Virtual Memory is a storage allocation scheme in which secondary memory can be addressed as though it were part of the main memory. The addresses a program may use to reference memory are distinguished from the addresses the memory system uses to identify physical storage sites and program-generated addresses are translated automatically to the corresponding machine addresses.
The size of virtual storage is limited by the addressing scheme of the computer system and the amount of secondary memory is available not by the actual number of the main storage locations.
It is a technique that is implemented using both hardware and software. It maps memory addresses used by a program, called virtual addresses, into physical addresses in computer memory.
All memory references within a process are logical addresses that are dynamically translated into physical addresses at run time. This means that a process can be swapped in and out of the main memory such that it occupies different places in the main memory at different times during the course of execution.
A process may be broken into a number of pieces and these pieces need not be continuously located in the main memory during execution. The combination of dynamic run-time address translation and the use of a page or segment table permits this.
If these characteristics are present then, it is not necessary that all the pages or segments are present in the main memory during execution. This means that the required pages need to be loaded into memory whenever required. Virtual memory is implemented using Demand Paging or Demand Segmentation.
The space used for virtual memory on a drive is known as "backing store," or "swap space." In the Windows world, it's usually implemented as a file, known as a "swap file." It's also possible to do this in Linux, but it's much more common to use a dedicated disk partition.
The upshot of virtual memory is that it's possible to run large programs by using more memory than the physical RAM in your machine. Virtual memory also allows developers to create applications without having to know how a computer's memory is organized.
The main downside historically with virtual memory has been that hard drives have been slower than RAM. If a machine doesn't have enough RAM, the system can keep swapping pages in and out endlessly, a process known as "thrashing." This is less of an issue on modern PCs with more RAM and faster SSDs displacing mechanical hard drives, but it's still something to be aware of.
