by Robert Lambert

Created on: April 12, 2009

I know that many people new to computing or who don't have an education in computer science can get confused about the difference between RAM and your hard drive. Both can be referred to as "memory," and they both store data, but they perform very different functions for the computer. RAM, or random access memory, is the component of your computer that stores whatever your computer is currently using. Right now your RAM contains your operating system, a web browser, this webpage, and any other programs you're running at the moment. Your hard drive also stores your operating system and your web browser, which may seem redundant, but the reason is simple: if you shut off your computer right now, everything in your RAM would disappear, while everything in your hard drive would remain.

The reason the hard drive can still store data when you turn off the power is that the data is stored on a physical disk (a hard drive is sometimes also called a hard disk). RAM, however, is actually made of circuits, which "store" an electrical signal. RAM needs to be powered in order to keep the signal going. So RAM is described "volatile" memory, meaning that it requires power to store information, whereas hard drives and other forms of permanent storage are called "non-volatile."

This might still seem redundant. If RAM can't store anything without power, why not just use a hard drive? The reason is speed. Reading information from a hard drive takes a long time, whereas reading from RAM is much faster. This makes a big difference because memory is accessed in very small pieces at a time. Your hard drive capacity is very likely measured in gigabytes, which are roughly billions of bytes, and each byte is 8 bits. The amount of data your processor can handle at one time is measured in bits. Your processor is constantly taking a small piece of data (usually a few bytes) called an instruction from RAM, then following that instruction, and then getting the next one. If it had to read your hard drive every time, your computer would never get anywhere.

However, if you need to transfer large amounts of data all at once, a hard drive can handle that. Therefore, when you start a program, that program is copied from your hard drive into your RAM so that your processor can run it. The program still exists on your hard drive, of course, so if you turn off the computer you won't lose that program, you'll just lose whatever you were doing with it at the time. So for example, if you were writing a

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