DDR vs. DDR2 vs. DDR3: Types Of RAM Explained

How do memory types differ? Here's a quick rundown on where DDR3 came from and what it means for your computer.

Shopping for most kinds of computer hardware is easy. If you pay even basic attention to the industry you're not going to have too much trouble following individual trends and understanding the basic specifications you'll find on desktops, laptops, or even components. But in your perusing the shelves at your local Best Buy or the e-stocks of an online retailer like Newegg.com, you may have noticed one unusual and even arcane-looking acronym come up time and time again: DDR3. What is it? What does it mean? And what happened to DDR and DDR2?

At its most basic, DDR3 is the current standard for system memory, aka RAM or, to get more specific, SDRAM. It's the fastest consumer RAM currently in widespread use, and the type you're most often going to want to buy (today, at any rate) if you want to upgrade your computer or if you're planning to buildone from scratch. DDR3 has all but replaced the older DDR and DDR2 in the marketplace, which is why these days DIMMs using those earlier technologies can be somewhat difficult to find and expensive to purchase.

But what exactly does the term "DDR3" mean? To understand that, you need to understand its history.

SDRAM, or synchronous dynamic random access memory, was developed in the early 1990s to solve a problem that began cropping up as computers became more powerful. Traditional DRAM used an asynchronous interface, which means it operated independently of the processor—which was not ideal if the memory couldn't keep up with all of the requests the processor made of it. SDRAM streamlined this process by synchronizing the memory's responses to control inputs with the system bus, allowing it to queue up one process while waiting for another. This way, computers couldexecute tasks much more quickly than had previously been possible, and was the memory standard in computer systems by the end of the 1990s.

It didn't take long after the introduction of SDRAM for hardware developers and regular users to determine that even this route had its limitations. The original SDRAM operated via a single data rate (or SDR) interface that, in spite of the type's overall advances compared with DRAM, could still only accept one command per clock cycle. As computers were becoming more popular and more complicated, and thus issuing more complex requests to the memory more frequently, this was slowing down performance.

Around 2000, a new interface method was developed. Called double data rate (or DDR), it let the memory transfer data on both the rising and falling edges of the clock signal, giving it the capability to move information nearly twice as quickly as with regular SDR SDRAM. There was another side benefit to this change as well: It meant memory could run at a lower clock rate (100-200MHz), using less energy (2.5 volts), and achieve faster speeds (transfer rates of up to 400 MTps).

As technology progressed and processors became still more powerful and demanding, DDR alone became insufficient. It was followed, in 2003, by DDR2, which refined the idea even further with an internal clock running at half the speed of the data bus; this meant it was about twice as fast as the original DDR (200-533MHz, with transfer rates up to 1,066MTps), but again used less power (1.8 volts). Naturally, DDR3 was next out of the gate (it debuted around 2007), with its internal clock cut in half again, its speed about twice that of DDR2 (400-1,066MHz, for a maximum transfer rate of 2,133MTps), and power usage reduced even more over its predecessor (to 1.5 volts).

(You may have already surmised the next logical step in memory technology. Indeed, DDR4 is already in development, and will probably begin appearing in consumer products around 2014, with wider adoption to follow gradually.It's expected to offer transfer rates of up to 4,266MTps, with voltage ranging from 1.05 to 1.2 volts.)

What's the down side to this constant improvement of memory? Unfortunately, you can't benefit from most of these advances without significantly upgrading—if not outright replacing—your current hardware: A DIMM that uses one kind of DDR interface will not work in a motherboard designed for another. Each type of memory is electrically incompatible with the others, starting with the number of pins on a chip (DDR desktop-style DIMMs have 184, and DDR2 and DDR3 each have 240), and DIMMs using each are keyed (or notched) differently so they can't even fit in the wrong kind of socket. It's therefore crucial that your existing hardware and the memory you want to add are of the same DDR type.

The good news is that because DDR3 is so prevalent today, you probably won't need to worry too much about this until DDR4 starts gaining ground in a couple of years. If your computer uses the DDR2 standard, there are definitely compatible chips out there, but expect to pay more and get less: A quick search on Newegg revealed an 8GB kit of high-speed G.SKILL DDR3 RAM available for $84.99; the same amount of slower DDR2 cost $149.99. And if your computer is stuck on the original DDR, you may want to consider a full-system upgrade. (A mere 2GB of that, also from G.SKILL and also slower, is $59.99.)

When you're purchasing memory, it's also best to verify that its speed matches that of your motherboard; otherwise, performance bottlenecks may result. At least this information is easy to determine from the memory's specs: If you have a motherboard that supports the DDR3 1333 standard (the "1333" references the memory's transfer rate, in MTps, not the actual speed of the memory, as is commonly assumed), you'll be fine as long as the memory uses that same designation. If the memory is slower (meaning, it has a lower DDR3 number) you won't be accessing your computer's full potential; if it's higher, you risk the processor not being able to keep up with it.

Neither, of course, is ideal: You'll get the best results when all of your computer's components are in harmony. Given the work and inspiration that went into DDR3, built on the platform of the types of memory that came before it, wouldn't it be a shame to not take advantage of the speed increases it offers if they're available to you?