Can you believe DDR3 has been present in home PC systems for three years already? It still has another two years as king of the hill before DDR4 will be introduced, and the industry currently isn't expecting volume shipments of DDR4 until 2015.
JEDEC isn't due to confirm the DDR4 standard until next year, but following on from the recent MemCon Tokyo 2010, Japanese website PC Watch has combined the roadmaps of several memory companies on what they expect DDR4 to offer.
Frequency abound! Voltage drops! But power still increases?
It looks like we should expect frequencies introduced at 2,133MHz and it will scale to over 4.2GHz with DDR4. 1,600MHz (10ns) could still well be the base spec for sever DIMMs that require reliability, but it's expected that JEDEC will create new standard DDR3 frequency specifications all the way up to 2,133MHz, which is where DDR4 should jump off.
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As the prefetch per clock should extend to 16 bits (up from 8 bits in DDR3), this means the internal cell frequency only has to scale the same as DDR2 and DDR3 in order to achieve the 4+GHz target.
The downside of frequency scaling is that voltage isn't dropping fast enough and the power consumption is increasing relative to PC-133. DDR4 at 4.2GHz and 1.2V actually uses 4x the power of SDRAM at 133MHz at 3.3V. 1.1V and 1.05V are currently being discussed, which brings the power down to just over 3x, but it depends on the quality of future manufacturing nodes - an unknown factor.
While 4.2GHz at 1.2V might require 4x the power it's also a 2.75x drop in voltage for a 32 fold increase in frequency: that seems like a very worthy trade off to us - put that against the evolution of power use in graphics cards for a comparison and it looks very favourable.
One area where this design might cause problems is enterprise computing. If you're using a lot of DIMMs, considerably higher power, higher heat and higher cost aren't exactly attractive. It's unlikely that DDR4 4.2GHz will reach a server rack near you though: remember most servers today are only using 1,066MHz DDR3 whereas enthusiast PC memory now exceeds twice that.