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The memory speed should always be dependant on the speed of the front side bus (FSB). When memory runs at the same speed as the FSB, it is said to be running in synchronous operation. When memory and FSB are clocked differently (lower or higher than), it is known as asynchronous mode.
Only Intel chipsets have implemented async modes that have any merit. If you are talking about the older i845 series of chipsets, running an async mode that runs the memory faster than the FSB is crucial to top system performance. And with the newer dual channel Intel chipset (i865/875 series) in an overclocked configuration, often you must run an async mode that runs the memory slower than the fsb for optimal results. The async modes in SiS P4 chipsets also work correctly.
When looking at the AMD-supporting chipsets async mode are to be avoided at all costs. AMD-supporting chipsets offer less flexibility in this regard due to poorly implemented async modes. Even if it means running our memory clock speed well below the maximum feasible for a given memory, an Athlon XP system will exhibit best performance running the memory in sync with the FSB.
To achieve synchronous operation, there is usually a Memory Frequency or DRAM ratio setting in the bios of your system that will allow you to manipulate the memory speed to a either a percentage of the FSB (ie. 100%) or a fraction (or ratio) ie. N/N where N is any integer available to you. Here are some examples:
200MHz FSB speed with 100% or 1:1 (FSB:Memory ratio) results in 200MHz memory speed (DDR400)
200MHz FSB speed with 120% or 5:6 (FSB:Memory ratio) results in 240MHz memory speed (DDR480)
250MHz FSB speed with 80% or 5:4 (FSB:Memory ratio) results in 200MHz memory speed (DDR400)
The first example is wholly acceptable for any AMD system, memory should be set this way at all times for best performance. Asynchronous FSB/Memory Speeds are horridly inefficient on AMD systems, but may well be the optimal configuration for P4 systems.
The second example shows running the Memory at higher asynchronous speeds. Assume we have a Barton 2500+ which has a FSB of 333 MHz (166 MHz X 2) and we also have PC3200 memory running at 400 MHz (PC3200). This is a typical scenario because many people think that faster memory running at 400 MHz will speed up their system. Or they fail to disable the SPD or Auto setting in their bios. There is NO benefit at all derived from running your memory at a higher frequency (Mhz) than your FSB on Athlon XP/Duron systems. In actuality, doing so has a negative effect.
Why does this happen? It happens because the memory and FSB can't "talk" at the same speeds, even though the memory is running at higher speeds than the FSB. The memory would have to "wait for the FSB to catch up", because higher async speeds forces de-synchronization of the memory and FSB frequencies and therefore increases the initial access latency on the memory path ---- causing about a 2 - 5% degradation in performance.
This is another ramification of the limiting effect of the AMD dual-pumped FSB. A P4's quad pumped FSB (along with the superior optimization of the async modes) allows P4's to benefit in some cases from async modes that run the memory faster than the FSB. This is especially true of single channel P4 systems. There still are syncronizaton losses inherent in an async mode on any system, but the adequate FSB bandwidth of the P4 allows the additional memory bandwidth produced by async operation to overcome these losses and produce a net gain.
The third example is most often used in situations where the memory is not able to keep up with the speed of the FSB. In such a situation, your best option, if using an AMD system, is to lower the FSB to a point where it is synchronous with the memory speed and increase the multiplier (CPU ratio) instead. It is also a common scenario, where a user has PC2100 (266Mhz) memory and tries to pair it with a 333Mhz FSB processor. While some PC2100 memory are capable of PC2700 and higher, many others may not overclock as well. The only real benefit on async modes on AMD system is the fact it comes in handy to overclockers for testing purposes; to determine their max FSB and to eliminate the memory as a possible cause for not being able to achieve a desired stable FSB speed.
Looking to the Intel side of the fence, async modes that run the memory slower than the FSB have merit because of how async modes are implemented in the Intel chipsets. This is extremely important, as we cannot change the CPU multiplier on modern Intel systems and therefore have to use and async mode to allow substantial overclocks on the majority of systems utilizing the current 200/800MHz fsb family of P4 processors.
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