DDR500 Support: Finally It Is Here!

As we have already shown above, K8 processors on E core revision haven’t got any better than their predecessors when it comes to working with four memory modules. However, the memory controller optimization by introducing enhanced workload distribution algorithms was not the only thing AMD engineers had been working on. They also added the support of new memory bus frequency dividers thus expanding the functionality of the integrated memory controller. As a result, CPUs on E core revision learned to support faster types of memory than the good old DDR400 SDRAM. And even though these memory modules haven’t been certified by JEDEC, which moved its primary focus to DDR2 SDRAM standards, these modules became pretty popular in the today’s market.

The memory manufacturers offering solutions for hardware enthusiasts have been offering memory modules with over 400MHz frequency for quite a while already. Until recently these solutions were mostly targeted for overclockers. AMD engineers, however, made them of interest to common users, too. The thing is that new Athlon 64, Athlon 64 FX, Athlon 64 X2 and Sempron processors based on E core revision can now support DDR466 and DDR500 SDRAM.

Before we start discussing how the owners of K8 processors on the new core can benefit from this new feature, we should say a few words about the data memory controller of these CPUs. This way we will be able to get a better idea of DDR466 and DDR500 SDRAM support peculiarities in new Athlon 64 processors.

The memory bus in K8 processors is clocked not exactly as the bus in Athlon XP or Pentium4. Athlon 64 processors and their modifications do not have the FSB bus because they are based on the so-called Direct Connect architecture. The memory controller of these CPUs is a part of the processor core that is why the CPU doesn’t use any busses or bus protocols to address the memory controller.

The working frequency of the K8 processors is based on the clock generator working frequency (equal 200MHz at default) and not on the frequencies of some busses. It means that the memory frequency in Athlon 64 based systems is not determined by the clock generator frequency or bus frequency but is based on the CPU clock rate and its clock frequency multiplier.

The basic ratio for the memory frequency in Athlon 64 systems looks as follows:

DRAM_frequency = CPU_frequency / ceil (CPU_multiplier / DRAM_frequency_divider)

Here DRAM_frequency is the frequency in question, CPU_frequency – processor working frequency, CPU_multiplier – its actual multiplier, and DRAM_frequency_divider – sets the memory controller work mode and is selected from the existing set. If you are into coding, you should be familiar with the ceil function: it computes the smallest integral value no loss than x.

The memory clock frequency is only determined by the CPU clock rate and the corresponding divider that is why Athlon 64 processors with different clock frequencies set slightly different working frequencies for the memory modules in the system. In particular, you can find the following table in the documents available on AMD’s official web-site:

You can easily compose this table yourself if you know what DRAM frequency dividers Athlon 64 CPUs support. Luckily this is no big secret. The CPUs on D core revision support 1/2, 3/5, 2/3, 7/10, 3/4, 5/6, 9/10 and 1/1 dividers. All these dividers ensure that the CPUs support memory working at any frequencies close to 100, 120, 133, 140, 150, 166, 180 and 200MHz. The CPUs on the new E core acquired two additional dividers, which are bigger than 1. They are 7/6 and 5/4. As a result, Athlon 64 on E core works fine with the memory supporting something close to 233 and 250MHz.