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page table

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page table

[′pāj ‚tā·bəl]
(computer science)
A key element in the virtual memory technique; a table of addresses where entries are adjusted for easy relocation of pages.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

virtual memory

Simulating more random access memory (RAM) than actually exists, which allows the computer to run larger programs as well as multiple programs concurrently. Actually developed back in the 1950s, virtual memory uses storage (HDD/SSD) to temporarily hold parts of the program that are in RAM. Because there is always more storage space than RAM space, virtual memory increases the computer's capacity to do work.

Hardware Is Required
All modern CPUs have memory management units (MMUs) that support virtual memory. They provide "page tables" that are used to translate between the program's "virtual" addresses and the "real" addresses in RAM and storage, which are constantly changing. Although a program initially loads as a contiguous block of code, it may wind up scattered around RAM. See Windows swap file.

Not a Virtual Machine
Virtual memory was the first use of the "virtual" term, and it meant "simulated." Another common simulation technology is the "virtual machine." However, "virtual memory" and "virtual machine" are different. Virtual memory operates in every computer, but a virtual machine is an optional technology widely used for running applications in datacenter servers. See virtual, virtual machine and cloud computing.


Memory Is Extended to Storage
The computer's RAM is divided into "pages," typically 4KB in size. When RAM fills up, pages not currently in use by open programs are written to storage in a "swap file." When instructions in a swapped out page are required again, some other page in RAM is written to storage to make room (see below).







Page Out, Page In
When real memory (RAM) is full and the current program needs instructions that are not in RAM, pages are swapped. In this example, program A needs a page from the disk, and a page from program C is swapped out to make room.
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References in periodicals archive
[T.sub.CR] Time duration of changing single item in page table, as well as its corresponding records (TLB) in other places except in cache.
Instead, dynamic remapping turns out more complicated, as it should not only change the items of page tables, but also reconcile all memory related, including exchanging content of two memory pages, invalidating caches and TLBs.
CacheSCDefender: VMM-based Comprehensive Framework against Cache-based Side-channel Attacks
(4) Complete the operation on the page table and return to Guest OS.
(1) Measure When Creating Page Table. When an executable subject carries out, this software creates a complete copy of program map on the hard disk, and the address space can come into being.
Research on dynamic integrity measurement model based on memory paging mechanism
Mutual exclusion within an SMP on page table state during TLB fault handling is achieved via a shared-memory lock.
The column labeled "L" is part of the state transition precondition and refers to the shared-memory lock necessary for mutual exclusion on page table state at the client SMP.
Multigrain Shared Memory
Under the previous optimizations, the transport of an fbuf from the originator to a receiver still requires two page table updates per page: one to remove write permissions from the originator when the fbuf is transferred, and one to return write permissions to the originator after the fbuf was freed by all the receivers.
The operating system's interrupt handler in turn looks up the address in the application's page table. If a valid mapping exists, the kernel provides the appropriate translation information to the network adaptor, paging in the page if necessary; otherwise, an access violation exception is raised in the offending application process.
Operating system support for high-speed communication
Accompanied by page table switching, the context switching can be monitored by VMM through the change of CR3.
A Multi-level Perception Security Model Using Virtualization
HPA) by the IOMMU based on the I/O page table [1][6][20].
IOMMU para-virtualization for efficient and secure DMA in virtual machines
The primary remaining areas of change are the new page table formats and the details of process context switching.
Porting OpenVMS from VAX to Alpha AXP
Several page table entries pointing to the same frame in the main memory.
Self-assessment procedure XX
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