Moscow State University

The Department of Computational Mathematics and Cybernetics

Generations of double data rate synchronous dynamic random access memory (DDR SDRAM)

Timofeev Kirill
Groupe 205

Moscow 2010

Introduction
Synchronous dynamic random access memory (SDRAM) is dynamic random access memory (DRAM) that has a synchronous interface. Traditionally, dynamic random access memory (DRAM) has an asynchronous interface, which means that it responds as quickly as possible to changes in control inputs. SDRAM has a synchronous interface, meaning that it waits for a clock signal  before responding to control inputs and is therefore synchronized with the computer's system bus. The clock is used to drive an internal finite state machine that pipelines incoming instructions. This allows the chip to have a more complex pattern of operation than an asynchronous DRAM, which does not have a synchronized interface.
Pipelining means that the chip can accept a new instruction before it has finished processing the previous one. In a pipelined write, the write command can be immediately followed by another instruction without waiting for the data to be written to the memory array. In a pipelined read, the requested data appears after a fixed number of clock pulses after the read instruction, cycles during which additional instructions can be sent. (This delay is called the latency and is an important parameter to consider when purchasing SDRAM for a computer.)
SDRAM is widely used in computers; from the original SDRAM, further generations of DDR (or DDR1) and then DDR2 and DDR3 have entered the mass market, with DDR4 currently being designed and anticipated to be available in 2012.

