| DDR3 RAM: System Memory Technology Explained |
| Articles - Featured Guides | |||||||||||||||||||||||||||||||
| Written by Olin Coles | |||||||||||||||||||||||||||||||
| Sunday, 11 May 2008 | |||||||||||||||||||||||||||||||
DDR3 Memory: Technology ExplainedThese are uncertain financial times we live in today, and the rise and fall of our economy has had direct affect on consumer spending. It has already been one full year now that DDR3 has been patiently waiting for the enthusiast community to give it proper consideration, yet it's success is still undermined by misconceptions and high price. Benchmark Reviews has been testing DDR3 more actively than anyone, which is why over fifteen different kits fill our System Memory section of reviews. Sadly, it might take an article like this to open the eyes of my fellow hardware enthusiast and overclocker, because it seems like DDR3 is the technology nobody wants bad enough to learn about. Pity, because DDR3 is the key to extreme overclocking.
First and foremost, DDR3 is not just a faster version of DDR2. In fact, the worst piece of misinformation I see spread in enthusiast forums is how DDR3 simply picks up speed where DDR2 left off... which is as accurate as saying an airplane picks up where a kite left off. DDR3 does improve upon the previous generation in certain shared areas, and the refined fabrication process has allowed for a more efficient integrated circuit (IC) module. Although DDR3 doesn't share the same pin connections or key placements, it does still share the DIMM profile and overall appearance. From a technical perspective however, this is where the similarities end.
For over six months now, I have personally devoted a large amount of time towards testing this new system memory standard. Sadly, most of my efforts have gone unappreciated; DDR3 was too far ahead of it's time to be adopted early on. Yet, even though DDR2 has clearly reached its limit, the cost of production combined with a wide-scale recession will further harm acceptance of the new format. But are you really missing anything? I could give you a simple 'yes', but that's what I've already been saying for many months now. Instead, I invite you learn about what you're losing... Features:
Improvements:
DDR3: EfficiencyEfficiency is a double-edged sword when we talk about DDR3, because aside from fabrication process efficiency there are also several architectural design improvements which create a more efficient transfer of data and a reduction in power. All of these items tie in together throughout this article, so for you to understand why DDR3 is going to be worth your money, you should probably also know why it's going to deliver more. Power ConsumptionSo lets begin with power: at the JEDEC JESD 79-3B standard of 1.5 V, DDR3 system memory reduces the base power consumption level by nearly 17% compared the 1.8 V specified base power requirement for DDR2 modules. Taking this one step further, consider that at the high end of DDR2 there are 1066 MHz memory modules demanding 2.2 V to properly function. Then compare this to the faster 1600 MHz DDR3 RAM modules operating at 1.5 V nominal and you'll begin to see where money can be saved on energy costs - conserving nearly 32% of the power previously needed. The reduced level of base power demand works particularly well with the 90 nm fabrication technology presently used for most DDR3 chips. In high-efficiency purposed modules, some manufacturers have reduced current leakage even more by using "dual-gate" transistors.
You might be wondering how big a difference 0.7 V can make on your power bill, and that's a fair concern. In reality, it's not going to be enough to justify the initial costs of the new technology, at least not for your average casual computer user who operates a single workstation in their home. But before you dismiss the power saving, consider who might really make an impact here: commercial and industrial industries. If the average user can see only a few dollars saved per month on utility costs, imagine the savings to be made from large data center and server farm facilities. Not only will the reduced cost of operation help minimize overhead expenses, but the improved design also reduces heat output. Many commercial facilities spend a double-digit portion of their monthly expenses on utilities, and the cost savings sustained from lower power consumption and reduced cooling expenses will have an enormous effect on overhead. If you can imagine things just one step further, you'll discover that reduced cooling needs will also translate into reduced maintenance costs on that equipment and prolong the lifespan of HVAC equipment. Voltage Notes for OverclockersAccording to JEDEC standard JESD 79-3B approved on April 2008, the maximum recommended voltage for any DDR3 module is must be regulated at 1.575 V (see reference documents at the end of this article). Keeping in mind that the vast majority of system memory resides in commercial workstations and mission-critical enterprise servers, if system memory stability is important then this specification should be considered the absolute maximum voltage limit. But there's still good news for overclockers, as JEDEC states that these DDR3 system memory modules must also withstand up to 1.975 V before any permanent damage is caused.
