Here’s a second example: Let’s say your word processor’s main program includes the code required to compose and print a document; but if you want to check your spelling, the spell checker program has to be loaded from the disk.
When that’s done, the main portion of the word processor is kept in memory and the second program is added without erasing the first one already there.
Now you have two programs in memory but only one of them is executing at any given time.
This is discussed in the section on Process Management later in this chapter.