The Fully Buffered DIMM (FBDIMM) is an alternative to using 4-rank RDIMMs to support high system memory capacity. FBDIMMs solve the stub bus timing challenges and can increase system memory capacity by allowing more DIMMs sockets in the system when physical space is not an issue (ie: high-end servers use 4-8 DIMM slots).

 

 

The example below shows a Dual-Processor motherboard using 8 slots

 

 

16GB/system =  8GB/processor  (eight 2GB DIMMs using 2 ranks of 512Mb DRAM)

 

FBDIMMs are more costly then RDIMMs and have extra power and cooling considerations because of the onboard Advanced Memory Buffer (AMB) chip, which contains a portion of the memory controller to support the serialized memory subsystem architecture, consumes ~5watts. 

 

FBDIMMs are intended for systems capable of many memory slots where system density is the main driving factor. However FBDIMMs, unlike 4-Rank RDIMMs, do not solve memory controller limitations, lower memory module costs, increase memory speed/bandwidth, reduce motherboard real-estate or decrease system power consumption. Also, FB-DIMMs consume quite a bit more power with commensurate heat (FBDIMM DRAM fans are a necessity, not an option) and the AMB chip can add latency as well (although, some recent versions, can make it as nearly the same as typical registered DIMMs. FBDIMM modules can also be more expensive the RDIMMs. Even the new FBD-800 low-voltage FBDIMMs, while taking care of performance and power/heat issues, don't solve the price problem. 4-Rank RDIMMs are better suited for high density applications where the number of memory slots are limited due to space constraints on the motherboard.

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