The term "virtual memory" refers to space allocated on a hard drive where data can be stored for rapid access. Virtual memory is slower than solid-state memory chips so it is typically used as backup memory in certain situations.
One important use of virtual memory is multitasking. When a computer user opens multiple programs at once, the data for these programs must be stored in memory for quick access. The more programs are open, the more memory is needed. When the computer's physical memory is full, the excess data is stored in virtual memory.
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In addition to multitasking, virtual memory allows programmers to create larger and more complex applications. When these programs are running, they occupy physical memory as well as virtual memory.
If computers only relied on memory chips, far less memory would be available and the usefulness of many software programs would be severely limited. Even though virtual memory is slower, it is still useful because it greatly expands a computer's functionality.
When virtual memory was first created, solid-state memory chips were much smaller and more expensive. Today's memory chips can store many gigabytes of data at very low cost. As memory chips continue to grow in capacity and prices fall, virtual memory is may be less useful in the future.