DDR3: Prefetch BufferThe SDRAM family has seen generations of change. JEDEC originally stepped in to define standards very early on in the production timeline, and subsequently produced DDR, DDR2 and now DDR3 DRAM (dynamic random access memory) implementations. You already know that DDR3 SDRAM is the replacement for DDR2 by virtue of its vast design improvements, but you might not know what all of those improvements actually are. In additional to the logically progressive changes in fabrication process, there are also improvements made to the architectural design of the memory. In the last section I extolled the benefits of saving power and conserving natural resources by using DDR3 system memory, but most hardware enthusiasts are not aware of how efficiency is now also extended into a new data transfer architecture introduced fresh in DDR3. One particularly important new change introduced with DDR3 is in the improved prefetch buffer: up from DDR2's four bits to an astounding eight bits per cycle. This translates to a full 100% increase in the prefetch payload; not just the small incremental improvement we've seen from past generations. Remember this important piece of information when I discuss CAS latency later on, because it makes all the difference. DDR3: SpeedEven in it's infancy, DDR3 offers double the standard JEDEC standard maximum speed over DDR2. According to JEDEC standard JESD 79-3B drafted on April 2008, DDR3 offers a maximum default speed of 1600 MHz compared to 800 MHz for DDR2. But this is all just ink on paper if you aren't able to actually notice an improvement in system performance. So far, we discussed how DDR3 is going to save the average enthusiast up to 37% of their energy costs consumed by system memory. We also added a much larger 8-bit prefetch buffer to the list of compelling features. So let's cinch it all together with a real ground-breaking improvement, because DDR3 has introduced a brand new system for managing the data bandwidth bus.
Completely new to DDR3 is the 'Fly-by' technology. In the past generations of SDRAM to include DDR2, system memory used a 'Star' topology to disseminate data across many branched signal paths. This improvement is similar to when automobiles first began using the front-wheel drive system to bypass the long drivetrain linkage which incrementally sapped power from the wheels. Essentially, the Fly-by data bus topology utilizes a single direct link between all DRAM components which allows the system to respond much quicker than if it had to address stubs. The reason DDR2 cannot develop beyond the point it already has isn't truly an issue of fabrication refinements, it is more specifically an issue with mechanical limitations. Essentially, DDR2 technology is no better prepared to reach higher speeds than a propeller airplane is capable of breaking the sound barrier; in theory it's possible, just not with the mechanical technology presently developed. At higher frequency the DIMM module becomes very dependant on signal integrity, and topology layout becomes an critical issue. For DDR2 this would mean that each of the 'T branches' in the topology must remain balanced, an effort which is beyond its physical limitation. With DDR3 however, the signal integrity is individually tuned to each DRAM module rather than balanced across the entire memory platform. Now both the address and control line travel a single path instead of the inefficient branch pattern T topology in DDR2. Each DDR3 DRAM module also incorporates a managed leveling circuit dedicated to calibration, and it is the function of this circuit to memorize the calibration data. The Fly-by topology removes the mechanical line balancing limitations of DDR2, and replaces it with an automatic signal time delay generated by the controller fixed at the memory system training. Although it is a rough analogy, DDR3 is very similar to the advancement of jet propulsion over prop-style aircraft, and an entirely new dimension of possibility is made available. There is a downside however, and this is primarily in the latency timings. In our next section, Benchmark Reviews will discuss how DDR3 can aid in overclocking, and why the higher latency will have little effect on the end result.
DDR3: Overclocker FunctionalitySo let's pause for a moment to recap what we've covered: DDR3 RAM modules can conserve up to 32% of the energy used on system memory, while at the same time saving money on maintenance costs for enterprise HVAC systems. The data prefetch buffer has doubled from only 4 bits per cycle to a full 8 bits with each pass. Finally, the Fly-by topology removes the mechanical limitations of physical line balancing by replacing it with an automatically controlled and calibrated signal time delay. Not just a speed improvement, like some would like you to think. XMPSo then, when was the last time enthusiasts were actually encouraged to overclock their system memory by the manufacturer? Better yet, when was the last time Intel endorsed the practice? To be fair, Intel processors have been capable of overclocks for quite some time already, but not nearly to the level of convenience introduced in XMP technology. XMP, or eXtreme Memory Profile is an automatic memory settings technology developed by Intel and Corsair to compete with Nvidia's SLI Memory and Enhanced Performance Profiles (EPP). It works very similar to EPP, with one major exception: XMP manages everything from the CPU multiplier, to voltage and front side bus frequencies. This makes overclocking one of the easiest thing possible, since it only requires an XMP compatible motherboard such as Intel's X48 series and an XMP enhanced set of system memory modules.
