Virtual memory is a memory management technique that uses secondary storage like hard disks to simulate a larger main memory for a process. It allows processes to have a larger address space than the actual physical memory size by swapping pages between main memory and secondary storage. This helps simplify memory management, protect processes from each other, and allows the operating system to share main memory among multiple processes. Virtual memory uses a memory hierarchy with different storage levels having varying sizes and speeds, and implements paging to map virtual addresses to physical addresses through page tables.

1. COMPUTER SCIENCE AND DATA PROCESSING
DEPARTMENT OF MATHEMATICS
IIT KHARAGPUR
Virtual Memory
Abir Naskar
13MA60R21

2. • Why virtual memory?
• Motivations.
• Memory hierarchy.
• Cache and Virtual memory.
• Paging.
• References.
OUTLINE
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3. • Extension of main memory.
• Secondary storage is used.
• Combination of hardware and software.
• Usually very large.
• Working set of pages resides in main memory, others are
in secondary memory.
VIRTUAL MEMORY
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4. • Main memory is expensive.
• Main memory size is less.
• All program are not active.
• Multiprogramming environment.
• Time sharing.
• Allow OS to share memory.
• Protect program from each other.
WHY VIRTUAL MEMORY
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5. • We use main memory as a cache of disk
memory.
- address space of process can exceed physical memory size.
• Simplify memory management.
- only active code and data is actually in memory.
• Provide protection.
- one process can’t interfere with another.
MOTIVATION
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6. LEVELS IN MEMORY HIERARCHY
Size 32B 32KB-4MB 128MB 30GB
Speed 3ns 6ns 60ns 8ms
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7. SYSTEM WITH PYHSICAL MEMORY ONLY
 Address generated by CPU point directly to
byte in physical memory.
 Example: Cray machines, early PCs,
embedded systems etc.
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8. SYSTEM WITH VIRTUAL MEMORY
o Hardware converts virtual address to
physical addresses via an OS-managed
look-up table (page table).
o Example : modern PCs, workstations,
servers etc.
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9. CACHE MEMORY VIRTUAL MEMORY
Is a hardware or processor Is simulated on hard drive maintained by OS
Data reads in unit called “block” Data reads in unit called “pages”
Stores memory access by CPU Storage swapping between active storage and
hard disk
Reduces the frequency that the CPU must wait for
data from the main memory
Reduces the amount of physical storage access
that is required
Performance-wise faster Performance-wise slower
CACHE MEMORY VS VIRTUAL MEMORY
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10. • It is a Memory Management Technique in which the memory is divided
into fixed size pages.
• Paging is used for faster access of data.
• Paging allows the physical address space of a process to be noncontiguous.
• A page is a sequence of N bytes where N is a power of 2.
• Page size should be high enough to amortize the high access time, and
page fault rate is small.
• These days, page sizes are at least 4KB.
• If 1Mb RAM, then 1MB/4KB=256 pages.
PAGING
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11. • Virtual address contains virtual page number and page offset.
• The number of pages addressable with virtual address is virtually unlimited
where number of pages addressable with physical address is limited.
• Mapping from virtual to a physical address.
ADDRESS MAPPING
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12. • A data may be or may not be present in main memory- if not present then
page fault occur.
• OS takes control when the valid bit is 0.
* OS creates pages for all the process in the disk.
* It also creates a data structure to record where each virtual page is stored in the disk.
* Another data structure to track which process and virtual address use physical page.
PAGE FAULT
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13. • Find a free frame to store a desire page.
* If there is free frame use it.
* Otherwise use a page replacement algorithm to find a victim page.
* Write the victim page on disk and change the page and page table accordingly.
• Read the desired page into free frame and change the page and page table.
• Restart the user process.
• Some page replacement algorithm:
FIFO replacement, Least Recently Used replacement, Least Frequently
Used replacement etc.
PAGE REPLACEMENT
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14. 13
• Page table is a data structure to store the mapping between virtual address
and physical address.
• Used to translate logical to physical addresses.
• Page table entries:
Address: pointer to the location of page in memory.
Valid bit: where page is pointed.
Dirty bit: set if at least one word of the page is modified.
Referenced bit: set if page has been referenced with either read or write.
protection bit: used to restrict access, like read only, system only access.
PAGE TABLE

15. • Applications run slower.
• It takes more time to switch between applications.
• Less hard drive space for your use.
DISADVANTAGES
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16. • Stallings W. (2006): “Operating Systems: Internals and Design Principles (7th Edition)”. prentice hall.
• Tanenbaum A. S. (2013): “Modern Operating System (3rd edition)”,
Pearson Education, pp.175-247.
• Cragon, Harvey G. (1996). Memory Systems
and Pipelined Processors. Jones and Bartlett Publishers.
ISBN 0-86720-474-5.
• URL: http://windows.microsoft.com/en-us/windows/what-is-virtual-memory#1TC=windows-7
• URL: http://www.cs.umd.edu/class/sum2003/cmsc311/Notes/Memory/virtual.html
REFERENCES
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17. Thank you…