This is a method of extending the available physical memory on a computer. In a virtual memory system, the operating system creates a pagefile, or swapfile, and divides memory into units called pages. Recently referenced pages are located in physical memory, or RAM. If a page of memory is not referenced for a while, it is written to the pagefile. This is called "swapping" or "paging out" memory. If that piece of memory is then later referenced by a program, the operating system reads the memory page back from the pagefile into physical memory, also called "swapping" or "paging in" memory. The total amount of memory that is available to programs is the amount of physical memory in the computer in addition to the size of the pagefile.