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  • Dwebtron - Monday, August 16, 2010 - link

    How did you know I was afraid to ask!! Reply
  • neslog - Monday, August 16, 2010 - link

    Thank you for a great article on memory and you are right, I was afraid to ask. Reply
  • landerf - Monday, August 16, 2010 - link

    I've found for the i7 platform the perfect ram setup is 1200 Mhz + cas5 or 6 timings, a 3:1 uncore ratio, and a B2B of 4. Not only does this perform well even in synthetics, it provides the "smoothest" intel experience. Something people who use amd and intel have been complaining about intel lacking. Check this chart and see how well that setup performs compared to all the conventional 2:1 setups. Reply
  • Servando Silva - Monday, August 16, 2010 - link

    Thanks for a great article. It will take me a while to read it carefully and fully understand it.
    Kris + Raju = Killer combo.
  • neslog - Monday, August 16, 2010 - link

    On page 8 you may want to change the wording in the last paragraph " Once you've had...
    to cordially invite[d] (you) to do some..."

    Thanks again for the article. I appreciate all the work that went into putting it together
  • elforeign - Monday, August 16, 2010 - link

    It's a site willing to go the extra mile like this to report and educate the masses that are truly worth the time to peruse and read the posted articles. I check this site daily because there is always something interesting to read. Thank you to all the staff who do a great job here! Reply
  • chizow - Monday, August 16, 2010 - link

    Just kidding....

    Or am I? :D
  • JarredWalton - Monday, August 16, 2010 - link

    There's obviously benefits to either direction. Reducing latency is definitely a priority, but something not mentioned in the text that bears repeating is that latency is a factor of clock speed as well as the various timings. While CAS 6 will always be better than CAS 7 at the same base clock (and likewise for the other timings), if you have a faster memory speed CAS 7 could end up being better.

    So here's the scoop:
    DDR3-1066 = 266MHz base clock, or 3.75ns per cycle.
    DDR3-1333 = 333MHz base clock, or 3.00ns per cycle.
    DDR3-1600 = 400MHz base clock, or 2.50ns per cycle.
    DDR3-2000 = 500MHz base clock, or 2.00ns per cycle.

    That gives this table in order of increasing latency, with rough pricing for 2x2GB. Based on pricing and latency, I've starred the best buys on Newegg:

    CAS 6 DDR3-2000 = 12.0ns. ($180)
    CAS 7 DDR3-2000 = 14.0ns. ($140)
    CAS 6 DDR3-1600 = 15.0ns. ($115) ***
    CAS 8 DDR3-2000 = 16.0ns. ($150)
    CAS 7 DDR3-1600 = 17.5ns. ($101) ***
    CAS 9 DDR3-2000 = 18.0ns. ($100) ***
    CAS 6 DDR3-1333 = 18.0ns. ($100) ***
    CAS 10 DDR3-2000 = 20.0ns. ($118)
    CAS 8 DDR3-1600 = 20.0ns. ($85) ***
    CAS 7 DDR3-1333 = 21.0ns. ($90)
    CAS 9 DDR3-1600 = 22.5ns. ($92)
    CAS 8 DDR3-1333 = 24.0ns. ($92)
    CAS 7 DDR3-1066 = 26.3ns. ($80)
    CAS 9 DDR3-1333 = 27.0ns. ($85)
    CAS 8 DDR3-1066 = 30.0ns. ($93)

    Notice how the total latency often comes in groups. The DDR3-1333 CL6, DDR3-1600 CL7, and DDR3-2000 CL9 are all priced around $100. If you buy any of these modules, there's a good chance (though YMMV) that you can tweak timings to run at whichever value makes you happiest. I'd probably err on the side of buying the higher speed rated modules, though, or at least grab the 1600MHz set.
  • Rick83 - Monday, August 16, 2010 - link

    Your pricing comparison is sadly missing one important factor:
    Operating voltage.
    I was at first surprised by the high cost of 1333/9, but I expect the voltage of that kit to be around 1.5, where most 1333/7 kits already clock in at 1.65.
    The 2000/9 kit probably also runs higher V's than the identically priced 1333/6?

    Lower voltages are usually preferred, as they give you a) more headroom and b) less heat at stock - with on-die controllers even less cpu heat.
  • JarredWalton - Monday, August 16, 2010 - link

    Oh, it's missing a lot more than just voltage information. :-) There are rebates on most memory kits right now, for instance. Still, I felt it was useful to highlight where the current "best deals" tend to fall.

