The essential idea of virtual memory is to pretend there is a very large amount of memory and to use a combination of disk files and real memory to implement it. A program works in a virtual memory space, which seems to it like a very large real memory. Its size is usually constrained only by the size of the MAR which dictates the maximum address that a program can feed to memory. This virtual memory is broken into a number of equal sized chunks, called pages.

By contrast, there is some smaller amount of real memory, called physical memory. It is broken into chunks of the same size as pages, only these chunks of physical memory are called frames.

Pages are copied into frames when they are referenced, a process called demand paging. The operating system and the hardware collaborate to do this copying behind the scenes automatically, without the programmer even knowing it is happening.