1.DDR SDRAM
Double data rate synchronous dynamic random access memory (DDR SDRAM) is a class of memory integrated circuits used in computers.
Compared to single data rate (SDR) SDRAM, the DDR SDRAM interface makes higher transfer rates possible by more strict control of the timing of the electrical data and clock signals. Implementations often have to use schemes such as phase-locked loops and self-calibration to reach the required timing accuracy. The interface uses double pumping (transferring data on both the rising and falling edges of the clock signal) to lower the clock frequency. One advantage of keeping the clock frequency down is that it reduces the signal integrity requirements on the circuit board connecting the memory to the controller. The name "double data rate" refers to the fact that a DDR SDRAM with a certain clock frequency achieves nearly twice the bandwidth of a single data rate (SDR) SDRAM running at the same clock frequency, due to this double pumping.
With data being transferred 64 bits at a time, DDR SDRAM gives a transfer rate of (memory bus clock rate) × 2 (for dual rate) × 64 (number of bits transferred) / 8 (number of bits/byte). Thus, with a bus frequency of 100 MHz, DDR SDRAM gives a maximum transfer rate of 1600 MB/s.
"Beginning in 1996 and concluding in June 2000, JEDEC developed the DDR (Double Data Rate) SDRAM specification (JESD79)." JEDEC has set standards for data rates of DDR SDRAM, divided into two parts. The first specification is for memory chips, and the second is for memory modules. DDR SDRAM (sometimes referred to as DDR1 SDRAM) has been superseded by DDR2 SDRAM and DDR3 SDRAM.
Specification standards
Chips and modules
Standard name -- Memory clock(MHz)--  Cycle time(ns)--  I/O bus clock(MHz)--  Data rate(MT/s)-- VDDQ(V)-- Module name-- Peak transfer rate(MB/s)-- Timings(CL-tRCD-tRP)
DDR-200 	100 	10 	100 	200 	2.5±0.2 	PC-1600 	1600    -
DDR-266 	133 	7.5 	133 	266 	2.5±0.2 	PC-2100 	2133    -
DDR-333 	166 	6 	166 	333 	2.5±0.2 	PC-2700 	2667	   -
DDR-400A 	200 	5 	200 	400 	2.6±0.1 	PC-3200 	3200 	2.5-3-3
DDR-400B	 -	-	 -	 -	   -		  -		 -	 3-3-3
DDR-400C 	 -	-	 -	 -	   -		  -		 -	 3-4-4
Note: All above listed are specified by JEDEC as JESD79F. All RAM data rates in-between or above these listed specifications are not standardized by JEDEC—often they are simply manufacturer optimizations using higher-tolerance or overvolted chips.
The package sizes in which DDR SDRAM is manufactured are also standardized by JEDEC.
There is no architectural difference between DDR SDRAM designed for different clock frequencies, for example, PC-1600, designed to run at 100 MHz, and PC-2100, designed to run at 133 MHz. The number simply designates the data rate at which the chip is guaranteed to perform, hence DDR SDRAM is guaranteed to run at lower and can possibly run at higher clock rates than those for which it was made. These practices are known as underclocking and overclocking respectively.
DDR SDRAM modules for desktop computers, commonly called DIMMs, have 184 pins (as opposed to 168 pins on SDRAM, or 240 pins on DDR2 SDRAM), and can be differentiated from SDRAM DIMMs by the number of notches (DDR SDRAM has one, SDRAM has two). DDR SDRAM for notebook computers, SO-DIMMs, have 200 pins, which is the same number of pins as DDR2 SO-DIMMs. These two specifications are notched very similarly and care must be taken during insertion if unsure of a correct match. DDR SDRAM operates at a voltage of 2.5 V, compared to 3.3 V for SDRAM. This can significantly reduce power consumption. Chips and modules with DDR-400/PC-3200 standard have a nominal voltage of 2.6 V.
Many new chipsets use these memory types in dual-channel configurations, which doubles or quadruples the effective bandwidth.
Chip characteristics
1. DRAM density
    Size of the chip in megabits. For example, 256 Mbit, or 32 MB chip. Nearly all motherboards only recognize 1 GB modules if they are low density 64M×8 modules. If high density 128M×4 1 GB modules are used, they most likely will not work. The JEDEC standard allows 128M×4 only for slower buffered/registered modules designed specifically for some servers, but some generic manufacturers do not comply.
