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K.Haught
Post subject: Everything You Need to Know About DDR Dual Channel  PostPosted: Oct 15, 2005 - 08:33 AM





General

By Gabriel Torres on May 4, 2005


Introduction


The system RAM memory prevents the PC of achieving its maximum capable performance. This happens because the processor (CPU) is faster than RAM memory and usually it has to wait for the RAM memory to deliver data. During this wait time the CPU is idle, doing nothing (that's not absolutely true, but it fits in our explanation). In a perfect computer, the RAM memory would be as fast as the CPU.


But it is very unprobable that RAM memory will one day reach CPU speed. Nowadays with processors running over 3 GHz, RAM memory is still stuck at 400 MHz (actually less than that as we will explain below).


Many years ago one idea was created to match CPU speed with memory speed, which is used until today. The processor has two speeds, one internal – which is the one labeled on the CPU, like 3 GHz, 3.2 GHz and so on – and one external, used to access the CPU's outside world, specially RAM.


But even with this technique the speeds don't match. Intel CPUs available today run externally at 400 MHz, 533 MHz, 800 MHz or even 1,066 MHz, while the RAM speed is still stuck at 400 MHz, at best (there are 533 MHz DDR2 memories emerging on the market but they are still rare).


Dual Channel Memory can help improve the RAM speed, because this technique doubles it. In order to use Dual Channel Memory, your motherboard has to be capable of supporting this technique and you will also need two equal memory modules. We'll explain this further, but first let's understand deeper the bottleneck problem.


External Bus Speeds - AMD Processors


As we said, the ideal scenario to get the best performance for you PC is to match the CPU external speed with the system's RAM speed. Having a RAM memory faster than the CPU external bus is not a problem, the problem is having a RAM memory slower than the CPU external bus – which happens a lot.


AMD CPUs use a technique called DDR or Double Data Rate. With this technique the CPU transfers two data per clock cycle, doubling the performance of the bus since usually just one data is tranfered per clock tick. This technique is used since the very first Athlon processor and then with Duron, Sempron and Athlon 64 processors.


Because of that, the advertised clock rate isn't the real external CPU clock. For example, Athlon XP 3200+ is said to have a 400 MHz external clock, but in fact its clock rate is 200 MHz transfering two data per clock, making it a processor with a performance similar if the CPU used an external 400 MHz clock but transfering just one data per clock.


Since it is hard do compare clocks when you don't know how much data is transferred per time, it is better to know the maximum transfer rate, given in megabytes per second. The formula to calculate it is rather simple: real clock x number of data transferred per clock x 64 / 8. 64 is used because the CPU communicates with the memory 64 bits per time, and we have to divide by eight to have the result in bytes.


Refer to the following table to know the maximum transfer rate of your AMD CPU. "External Clock" is the clock speed advertised by the manufacturer, while "Real Clock" is the real clock signal speed used by the CPU.


All Athlon 64 uses a 400 MHz external bus. To known the bus speed of your Athlon XP or Sempron processor, please read our tutorial on that subject. Duron processors up to 1.3 GHz use a 200 MHz bus, while models from 1.4 GHz on use a 266 MHz bus.




External Clock
Real Clock
Maximum Transfer Rate
200 MHz
100 MHz
1,600 MB/s
266 MHz
133 MHz
2,100 MB/s
333 MHz
166 MHz
2,700 MBs
400 MHz
200 MHz
3,200 MB/s


Another – and better – way of knowing your CPU external bus speed is just running CPU-Z software, available on our download section. http://www.hardwaresecrets.com/page/download_id Run it and check the external clock speed of your CPU on the "Bus Speed" field and check what is its maximum transfer rate on the table above.


CPU-Z AMD


Figure 1: Running CPU-Z on an Athlon XP system. This CPU uses a 333 MHz external bus.


As you can see on Figure 1, the CPU clock speed may not be exactly the same as shown on our table, some slight differences may occur. For example, the external bus from our CPU is running at 333.8 MHz which makes it a 333 MHz CPU.


So, once you know the maximum external transfer rate of your CPU it is just a matter of matching the memory transfer rate with it. Keep reading.


External Bus - Intel Processors


Intel CPUs use a technique called QDR or Quad Data Rate, which transfers four data per clock cycle. With this technique the CPU achieves an external performance four times greater than if it was transfering just one data chunk. Because of that, the clock advertised by Intel is four times greater than the real clock used by the CPU, as you can see on the table below. It is still a mystery for us if the very high-end 1,066 MHz CPU uses a 400 MHz x 4 bus or a 200 MHz x 8 bus configuration. We believe on the latter, due to the physical difficulties involved in increasing the CPU external clock rate. So, a Pentium 4 processor with 533 MHz bus runs at 133 MHz but achieves a performance "as if" it was running at 533 MHz.


