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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".
It was first announced by Digital Equipment Corporation (DEC) as VAX/VMS (Virtual Address eXtension/Virtual Memory System [15]) alongside the VAX-11/780 minicomputer in 1977. [16] [17] [18] OpenVMS has subsequently been ported to run on DEC Alpha systems, the Itanium-based HPE Integrity Servers, [19] and select x86-64 hardware and hypervisors. [20]
A modern computer operating system usually uses virtual memory to provide separate address spaces or separate regions of a single address space, called user space and kernel space. [1] [a] Primarily, this separation serves to provide memory protection and hardware protection from malicious or errant software behaviour.
The memory subsystem manages the physical memory and the virtual memory of the system (both part of the hardware resource). The virtual memory extends physical memory by using extra space on a peripheral device, usually disk. The memory subsystem is responsible for moving code and data between main and virtual memory in a process known as ...
Single-level storage (SLS) or single-level memory is a computer storage term which has had two meanings. The two meanings are related in that in both, pages of memory may be in primary storage or in secondary storage (disk), and that the physical location of a page is unimportant to a process.
A hypervisor is an operating system that runs a virtual machine. The virtual machine is unaware that it is an application and operates as if it had its own hardware. [14] [29] Virtual machines can be paused, saved, and resumed, making them useful for operating systems research, development, [30] and debugging. [31]
In some respects, a system virtual machine can be considered a generalization of the concept of virtual memory that historically preceded it. IBM's CP/CMS , the first systems to allow full virtualization , implemented time sharing by providing each user with a single-user operating system, the Conversational Monitor System (CMS).
A memory-mapped file is a segment of virtual memory [1] that has been assigned a direct byte-for-byte correlation with some portion of a file or file-like resource. This resource is typically a file that is physically present on disk, but can also be a device, shared memory object, or other resource that an operating system can reference through a file descriptor.