This is important to enable the user to run many programs at the same time. Currently unused areas of RAM are swapped out to the harddisk. By doing this the currently running application has access to all resources it needs. There are four things that go wrong and make the memory management fail Program Errors Many applications today have programming errors. The Operating System itself has programming errors.
Many people think that it is impossible to make programs without errors.
Most errors are no problem. A user never sees them. Sometimes one of these errors leaves some amount of memory allocated, even if it is not needed any more. This is bad as RAM is a valuable resource and this resource is thus wasted. However Windows has no chance to detect these errors and must still assume that all this RAM is needed somehow.