Virtual Memory, Swap Space And Paging Virtual memory is the ability to map a (large) virtual address space into a set of (small) physical memory pages and disk space. The system keeps a virtual memory map. At a given time, only a small number of virtual memory segments is actually mapped into real physical memory pages. The rest is mapped into special disk areas (known as 'swap space'), or not mapped at all. When a process accesses a virtual address, the CPU translates this into a real memory page. If no page was mapped for this address yet, a 'page fault' exception is invoked, and an OS handler allocates a new page for the virtual memory segment containing that virtual address.