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In order to really maximize performance from your memory, you'll need to gain access to your system's bios. There is usually a Master Memory setting, often rightly called Memory Timing or Interface, which gives usually gives you the choice to set your memory timings by SPD or Auto, preset Optimal or Aggressive timings (e.g. turbo or ultra), and lastly an Expert or Manual setting that will enable you to manipulate individual memory timing settings to your liking.
Are the gains of the perfect, hand-tweaked memory timing settings worth it over the automatic settings? If you're just looking to run at stock speeds and want absolute stability, then the answer to that question would probably be no. You're better off going by SPD or Auto. You generally do not have a choice if you overclock or want to tweak more performance out of your system.
Q: Ok so I want to tweak, What do I do?
When tweaking your memory, the first step is to deactivate the automatic RAM configuration ---- SPD or Auto. When this function is activated, the motherboard reads the SPD chip (explained in next topic) on the memory module to obtain information about the timings and clock speed and to adjust the settings accordingly. However, these settings, which the RAM manufacturer stores in this chip, are very conservative in order to ensure stable operation on as many systems as possible. With a manual configuration, you can customize your settings for your own system - in most cases, the RAM modules will remain stable even when they exceed the manufacturer's specifications.
As a general rule, a lower number (or timing) will result in improved performance. After all, if it takes fewer cycles to complete an operation, then it can fit more operations within X amount of time. You might ask: Why can't we use 1 or even 0 values for memory timings? Good Question. There's a good answer. JEDEC specifies that it's not possible for current DRAM technology to operate as it should under such conditions. Depending on motherboard, you might be able to squeeze '1' on certain timings, but will very likely result memory errors and instability. And even if it doesn't't, it is unlikely to result in a performance gain.
If you are not planning on overclocking the clock speed of your RAM or if you have fast RAM rated at speeds above that of your current FSB, it may be possible to just lower the latencies for a nice performance gain. Memory Latencies can vary depending on the performance of RAM chips used. Not all memory modules will exhibit the ability to use certain latencies without producing errors. So testing, trial and error, is required.
Here are general guidelines to follow while "tweaking":
lower figures = better performance, but lower overclockability and possibly diminished stability.
higher figures = lesser performance, but increased overclockability and more stability -- to an extent
tRCD & tRP are usually equal numbers between 2 and 4. In tweaking for more overclockability, lower tRCD first between these two
CAS is not most critical of the various timings, unlike what is taught by many.
In general, the importance of CAS when placed against tRP and tRCD is nominal. Reducing CAS has a relatively minor effect on memory performance, while lower tRP & tRCD values result in a much more substantial gain. In other words if you had to choose, 3-3-2.5 would be better than 4-4-2.0 (tRCD-tRP-CAS)
CAS should be either 2.0 or 2.5. Many systems, most nforce2, fail to boot with a 3.0 setting or have stability problems.
tRAS should always be no less than the sum of CAS & tRCD see below
tRC is usually no less than the sum of tRAS and tRP. So if you have a tRAS of 11 and tRP of 2....then tRC should be 13. tRFC should be tRC + 2.
tRAS is unique, in that lowering it can lead to problems and lesser performance. tRAS is the only timing that has NO effect on real performance, if it is configured as it should. Sure a high tRAS on the nForce2 chipsets shows a better sandra score. But real-life performance is the same with different tRAS settings as long as tRAS is no less than the sum of tRCD and CAS Latency. Lower than this sum, can and will negatively affect your system's performance.
This document from Mushkin outlines how tRAS should be a sum of tRCD, CAS, and 2. For example, if you are using a tRCD of 2, and a CAS of 2 on your RAM, then you should set tRAS to 6. At values lower than that theory would dictate catastrophic consequences for data integrity including Hard drive addressing schemes --- truncation, data corruption, etc --- as a cycle or process would be ended before it's done. How is it possible for memory timings to affect my hard drive? When the system is shut down or a program is closed, physical ram data that becomes corrupted may be written back to the hard drive and thats where the consequences for the hard drive come in. Also lets not forget when physical ram data is translated by the operating system to virtual memory space located on the hard drive.
While it's important to consider the advice of experts like Mushkin, your own testing is still valuable. Systems both AMD & Intel can indeed operate with stability with 2-2-2-5 timings, and even exhibit a performance gain as compared to the theoretically mandated 2-2-2-6 configuration. The most important thing in any endeavor is to keep an open mind, and don't spare the effort. Once you've tried both approaches extensively it will be clear to you which is superior for your particular combination of components.