# What's the difference between DDR2 and DDR3, anyway?



## Impasse (Dec 26, 2009)

The only one I can see is the clock speeds, from 667 and 800 MHz to 1066 and 1333 MHz.


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## Runefox (Dec 27, 2009)

The base clock speeds are actually not _too much_ higher (peaks at I/O 800MHz vs 533MHz; Modules themselves are still 100MHz-200MHz), but the effective clocks are. Power requirement and voltage are a good deal lower with DDR3 over DDR2. DDR3 supports a triple-channel arrangement in addition to dual channel. DDR3 has significantly (yet imperceptibly) higher measured latency timings, but real-world latency is actually similar to DDR2 (the measure for DDR3 latency uses shorter clock cycles than DDR2), and it more than makes up for it with significantly higher bandwidth.

... I think that went well. I only had to refer to Wiki for the clock rates. ... Wait, that means I have no life and am doomed to reciting computer crap from memory for the rest of my existence.


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## Janglur (Dec 27, 2009)

They have a different pin count, and mildly different electrical design.

DDR2 ranges typically from 400 MHz to 1066 MHz (up to 1200), and CAS3 to CAS6 (5.6ns and up)

DDR3 ranges typically from 667 MHz to 2000 MHz (up to 2200), and CAS5 to CAS10 (3.6ns and up)


Keep in mind the CAS latency when buying memory.  For example:  1066 MHz CAS7 is slower than 800 MHz CAS5.  (6.56ns vs. 6.25ns)  A good approximation math is to divide the CAS latency by the effective speed in MHz, it gives you the rough response time in microsends (IE, 0.0056565656 is 5.65ns)

It took several years for DDR3 to actually beat DDR2, because even at higher clock speeds the CAS latency slowed it down.

The CAS latency vs. the frequency is similar to Top Speed versus Acceleration.
If you are doing a lot of single, very large transfers, you want bandwidth.  While for lots of small transfers, latency adds up, and you want the lowest timings possible.
Generally, you want to balance Bandwidth and CAS equally.  If you know your computer uses very little memory, focus heavier on timings.  Gamers want a balance, and servers/workstations want bandwidth.



CAS is the Column Address Strobe.  It dictates how long it takes for memory to access a column.  Then the RAS accesses the required row, to load the specific data.  Kind of like how an excel spreadsheet works. The CAS is the function called the most often (effectively every access), while the others are called less often.  You'll see timings listed like so:
5-5-5-15-30-2T
The first number is typically CAS (sometimes called CL).  The next two are RCD and RP, 'RAS to CAS delay', and 'RAS precharge'.  The RCD is how long the chip waits after doing a RAS, before it does a CAS.  And the RAS precharge is how long the chip spends 'charging' the power before doing a RAS.  The RAS is typically the first three combined, though this isn't always the case or required, it'd just a good general rule to prevent instability caused by the chip not having adequate time to prepare.  The last number is the Bank Cycle Time, or RC.  This is how long it takes to change from one memory bank/chip to another.  It is typically all of the the first four added together, however good stability practices often suggest twice the highest of the four, +1.  This can actually be safely lower, but instability may occur.
As a general rule, RAM has SPD chips stating what they're officially rated for by the JEDEC.

The 2T/1T is how many accesses/commands it takes per cycle, 1 transaction or 2 transactions.  1T is better by a good margin (10-25%) but is extremely rare, and usually only possible with high quality, dual-channel kits or underclocked memory.  It usually results in instability.  It was more common on DDR than it is DDR2, and almost nonexistant on DDR3.  Having more sticks, and larger sticks, reduces the likelyhood 1T will work.  The more sticks, reguardless of size, the more stress it puts on the memory controller.  And the larger the sticks, the more effort and wait time is required to access through the many banks.