2. DRAM organization
    Written in the form of 64M×4, where 64M is a number of storage units (64 million), x4 (pronounced "by 4") — number of bits per chip, which equals the number of bits per storage unit. There are ×4, ×8, and ×16 DDR chips. The ×4 chips allow the use of advanced error correction features like Chipkill, memory scrubbing and Intel SDDC, while the ×8 and ×16 chips are somewhat more expensive.
Module characteristics
1. Capacity
2. Number of DRAM Devices
    The number of chips is a multiple of 8 for non-ECC modules and a multiple of 9 for ECC modules. Chips can occupy one side (single sided) or both sides (dual sided) of the module. The maximum number of chips per DDR module is 36 (9×4) for ECC and 32 (8x4) for non-ECC.
3. ECC vs non-ECC
    Modules that have error correcting code are labeled as ECC. Modules without error correcting code are labeled non-ECC.
4. Number of DRAM Ranks
(also known as rows or sides) Any given module can have 1, 2, or 4 ranks, but only 1 rank of a module can be active at any moment in time. When a module has two or more ranks, the memory controller must periodically switch between them by performing close and open operations. Do not confuse rows in this context with rows used to describe internal chip architecture (that is why the term rank is to be preferred). The term sides is also confusing because it incorrectly suggests that this is tied to the physical placement of chips on the module.
5. Timings
    CAS latency (CL), clock cycle time (tCK), row cycle time (tRC), refresh row cycle time (tRFC), row active time (tRAS).
6. Buffering
    registered (or buffered) vs unbuffered
7. Packaging
    Typically DIMM or SO-DIMM
Module and chip characteristics are inherently linked.
Total module capacity is a product of one chip's capacity by the number of chips. ECC modules multiply it by 8/9 because they use one bit per byte for error correction. A module of any particular size can therefore be assembled either from 32 small chips (36 for ECC memory), or 16(18) or 8(9) bigger ones.
DDR memory bus width per channel is 64 bits (72 for ECC memory). Total module bit width is a product of bits per chip by number of chips. It also equals number of ranks (rows) multiplied by DDR memory bus width. Consequently a module with greater amount of chips or using ×8 chips instead of ×4 will have more ranks.
Example: Variations of 1 GB PC2100 Registered DDR SDRAM module with ECC Module size(GB) Number of chips Chip size(Mbit) Chip organization Number of ranks
1 		36 			256 			64M×4 			2
1 		18 			512 			64M×8 			2
1 		18 			512 			128M×4 			1
This example compares different real-world server memory modules with a common size of 1 GB. One should definitely be careful buying 1 GB memory modules, because all these variations can be sold under one price position without stating whether they are ×4 or ×8, single or dual ranked.
There is a common belief that number of module ranks equals number of sides. As above data shows, this is not true. One can find 2-side/1-rank or 2-side/4-rank modules. One can even think of a 1-side/2-rank memory module having 16(18) chips on single side ×8 each, but it's unlikely such a module was ever produced.
History
Double data rate (DDR) SDRAM specification
From JEDEC Board Ballot JCB-99-70, and modified by numerous other Board Ballots, formulated under the cognizance of Committee JC-42.3 on DRAM Parametrics.
Standard No. 79 Revision Log:
    * Release 1, June 2000
    * Release 2, May 2002
    * Release C, March 2003 – JEDEC Standard No. 79C.
"This comprehensive standard defines all required aspects of 64Mb through 1Gb DDR SDRAMs with X4/X8/X16 data interfaces, including features, functionality, ac and dc parametrics, packages and pin assignments. This scope will subsequently be expanded to formally apply to x32 devices, and higher density devices as well."
MDDR
MDDR is an acronym that some enterprises use for Mobile DDR SDRAM, a type of memory used in some portable electronic devices, like mobile phones, handhelds, and digital audio players. Through techniques including reduced voltage supply and advanced refresh options, Mobile DDR can achieve greater power efficiency.

