Editing Virtual memory (section)

===Pinned pages===
Operating systems have memory areas that are ''pinned'' (never swapped to secondary storage). Other terms used are ''locked'', ''fixed'', or ''wired'' pages.  For example, [[interrupt]] mechanisms rely on an array of pointers to their handlers, such as [[I/O]] completion and [[page fault]]. If the pages containing these pointers or the code that they invoke were pageable, interrupt-handling would become far more complex and time-consuming, particularly in the case of page fault interruptions. Hence, some part of the page table structures is not pageable.

Some pages may be pinned for short periods of time, others may be pinned for long periods of time, and still others may need to be permanently pinned. For example:
* The paging supervisor code and drivers for secondary storage devices on which pages reside must be permanently pinned, as otherwise paging would not even work because the necessary code would not be available.
* Timing-dependent components may be pinned to avoid variable paging delays.
* [[Data buffer]]s that are accessed directly by peripheral devices that use [[direct memory access]] or [[I/O channel]]s must reside in pinned pages while the I/O operation is in progress because such devices and the [[Bus (computing)|buses]] to which they are attached expect to find data buffers located at physical memory addresses; regardless of whether the bus has a [[IOMMU|memory management unit for I/O]], transfers cannot be stopped if a page fault occurs and then restarted when the page fault has been processed. For example, the data could come from a measurement sensor unit and lost real time data that got lost because of a page fault can not be recovered.

In IBM's operating systems for [[System/370]] and successor systems, the term is "fixed", and such pages may be long-term fixed, or may be short-term fixed, or may be unfixed (i.e., pageable). System control structures are often long-term fixed (measured in wall-clock time, i.e., time measured in seconds, rather than time measured in fractions of one second) whereas I/O buffers are usually short-term fixed (usually measured in significantly less than wall-clock time, possibly for tens of milliseconds). Indeed, the OS has a special facility for "fast fixing" these short-term fixed data buffers (fixing which is performed without resorting to a time-consuming [[Supervisor Call instruction]]).

[[Multics]] used the term "wired". [[OpenVMS]] and [[Microsoft Windows|Windows]] refer to pages temporarily made nonpageable (as for I/O buffers) as "locked", and simply "nonpageable" for those that are never pageable.  The [[Single UNIX Specification]] also uses the term "locked" in the specification for  {{code|lang=c|mlock()}}, as do the {{code|lang=c|mlock()}} [[man pages]] on many [[Unix-like]] systems.

====Virtual-real operation====
In [[OS/VS1]] and similar OSes, some parts of systems memory are managed in "virtual-real" mode, called "V=R". In this mode every virtual address corresponds to the same real address. This mode is used for [[interrupt]] mechanisms, for the paging supervisor and page tables in older systems, and for application programs using non-standard I/O management. For example, IBM's z/OS has 3 modes (virtual-virtual, virtual-real and virtual-fixed).{{citation needed|date=September 2022}}