Editing Virtual memory (section)

===Page tables===
[[Page table]]s are used to translate the virtual addresses seen by the application into [[physical address]]es used by the [[Computer hardware|hardware]] to process instructions;<ref>{{cite book|last1=Sharma|first1=Dp|title=Foundation of Operating Systems|date=2009|publisher=Excel Books India|isbn=978-81-7446-626-6|page=62|url=https://books.google.com/books?id=AjWh-o7eICMC&pg=PA62|access-date=18 July 2017}}</ref> such hardware that handles this specific translation is often known as the [[memory management unit]]. Each entry in the page table holds a flag indicating whether the corresponding page is in real memory or not.  If it is in real memory, the page table entry will contain the real memory address at which the page is stored.  When a reference is made to a page by the hardware, if the page table entry for the page indicates that it is not currently in real memory, the hardware raises a [[page fault]] [[trap (computing)|exception]], invoking the paging supervisor component of the [[operating system]].

Systems can have, e.g., one page table for the whole system, separate page tables for each address space or process, separate page tables for each segment; similarly, systems can have, e.g., no segment table, one segment table for the whole system, separate segment tables for each address space or process, separate segment tables for each ''region'' in a tree{{efn|On [[IBM Z]]<ref>{{cite book
 |       title = z/Architecture - Principles of Operation
 |          id = SA22-7832-13
 |     edition = Fourteenth
 |        date = May 2022
 |     section = Translation Tables
 | section-url = http://publibfp.dhe.ibm.com/epubs/pdf/a227832d.pdf#page=152
 |       pages = 3-46-3-53
 |         url = http://publibfp.dhe.ibm.com/epubs/pdf/a227832d.pdf
 |   publisher = [[IBM]]
 | access-date = January 18, 2023
 }}
</ref> there is a 3-level tree of regions for each address space.}} of region tables for each address space or process. If there is only one page table, different applications [[multiprogramming|running at the same time]] use different parts of a single range of virtual addresses. If there are multiple page or segment tables, there are multiple virtual address spaces and concurrent applications with separate page tables redirect to different real addresses.

Some earlier systems with smaller real memory sizes, such as the [[SDS 940]], used ''[[Page address register|page registers]]'' instead of page tables in memory for address translation.