Starting in the late 1960s and early 1970s, there arose the problem that software programs and their data would take up more memory space than was available in the primary storage (RAM). It became necessary to think of ways to break up such a larger program and its data into parts, with less-relevant parts being held in some auxiliary storage. If such additional memory is available, such as in the form of secondary storage on disk, then there are three major ways of doing this:

• Overlays--sections of the application code or data are stored on secondary storage (typically hard drives), and the application has to load them whenever required. This was a common method in personal computers under DOS right up to the early 1990s, but has the disadvantage that applications are slowed down by drive access speeds.
• Swapping--this is slightly similar to the use of overlays, but...