Since we haven’t yet discussed FB-DIMM (Fully Buffered DIMM) memory modules in any of our previous reviews, we would like to say a few words about this memory type today.
The need for high memory capacities brought FB-DIMM support into server and workstation platforms. The thing is that the memory controller cannot work with a lot of traditional memory modules because of pretty high electrical load that is required for each of the memory modules to function stably at high speeds. For example, to support 64GB of DDR2 SDRAM you will need to install 16 memory modules, so the memory controller in your system is very unlikely to cope successfully with all those DIMMs even if they are Registered ones. That is why Intel decided to give up the traditional parallel memory bus design and switch to serial bus. FB-DIMM modules are part of this concept: each module like that has DDR2 chips connected to a serial bus via the AMB (Advanced Memory Buffer) chip installed on each modules like that.
Like everything else, this approach also has some disadvantages. Firstly, the memory subsystem built with FB-DIMM features much higher latency. There is an additional AMB controller that stands in the way of data transferred between the DDR2 SDRAM chips and the processor. Besides, AMB is a pretty complex device that consumes quite a bit of power and heats up tangibly during operation. So all FB-DIMM modules consume about 3-6W each and should be equipped with heat-spreaders.
Our system featured four 1GB modules from Samsung, one in each channel.
As you can see from the stickers, these modules are designed to run at 667MHz with 5-5-5-11 timings.
The same info is reported in the modules SPD: