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1. SOLAIMAN HOSSAIN TUHIN
ID : 173-15-1639
CSE IN DAFFODIL INTERNATIONAL UNIVERSITY

2. Topic : Virtual Memory
What is Virtual Memory?
-Virtual memory is a memory management capability of an operating system
Operating System. which uses hardware and software to allow a computer to
compensate for physical memory shortages, by temporarily transferring data
from random access memory (RAM) to disk storage.
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3. Virtual Memory
 Background
 Demand Paging
 Page Replacement
 Allocation of Frames
 Trashing
 Demand Segmentation
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4. Background
• Virtual Memory- 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.
-Need to allow pages to be swapped in and out.
• Virtual memory can be implemented via:
-Demand Paging
-Demand segmentation 4

5. 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 5

6. Page Replacement
• Prevent over-allocation of memory by modifying page-fault service
routine to include page replacement.
• Use modify bit to reduce overhead of page transfers-only modified pages
are written to disk.
• Page replacement completes separation between logical memory &
physical memory-large virtual memory can be provided on a smaller
physical memory.
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7. Thrashing
• If a process does not have “enough” pages, the page-fault rate is very high. This leads to:
- low CPU utilization.
- operating system thinks that it needs to increase the degree of multiprogramming.
- another process added to the system.
• Thrashing = a process is busy swapping pages in and out.
Video Link:
https://drive.google.com/file/d/1VPyZpezqgXHFMGVu0qPpn4JFHlw6uFN7/view?usp=sharing
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8. THANK YOU
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