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How to choose the right DDR3 memory

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Last response: in Motherboards
November 12, 2009 12:53:35 AM

I am going to build a new system and what's in my mind are Phenom II x3 720 and GIGABYTE GA-MA785GMT-UD2H. I don't know how to determine what memory speed should I go for though. The motherboard's spec shows it supports 1066/1333/1666/1800(OC) DDR3 memory and its FSB is 2600 MHz. I thought the state of the art FSB is 1333 MHz though. What is the base frequency and multiplier to achieve 2600 MHz FSB? And what FSB and memory frequency multiplier should I use to achieve 1066/1333/1666/1800 MHz memory frequency? Many thanks.

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a c 178 V Motherboard
a b } Memory
November 12, 2009 1:35:51 AM

You have a lot to learn!

First of all, AMDs don't have an FSB (front side bus: ); they use HyperTransport, and have their memory controllers on-die, with the CPU; second, RAM speed won't matter much:
and note: "Socket-AM3 CPUs will only officially support one DIMM per channel running at 1333MHz. You'll need to run at 1066MHz speeds if you're running four DIMMs."

I don't know where you got the 2600MHz number from; that processor's cores run at 2.8GHz, which is normally a 14 multiplier of a 200MHz host clock; with the host clock at 200, your available memory multipliers are 4x (800), 5.33 (1066), 6.66 (1333), and 8x (1600); EPP/XMP is not supported, so to get above 1066, you will need to set the memory up manually...

I suggest you read the whole Anandtech article, to see what you're paying for before you spend the money and give yourself a major headache; and follow up by reading the manual here:
especially towards the middle regarding BIOS setup...

BTW - the AMD CPU spec is here:
November 12, 2009 10:14:12 PM

Thank you very much for the help. After reading the Anandtech article, I decide to choose the AM3 or AM2+ platform purely based on price. However, surprisingly, I don't see much difference in price between the two either. If AM3 is limited to 1333 MHz memory speed, how about intel LGA1156 cpus? I also reconsidered Intel solution and below are the four choices (prices are from newegg and microcenter):

Phenom II X4 based on AM3

motherboard Gigabyte UD2H 89 USD
cpu Phenom II X4 955 175 USD
memory A-DATA 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 80 USD

Phenom II X4 based on AM2+

motherboard Gigabyte US2H 80 USD
cpu Phenom II X4 955 175 USD
memory A-DATA 2GB 240-Pin DDR2 SDRAM DDR2 800 (Qty 2) 96 USD

Intel Socket 1156
motherboard GIGABYTE GA-P55-UD3R LGA 1156 139 USD
cpu Core i5 750 150 USD
memory A-DATA 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 80 USD

Intel Socket 775
motherboard GIGABYTE GA-EP45-UD3L LGA 775 Intel P45 75 USD
cpu Core 2 Quad 9550 170 USD
memory A-DATA 2GB 240-Pin DDR2 SDRAM DDR2 800 (Qty 2) 96 USD

There really isn't much difference in price between the four and the main usage of the computer will be gaming and some picture editing. Which one is better?

For intel system, if the FSB is 1333 MHz, the memory frequency is 667 MHz and if FSB is overclocked to 1600 MHz, the memory frequency is 800 MHz, is that correct? Then what the FSB needs to be for memory running at 1600 MHz? Or can I simply tune the memory frequency multiplier without increasing the bus frequency just as on the AMD platform? Any suggestion for other budget motherboard instead of the Gigabyte products I select? Overclocking ability will be a concern here even though this will be my first DIY system.

Thank you very much!

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a c 178 V Motherboard
a b } Memory
November 12, 2009 11:34:13 PM

First, a bit of detail about memory, mostly aimed at the 775 platforms:

You can always use faster RAM with any modern CPU/MOBO setup - you're just likely to have to set it up manually in the BIOS to take advantage of it. Pretty much all DDR2 ram is actually DDR2/800; they 'speed-bin' it, i.e., test and select the sticks that will work at either lower (faster) latencies, or higher (faster) speeds, or both, and sell it at a premium as 2/1066, 2/1200, and so on. JEDEC spec'd RAM has a little EEPROM chip in it that stores the set-up information/tables for running it at 800 at various FSB (Front System Bus) speeds - has the preferred memory multiplier and timing info - this is called an SPD (Serial Presence Detect) just to confuse us; faster, higher rated sticks may (but don't necessarily) contain another set of tables (called an EPP - this one makes sense - Extended Performance Profile) that will tell the BIOS what multiplier/latencies to use at its higher rated speed - BUT - not all BIOS are created equal: some will read this EPP automatically, and set the RAM at the higher speed; some will require intervention (on a lot of GBs, it's "Load Optimized Defaults" [but, to keep it more confusing - not all BIOS with the "Load Optimized Defaults" fuction actually use it to set the EPP]), and some just plainly don't know the EPP exists (if it does) and you have to set the higher speed manually!

