Overclocking DDR Memory: Truths and Misconceptions
A Buyers Guide

Date Written: September 23, 2002
Written By: Joey C. aka Chong345

Introduction:

With today's technology changing so quickly it is very easy to get behind. But when you hear about the fastest memory or CPU you think to yourself, "I have got to have it." Well, in truth, when looking at purchasing DDR based memory, there are a lot of things that you have to take into consideration. I am writing this article due to the fact that I frequently browse hardware forums and sites and one of the more notable questions that comes up all the time is, "What is the fastest memory?" or "What kind of memory should I buy?" People typically reply back and say what the faster modules are, or their opinion of different types of memory. What a lot fail to say are the other interesting points that one needs to know in order to maximize his / her system's performance, and at the same time spend money wisely.

DDR memory is available in a wide variety of different speeds at the moment. Here is a short list of some of the more common types:

• PC1600 - DDR200 MHz (100x2)
• PC2100 - DDR266 MHz (133x2)
• PC2700 - DDR333 MHz (166x2)
• PC3000 - DDR366 MHz (183x2)
• PC3200 - DDR400 MHz (200x2)
• PC3500 - DDR433 MHz (216x2)

Many manufacturers produce these modules, Corsair, Kingston, Samsung, Mushkin, Geil, XtremeDDR, and Crucial just to name a few. Each of these companies produces memory that is "rated" at a certain speed. They stand by their product and claim that the memory will run as fast as its rated speed. As I stated before there are many other factors though. For instance, if you are using a DDR333 motherboard, then your motherboard definitely supports DDR333 or PC2700. Anything higher than PC2700 you will have to overclock to get it to run at that speed. To put it simply, just because you buy PC3500 doesn't mean that you will be running 433 MHz (DDR). You have to overclock to that speed because the current standard only supports up to PC2700. To overclock your memory you must raise the front side bus which will typically overclock your AGP/PCI slots, CPU, and DIMM slots. Anytime you overclock, you run the risk of damaging parts or you might have issues with stability and performance of the rest of your system. Selection of memory will be directly influenced by the components in the rest of your system.

The main component that you need to take into consideration is the motherboard. It will have to be able to handle the speeds that you want from your memory. So the first thing to think about when purchasing DDR memory is, "Will my motherboard be able to handle the speeds that I want to run at, and does it provide options in the BIOS that I will need to overclock my memory?" I would suggest reading up on your motherboard and finding out the results that others have had. This is key and will greatly aid in deciding which memory you should purchase.

Choosing Memory For AMD Systems:

Let's start with the AMD platform. When picking DDR memory for an AMD based system you need to be really picky about what you buy. There are very few AMD based motherboards at the moment that can run at DDR400 speeds. Most support 333 MHz but some are just better overclockers than others. The good thing about the AMD platform is that you can unlock the multiplier on the chip. The multiplier x FSB (front side bus) gives you your CPU speed. For instance 133 FSB with a chip that has a 10 multiplier would give you 1,330 MHz (133x10) or 1.33 GHz. Since you are raising the FSB to overclock your memory, your CPU will come into play. It will be getting overclocked and at some point it will not be able to overclock any higher. This could stop you from maximizing you RAM. So by unlocking the CPU and lowering the multiplier, you could run the processor at a speed you know it can handle but overclock the FSB even higher to maximize the memory bandwidth. So let's say you don't want to overclock your CPU but you want to get your memory to run at its maximum, which just happens to be 400 MHz (200FSBx2). Using the same CPU as the one in the first example and considering you have a good motherboard, you could use the 6.5 multiplier and run your CPU at 1.3 GHz (200x6.5). Issues that this high of a FSB are mainly PCI/AGP clocks as they are now running out of specification which cause instabilities, video distortions, or glitches (AGP tearing), hard drive issues, and the list just goes on. Luckily most 333 motherboards provide a 1/5 divider which allows you to put your AGP/PCI closer to spec. At 133 FSB on a 333 motherboard your AGP/PCI clocks run at 66/33. This is uses a ¼ divider for the PCI. At 166  FSB you could use the 1/5 divider to bring your AGP/PCI back to spec, but anything higher will make you run out of spec again. So when trying to run at speeds over PC2700 you might run into issues with your AGP/PCI clocks. In short your video card, hard drives, soundcard, etc. will determine how high you can go as well the memory.