The XMP Specification was first officially introduced by Intel on March 23rd, 2007 to enable a enthusiast performance extension to the traditional JEDEC SPD specifications. It is very common for Intel Extreme Memory Profiles to offer two different performance profiles. Profile 1 is used for the hardware enthusiast or for certified settings and is the profile that is tested under the Intel Extreme Memory Certification program. Profile 2 is designed to host the Extreme or Fastest possible settings that have no guard band and may or may not work on every system. It should also be noted that XMP settings are not always defined as overclocked or over-volted components. In some less common cases, Extreme Memory Profiles can be used to define conservative power saving settings or reduced (faster) latencies timings. CAS Latency TimingCAS latency timing is not something new to DDR3, and it is one of the few items that remains unchanged in the new format. You may wonder why I used the term "unchanged", when every enthusiast in every web forum world-wide has jumped on their soapbox and chastised anyone considering DDR3 because of the higher latencies. The simple fact is that you cannot extend base frequencies without also extending the CAS delay, and DDR3 is actually requires less latency in comparison. As a quick refresher, you might recall that 1066MHz DDR2 began with CL5 and CL6 latencies, and eventually improved to CL4 in rare cases of special IC module binning. So it should be considered a vast improvement in comparison that 1333 MHz DDR3 can achieve CL5, and some 1800 MHz DDR3 modules such as Corsair's PC3-14400 kit which have received careful parts binning can operate on CL7 timings. Putting this arguement into greater perspective, drift back to the first days of DDR2. I can still remember the complaints back then, although to a lesser extent, about the increased latency. Back in those days, 400 MHz DDR was often times seen with CL2 timings, so keep that in mind when you look at the 800 MHz DDR2 presently available at a 100% latency increase to CL4 timings. In comparison, the CL7 timings of 1600 MHz DDR3 are still ahead of the curve by 25%, or even up to 50% faster latencies with OCZ's CL6 DDR3. The bottom line is that enthusiasts need to hone in on the truth behind the technology, and ignore the self-serving ignorance that often runs rampant in most technology forums. The same person who condescends the idea of using DDR3 is also the same person who doesn't know the reason for the difference in architecture. The reality of the matter is that DDR3 is actually a better memory in terms of latency timings, especially compared against DDR2. So now imagine how tight the timings will be once the now infantile manufacturing process evolves from 90 nm to 70 nm; these latency timings will only get better.
Final Opinion on DDR3 RAMWhen I first began this article, it felt to me like this kind of information should be required reading for anyone who considers themselves a hardware enthusiasts or overclocker. Even after discussing the topic with some of my colleges, it was clear that the misconceptions had already entrenched themselves deep into the everyman. I can't give up hope, not yet, because if you've made it this far into the article then you've probably picked up a thing or two about the technology. Retracing my key points, there are a few important major features worth mentioning again for those who like skipping to the end (statistically 70% of visitors). To begin with, DDR3 RAM modules can conserve up to 32% of the energy used on system memory, while at the same time saving money on maintenance costs for facilities HVAC systems. Next on this list is the data prefetch buffer; which has doubled from only 4 bits per cycle to a full 8 bits with each pass. Then comes the new Fly-by topology that removes the mechanical limitations of physical line balancing by replacing it with an automatically controlled and calibrated signal time delay. After that comes latencies which are lower in rate than the previous curve, and in some cases offer 50% better timings per MHz. Finally, we have all of the extra perks. The first few perks are more to a technical advantage than anything else. At the beginning of this article I listed the introduction of an asynchronous reset pin, which gives DDR3 the ability to complete a device reset without interfering with the operation of the entire computer system. Additionally, DDR3 can also complete a partial refresh, so energy isn't wasted on refreshing memory that isn't active. The concept that appears to be gaining momentum is onboard AI for the memory modules. For instance, the JEDEC standard allows for an optional on-die thermal sensor that can be used to detect a nearing temperature threshold for the memory, and shorten the refresh intervals if necessary. This fail-safe offers the memory an opportunity to reduce temperatures and consume less power. I consider another major perk to be the XMP eXtended Memory Profile, which I have personally seen in action. One simple decision to enable the profile (or particular profile if more than one exists), and your system is automatically adjusted for a pre-defined overclock - voltages and all. This is going to be a great feature for anyone who just isn't ready to burn up their investment while trying to discover the mystery overclocking sweet spot.
