32 dynamic linking nd overlays
- 1. Storage Management • Dynamic linking • Overlays 1
- 2. HOME PREVIOUS TOPIC NEXT PREVIOUS QUESTION PAPERS FOR OS CPP TUTORIALS 2
- 3. Recap In the last class, you have learnt • Address binding • Concepts of Dynamic loading • Advantages of Dynamic loading 3
- 4. Objectives On completion of this class, you will be able to know • Dynamic linking • Overlays 4
- 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 5
- 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 6
- 7. Dynamic Linking Secondary storage printf( ) Memory { • main( ) ------- ; { ------- ; ---- ; ------- ; ---- ; } printf( ); // stub is linked dynamically ---- ; } 7
- 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 8
- 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 9
- 10. Dynamic Linking Disadvantages • Cannot detect the needed routine in another process memory space • Support from the operating system is required 10
- 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 11
- 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 12
- 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) 13
- 14. Overlays for a Two-Pass Assembler Example 1 Fig - 1 14
- 15. Example: 2 15
- 16. Secondary Storage Overlays Main Memory Overlay 1 0K Main Program Overlay 2 5k Overlay Manager 7k Overlay Area Overlay 1 3 2 Overlay 3 12k 16
- 17. Overlays • Overlay driver / manager reads the overlays into the memory • We can load the given two pass assemblers into memory and execute them 17
- 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 18
- 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 19
- 20. Summary In this class, you have learnt • Dynamic linking -advantages and disadvantages • Overlays -advantages and disadvantages 20
- 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? 21
- 22. Quiz 1. Concept of Dynamic linking is similar to a) Dynamic loading b) Address binding c) Both d) None 22
- 23. Quiz 2. _________is postponed till execution in dynamic linking a) Linking b) Loading c) Both d) None 23
- 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 24
- 25. Quiz 4. Support from the operating system is required a) True b) False 25
- 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 home
- 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