2.DDR2 SDRAM
DDR2 SDRAM is a double data rate synchronous dynamic random access memory interface. It supersedes the original DDR SDRAM specification and the two are not compatible. In addition to double pumping the data bus as in DDR SDRAM (transferring data on the rising and falling edges of the bus clock signal), DDR2 allows higher bus speed and requires lower power by running the internal clock at half the speed of the data bus. The two factors combine to require a total of four data transfers per internal clock cycle.
With data being transferred 64 bits at a time, DDR2 SDRAM gives a transfer rate of (memory clock rate) × 2 (for bus clock multiplier) × 2 (for dual rate) × 64 (number of bits transferred) / 8 (number of bits/byte). Thus with a memory clock frequency of 100 MHz, DDR2 SDRAM gives a maximum transfer rate of 3200 MB/s.
Since the DDR2 internal clock runs at half the DDR internal clock rate, DDR2 memory operating at the same external data bus clock rate as DDR results in DDR2 being able to provide the same bandwidth but with higher latency. Consequently, DDR2 RAM possesses inferior performance. Alternatively, DDR2 memory operating at twice the external data bus clock rate as DDR may provide twice the bandwidth with the same latency. The best-rated DDR2 memory modules are at least twice as fast as the best-rated DDR memory modules.
Overview
Like all SDRAM implementations, DDR2 stores memory in memory cells that are activated with the use of a clock signal to synchronize their operation with an external data bus. Like DDR before it, the DDR2 I/O buffer transfers data both on the rising and falling edges of the clock signal (a technique called "double pumping"). The key difference between DDR and DDR2 is that for DDR2 the memory cells are clocked at 1 quarter (rather than half) the rate of the bus. This requires a 4-bit-deep prefetch queue, but, without changing the memory cells themselves, DDR2 can effectively operate at twice the bus speed of DDR.
DDR2's bus frequency is boosted by electrical interface improvements, on-die termination, prefetch buffers and off-chip drivers. However, latency is greatly increased as a trade-off. The DDR2 prefetch buffer is 4 bits deep, whereas it is two bits deep for DDR and eight bits deep for DDR3. While DDR SDRAM has typical read latencies of between 2 and 3 bus cycles, DDR2 may have read latencies between 4 and 6 cycles. Thus, DDR2 memory must be operated at twice the data rate to achieve the same latency.
Another cost of the increased bandwidth is the requirement that the chips are packaged in a more expensive and more difficult to assemble BGA package as compared to the TSSOP package of the previous memory generations such as DDR SDRAM and SDR SDRAM. This packaging change was necessary to maintain signal integrity at higher bus speeds.
Power savings are achieved primarily due to an improved manufacturing process through die shrinkage, resulting in a drop in operating voltage (1.8 V compared to DDR's 2.5 V). The lower memory clock frequency may also enable power reductions in applications that do not require the highest available data rates.
According to JEDEC the maximum recommended voltage is 1.9 volts and should be considered the absolute maximum when memory stability is an issue (such as in servers or other mission critical devices). In addition, JEDEC states that memory modules must withstand up to 2.3 volts before incurring permanent damage (although they may not actually function correctly at that level).
Specification standards
Chips and modules
For use in computers, DDR2 SDRAM is supplied in DIMMs with 240 pins and a single locating notch. Laptop DDR2 SO-DIMMs have 200 pins and often come identified by an additional S in their designation. DIMMs are identified by their peak transfer capacity (often called bandwidth).
Standard name--Memory clock(MHz)--Cycle time(ns)--I/O bus clock(MHz)--Data rate(MT/s)--Module name--Peak transfer rate(MB/s)--Timings(CL-tRCD-tRP)
DDR2-400B	100 	10 	200 	400 	PC2-3200 	3200 	3-3-3
DDR2-400C 	 -	-	 -	 -	   -		  -	4-4-4
DDR2-533B	133 	71/2 	266 	533 	PC2-4200* 	4266 	3-3-3
DDR2-533C 	 -	-	 -	 -	   -		  -	4-4-4
DDR2-667C	166 	6 	333 	667 	PC2-5300* 	5333 	4-4-4
DDR2-667D 	 -	-	 -	 -	   -		  -	5-5-5
DDR2-800C	 200 	5 	400 	800 	PC2-6400 	6400 	4-4-4
DDR2-800D	 -	-	 -	 -	   -		  -	5-5-5
DDR2-800E 	 -	-	 -	 -	   -		  -	6-6-6
DDR2-1066E	266 	33/4 	533 	1066 	PC2-8500* 	8533 	6-6-6
DDR2-1066F  -	-	 -	 -	   -		  -	7-7-7
* Some manufacturers label their DDR2 modules as PC2-4300, PC2-5400 or PC2-8600 instead of the respective names suggested by JEDEC. At least one manufacturer has reported this reflects successful testing at a higher-than standard data rate whilst others simply round up for the name.
Debut
DDR2 was introduced in the second quarter of 2003 at two initial clock rates: 200 MHz (referred to as PC2-3200) and 266 MHz (PC2-4200). Both performed worse than the original DDR specification due to higher latency, which made total access times longer. However, the original DDR technology tops out at a clock rate around 200 MHz (400 MT/s). Higher performance DDR chips exist, but JEDEC has stated that they will not be standardized. These modules are mostly manufacturer optimizations of highest-yielding chips, drawing significantly more power than slower-clocked modules, and usually do not offer much, if any, greater real-world performance.
DDR2 started to become competitive with the older DDR standard by the end of 2004, as modules with lower latencies became available.
Backward compatibility
DDR2 DIMMs are not designed to be backward compatible with DDR DIMMs. The notch on DDR2 DIMMs is in a different position from DDR DIMMs, and the pin density is higher than DDR DIMMs in desktops. DDR2 is a 240-pin module, DDR is a 184-pin module. Notebooks have 200-pin modules for DDR and DDR2, however the notch on DDR modules is in a slightly different position than that on DDR2 modules.
Higher performance DDR2 DIMMs are compatible with lower performance DDR2 DIMMs; however, the higher performance module runs at the lower module's frequency. Using lower performing DDR2 memory in a system capable of higher performance results in the bus running at the rate of the lowest performance memory in use; however, in many systems this performance hit can be mitigated to some extent by setting the timings of the memory to a lower latency setting.