    I personally wouldn't touch the ultra-expensive $150+ stuff, but up to $115 has potential at least. For a lower voltage kit, G.Skill has an ECO line rated at DDR3-1600 7-8-7-24-2N and 1.35V for $103. Worth a look at least....
  • JarredWalton - Monday, August 16, 2010 - link

    Note: I screwed up my table above. DDR3 is two bits per clock, so the base clocks are all twice what I listed, which means latency for CAS is half what I listed. Sorry. Got things confused with GDDR5. :-) The relative latency is still the same, of course, which is the main point. Reply
  • JarredWalton - Monday, August 16, 2010 - link

    Side note number two: And of course, CAS Latency isn't the be-all, end-all. According to benchmarks by Raja, DDR3-2000 at 6-9-6 timings often trails RAM at 7-8-7, as the tRCD difference becomes more pronounced in some cases. Reply
  • Rajinder Gill - Monday, August 16, 2010 - link

    Sorry I should have said 7-7-8 vs 6-9-8. This happens when the number of random access requests are high (fewer back to back reads). Benchmarks like WinRar and Super Pi (synthetic) are mainly the ones that show this.

  • Drag0nFire - Friday, August 20, 2010 - link

    I've had great experience with the ECO line. Put the 2x2 kit you mentioned in two computers so far, and it's been great. Feels like a steal to get such high speed and low voltage at such a great price. Reply
  • kalniel - Monday, August 16, 2010 - link

    Thanks for taking the time to write the article - the cycle time-line figures are very helpful, but I'm struggling to understand it correctly.

    Take fig. 5. There doesn't seem to be a Read to Precharge Delay. If we follow the recommendation of CL+tBurst = tRCP + tRP then won't there be a delay of 4T after the Data Read Burst before the RAS Precharge starts, giving a Row Cycle Time of 26 rather than 24?
  • kjboughton - Monday, August 16, 2010 - link

    tRTP may very well be 4T but the minimum RAS Active Time (tRAS) is 18T. The precharge is precluded from occuring until this period has expired making the clock at T + 18 the first opportunity to precharge the bank. Add to this the RAS Precharge (tRP) and you have the Row Cycle Time (tRC = tRAS + tRP) - the minimum time any single row MUST remain open before it can be closed (and before another page in the same bank can be accessed).

    Does this help?
  • kalniel - Monday, August 16, 2010 - link

    I thought the Read to Precharge Delay was there precisely to ensure you waited the minimum RAS active time before precharging the bank. Are you saying that the tRTP doesn't apply if you've already finished tRCD+CL+tBurst within tRAS so can start precharging as soon as minimum RAS active time is achieved?

    In other words, tRTP doesn't have a bearing on a single burst per page, but is there to help synchronise auto-precharge reads within the same page?

    My ignorance may be beyond redemption!
  • kjboughton - Monday, August 16, 2010 - link

    Read to Precharge Delay (tRTP) is the minimum wait time from a READ (column access) to bank PRECHARGE.

    RAS Active Time (tRAS) is the minimum wait time from an ACTIVATE (row access) to bank PRECHARGE.

    Both times must be satisfied before the bank can be precharged. Perhaps I wasn't quite clear enough on this point. I hope this clears things up.
  • kalniel - Monday, August 16, 2010 - link

    I think I've got it now, thanks. My brain saw the relevant diagram and screamed 'Cthulu' instead. Reply
  • mupilot - Monday, August 16, 2010 - link

    Nice article, its very in depth, but easy to read! I'll have to read through a couple times if I truly want to understand how my memory works so I can actually understand what I'm doing when I overclock my memory and adjust the timings. Reply
  • Cool Mike - Monday, August 16, 2010 - link

    I find memory a bit confusing. What type of memory would be best (or even work) on an AMD system - for example the Asus M4N98TD EVO. Will the lower voltage modules work? I'd like to get some pretty decent memory, but almost all of the discussions are around Intel based chipsets. Any help would be appreciated. Reply
  • vajm1234 - Monday, August 16, 2010 - link

    ---- well i too wanna see a comparison how AMD works against the intel Reply
  • PrinceGaz - Monday, August 16, 2010 - link

    Whilst the text for the Mistake Counter is incorrect, the bit count in the table does match the text-- the PARAMS2 register holds the 9, not 8 MSBs of each parameter (the internal values being 13 bit, not 12 bit), and the text and diagram both agree on this. Reply
  • PrinceGaz - Monday, August 16, 2010 - link

    Oh and by the way, thankyou for an extremely informative article which has refreshed (hehe) and added to what I knew about how RAM operates and the effects of the main timings. Reply
  • Icepop66 - Monday, August 16, 2010 - link

    Thanks for touching on some of the important aspects of sdram architecture and function and how they're related. My understanding is better now. Thanks for consistently good articles and reviews. Reply
  • Gary Key - Monday, August 16, 2010 - link

    Rampage III Extreme -

    BIOS 0878 -
    1. Opened Memory Timing Selections for Ultimate Tweaking
    2. General Performance Enhancements for Overclocking
    3. AnandTech Memory Article BIOS - Support provided by ASUS USA Tech Support (that would be me).
  • lowenz - Monday, August 16, 2010 - link

    Simply AWESOME article! Reply
  • Muhammed - Monday, August 16, 2010 - link

    You didn't mention the fact that (for example) DDR3 1600MHz actually works @200 MHz , and by fetching 8-bits per clock , it can effectively work like a 1600MHz RAM .

