Will we ever see unbuffered DDR4?


Dec 24, 2005
So, DDR3 is about to come out with Intels Bearlake boards, but lets step back for a minute.

Remember the Athlon XP days with Single Channel memory controllers? at best you had 3 DIMMs per channel, but with 3 you could usually only run at 333MT/s (166MHz) as it was unstable at 400MHz.

Now we have DDR2, Dual channel, and we never have more than 2 DIMMs per channel. This is because even with on-die termination, it just wouldn't be stable otherwise.

Now with DDR3, we are told that we will only be allowed 1 DIMM per channel if we are running at 1600MT/s (800MHz) speeds or greater. Of course although JEDEC says DDR3 will be 800-1600MT/s, it also said DDR2 would be 400-800, and we have 1066, so we will probably see 2133MT/s DDR3 too. This would then also be 1 DIMM per channel

Don't worry, you will still have 4 DIMM slots (2 per channel) available if you want to run at 1333MT/s or slower.

This does put a dampener on having a lot of high speed memory. While 2 sticks is enough for most people, I have 4 sticks of 1GB, and more and more people are adopting this config. We CANNOT simply add more memory channels. The signal trace count for dual channel DDR2/3 is already insane, there just isn't room on the motherboard for more Parallel memory interfaces.

Where will DDR4 start then? It seems that at its lowest speeds we'll be limited to 2 DIMMs, and it maximum speeds will probably be limited by signal degradation.

The solution is of course a Serial memory interface, and one already exists!

FB-DIMMs. With only 67 active pins, FB-DIMMs allow us to cram Quad Channel interfaces in. the Buffer itself keeps the signal more stable, and brings server class error correction in. As the northbridge memory controller only ever sees the buffer, the system is fully compatible with future memory standards - DDR3 FB-DIMMs will work in current boards, and there is no reason why DDR4 ones cant.

Yes, we have a slight latency increase, but we enable higher maximum stable transfer speeds, and we enable Quad channel interleaved interfaces, which should double the maximum throughput.

Yay for FB-DIMM!