3.DDR3 SDRAM
In computing, DDR3 SDRAM or double-data-rate three synchronous dynamic random access memory is a random access memory interface technology used for high bandwidth storage of the working data of a computer or other digital electronic devices. DDR3 is part of the SDRAM family of technologies and is one of the many DRAM (dynamic random access memory) implementations.
DDR3 SDRAM is an improvement over its predecessor, DDR2 SDRAM, and the two are not compatible. The primary benefit of DDR3 is the ability to transfer at twice the data rate of DDR2 (I/O at 8× the data rate of the memory cells it contains), thus enabling higher bus rates and higher peak rates than earlier memory technologies. In addition, the DDR3 standard allows for chip capacities of 512 megabits to 8 gigabits, effectively enabling a maximum memory module size of 16 gigabytes.
With data being transferred 64 bits at a time per memory module, DDR3 SDRAM gives a transfer rate of (memory clock rate) × 4 (for bus clock multiplier) × 2 (for data rate) × 64 (number of bits transferred) / 8 (number of bits/byte). Thus with a memory clock frequency of 100 MHz, DDR3 SDRAM gives a maximum transfer rate of 6400 MB/s.
DDR3 is a DRAM interface specification; the actual DRAM arrays that store the data are the same as in any other type of DRAM, and have similar performance.
Overview
DDR3 memory provides a reduction in power consumption of 30% compared to DDR2 modules due to DDR3's 1.5 V supply voltage, compared to DDR2's 1.8 V or DDR's 2.5 V. The 1.5 V supply voltage works well with the 90 nanometer fabrication technology used in the original DDR3 chips. Some manufacturers further propose using "dual-gate" transistors to reduce leakage of current.
According to JEDEC the maximum recommended voltage is 1.575 volts and should be considered the absolute maximum when memory stability is the foremost consideration, such as in servers or other mission critical devices. In addition, JEDEC states that memory modules must withstand up to 1.975 volts before incurring permanent damage, although they are not required to function correctly at that level.
The main benefit of DDR3 comes from the higher bandwidth made possible by DDR3's 8-burst-deep prefetch buffer, in contrast to DDR2's 4-burst-deep or DDR's 2-burst-deep prefetch buffer.
DDR3 modules can transfer data at a rate of 800–2133 MT/s using both rising and falling edges of a 400–1066 MHz I/O clock. Sometimes, a vendor may misleadingly advertise the I/O clock rate by labeling the MT/s as MHz. The MT/s is normally twice that of MHz by double sampling, one on the rising clock edge, and the other, on the falling. In comparison, DDR2's current range of data transfer rates is 400–1066 MT/s using a 200–533 MHz I/O clock, and DDR's range is 200–400 MT/s based on a 100–200 MHz I/O clock. High-performance graphics was an initial driver of such bandwidth requirements, where high bandwidth data transfer between framebuffers is required.
DDR3 prototypes were announced in early 2005. Products in the form of motherboards appeared on the market in June 2007 based on Intel's P35 "Bearlake" chipset with DIMMs at bandwidths up to DDR3-1600 (PC3-12800). The Intel Core i7, released in November 2008, connects directly to memory rather than via a chipset. The Core i7 supports only DDR3. AMD's first socket AM3 Phenom II X4 processors, released in February 2009, were their first to support DDR3.
DDR3 DIMMs have 240 pins, are electrically incompatible with DDR2 and have a different key notch location DDR3 SO-DIMMs have 204 pins.
GDDR3 memory, having a similar name but being from an entirely dissimilar technology, has been in use for graphic cards. GDDR3 has sometimes been incorrectly referred to as "DDR3".
Extensions
Intel Corporation officially introduced the eXtreme Memory Profile (XMP) Specification on March 23, 2007 to enable enthusiast performance extensions to the traditional JEDEC SPD specifications for DDR3 SDRAM.
Modules
JEDEC standard modules
Standard name--Memory clock(MHz)--Cycle time(ns)--I/O bus clock(MHz)--Data rate(MT/s)--Module name--Peak transfer rate(MB/s)--Timings(CL-tRCD-tRP)
DDR3-800D	100 	10 	400 	800 	PC3-6400 	6400 	 5-5-5
DDR3-800E 	 -	-	 -	 -	   -		  -	 6-6-6
DDR3-1066E	133 	71/2 	533 	1066 	PC3-8500 	8533 	 6-6-6
DDR3-1066F	 -	-	 -	 -	   -		  -	 7-7-7
DDR3-1066G 	 -	-	 -	 -	   -		  -	 8-8-8
DDR3-1333F*	166 	6 	667 	1333 	PC3-10600 	10667 7-7-7
DDR3-1333G	-	-	 -	 -	   -		  -	 8-8-8
DDR3-1333H	-	-	 -	 -	   -		  -	 9-9-9
DDR3-1333J* -	-	 -	 -	   -		  -	 10-10-10
DDR3-1600G*	200 	5 	800 	1600 	PC3-12800 	12800 8-8-8
DDR3-1600H	-	-	 -	 -	   -		  -	 9-9-9
DDR3-1600J 	-	-	 -	 -	   -		  -	 10-10-10
DDR3-1600K 	-	-	 -	 -	   -		  -	 11-11-11
DDR3-1866J*233 	42/7 	933 	1866 	PC3-14900 	14933 10-10-10
DDR3-1866K	-	-	 -	 -	   -		  -	 11-11-11
DDR3-1866L	-	-	 -	 -	   -		  -	 12-12-12
DDR3-1866M*	-	-	 -	 -	   -		  -	 13-13-13
DDR3-2133K*	266 	33/4 	1066 	2133 	PC3-17000 	17066 11-11-11
DDR3-2133L	-	-	 -	 -	   -		  -	 12-12-12
DDR3-2133M	-	-	 -	 -	   -		  -	 13-13-13
DDR3-2133N*	-	-	 -	 -	   -		  -	 14-14-14
* optional
Feature summary
DDR3 SDRAM components
•	Introduction of asynchronous RESET pin
•	Support of system-level flight-time compensation
•	On-DIMM mirror-friendly DRAM pinout
•	Introduction of CWL (CAS write latency) per clock bin
•	On-die I/O calibration engine
•	READ and WRITE calibration
DDR3 modules
•	Fly-by command/address/control bus with on-DIMM termination
•	High-precision calibration resistors
•	Are not backwards compatible—DDR3 modules do not fit into DDR2 sockets; forcing them can damage the DIMM and/or the motherboard
Technological advantages compared to DDR2
•	Higher bandwidth performance, up to 2133 MT/s standardized
•	Slightly improved latencies as measured in nanoseconds
•	Higher performance at low power (longer battery life in laptops)
•	Enhanced low-power features
Market penetration
Although DDR3 was launched in 2007, DDR3 sales are not expected to overtake DDR2 until the end of 2009, or possibly early 2010, according to Intel strategist Carlos Weissenberg, speaking during the early part of their roll-out in August 2008 (the same view had been stated by market intelligence company DRAMeXchange over a year earlier in April 2007.) The primary driving force behind the increased usage of DDR3 has been new Core i7 processors from Intel and Phenom II processors from AMD, both of which have internal memory controllers: the latter recommends DDR3, the former requires it. IDC stated in January 2009 that DDR3 sales will account for 29 percent of the total DRAM units sold in 2009, rising to 72% by 2011.
Successor
It was revealed at the Intel Developer Forum in San Francisco 2008 that the successor to DDR3 will be known as DDR4. It is currently in the design stage, and is expected to be released in 2012. When released, it is expected to run at 1.2 volts or less, versus the 1.5 volts of DDR3 chips and have in excess of 2 billion data transfers per second.