    DDR2 @800 MHz , actually works at 200MHz too but fetching 4-bits per clock .
    DDR@400 MHz , actually works at 200MHz too , fetching 2-bits per clock .
  • MrBrownSound - Monday, August 16, 2010 - link

    Thank you for helping me understand. Yes I was afraid to ask, or maybe I just felt I didn't need to know. I was wrong. Reply
  • ekoostik - Tuesday, August 17, 2010 - link

    Great article. Going to take me a few more reads. One question - why no mention of Command Rate (and I double checked the Memory Scaling on Core i7 article, absent there too)? CR is often included in RAM specs, e.g. 9-9-9-24-2T, but never fully discussed if mentioned at all. Is it just not important anymore? Reply
  • Muhammed - Tuesday, August 17, 2010 - link

    Ok I managed to royally confuse my self !

    What I know is that DDR3 operates at 1/8 the rated frequency , that means in case of DDR3 @800MHz , the internal memory operations are actually running at 100MHz , but the memory is able to fetch 8-words every clock cycle .

    So 100MHz X (8 words ) = 800 Word per second as data rate , then the manufacturer misleadingly label the RAM module as a 800MHz part .

    so the real benefit of DDR3 over DDR or DDR2 comes not from increased operating frequency , but from higher bandwidth .

    To stress that fact , I mention DDR2 @800MHz , it operates at 200Mhz (internal clock) , however it only fetches 4-words every clock cycle , (200X4 = 800 Words).

    When DDR3 operates at 200MHz (internal operations) like DDR2 , it fetches double the data , effectively managing 1600 words per second .

    NOW , in your article .. you mention the base clock (I/o Bus) and you mention the double data rate , I know the I/O Bus clock is always 2 or 4 times the internal clock , so DDR3 @ 100MHz , has a 400MHz I/O bus .. but I couldn't understand the I/O bus function and it's relation in data transmission and data rate .

    I am missing something here , could you enlighten me ?
  • Edison5do - Tuesday, August 17, 2010 - link

    I Really was Affraid..!! Reply
  • Edison5do - Tuesday, August 17, 2010 - link

    Technical Reading !!! LOve This Reply
  • hasherr - Wednesday, August 18, 2010 - link

    Great article. But what i dont get is how the hell motherboard knows all those timings? In SPDs there are like N timings described, isnt there really more? At least in bios settings i see more.

    Another thing. I buy Kingston 1800 MT/s module, with SPDs up to 1333 MTs. I overclock and make it run @ rated 1800mt/s speed. All timings are on auto. How the hell mobo/bios guess all of them :)?
  • ClagMaster - Wednesday, August 18, 2010 - link

    Afraid to Ask ?

    After perusing through this I find myself afraid to read.

    Comprehensive article for a novice EE
  • just4U - Wednesday, August 18, 2010 - link

    Great article and ...

    " ....should be to focus development on reducing absolute minimum latency requirements for timings such as CAS and tRCD, rather than chasing.."

    I hope the memory makers and shakers out there read that!!
  • lyeoh - Saturday, August 21, 2010 - link

    The mistake counter bit counts seem OK to me. In what way are they wrong?

    There are 9 MSB (most significant bits) in the table.

    Yes there are 13 bits in the counter, but the 9 bits in the table only refer to the 9 "top bits" of those 13 bits.

    For example, if I have an 8 bit counter but 4 bits in some table only refer to the 4 most significant bits, then that means that you'd only see all zeroes in those 4 bits when the counter has values from 0 to 15 (0x0 to 0xF). When the counter has values from 240 to 255 (0xF0 to 0xFF), you'd see all ones in those 4 bits.

    As for the description, I don't know the details of how the stuff works, so I don't know whether it's wrong or not.
  • dia - Saturday, August 21, 2010 - link

    Read the explanation here:
    Page 79.

    To quote:

    This field is the upper 8 MSBs of a 12-bit counter. This counter adapts the
    interval between assertions of the page close flag. For a less aggressive page
    close, the length of the count interval is increased and vice versa for a more
    aggressive page close policy."

    Now look at the left hand column, it shows 8:0. That's 9 bits! It's a 13 bit counter.

