Well, virtual memory really doesnt exist. Its not on a card. Its on your disk, probably. But it still doesnt “exist”, not as memory, anyway.

Here’s a very crude example. (There are many issues with virtaul memory that are not addressed by this example.)

Lets say that you had a laptop with only 100 bytes of real, honest-to-goodness memory, the kind that comes on those long, 3″-4″ cards, which plug directly to your motherboard. But all you have is 100 bytes.

Lets say also that you had a game you wanted to play on this machine, but the game was 200 bytes in size.

Even though the game is bigger than you’re “real memory”, you can still play it, on computers and operating systems that support the “IDEA” of virtual memory. Which, in this example is simple:

When you ask to play the game, the computer loads as much as it can into real memory. Meaning, 100 bytes are loaded, and 100 bytes stay out on your hard drive, because there is no more room in real memory.

You start to play the game. At some point, the game software needs to access the second half of itself, the half that is still on the disk. What the os/computer does is it writes a copy of the 100 bytes in memory out to the disk in a special file that some people call a “swap file” (not a technically accurate name, by the way, but thats what folks call it). It then loads the second part of your program, the 100 bytes that wouldnt fit in the first place, into memory, and continues executing the second half of the game.

The 100 bytes written to disk is sometimes said to be “100 bytes of virtual memory.” And you have as much virtual memory on your system as you have space in the special file.

Thats a very rough description of virtual memory.