demand-paged virtual memory - définition. Qu'est-ce que demand-paged virtual memory
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Qu'est-ce (qui) est demand-paged virtual memory - définition

OPERATING SYSTEM LEVEL MEMORY MANAGEMENT TECHNIQUE
Virtual storage; Virtual Memory; /dev/drum; Memory maps; VSIZE; Virtual memoy; Paged virtual memory; Memoria virtual; Virtual address translation
  • The University of Manchester [[Atlas Computer]] was the first computer to feature true virtual memory.
  • solid state memory]]}} to form a large range of contiguous addresses.

virtual memory         
Virtual memory is a computing technique in which you increase the size of a computer's memory by arranging or storing the data in it in a different way. (COMPUTING)
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virtual storage         
Virtual storage is the same as virtual memory
. (COMPUTING)
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virtual memory         
<memory management> A system allowing a computer program to behave as though the computer's memory was larger than the actual physical RAM. The excess is stored on hard disk and copied to RAM as required. Virtual memory is usually much larger than physical memory, making it possible to run programs for which the total code plus data size is greater than the amount of RAM available. This is known as "demand paged virtual memory". A page is copied from disk to RAM ("paged in") when an attempt is made to access it and it is not already present. This paging is performed automatically by collaboration between the CPU, the memory management unit (MMU), and the operating system kernel. The program is unaware of virtual memory, it just sees a large address space, only part of which corresponds to physical memory at any instant. The virtual address space is divided into pages. Each virtual address output by the CPU is split into a (virtual) page number (the most significant bits) and an offset within the page (the N least significant bits). Each page thus contains 2^N bytes (or whatever the unit of addressing is). The offset is left unchanged and the {memory management unit} (MMU) maps the virtual page number to a physical page number. This is recombined with the offset to give a physical address - a location in physical memory (RAM). The performance of a program will depend dramatically on how its memory access pattern interacts with the paging scheme. If accesses exhibit a lot of locality of reference, i.e. each access tends to be close to previous accesses, the performance will be better than if accesses are randomly distributed over the program's address space thus requiring more paging. In a multitasking system, physical memory may contain pages belonging to several programs. Without demand paging, an OS would need to allocate physical memory for the whole of every active program and its data. Such a system might still use an MMU so that each program could be located at the same virtual address and not require run-time relocation. Thus virtual addressing does not necessarily imply the existence of virtual memory. Similarly, a multitasking system might load the whole program and its data into physical memory when it is to be executed and copy it all out to disk when its timeslice expired. Such "swapping" does not imply virtual memory and is less efficient than paging. Some application programs implement virtual memory wholly in software, by translating every virtual memory access into a file access, but efficient virtual memory requires hardware and operating system support. (2002-11-26)

Wikipédia

Virtual memory

In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very large (main) memory".

The computer's operating system, using a combination of hardware and software, maps memory addresses used by a program, called virtual addresses, into physical addresses in computer memory. Main storage, as seen by a process or task, appears as a contiguous address space or collection of contiguous segments. The operating system manages virtual address spaces and the assignment of real memory to virtual memory. Address translation hardware in the CPU, often referred to as a memory management unit (MMU), automatically translates virtual addresses to physical addresses. Software within the operating system may extend these capabilities, utilizing, e.g., disk storage, to provide a virtual address space that can exceed the capacity of real memory and thus reference more memory than is physically present in the computer.

The primary benefits of virtual memory include freeing applications from having to manage a shared memory space, ability to share memory used by libraries between processes, increased security due to memory isolation, and being able to conceptually use more memory than might be physically available, using the technique of paging or segmentation.