Computers are sometimes able to use storage area that is not actually RAM as memory. This is then known as virtual memory, and has a variety of other names, some of which will be detailed below. Typically, this virtual memory has come from space on the hard drive, although recently more USB flash drives are being used for the same purpose.

Virtual memory is not as preferred as the actual RAM chips. Mainly, its speed cannot compare to that of legitimate memory. More wires need to be passed through in order to access data from a hard drive, for instance, than to get it from the memory chips, which are right next to the computer's motherboard. Additionally, the hard drive may need to spin up, further slowing the data. USB memory sticks would not need to do this, as they are solid state, however, they are slowed by going through the USB port.

Below are several implementations of virtual memory in different operating systems:

Windows

Virtual memory first came to Microsoft Windows in version 3.0. In the NT versions (including XP and Vista), a file called pagefile.sys is used for paging (a name for the process). By default, the pagefile can expand beyond its initial space allotment, and eventually become quite fragmented. This may lead to a drop in performance.

Linux

In Linux, swap is used as the term for the area of the disk used for this process, as well as the process itself. For instance, the location might be called swap space. This can be set up either as a swap file or swap partition. Mac

In Mac OS X, virtual memory can be had through either swap files or swap partitions, as in Linux operating systems.

In general, swap space can be usable for when the amount of system memory is not sufficient. However, due to its limitations, an actual upgrade to the computer's RAM may be more efficient than simply allowing for a larger swap partition. Still, even in the case of an acceptable level of memory, adding a small swap file or partition may be useful if only for when a program unexpectedly starts hogging the system's resources.