Double Data Rate

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A very commonly used type of memory in today's PCs is Double Data Rate (or DDR) RAM. What sets DDR memory apart from Single Data Rate (SDR) memory is how information is written to it and read from it. RAM generally is written to, (and read from), in concert with a “clock signal”. The clock signal must be a regular, consistent electrical pulse or the system will destabilize and stop working.

So, imagine the clock signal as a child on a swing. At the forward peak of the swing, she is close enough to her friend to whisper a few words, but on the backward swing, they are too far apart to talk in private. This is equivalent to SDR memory: you get across one message per cycle.

If the child and her friend want to communicate faster, they could try making the swing go faster. This isn't really easy to do; the child would have to swing her legs more than twice as fast. A better option is to have another friend standing near the backside peak of the swing. Now the swinger can give one message at the front peak of her swing, and another on the back peak, effectively doubling the rate at which she can communicate without working any harder. This is what DDR memory does. It squeezes in two memory accesses per clock cycle. Since you are sending data twice per clock cycle, rather than pushing the clock faster, you don't expend as much energy. This is a good thing for laptops, and keeps the overall costs down.

DDR memory (and its successor, DDR2) is commonly used in personal computers, laptops, and even video cards. Usually found in DIMM configurations, it can be purchased in various speeds, labeled using the version (DDR RAM begins with a “PC” prefix, DDR2 begins with “PC2”), and the speed (bandwidth measured in Gigabytes per Second).

The following types of memory are fairly common and should be available from your local computer shop or online store:
PC-1600, PC-2100, PC-2700, PC-3200, PC2-3200, PC2-4200, PC2-5300, and PC2-6400. Keep in mind that each system may have its own restrictions or requirements, so refer to your users manual before upgrading memory.

Next Page: Dual In-Line Memory Module

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