    If it were a 12 bit counter the maximum permissible selection value would be 4095 and not 8191.
  • datasegment - Saturday, August 21, 2010 - link

    Quick fyi - 8k is not 8196, it is 8192 :) Reply
  • bowhe - Tuesday, October 26, 2010 - link

    Thanks for these great articles!

    What I didn't understand yet:
    You state "Installing more than one DIMM per channel does not double the Memory Bus bandwidth, as modules co-located in the same channel must compete for access to a shared 64-bit sub-bus; however, adding more modules does have the added benefit of doubling the number of pages that may be open concurrently (twice the ranks for twice the fun!)". This sounds very positive, but:

    Some system manufacturers state that with 3 dimms the memory frequency can be for example 1333MHz, but with 6 dimms it needs to drop to 800MHz. Why does the frequency need to drop when using 6 versus 3 dimms? Does this apply to high end boards like the Gigabyte-X58A-UD9?

    Some manufacturer states in a small side note of a 24GB kit (6x4GB) that the stated frequency/timing is only guaranteed when using 3 dimm slots. This leads me to think that any 3 dimms of the set can do the stated timing, but when all are used something inherent in the design or interaction of the i7 processor, motherboard and dimm prevents the use of stated frequency/timings? What is it?

    Can one overcome these limitations by adjusting voltages in a high end board like the Gigabyte-X58A-UD9? (without use of extreme cooling <32F/0C)

    Thanks a lot!
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  • cochleoid - Tuesday, March 12, 2013 - link

    "When associated in groups of two (DDR), four (DDR2) or eight (DDR3), these banks form the next higher logical unit, known as a rank. "

    This mislead me. DDR2 may have coincidentally introduced 3 bit banks - allowing for 8 bank chips - but a typical old SDRAM (no DDR) chip had 4 banks.

    "We can now see why the DDR3 core has a 8n-prefetch (where n refers to the number of banks per rank) as every read access to the memory requires a minimum of 64 bits (8 bytes) of data to be transferred. This is because each bank, of which there are eight for DDR3, fetches no less than 8 bits (1 byte) of data per read request - the equivalent of one column's worth of data. Whether or not the system actually makes use of all 8 bytes of transferred data is irrelevant. Any delivered data not actually requested can be safely disregarded as it's just a copy of what is still retained in memory."

    This threw me off even more. What's happening is that the data at 8 consecutive (or otherwise close, depending on the burst mode) column addresses is being bursted on each read. "n" refers to the width of the memory chip, or the size of the "word" at a particular column address. "n" does not have any relation to the number of banks in a rank.

    8 8bit-wide DDR3 chips would make a total module width of 64 bits or 8 bytes at each column address. 8 column addresses would be 64 bytes (not 8 bytes, as the article seems to suggest), which actually corresponds to the cacheline size on most PCs.

    SDRAM could burst in sizes of 1,2,4,8
    DDR could burst in sizes of only 2,4,8
    DDR2 could burst in sizes of only 4,8
    DDR3 can burst only in 8.
    (All of these could burst in 8, filling the 64 byte cachline in one read operation. The difference with the generations of DDR has been a larger minimum wait in interface clock cycles as the interface got faster and the row accesses remained sluggish.)
    The internal clock of SDRAM has been limited by the speed of row accesses. What the 2n,4n,8n prefetches are doing is transferring more of this data available in an open row out at higher interface speeds with the rest of the system. It has nothing to do with the banks.

    SDRAM chips were segmented into independently operating banks so that parallel operations on interleaved banks could be synchronized or pipelined. 2n, 4n, and 8n prefetch buffering can be applied without independently operating banks.
  • ricardo_sa - Saturday, March 26, 2016 - link

    Thanks for the detailed explanation. You really saved my day. Ive read this article some time ago to help me understand how a DDR3 worked (theres few detailed explanations on google) and it turned out to be the worst mistake possible. I got the concepts wrong because of the incompetence of the publisher and lost a lot of time dealing with that 8 Bank misconception about the 64 bits.

    So it turns out one can only write a burst at 1 bank at a time, am i right? Otherwise you could access all the 8 banks in one single write/read....
  • Huendli - Friday, March 13, 2015 - link

    Thanks for this interesting read with much attention to detail!

    "a top priority [...] should be to focus development on reducing absolute minimum latency requirements for timings such as CAS and tRCD, rather than chasing raw synthetic bandwidth figures or setting outright frequency records at the expense of unduly high random access times."

    The latter's exactly what happened. DDR3-1600 modules with CL7 timings were widely available at the time this article had been written. Nowadays, you only get ridiculously-named bars with equally-ridiculously monstrous heatspreaders, but more bandwidth and worse timings than ever.

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