While all parts of a computer serve a function, random access memory, or RAM, is where the real work takes place. A computer serves no useful purpose without RAM, and RAM directly affects the computer's speed. One of the biggest reasons people upgrade their computers or replace them is for more RAM and increased speed.
What RAM does
RAM is the work space in your computer. When you pull up a file, the processor makes a copy of that file from the hard drive, puts it in RAM, and that's the one you work on. When you save that file, the work you have in RAM is copied down onto the read-only memory, or ROM, of the hard drive. In addition, the whatever program you're using runs in RAM, along with much of the operating system.
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System and Speed
RAM comes in the form of detachable integrated circuit strips, called "dual in-line memory modules" or DIMMs. A DIMM is usually 5 inches long and mounted to the motherboard of your computer. Generally, the more RAM you have installed in the computer, the faster it will run. More RAM increases the size of that work space, allowing you to do more complex things, or just to do more things. A person who multi-tasks and simultaneously runs a multi-tabbed Web browser, mail reader, word processing program, and music player will notice quickly a shortfall in RAM.
An older computer will run, but it probably won't be as fast as an out-of-the-box model. Although the user can opt for lighter-weight programs, they will bog down in time. If swap space is set up on the hard drive, the drive will work extra hard to take up the slack.
Complexity Requires More RAM
Long-time computer users may remember when RAM was measured in kilobytes instead of gigabytes as it is in 2010. But as computers evolved since the late-1980s, so has software--and the users' needs. A DOS system running text-based programs doesn't require as much memory as the latest Windows or Linux operating systems running videos and sound applications.
There are several ways to upgrade RAM in a computer. The surest way is to physically add more or bigger RAM chip. Fortunately, memory is fairly cheap. Before you try to do this, make sure your computer can be upgraded--most come with a limit on how much RAM they can use. In a pinch, you can set disk space aside as a swap file or partition, with the swap space acting as additional RAM.