4.DDR4 SDRAM
In computing, DDR4 SDRAM or double-data-rate four synchronous dynamic random access memory is a random access memory interface technology used for high bandwidth storage of the working data of a computer or other digital electronic devices. DDR4 is part of the SDRAM family of technologies and is one of the many DRAM (dynamic random access memory) implementations.
DDR4 SDRAM will be the successor to DDR3 SDRAM. It was revealed at the Intel Developer Forum in San Francisco, 2008, that it is currently in the design phase and has an expected release date of 2012.
The new chips are expected to run at 1.2 V or less, versus the 1.5 V of DDR3 chips and have in excess of 2 billion data transfers per second. They are expected to be introduced at clock speeds of 2133 MHz, estimated to rise to a potential 3.2 GHz and lowered voltage of 1.0 V by 2013.
In February 2009, Samsung validated 40 nm DRAM chips, considered a "significant step" towards DDR4 development. As of 2009, current DRAM chips are only migrating to a 50 nm process.
Bandwidth
	It looks like we should expect frequencies introduced at 2,133 MHz and it will scale to over 4.2 GHz with DDR4. 1,600 MHz (10ns) could still well be the base spec for several DIMMs that require reliability, but it's expected that JEDEC will create new standard DDR3 frequency specifications all the way up to 2,133 MHz, which is where DDR4 should jump off.
Processes
The designers of DDR4 memory are looking forward 1.2V and 1.1V voltage settings for the new memory type and are even considering 1.05V option to greatly reduce power consumption of the forthcoming systems. It is expected that manufacturers of dynamic random access memory (DRAM) will have to use advanced fabrication technology to make the DDR4 chips. The first chips are likely to be made using 32 nm or 36 nm process technologies.
Power Consumption
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 4.2 GHz and 1.2V actually uses 4x the power of SDRAM at 133 MHz 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 4.2 GHz 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 4.2 GHz will reach a server rack near you though: remember most servers today are only using 1,066 MHz DDR3 whereas enthusiast PC memory now exceeds twice that.
Conclusion

Now DDR is one of the most popular synchronus memory type, using in computers. These are four types of generations of DDR: DDR1, DDR2, DDR3 and DDR4 currently being designed to be available in 2012. The development of DDR memory is continuing and soon may be we will see DDR5 memory with more than 10GHz clock speed and working on 1V voltage or DDR will 'dead' and companies will develop other memory types, which are more convenient for market than DDR.

References

1.	http://en.wikipedia.org/wiki/SDRAM
http://en.wikipedia.org/wiki/DDR_SDRAM
http://en.wikipedia.org/wiki/DDR2_SDRAM
http://ru.wikipedia.org/wiki/DDR3_SDRAM
http://en.wikipedia.org/wiki/DDR4_SDRAM