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, allowing the computer to run larger programs and multiple programs concurrently. A common function in most every OS and hardware platform, virtual memory uses storage (hard drive or SSD) to temporarily hold what was in RAM.

Virtual memory allows multiple programs to load in RAM at the same time. Each application addresses RAM starting at zero, but virtual memory takes control of the RAM addressing and lets each application function as if it had unlimited RAM.

Note that virtual "memory" and virtual "machine" are not the same. Virtual memory is used all the time, whereas a virtual machine is an optional approach for running applications and pertains mostly to servers (see virtual machine).

Virtual Memory Pages
The computer's real memory (RAM) is broken up into smaller segments, called "pages," typically 4KB in size. When RAM fills up, pages not currently in use by open applications are written to storage in a virtual memory "swap file." When any swapped out page in storage is required again, once again a page in RAM is written to storage to make room, and the required page in storage is retrieved.

RAM is the computer's workspace, and since there is often several hundred times more storage space than RAM space, virtual memory dramatically increases the computer's capacity to do work. However, there is a penalty. When a user has too many open programs, there can be excessive amounts of page swapping, causing applications to slow down. In addition, switching between applications is no longer instantaneous (see thrashing).

Hardware Is Required
Virtual memory can be implemented in software only, but efficient operation requires specialized hardware circuits. All modern, general-purpose 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 may change at any time. Although a program may initially load as a contiguous block of code, it can wind up in pages randomly scattered around RAM.

Virtual memory claims are sometimes made for specific applications that bring additional parts of the program in as needed; however, true virtual memory is built into the operating system and hardware and works with all applications. See Windows swap file.


Memory Is Extended to Storage
Virtual memory allows more programs to be opened simultaneously by using the hard drive or solid state drive (SSD) as temporary storage of RAM pages.







Page Out, Page In
When RAM (real memory) 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 ?
For example, within an abstract address design, there might be a single address table. Each unique address exists as a row within the address table once.
16K MAC address table and jumbo frame support up to 9KB
What is below describes particular format problems unlikely to happen in PE programs compiled with a 32-bit compiler (such as Microsoft and Borland compiler) [20]: (1) code execution starting in the last section, (2) suspicious section characteristics, (3) suspicious code redirection, (4) suspicious code section name, (5) the entry point not pointing into any of the sections, (6) import address table not patched, (7) multiple PE header, (8) incorrect size of code in header, and so forth.
12.8Gbps switch fabric, non-blocking switching performance, 8K MAC address table
The switches deliver gigabit copper connectivity with 16 of 24 auto-MDIX ports backed by an 8K MAC address table to ensure support for large networks.
To browse through your folders for the address table, click on the down arrow next to My Data Sources.
For example, Economic Development utilizes the Census tables and the Police department imports the address table from the street centerline dataset.
Because this table uses the same names as the address table, you can easily find the address and phone number for a name by connecting information in the two tables.
A large portion of code size reductions at link time in squeeze comes from the application of this optimization to computations of a hardware register called the global pointer (gp) register which points to a collection of 64-bit constants called a global address table. The Alpha processor, on which squeeze is implemented, is a 64-bit architecture with 32-bit instructions.
NETWORX 24 is a 1u high density, fast Ethernet workgroup switch that has been designed with N-Way auto negotiation and features an 8, 192 entry MAC address table, 6Mb of memory buffer in/out and with a full metal casing that makes it suitable for the desktop or wall mounting.
This is achieved with an address table, which can be stored in memory or on disk.

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