In Virtual Memory Systems the pages are not loaded in the
memory until they are "demanded" by a process; hence the name, Demand
Paging. Demand paging allows the various parts of a process to be brought into
physical memory as the process needs them to execute.
In virtual memory systems, demand paging is a type of
swapping in which pages of data are not copied from disk to RAM until they are
needed. In contrast, some virtual memory systems use anticipatory paging in
which the operating system attempts to anticipate which data will be needed
next and copies it to RAM before it is actually required. For a process to
execute, all the structures for data, text, and so on have to be set up. Pages
are not loaded in memory until they are "demanded" by a process.
Thus we get the term "demand paging." Demand paging allows the
various parts of a process to be brought into physical memory as the process
needs them to execute.
