Articles: Memory
 

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DDR3-1600 vs. DDR2-1067 and DDR2-800 SDRAM

It turns out DDR3-1600 SDRAM faces a pretty serious problem. The today’s LGA775 systems, even those based on the latest Intel P45 chipset, officially do not support DDR3-1600 SDRAM. And it is not just a missing formal mention ion the specifications of the chipset and mainboards based on it. Things are in fact much worse: the memory controller in the chipsets for LGa775 processors supporting DDR3 memory cannot clock the memory at frequencies more than 4x higher than the FSB speed. And its means that the top memory type on the list of supported ones will be DDR3-1333 SDRAM even for CPUs with 1333MHz Quad Pumped Bus, and as we have just seen, this memory doesn’t improve the performance in any significant way.

The way-out of this situation is actually pretty evident: you can clock the memory at 1600MHz by setting the FSB at 400MHz. Unfortunately, there is only one CPU in the market today that supports this FSB frequency. It is Core 2 Extreme Q9770, which price exceeds all reasonable boundaries, that is why it is of no immediate interest to most users out there. In this case, there is practically only one solution remaining for those willing to use DDR3-1600 SDRAM: overclocking.

Of course, the need to increase the FSB speed past the nominal values does set certain limitations for the use of DDR3-1600 SDRAM. However, these limitations will definitely not discourage us from checking this memory inside out. Especially since contemporary system allow increasing the FSB to 400MHz without any difficulties. Since this frequency is in fact nominal for certain existing LGA775 processors, chipsets and mainboards should work just fine in these conditions. As for the CPUs, processors with 1333MHz Quad Pumped Bus can easily cope with 33% FSB frequency increase from 333MHz to 400MHz. In most cases, contemporary 45nm processors overclocked only that far will work stably without any additional voltages adjustment or special cooling solutions.

To illustrate this statement I would like to add that Core 2 Duo E8600 processor we used in our today’s test worked perfectly fine at 4.0GHz when the FSB frequency was raised to 400MHz. Experiments showed that this overclocking didn’t require any increase of the processor core voltage.

However, if you are not one of the overclocking fans, you can always lower the processor multiplier when increasing the FSB frequency. In this case the resulting CPU clock speed will not exceed its nominal value.

Note that at 400MHz FSB the processor bus bandwidth increases to 12.8 GB/s. The memory subsystem built on DDR2-667 or slower memory modules cannot provide the same bandwidth, and DDR2-800 can only guarantee that the processor bus bandwidth and memory bus bandwidth will match. Therefore, LGA775 chipsets do not support slower memory than DDR2-800 SDRAM at 400MHz FSB. And it means that you will not be able to increase the FSB frequency any further with this type of memory in your system. This is actually the major drawback of DDR2-800 SDRAM: it may not just limit the system performance at 400MHz FSB, but can also turn into a serious problem during further overclocking attempts.

Moreover, slow memory in systems with 400MHz FSB (and higher) may cause another problem. Low frequency of the memory subsystem requires higher setting of the Performance Level parameter – the chipset uses this latency to synchronize the processor and memory busses. This factor also affects the performance. Fast DDR3-1600 SDRAM supports Performance Level 6 or 7, while DDR2-800 SDRAM requires this parameter to be set at 8 or 9. Faster DDR2-1067 SDRAM is a little better in this respect: it allows lowering Performance Level to 7. The BIOS’s of contemporary mainboards are very well familiar with this chipset peculiarity that is why this factor is yet another reason for performance differences between systems using different memory types.

So, it looks like the use of faster DDR3 memory may really pay back in systems with 400MHz FSB. Let’s take a look at the results of our experimental test session performed in a system with Intel Core 2 Duo E8600 processor overclocked to 4.0GHz by raising the FSB frequency to 400MHz. Dual-channel 4GB memory kits we are going to discuss here worked in the following modes:

  • DDR2-800, 5-5-5-15 timings, Performance Level 9;
  • DDR2-800, 4-4-4-12 timings, Performance Level 8;
  • DDR2-1067, 5-5-5-15 timings, Performance Level 7;
  • DDR3-1600, 9-9-9-27 timings, Performance Level 7;
  • DDR3-1600, 8-8-8-24 timings, Performance Level 7;
  • DDR3-1600, 7-7-7-20 timings, Performance Level 6;
  • DDR3-1600, 7-6-6-18 timings, Performance Level 6.

The above listed DDR3-1600 timings configurations were selected basing on the specifications of solutions available in the market these days. It is also important to keep in mind that DDR3-1600 memory with CAS Latency 7 is the most widely spread type of memory. Therefore, we will focus on this type of SDRAM when analyzing the advantages of fast memory subsystem.

The latency of the memory subsystem based on DDR3-1600 is close to that of DDR2-1067. However, the bandwidth tests show that DDR3 boasts a serious advantage. This is a typical situation that is hardly surprising. What you should in fact pay special attention to, is the fact that transition from DDR2-800 to DDR2-1067 provides a much greater parameters improvement than transition from DDR2-1067 to DDR3-1600. You may consider this fact a perfect illustration of how insufficient the bandwidth of DDR2-800 is in a system with 1600MHz Quad Pumped Bus.

 
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