The purpose of (66) is to enlarge the (67) , the set of addresses a program can utilize. For example, it might contain twice as many addresses as main memory. A program using all of it, therefore, would not be able to fit in main memory all at once. Nevertheless, the computer could execute such a program by Copying into main memory those portions of the program needed at any given point during execution.
To facilitate copying (66) into real memory, the operating system divides it into pages, each of which contains a fixed number of addresses. Each page is stored on a disk until ii is needed. When the page is needed, the operating system copies it from disk to main memory, translating the (68) into real addresses. The process of translating virtual addresses into real addresses is called (69) . The copying of virtual pages from disk to main memory is known as (70) or swapping.

A．drawing

B．mapping

C．picturing

D．scanning