Dynamic linking and overlays are techniques for improving memory utilization in operating systems. Dynamic linking postpones linking of library routines until execution using stubs. This allows better memory usage and automatic use of new library versions. Overlays improve memory usage for large programs by loading only required parts into memory at a given time using an overlay manager. Both have advantages of improved memory usage but overlays require complex programming and are slower.

1. Storage Management
• Dynamic linking
• Overlays
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3. Recap
In the last class, you have learnt
• Address binding
• Concepts of Dynamic loading
• Advantages of Dynamic loading
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4. Objectives
On completion of this class, you will be able to
know
• Dynamic linking
• Overlays
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5. 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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6. 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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7. Dynamic Linking
Secondary storage
printf( )
Memory
{
• main( )
------- ;
{ ------- ;
---- ; ------- ;
---- ; }
printf( ); // stub is linked dynamically
---- ;
}
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8. Dynamic Linking
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
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9. Dynamic Linking
Advantages
• All programs that reference the library will
automatically use new version
• Can allow multiple processes to access the
same memory address
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10. Dynamic Linking
Disadvantages
• Cannot detect the needed routine in another
process memory space
• Support from the operating system is required
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11. 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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12. 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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13. Overlays
• 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)
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14. Overlays for a Two-Pass Assembler
Example 1
Fig - 1
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15. Example: 2
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16. Secondary Storage
Overlays
Main Memory Overlay 1
0K
Main Program
Overlay 2
5k
Overlay Manager
7k
Overlay Area
Overlay 1
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2 Overlay 3
12k
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17. Overlays
• Overlay driver / manager reads the overlays
into the memory
• We can load the given two pass assemblers
into memory and execute them
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18. Overlays
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
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19. Overlays
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
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20. Summary
In this class, you have learnt
• Dynamic linking
-advantages and disadvantages
• Overlays
-advantages and disadvantages
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21. 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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22. Quiz
1. Concept of Dynamic linking is similar to
a) Dynamic loading
b) Address binding
c) Both
d) None
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23. Quiz
2. _________is postponed till execution in
dynamic linking
a) Linking
b) Loading
c) Both
d) None
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24. Quiz
3. _________ is a small piece of code that
indicates how to load the library routine
a) Address
b) Program
c) Stub
d) None
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25. Quiz
4. Support from the operating system is required
a) True
b) False
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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
9CM402.32 26

27. Quiz
6. ________reads the overlays into the memory
a) Linking driver
b) Operating system
c) Overlay driver
d) None
9CM402.32 27

28. Other subject materials
• Web designing
• Micro processors
• C++ tutorials
• java
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29. Quiz
7. Programming design of overlay structure is
_______
a) Simple
b) Complex
c) None
9CM402.32 29

30. Quiz
8. Overlays are limited to _____________
a) Minicomputers
b) Microcomputers
c) Mainframe computers
d) None
9CM402.32 30