The computer has physical memory or RAM attached. RAM has size limitations. When you turn on the computer, the program loads in RAM. When you open a file, it also loads in RAM. If the file size is large, that is, working with a large amount of data causes problems. In this case, some of the secondary memory hard disk space in the Windows operating system can be used as physical memory. This is called virtual memory. Virtual memory management technique that can swap information from a computer's physical memory (RAM) to a hard disk. This technique allows application programs to use much more memory than physical memory. The amount of virtual memory depends on the amount of hard disk space available. To copy virtual memory to real memory, the operating system divides virtual memory into pages , each of which contains a certain number of addresses. Each page is stored on disk until it is needed. When a page is needed , the operating system copies it from disk to main memory (RAM) , translating the virtual address to the real address. The process of translating a virtual address into a real address is called mapping. Copying a virtual page from disk to main memory is called paging or shopping.