Addresses in the virtual address space that programmers see are mapped to physical addresses by the operating system and the memory management unit (MMU), which is a part of the processor. This mapping or translation happens each time we access a memory address.
This extra layer of indirection makes it possible for the operating system to use physical memory for the parts of a process that are currently being used and back up the rest of the virtual memory on-disk. In this sense, we can see the physical main memory as a cache for the virtual memory space, which resides on secondary storage. The areas of the secondary storage that are used for backing up memory pages are usually called swap space, swap file, or, simply, ...