As for importance, CAS is as important to consider as the actual frequency of the memory.  However the RAS to CAS delay and RAS precharge make less of an impact.  5-6-6-17-34-2T performs only 1-5% slower than 5-5-5-15-30-2T, and isn't noticeable.  5-5-6-16-32-2T is only about 2% tops slower than 5-5-5-15-30-2T.  5-5-5-35-2T is less than 1% slower than 5-5-5-15-30-2T.

There are other timings as well, but these are rarely published and not very relevent to performance, and are best left to 'auto'.


That answer all your questions?  Probably raises a lot more.  =3


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## quayza (Dec 27, 2009)

I love DDR


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## Shindo (Dec 27, 2009)

inb4 dance dance rev....

nvm


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## quayza (Dec 27, 2009)

Left,left,right,right,up,down,stomp. 
Yay high score


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## Aden (Dec 27, 2009)

quayza said:


> Left,left,right,right,up,down,stomp.
> Yay high score



Stop posting


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## Sam (Dec 28, 2009)

Aden said:


> Stop posting



That time of the week again?


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## Runefox (Dec 28, 2009)

Sorry, I have to.



Sam said:


> That time of the week again?



Are you implying that Aden begins a menstrual cycle every week? Eternal PMS?


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## Sam (Jan 1, 2010)

Runefox said:


> Sorry, I have to.
> 
> 
> 
> ...



Perhaps........


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## Yaps (Jan 3, 2010)

Impasse said:


> The only one I can see is the clock speeds, from 667 and 800 MHz to 1066 and 1333 MHz.



That is pretty much it...


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## CaptainCool (Jan 3, 2010)

Yaps said:


> That is pretty much it...



wrong. see runefox's first post. there arent that many differences but they still make DDR3 quite a lot more powerful than DDR2


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## yak (Jan 3, 2010)

The way I see it, even if DDR3 is questionably faster then DDR2 an average Joe will not have a CPU powerful enough to utilize the even two thirds of the bandwidth offered by DDR2. 
What typically concerns people the most in their PC is why it's slow and how to make it faster. Usually the root cause of the problem lies somewhere else, and upgrading their RAM would bring them one of the least significant improvements in their PC speed that there can be.
That, and DDR3 is more expensive then DDR2.


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## jagdwolf (Jan 3, 2010)

power consumption and heat generation is less in the ddr3 giving you a potentially extended life of the stick. not that in this day and age that really matters, most things are out of date before they hit the shelf.


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## Runefox (Jan 4, 2010)

jagdwolf said:


> power consumption and heat generation is less in the ddr3 giving you a potentially extended life of the stick. not that in this day and age that really matters, most things are out of date before they hit the shelf.



The flow of change in PC technology hasn't changed much over the past few years - DDR3 is pretty much one of the only major changes, and it's not terribly different when you get down to it. Both AMD and Intel are basically rehashing the same stuff they've been producing since 2005-2006 (including the Core i7, which while architecturally nice and all, is pretty much an evolution of the original Core 2 series with some HyperTransport (excuse me, QuickPath) and Hyperthreading support (which gives the user eight cores in a world where two cores is the most anything's ever optimized for, if it's optimized at all). For the most part, any dual or quad core AMD or Intel processor that wasn't budget-minded is probably fast enough for most people, including gamers. Graphics cards are really the only place you could make a serious case for, and even then people are still using old 8800's and 7900's and doing just fine.

That said, the major advantage for DDR3's lower power consumption are that laptops can achieve a longer battery life over their DDR2 cousins, and that tweakers are in theory able to pull some extra horsepower out of it compared to comparable DDR2 stuff (along with the extra bandwidth). In general, I think you'd be hard-pressed to find much difference between PC2-6400 and PC3-8500, but it's the same situation as with DDR2 - You didn't see a big jump between PC-3200 and PC2-4300. While you might notice some difference with something like PC3-10600 or PC3-12800, it's going to be a small gap.

Then again, I'm really tired. This post might not actually make any sense at all.


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