Info: What are FB-DIMMs? - Overclock.net

HrnyGoat · 09-14-06

FB-DIMM stands for Fully-Buffered Dual Inline Memory Module. It is a memory standard that was developed by Intel for server use.

Why FB-DIMM?

Current memory technology uses a parallel memory bus between the memory controller and the memory itself. Parallel buses can transmit a large amount of data with a relatively low clockspeed. However, they require a large number of data pathways. 240 pins are need per channel for DDR2. This takes up a significant amount of motherboard space, so the more pathways, the higher the cost and the less room for other components.

This is where FB-DIMM technology comes in. It uses a highly-clocked serial bus to transmit data. The number of data pathways are reduced significantly, down to 69 pins.

Also, with a parallel bus, all the traces (data paths) have to be exactly the same length. That is why you see all those squiggly lines running from the RAM on motherboards and graphics cards. This is because all the signals must reach the destination at the exact same time. With the serial bus that FB-DIMM uses, the traces do not have to be the same length, further saving motherboard space.

Another reason is that as the clock speed goes up, the number of devices a parallel bus can handle decreases. So if we were to keep increasing the bus speed, there would be fewer and fewer sticks of RAM that could be used. On a serial bus, the number of devices can stay the same, regardless of clockspeed. FB-DIMM technology can currently handle 8 devices (RAM sticks) per channel.

The technology behind FB-DIMM

FB-DIMM isnt the first memory technology to use a parallel bus. RDRAM was introduced for PC's in 1999. However, due to a non-standard memory chip design, the cost was excessive and quickly lost favor to DDR SDRAM, which was considerably cheaper.

Intel addressed this issue by using standard DDR2 memory chips and putting a buffer on the memory module. The buffer converts the serial memory requests into parallel requests and sends them to the memory chips. Then parallel data from the memory chips is serialized and sent to the memory controller. Intel refers to this buffer as an AMB, or Advanced Memory Buffer.

The memory bus itself runs at 6 times the speed of the memory chips themselves. It consists of 24 lanes; 14 read and 10 write. The reason is that memory is read more often than it is written to, so there are more lanes to read than write.

If there is more than one memory module per channel, the memory controller will send data to the first module's AMB. If the data is not destined for the first module, it is relayed onto the next module's AMB and so on.

FB-DIMM technology is scalable, and can support up to six channels with 8 modules per channel, for a total of 48 DIMMs. With a memory density of 4GB per DIMM, that adds up to 192 gigabytes. It is also future-proof, supporting DDR3 and beyond without needing a new memory controller.

There are some disadvantages to using FB-DIMM's, such as higher latancies that are inherent with serial technology. Due to the AMB, the memory modules put out more heat and must be cooled with a passive heatsink. This also results in more power consumption.

Despite these disadvantages, FB-DIMM technology may represent a major advance in the server/workstation market.

*Images courtesy of Intel