World Library  
Flag as Inappropriate
Email this Article

Demand paging

Article Id: WHEBN0001310997
Reproduction Date:

Title: Demand paging  
Author: World Heritage Encyclopedia
Language: English
Subject: Memory management, Virtual memory, Memory-mapped file, Memory management (operating systems), SUNMOS
Collection: Virtual Memory
Publisher: World Heritage Encyclopedia

Demand paging

In computer operating systems, demand paging (as opposed to anticipatory paging) is a method of virtual memory management. In a system that uses demand paging, the operating system copies a disk paging into physical memory only if an attempt is made to access it and that page is not already in memory (i.e., if a page fault occurs). It follows that a process begins execution with none of its pages in physical memory, and many page faults will occur until most of a process's working set of pages is located in physical memory. This is an example of a lazy loading technique.


  • Basic concept 1
  • Advantages 2
  • Disadvantages 3
  • See also 4
  • References 5

Basic concept

Demand paging follows that pages should only be brought into memory if the executing process demands them. This is often referred to as lazy evaluation as only those pages demanded by the process are swapped from secondary storage to main memory. Contrast this to pure swapping, where all memory for a process is swapped from secondary storage to main memory during the process startup.

Commonly, to achieve this process a page table implementation is used. The page table maps logical memory to physical memory. The page table uses a bitwise operator to mark if a page is valid or invalid. A valid page is one that currently resides in main memory. An invalid page is one that currently resides in secondary memory. When a process tries to access a page, the following steps are generally followed:

  • Attempt to access page.
  • If page is valid (in memory) then continue processing instruction as normal.
  • If page is invalid then a page-fault trap occurs.
  • Check if the memory reference is a valid reference to a location on secondary memory. If not, the process is terminated (illegal memory access). Otherwise, we have to page in the required page.
  • Schedule disk operation to read the desired page into main memory.
  • Restart the instruction that was interrupted by the operating system trap.


Demand paging, as opposed to loading all pages immediately:

  • Only loads pages that are demanded by the executing process.
  • As there is more space in main memory, more processes can be loaded reducing context switching time which utilizes large amounts of resources.
  • Less loading latency occurs at program startup, as less information is accessed from secondary storage and less information is brought into main memory.
  • As main memory is expensive compared to secondary memory, this technique helps significantly reduce the bill of material (BOM) cost in smart phones for example. Symbian OS had this feature.


  • Individual programs face extra latency when they access a page for the first time.
  • Programs running on low-cost, low-power embedded systems may not have a memory management unit that supports page replacement.
  • Memory management with page replacement algorithms becomes slightly more complex.
  • Possible security risks, including vulnerability to timing attacks; see Percival 2005 Cache Missing for Fun and Profit (specifically the virtual memory attack in section 2).
  • Thrashing which may occur due to repeated page faults.

See also


  • Tanenbaum, Andrew S. Operating Systems: Design and Implementation (Second Edition). New Jersey: Prentice-Hall 1997.
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.

Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.