which of the following are the situations ,when entire program is not required to be loaded fully in main memory code example

Example: virtual memory in os

Virtual Memory is a storage mechanism which offers user 
an illusion of having a very big main memory. It is done
by treating a part of secondary memory as the main memory. 
Therefore, instead of loading one long process in the 
main memory, the OS loads the various parts
of more than one process in the main memory.