A stub is a small piece of code that indicates

how to load the library routine

• Stub is executed

• It replaces itself with the address of the routine

and executes the routine

• Next time when that code segment is reached

the library routine is executed directly

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Dynamic Linking

printf( ){ ------- ; ------- ; ------- ;}

Memory• main( )

{

---- ;

---- ;

printf( );

---- ;

}

// stub is linked dynamically

Secondary storage

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Advantages

• Better memory utilization

• Without this facility the system libraries must

be included in the executable image

• A library may be replaced by a new version

Dynamic Linking

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Advantages

• All programs that reference the library will

automatically use new version

• Can allow multiple processes to access the

same memory address

Dynamic Linking

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Disadvantages

• Cannot detect the needed routine in another

process memory space

• Support from the operating system is required

Dynamic Linking

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Overlays

• Overlay is needed when a process is larger

than, the amount of memory allocated to it

• Keep in memory only those instructions and

data that are needed at any given time

• Other instructions and data are loaded into the

memory space

– occupied by the instructions that are no longer

needed

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Overlays

Example 1:

• Consider a two pass assembler

• Pass1 : Constructs symbol table ( 70K )

• Pass2 : Generates machine language code ( 80K )

• Common subroutines ( 30K )

• Symbol table ( 20K)

• To load every thing at once we require 200K of memory

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• But only 150K of memory is available

• Pass1 and Pass2 need not be in memory at

same time

• So define two overlays:

• Overlay A: Symbol table, Common routines

and Pass1 (120K )

• Overlay B: Symbol table, Common routines

and Pass2 (130K)

Overlays

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Overlays for a Two-Pass Assembler

Fig - 1

Example 1

Example: 2

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Overlays

Overlay Area

Overlay Manager

Main Program

Overlay 1

Overlay 2

Overlay 3

Secondary Storage

Overlay 1Overlay 2Overlay 3Overlay 1

0K

5k

7k

12k

Main Memory

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• Overlay driver / manager reads the overlays

into the memory

• We can load the given two pass assemblers

into memory and execute them

Overlays

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Advantages

• No special support is required from the operating

system

• Can execute programs larger than the size of the

memory allocated to it

• Implemented by user

Overlays

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Disadvantages

• Programming design of overlay structure is

complex

• Process execution is slower due to extra I/O

• Special relocation and linking algorithms are

needed to construct overlays

• Overlays are limited to microcomputers

Overlays

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Summary

In this class, you have learnt

• Dynamic linking

-advantages and disadvantages

• Overlays

-advantages and disadvantages

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Frequently Asked Questions

• What is meant by dynamic linking?

• Explain the concept of dynamic linking

• What are the advantages & disadvantages of dynamic

linking?

• What is meant by overlay?

• Explain the concepts of overlays

• What are the advantages & disadvantages of overlays?

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Quiz

1. Concept of Dynamic linking is similar to

a) Dynamic loading

b) Address binding

c) Both

d) None

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2. _________is postponed till execution in

dynamic linking

a) Linking

b) Loading

c) Both

d) None

Quiz

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3. _________ is a small piece of code that

indicates how to load the library routine

a) Address

b) Program

c) Stub

d) None

Quiz

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4. Support from the operating system is required

a) True

b) False

Quiz

9CM402.32 26

Quiz

5. _________is needed when process is larger

than amount of memory allocated to it

A) Address binding

b) Dynamic loading

c) Overlays

d) Dynamic linking

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6. ________reads the overlays into the memory

a) Linking driver

b) Operating system

c) Overlay driver

d) None

Quiz

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9CM402.32 29

7. Programming design of overlay structure is

_______

a) Simple

b) Complex

c) None

Quiz

9CM402.32 30

8. Overlays are limited to _____________

a) Minicomputers

b) Microcomputers

c) Mainframe computers

d) None

Quiz