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Virtual memory

Virtual memory combines active RAM and inactive memory on DASD[a] to form a large range of contiguous addresses.

In computing, virtual memory, or virtual storage[b] is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine"[3] which "creates the illusion to users of a very large (main) memory".[4]

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.
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  1. ^ "SYSTEM COMPONENTS: Dynamic Relocation" (PDF). System/360 Model 67 Time Sharing System Preliminary Technical Summary (PDF). IBM. 1966. p. 21. C20-1647-0.
  2. ^ "BCP (Base Control Program)" (PDF). z/OS Version 2 Release 4 z/OS Introduction and Release Guide (PDF). IBM. September 22, 2020. p. 3. GA32-0887-40.
  3. ^ Bhattacharjee, Abhishek; Lustig, Daniel (2017). Architectural and Operating System Support for Virtual Memory. Morgan & Claypool Publishers. p. 1. ISBN 9781627056021. Retrieved October 16, 2017.
  4. ^ Haldar, Sibsankar; Aravind, Alex Alagarsamy (2010). Operating Systems. Pearson Education India. p. 269. ISBN 978-8131730225. Retrieved October 16, 2017.

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