Another perk is the increased front side bus speed which allows for extremely high overclocks and excellent bandwidth throughput. Some will argue that this comes at the expense of high latency; but let's be realistic. You can't reach 100 MPH in a car with out traveling a long distance. This analogy is just as true to system memory as it is to cars: the faster you want to your top speed to be farther you'll have to travel before you'll reach it. There are other benefits to the new standard, but the last of the major differences is in the capacity. DDR3 allows for chip capacities of 512 megabits to 8 gigabits, effectively enabling a maximum memory module size of 16 gigabytes. This should (hopefully) help move the computing world into 64-bit computing with a more compelling force. I hope. DisadvantagesWith every action comes an equal and opposite reaction. I am constantly reminded of this, because whenever I'm feeling especially good about something there will always be something to bring me right back down. When you compare DDR3 to previous SDRAM generations, it inherently claims a higher CAS latency. The higher timings may be compensated by higher bandwidth which increases overall system performance, but they aren't nullified. Additionally, this is new technology and it wears the new technology price tag. DDR3 generally costs more if you compare the price per megahertz ratio, this was also the case when DDR2 replaced DDR years ago. In fact, I still have the receipt for a nearly $400 set of Corsair Dominator 1066 MHz DDR2 from just under two years ago. For that some amount today, I could get a lot more performance for my dollar. There are also a few technical difficulties which must be overcome in order to take advantage of DDR3. For example, to achieve a maximum memory efficiency on the system level, the systems front side bus frequency must also extend to that level. In most cases, it's best to have the front side bus operate at a matching memory frequency. Now obviously this isn't going to be a problem as 1600 MHz FSB processors become mainstream, it still places burden on the processor and motherboard chipset to make accommodations. But we're not quite out of the woods yet... a higher operating frequency also means more signal integrity issues. Both motherboard and memory module design engineers now have to overcome new technologies and purchase test equipment for verification which often takes very expensive equipment just to look at the specific performance routines. In the end, the lab facilities costs will be passed along to you know who. This might explain how $300+ DDR3 motherboards have become such a common sight. Potential ConcernsSystem memory has had the opportunity to evolve and improve, but it hasn't been alone. Processor and motherboard technology have also moved forward at what might might be considered a faster rate of development. Just as the speed of system memory has increased, the amount of onboard processor cache memory has also increased. As I write this article, I have a set of DDR3 memory modules running at 2000 MHz, and an Intel E8200 processor with 6 MB of cache buffer. It seems that at some point in the upcoming wave of product evolution, my computer may not see the need to call on system memory unless I'm utilizing a graphics-intensive application. If the trend continues, as it likely will, we might not see any benefit from the ever-increasing operating frequency of system memory because the processor will have large amount of buffer operating at a far faster speed. Another concern is scalability and expansion. While I admire the brilliance of JEDEC to bring a more efficient module into mainstream use, I sometimes wonder how they come about other decisions. One key issue that may become a problem down the road is the specification calling for a maximum of 2 two rank modules per channel at 800-1333 MHz frequencies. It get's worse, only one memory slot is allowed at the present-specification top operational frequency of 1600 MHz. All in all, DDR3 isn't perfect. It's unquestionably better than its predecessor, but I think my points have illustrated that the good also comes with the bad. For the past year our concentration here at Benchmark Reviews has constantly centered around DDR3, as if it's a new toy to play with. But it's not; DDR3 is here to stay, and whether you want it to or not the market will soon be treating DDR2 the same way it presently treats DDR. You can cling on to your old technology, but at this point that would also be like reverting to AGP discrete graphics... which also cost a lot less than PCI Express. But that's for another article. Questions? Comments? Benchmark Reviews really wants your feedback. We invite you to leave your remarks in our Discussion Forum.
References:Intel XMS Technology Standards: https://download.intel.com/personal/gaming/367654.pdf JEDEC DDR3 SDRAM Standards JESD 79-3B: https://www.jedec.org/download/search/JESD79-3B.pdf JEDEC Specialty DDR2-1066 SDRAM Standards JESD 208: https://www.jedec.org/download/search/JESD208.pdf JEDEC DDR2 SDRAM Standards JESD 79-2E: https://www.jedec.org/download/search/JESD79-2E.pdf Related Articles:
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Comments
Cheers
I was not refering to any of the above you mention! As I do not understand "memory timing" Ihope this explains it better.