On process startup only first page is loaded into physical memory. Pages are then loaded as referenced. Saves memory. May cause frequent page faults until process has its working set in physical memory.

OS may adjust its policy (demand / prepaging) dependent on :

• Available free physical memory
• Process types and history
• Mapped per page
• Page size (usually 4Kbytes)
• Two phase execution

Virtual memory is allocated(mmap); just registered in management table. At actual access, physical momory is allocated for the page.