Demand paging is a virtual memory technique where pages are brought into memory only when needed, reducing I/O and memory usage compared to swapping. When a process references a page not currently in memory, the system checks permissions and retrieves the page from disk if valid. It finds a free frame, loads the page, updates the page table, and restarts the faulting instruction. This lazy loading approach brings pages in only as needed rather than pre-loading the entire process address space.

1. Storage Management
• Demand Paging
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2. Recap
In the last class, you have learnt:
Virtual Memory
• Advantages of virtual memory
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4. Objective
On completion of this class you will be able to
know
• Demand Paging
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5. Demand Paging
• Demand paging system is similar to a paging
system with swapping
• A lazy swapper swaps a page into memory only
when it is needed
• A process is a sequence of pages rather than
one large contiguous address space
• In demand paging instead of swapper the term
pager is used
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6. Demand Paging
• Bring a page into memory only when it is needed
– Less I/O needed
– Less memory needed
– Faster response
– More users
• Page is needed ⇒ reference to it
– invalid reference ⇒ abort
– not-in-memory ⇒ bring to memory
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7. Demand Paging
•When a page is referenced, and that page isn’t in memory,
• then get the page from disk and re-execute the statement
•Here’s migration between memory and disk
Program A
Program B
Physical Disk
Memory
Fig 1
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8. Transfer of a Paged Memory to Contiguous
Disk Space
Fig 2
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9. Valid-Invalid Bit
• With each page table entry a valid–invalid bit is associated
(1 ⇒ in-memory, 0 ⇒ not-in-memory)
• Initially valid–invalid but is set to 0 on all entries
• Example of a page table snapshot Page table
• During address translation, if valid– Frame # valid-invalid bit
1
invalid bit in page table entry is 1
0 ⇒ page fault 1
1
0

0
0
Fig 3
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10. Page Table When Some Pages Are Not in
Main Memory
Fig 4
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11. Demand Paging
Steps in handling a page fault
• The process has touched a page not currently in
memory
• Check an internal table for the target process to
determine if the reference was valid ( do this in
hardware )
• If it was valid, but page isn't resident, then try to get it
from secondary storage
• Find a free frame; a page of physical memory not
currently in use (May need to free up a page )
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12. Demand Paging
• Schedule a disk operation to read the desired page into
the newly allocated frame
• When memory is filled, modify the page table to show
the page is now resident
• Restart the instruction that failed
• Do these steps using fig 5 which you can see on the next
slide
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13. Demand Paging
Fig 5
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14. Frequently Asked Questions
• What is demand paging ?
• What is page fault ?
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15. Summary
In this class, you have learnt
• Demand paging is used to get a page into
memory only when it is needed
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16. Quiz
1. Demand paging system is similar to a paging
system with
a) Segmentation
b) Swapping
c) None
2. In demand paging instead of swapper the term
__________is used
a) Page offset
b) Hole
c) Pager
d) None
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17. Other subject materials
• Web designing
• Micro processors
• C++ tutorials
• java
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