VIRTUAL MEMORY
Virtual memory is a common part of
most operating systems on desktop computers. It has become so common
because it provides a big benefit for users at a very low cost.
Most computers today have something
like 64 or 128 megabytes of RAM (random-access memory) available for use
by the CPU (central processing unit). Often, that amount of RAM is not
enough to run all of the programs that most users expect to run at once.
For example, if you load the Windows operating system, an e-mail
program, a Web browser and word processor into RAM simultaneously, 64
megabytes is not enough to hold it all. If there were no such thing as
virtual memory, your computer would have to say, "Sorry, you cannot
load any more applications. Please close an application to load a new
one." With virtual memory, the computer can look for areas of RAM
that have not been used recently and copy them onto the hard disk. This
frees up space in RAM to load the new application. Because it does this
automatically, you don't even know it is happening, and it makes your
computer feel like is has unlimited RAM space even though it has only 32
megabytes installed. Because hard-disk space is so much cheaper than RAM
chips, virtual memory also provides a nice economic benefit.
The area of the hard disk that stores
the RAM image is called a page file. It holds pages of RAM on the hard
disk, and the operating system moves data back and forth between the
page file and RAM. (On a Windows machine, page files have a .SWP
extension.)
Of course, the read/write speed of a
hard drive is much slower than RAM, and the technology of a hard drive
is not geared toward accessing small pieces of data at a time. If your
system has to rely too heavily on virtual memory, you will notice a
significant performance drop. The key is to have enough RAM to handle
everything you tend to work on simultaneously. Then, the only time you
"feel" the slowness of virtual memory is in the slight pause
that occurs when you change tasks. When you have enough RAM for your
needs, virtual memory works beautifully. When you don't, the operating
system has to constantly swap information back and forth between RAM and
the hard disk. This is called thrashing, and it can make your computer
feel incredibly slow.
