Virtual Address Space and Physical Storage

The virtual address space of each process is much larger than the total physical memory available to all processes. To increase the size of physical storage, the system uses the disk for additional storage. The total amount of storage available to all executing processes is the sum of the physical memory and the free space on disk available to the paging file, a disk file used to increase the amount of physical storage. Physical storage and the virtual address space of each process are organized into pages, units of memory, whose size depends on the host computer. For example, on x86 computers the host page size is 4 kilobytes.

To maximize its flexibility in managing memory, the system can move pages of physical memory to and from a paging file on disk. When a page is moved in physical memory, the system updates the page maps of the affected processes. When the system needs space in physical memory, it moves the least recently used pages of physical memory to the paging file. Manipulation of physical memory by the system is completely transparent to applications, which operate only in their virtual address spaces.