World Library  
Flag as Inappropriate
Email this Article


Article Id: WHEBN0005587875
Reproduction Date:

Title: Read-modify-write  
Author: World Heritage Encyclopedia
Language: English
Subject: Arbiter (electronics), WikiProject Spam/LinkReports/, Load-link/store-conditional, Motorola 68020, Test-and-set
Collection: Computer Memory, Concurrency Control
Publisher: World Heritage Encyclopedia


In computer science, read-modify-write is a class of atomic operations (such as test-and-set, fetch-and-add, and compare-and-swap) that both read a memory location and write a new value into it simultaneously, either with a completely new value or some function of the previous value. These operations prevent race conditions in multi-threaded applications. Typically they are used to implement mutexes or semaphores. These atomic operations are also heavily used in non-blocking synchronization.

Maurice Herlihy (1991) ranks atomic operations by their consensus numbers, as follows:

It is impossible to implement an operation that requires a given consensus number with only operations with a lower consensus number, no matter how many of such operations one uses.[2] Read-modify-write instructions often produce unexpected results when used on I/O devices, as a write operation may not affect the same internal register that would be accessed in a read operation.[3]

This term is also associated with RAID levels that perform actual write operations as atomic read-modify-write sequences.[4] Such RAID levels include RAID 4, RAID 5 and RAID 6.

See also


  1. ^ "Writing Lock-Free Code: A Corrected Queue" by Herb Sutter: "Compare-and-swap (CAS) is ... widely available ... However, some systems instead provide the equivalently powerful load-linked/store-conditional (LL/SC) instead."
  2. ^ Herlihy, Maurice (January 1991). "Wait-free synchronization" (PDF). ACM Trans. Program. Lang. Syst. 13 (1): 124–149.  
  3. ^ Massmind: "The read–modify–write problem"
  4. ^ "Basic RAID Organizations". Retrieved 2013-10-04. 

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.