Virtual memory allows treating main memory as a cache for pages on disk. It implements the separation of user logical memory from physical memory, allowing logical address spaces to be much larger than physical memory. Virtual memory can be implemented via demand paging or demand segmentation and gives advantages like higher multiprogramming and allowing very large logical address spaces by paging pages between disk and memory as needed.

1. Storage Management
• Virtual Memory
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CPP TUTORIALS
9CM402.39 2

3. Recap
In the last class, you have learnt:
• Paged Segmentation
• MULTICS Address Translation Scheme
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4. Objective
On completion of this class, you will be able to
know
• Virtual Memory Techniques
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5. Virtual Memory Techniques
• RAM is expensive ( but fast ), disk is cheap (but
slow)
• Need to find a way to use the cheaper memory
– Store memory that isn’t frequently used on disk
– Swap pages between disk and memory as needed
• Treat main memory as a cache for pages on
disk
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6. Virtual Memory
• It is separation of user logical memory from
physical memory
– Only part of the program needs to be in memory for
execution
– Logical address space can therefore be much larger
than physical address space
– Allows address spaces to be shared by several
processes
– Allows for more efficient process creation
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7. Virtual Memory
• Virtual memory can be implemented via:
– Demand paging
– Demand segmentation
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8. Virtual Memory
• It is a conceptual separation of user logical memory from
physical memory
• Thus we can have large virtual memory on a small physical
memory
Individual Pages
Physical Disk
Virtual Memory
Memory Map Memory
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9. Virtual Memory
• Programs can be larger than memory
– Program loaded into memory as needed
– Active program and data “swapped” to a disk until
needed
• Memory space treated uniformly
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10. WHY VIRTUAL MEMORY?
• Previously entire logical space was required for the
process to be in memory
• Before the process could run
• The alternatives to this
• Most code / data isn't needed at any instant, or even
within a finite time
• We can bring it in only as needed
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11. Virtual Memory
Advantages
• Gives a higher level of multiprogramming
• The program size isn't constrained (thus the
term 'virtual memory'). Virtual memory allows
very large logical address spaces
• Swap sizes smaller
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12. Summary
In this class, you have learnt
• Virtual Memory
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13. Frequently Asked Questions
• Explain Virtual Memory concepts with a neat
sketch
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14. Quiz
1. __________separation of user logical memory
from physical memory
a) Segmentation
b) Virtual memory
c) None
2. ____________gives a higher level of
multiprogramming
a) Page offset
b) Hole
c) Virtual memory
d) None 14

15. Quiz
3. Logical address space can be much larger than
a) Page number
b) MULTICS
c) Physical address space
d) None
4. __________allows very large logical address
spaces
a) External fragmentation
b) Virtual memory
c) Compaction
d) None 15

16. Other subject materials
• Web designing
• Micro processors
• C++ tutorials
• java
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