Editing Operating system (section)

====Virtual memory====
{{Main|Virtual memory}}
{{Further|Page fault}}
[[File:Virtual memory.svg|thumb|250px|Many operating systems can "trick" programs into using memory scattered around the hard disk and RAM as if it is one continuous chunk of memory, called virtual memory.]]

The use of virtual memory addressing (such as paging or segmentation) means that the kernel can choose what memory each program may use at any given time, allowing the operating system to use the same memory locations for multiple tasks.

If a program tries to access memory that is not accessible{{efn|There are several reasons that the memory might be inaccessible
* The address might be out of range
* The address might refer to a page or segment that has been moved to a backing store
* The address might refer to memory that has restricted access due to, e.g., [[Memory protection#Protection keys|key]], [[Memory protection#Protection rings|ring]].}} memory, but nonetheless has been allocated to it, the kernel is interrupted {{See above|{{Section link||Memory management}}}}. This kind of interrupt is typically a [[page fault]].

When the kernel detects a page fault it generally adjusts the virtual memory range of the program which triggered it, granting it access to the memory requested. This gives the kernel discretionary power over where a particular application's memory is stored, or even whether or not it has been allocated yet.

In modern operating systems, memory which is accessed less frequently can be temporarily stored on a disk or other media to make that space available for use by other programs. This is called [[paging|swapping]], as an area of memory can be used by multiple programs, and what that memory area contains can be swapped or exchanged on demand.

Virtual memory provides the programmer or the user with the perception that there is a much larger amount of RAM in the computer than is really there.<ref name="Operating System">{{cite book|last=Stallings|first=William|title=Computer Organization & Architecture|year=2008|publisher=Prentice-Hall of India Private Limited|location=New Delhi|isbn=978-81-203-2962-1|page=267}}</ref>