The Windows NT virtual memory system extends the server's physical memory by using space on disk. The virtual memory system reserves physical memory for items that are used frequently or are costly to retrieve, and moves unreferenced items to disk. If a process or service needs code or data on disk, the system retrieves the requested pages and delivers them to the process' working set in physical memory — all without intervention from the process. The system will also retrieve contiguous pages, if there is enough physical memory to hold them.
The challenge of a virtual memory system is to minimize disk I/O, which is slow, mechanical, and laborious compared to the speed at which items are written to and retrieved from physical memory. Minimizing disk I/O requires that the system keep as many items as possible in physical memory.