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Kingston always packs plenty of information into its model codes but the KVR800D2D8F5K2/2G kit I'm looking at today really does take the biscuit. Unfortunatley, to fully get to the bottom of what makes these modules tick we're going to have to grab that bull by its horns.
From the front the KVR stands for Kingston ValueRAM, the 800MHz speed is self-explanatory, D2 is DDR2, D8 is Dual Rank x8 i.e. the number of memory chips and their layout, the 5 is CAS Latency 5, K2 is a Kit of two modules and the 2G denotes a capacity of 2GB. Take those final two pieces of information and you'll see that this 2GB kit consists of two 1GB modules. However, the key part of the model code is the F, which stands for Fully Buffered and that's the novel feature in this memory.
Intel started talking about Fully Buffered DIMM (FB-DIMM) for servers in 2004 at just about the same time that desktop users were getting used to the idea of moving from DDR to DDR2 memory. FB-DIMM addressed a specific problem with DDR memory as it is tricky to load a motherboard with more than two modules of DDR. You may recall the popular nForce chipsets for Socket A Athlon processors supported three memory slots, which always seemed like an odd choice, but with DDR it was the practical limit.
Getting your system memory to communicate effectively with the Northbridge in the case of Pentium 4 and Core 2 is difficult and fans of AMD face a similar battle with the Athlon 64 and Phenom, although the processor talks directly to the memory without the involvement of the chipset in this case.
We PC merchants haven't much cared that our motherboards are effectively limited to two modules of memory as 32-bit Windows XP is limited to 4GB of RAM, anyway. When you load your system with decent graphics the memory limit can then drop to 3GB so most of us have got used to using 2x1GB or even 2x512MB to get by. This is no good for the server market, however, where 8GB, 16GB or even 32GB of RAM can be needed.
To handle this amount of memory you need to build an incredibly expensive multi-layered motherboard so Intel decided to help out with FB-DIMM. Instead of the usual DDR/DDR2/DDR3 topography where the memory controller talks to every memory chip on every memory module FB-DIMM adds a memory controller called the AMB (Advanced Memory Buffer) to the centre of each module. With FB-DIMM the memory controller in your CPU or chipset talks to the AMB which communicates with the memory chips on the module and also to the next FB-DIMM on the motherboard. It's a big change from a parallel layout to serial that simplifies the communication path to the memory and which allows the motherboard manufacturer to load the system with memory while simultaneously reducing manufacturing costs.
By my count that's a win-win-win situation however there is a sting in the tail.
Although the memory chips on a FB-DIMM module are regular DDR2 chips there is the consideration that the memory manufacturer has to add the AMB chip so costs are necessarily slightly higher in that department. More significantly the FB-DIMM module has two separate power feeds with the usual 1.8V feed for the memory and a new 1.5V supply for the AMB. That doesn't sound like much but where DDR2 modules are generally cool to the touch an FB-DIMM module carries a warning sticker that it gets hot. Once you've installed DDR2 you can pretty much forget about it, but not actively cooling FB-DIMMs would likely lead to a very short lifespan for your PC.
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