As you can see when running an AMD machine and trying to maximize memory, there are a lot of other issues that that you have to deal with. Major concerns are the AGP/PCI clocks and the CPU speed. Also to really maximize the RAM it's nice to have voltage options up to 3.2 volts on the VDIMM such as the Epox 8K3A. I like AMD machines myself but they just cannot seem to overclock the memory like the Intel Platforms do.

Choosing Memory For Intel Systems:

Picking RAM for an Intel machine is not any easier than picking it for an AMD rig but you can overclock easier in some aspects and harder in other when compared to AMD. Unlike the AMD, Pentium 4 multipliers cannot be unlocked. So you can only overclock the RAM by upping the front side bus and the rest of the system. The good thing is that on an Intel platform you can lock the AGP/PCI clocks in at 66/33, this is a very nice feature that I like. You don't ever have to worry about if your video card, sound card, hard drive, etc. is giving out. Other things like we discussed earlier like voltage options, etc still will play a part in deciding process.

Okay since we cannot unlock the multiplier what can we do to make the ram run faster? Well luckily Northwood P4's are very good overclockers to begin with. They are able to achieve high FSBs on good air-cooling. Also there are options on the Intel platform motherboards such as RAM ratios. For instance, I have an Abit IT7, let's say I can overclock my CPU to roughly 160 FSB with good air-cooling using a P4 1.6A. This would mean the chip is at about 2.56 GHz and the RAM is set at 1:1 (FSB:RAM) so I would be running it like the AMD rig. It would be 160x2 so it would be 320 MHz DDR but my IT7 has a 3:4 memory ratio. So at 160 FSB the memory would be at 213x2 which would be 426 MHz! This is great for people who want to use PC3500 and run some really great memory speeds. Now you run into issues again though. Let's say you have really good cooling for instance. Something that lets you overclock to say 190 FSB. If you were to use a 3:4 memory ratio the memory would be at 506 MHz (253x2). Let's also say that you have some PC3500 that you want to use, well there are not many sticks of RAM that can run at 506 MHz. But if you were to use the 1:1 memory ratio then you memory would be doing 380 MHz. You would be underclocking you memory below its rated specifications. So what do you do? Well you could get a new chip with a bigger multiplier that will overclock to 160's FSB with serious cooling such as a 2.8 with its 21 multiplier. Or you could run your CPU lower than what you max out at so that you can use the 3:4 divider. This is the exact spot I am in at the moment. In my situation I can run my CPU at 193FSB but I have to use the 1:1 ratio. I don't want to go back to a lower FSB because I like my CPU to be maxed out. So what am I going to do? I am going to get a chip with the larger multiplier so I can have super fast CPU speed as well as memory speed.

Conclusions & Other Insights:

So you see there is a lot to Intel platforms as well as AMD. You have some good benefits and some bad for each. You have to choose your RAM wisely considering what kind of chip you have and what FSB you can run. I've seen some people get in a situation where their CPU was a bad overclocker so they could not max out the RAM, so they got extreme cooling and were able to overclock the CPU a lot more but got stuck in the situation that I am in.

When purchasing memory you have to find the perfect balance of everything if you want your components to be running at their full potential. This is called the sweet spot. Both AMD and Intel machines have a sweet spot where you have the perfect balance of memory bandwidth as well as CPU speed.

Now that we have covered the AMD and Intel platforms separately by what each machine has to offer and how it affects your choice, now let's talk about the last thing that applies to both. Memory timings are just as important as overall speed. Some people buy PC3500 and expect it to run at 3500 speeds and forget that not only is there a rated speed, but every stick of RAM has its own memory timings. For instance, some are CAS 2.5 3-6-3 2T and some are CAS 2 3-6-3 1T. These timings will influence how well you can overclock the memory as well. Since this is not a overclocking guide I will not go into what the timings mean but I just want the buyer to keep in mind that his memory is rated to run at a specific speed at specific memory timings, anything faster and you are overclocking your memory just like any other component, which mileage can vary.

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