virtual memory

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(n.) The address space available to a process running in a system with a memory management unit (MMU). Hard disk space is used to virtually simulate internal RAM. Virtual memory may be much larger than the amount of physical RAM installed. The virtual address space is divided into pages. Paging allows the excess to be stored on disk and copied to RAM as required. This makes it possible to run programs whose code plus data size exceeds the amount of available RAM. A page is copied from disk to RAM when an attempt is made to access it and when it is not already present. This paging is performed automatically by collaboration between the CPU, the MMU, and the operating system.