Now, you have control over the basic system clock (I'm going to cal it B_CLK), once you start manually timing the MOBO through the BIOS. B_CLK times four is your FSB (once again, Front System Bus); B_CLK times your memory multiplier is your DRAM rate; B_CLK times your CPU's multiplier is your CPU frequency.

Examples: if you set your system clock to 333, you will need a 2.4 memory multiplier (333 x 2.4 = 799.blahblahblah) to run your RAM at 800, and if the CPU multiplier is, say, 8.5, you will get a CPU clock of 2.83GHz; at that same B_CLK you would need a memory multiplier of 3.2 (3.2 x 333 = 1065.6) to take advantage of 1066 RAM. Now, lots of CPUs that are rated at a 1333 nominal FSB will run a lot faster, sometimes with a little more 'oomph' from a voltage increase; for example, I run a Q9550 that is rated at 1333 FSB (333 B_CLK) times an eight point five multiplier, for a 2.83GHz speed. It will comfortably run with the B_CLK well over 450 - and here's where faster RAM comes in. The smallest RAM multiplier available from a MCH (Memory Control Hub - or 'NorthBridge') is 2.0, but, with a 2.0 multiplier, that means at a 450 clock, your RAM will need to run at 900 (again, 450 B_CLK x 2 = 900), which most 800 RAM just won't do! This is referred to as a 'RAM limited bus', meaning the CPU can't run a B_CLK any higher than (roughly) half the RAM's available speed - and thus, the need for faster RAM. Mind you, this only applies if you both can, and intend to, run your FSB above 1600 (once again, a B_CLK of 400+ times 4 gives you a 1600+ FSB)...

To further complicate matters, people often misunderstand the actual quantitative speed improvements inherent in faster ram... Here's the mistake: 1066 is 33% higher than 800 ([1066-800]/800 = 266/800 = .33), so 1066 RAM must be a third faster than 800, right? Not so! You have to figure in latencies. Most 800 will run at 4-4-4-12, while most 1066 is rated at 5-5-5-15, or, even worse, 5-5-5-18. Here's how to appraise the situation in reality: at 800 MHz, a RAM bus cycle is 1.25 nSec long (1000/800); at 1066 (1000/1066), it is roughly .938 nSec long - so, with an 800 stick at a 4 average latency, a RAM bus transaction takes 1.25 (cycle time) times 4 (latency), or 5nSec, while at 1066 it is .938 (cycle time) times 5 (latency), for a transaction time of (roughly) 4.7nSec - so you see, by going to nominally 33% faster RAM, you actually gain three tenths of a nSec per transaction: .3 (transaction gain) over 5(transaction total) = .06, for a real-world improvement of 6%

With 775s, as with any processoryou have a limited selection of memory multipliers - mainly, because they are not the decimal multipliers you see in the BIOS - they are integer divider ratios built into the northbridges. They are organized by 'straps' - the 'straps' are sets of northbridge timings - much like memory latencies, the faster you go, the 'looser' the timings have to be... There are four straps, corresponding to the Intel FSB ratings: 200 (800FSB), 266 (1066FSB), 333 (1333FSB), and 400 (1600FSB - Intel actually does make a 1600 FSB CPU - the QX9775 - but, I think, it's over $1500 a pop!); each strap has its own set of available memory multipliers (ratios).

Here is a chart illustrating 'walking up' the processor frequency, by changing multipliers to keep the memory 'in-band'...

There is a pretty thorough discussion here:
of memory useage by the 1156/1366 platforms, and how to evaluate 'bang for your buck'...
Give it all a read, digest slowly, and give a holler for more...
December 3, 2009 1:34:55 AM

Thank you very much, Bilbat. I just bought all the parts including a gigabyte 790gpt ud3h motherboard, a Phenom ii x3 720 BE cpu and 4G (2Gx2) G.Skill DDR3 1333 memory. Hope they will work together smoothly.
December 3, 2009 2:43:09 AM

The pros are bragging rights and a little bit more memory bandwidth (memory controllers on even the latest motherboard chipsets aren't getting much more out of DDR3 than DDR2), cooler operating temperatures, and bragging rights. Cons are finding the memory at a low price.