Overclocking Intel's New 45nm QX9650: The Rules Have Changed

Select a Memory Divider and Set Some Timings

The latest generation of Intel memory controllers provides a much more expansive choice in memory dividers than ever before. That said, there are only three that we ever use, the most obvious of these being 1:1. Setting 1:1, simply put, means that the memory runs synchronously with the FSB. Keep in mind though that the FSB is quad-pumped (QDR) and memory is double data rate (DDR). For example, setting an FSB of 400MHz results in a 1.6GHz (4 x 400) effective FSB frequency at DDR-800 (2 x 400), assuming your memory is running 1:1. Selecting 5:4 at an FSB of 400MHz sets a memory speed of DDR-1000 (5/4 x 2 x 400). The other two dividers we would consider using besides 1:1 are 5:4, and in the case of DDR3, 2:1.

Regrettably, there are rarely any real performance gains by moving to memory ratios greater than 1:1. While it is true that many synthetic benchmarks will reward you with higher read and copy bandwidth values, the reality of the situation is that few programs are in fact bottlenecked with respect to total memory throughput. If we were to take the time to analyze what happens to true memory latency when moving from DDR2-800 CAS3 to DDR2-1000 CAS4, we would find that overall memory access times might actually increase. That may seem counterintuitive to the casual observer and is a great example of why it's important to understand the effect before committing to the change.

Start your next phase of tuning by once again entering the BIOS and selecting a memory divider. As mentioned earlier, even though there are many choices in dividers you will do best to stick to either 1:1 or 5:4 when using DDR2 and 2:1 when running DDR3. Next set your primary timings - typically, even the worst "performance" memory can handle CAS3 when running at about DDR2-800, CAS4 to about DDR2-1075, and CAS5 for anything higher. These are only approximate ranges though and your results will vary depending on the design of you motherboard's memory system layout, the quality of your memory, and the voltages you apply. You may find it easiest to set all primary memory timings (CL-tRCD-tRP) to the same value when first testing (i.e. 4-4-4, 5-5-5, etc.), and as a general rule of thumb, cycle time (tRAS) should be set no lower than tRCD + tRP + 2 when using DDR2 - for DDR3 try to keep this value between 15 and 18 clocks inclusive.

Failure of the board to POST (Power On Self-Test) after adjusting memory settings is a strong indication that either: A) you've exceed the memory's maximum possible frequency - choose a divider that results in a lower memory speed; B) the timings are too tight (low) for the attempted speed - loosen the values and try again; or C) the particular frequency/timing combination is possible, but not at the voltage currently applied - raise the memory voltage. Not all failure to POST conditions will have a solution. Some motherboards simply refuse to run certain memory dividers and we're finding more and more memory modules these days that are just flat out incapable of running the tighter timings possible with the previous generation's products.

Booting to the Windows desktop is always a pretty good indication that you are at least close to final stable values when it comes to memory. Again, start Prime95 and run at least 30 minutes of the blend test. Failures, especially rounding errors, are strong indications of memory problems. If you encounter errors, reset the system and increase the memory voltage by a small amount, always remembering to stay within specified limits. If you continue to experience errors, regardless of memory voltage, then you should loosen the primary timings and continue the testing. Once you have managed to "prime" for 30 minutes or more you can move on to the final phase - overclocking the CPU.