Since it is hard do compare clocks when you don't know how much data is transferred per time, it is better to know the maximum transfer rate, given in megabytes per second. The formula to calculate it is rather simple: real clock x number of data transferred per clock x 64 / 8. 64 is used because the CPU communicates with the memory 64 bits per time, and we have to divide by eight to have the result in bytes.




External Clock
Real Clock
Maximum Transfer Rate
400 MHz
100 MHz
3,200 MB/s
533 MHz
133 MHz
4,264 MB/s
800 MHz
200 MHz
6,400 MBs
1,066 MHz
200 MHz (our guess)
8,528 MB/s


So you need to known your CPU external clock rate to know which is its maximum transfer rate. The best way to check this is running CPU-Z software, available on our download section. http://www.hardwaresecrets.com/page/download_id Run it and check the external clock speed of your CPU on the "Bus Speed" field and check what is its maximum transfer rate on the table above.


CPUZ Intel


Figure 2: Running CPU-Z on a Pentium 4 system. This CPU uses a 533 MHz external bus.


As you can see on Figure 2, the CPU clock speed may not be exactly the same as shown on our table, some slight differences may occur. For example, the external bus from our CPU is running at 535 MHz which makes it a 533 MHz CPU.


As we said before, once you know the maximum external transfer rate of your CPU it is just a matter of matching the memory transfer rate with it. As you can see by comparing AMD table with Intel table, Intel CPUs face more problem matching the memory speed, since Intel CPUs have a higher maximum transfer rate compared to AMD CPUs.


Let's talk now about memory speeds.


Memory Speeds


On the table below you see all official speeds for SDRAM, DDR-SDRAM and DDR2. DDR and DDR2 transfer two data per clock cycle, so its rated clock isn't its real clock, as it occurs with the external bus of CPUs from AMD. We listed DDR2-667 and DDR2-800 just as a reference, since these memory types are not available on the market yet.




Memory
Tecnology
Rated Clock
Real Clock
Maximum Transfer Rate
PC66
SDRAM
66 MHz
66 MHz
533 MB/s
PC100
SDRAM
100 MHz
100 MHz
800 MB/s
PC133
SDRAM
133 MHz
133 MHz
1,066 MB/s
DDR200
DDR-SDRAM
200 MHz
100 MHz
1,600 MB/s
DDR266
DDR-SDRAM
266 MHz
133 MHz
2,100 MB/s
DDR333
DDR-SDRAM
333 MHz
166 MHz
2,700 MB/s
DDR400
DDR-SDRAM
400 MHz
200 MHz
3,200 MB/s
DDR2-400
DDR2-SDRAM
400 MHz
200 MHz
3,200 MB/s
DDR2-533
DDR2-SDRAM
533 MHz
266 MHz
4,264 MB/s
DDR2-667
DDR2-SDRAM
667 MHz
333 MHz
5,336 MB/s
DDR2-800
DDR2-SDRAM
800 MHz
400 MHz
6,400 MB/s


As we've been saying, in order to achieve the top performance your PC is capable of giving you, you should match the memory speed with the CPU speed (it is ok if your memory is faster than your CPU external bus, the problem is the other way around).


Actually, if you have an AMD CPU you won't have problems. For instance, if you have an Athlon XP with 400 MHz (3,200 MB/s transfer rate) external bus just install DDR400 memories on your computer and it will work great since both the CPU and the memory will be running at the same speed grade (3,200 MB/s). You can use DDR Dual Channel to improve the performance, anyway, as we will be talking about on the next page.


Intel processors are a problem. You don't even need to have a state-of-the-art PC to have memory bottleneck. The 2.4 GHz Pentium 4 from Figure 2 would need a 4,264 MB/s memory and from the table above you can see that only DDR2-533 can provide that. If you have a newer CPU with 800 MHz external clock rate, you would need DDR2-800 memories on your PC – a memory type that was not even lauched yet!


That's why Intel is pushing DDR Dual Channel. With this technique memory bandwidth is doubled and you can match the CPU speed with memory speed.


So, specially with Intel processors, if you don't use Dual Channel Memories your PC will achieve a performance lower than it is capable of.


DDR Dual Channel


On DDR Dual Channel configuration, memory is acessed 128 bits at a time, instead of 64 bits, which is the normal rate. Therefore, the transfer rate is doubled. In order to use this configuration on your PC, you need to have a motherboard with this feature and two identical memory modules correctly installed (same capacity and same speed – same manufacturer and same model is highly adviced). The installation is tricky because it is not just a matter of inserting the memory modules in any of the memory sockets available. We will be covering the installation on next page.


So, with Dual Channel the memory speed doubles as you can check on the table below.




Memory
Rated Clock
Maximum Transfer Rate
Dual Channel Maximum Transfer Rate
DDR200
200 MHz
1,600 MB/s
3,200 MB/s
DDR266
266 MHz
2,100 MB/s
4,200 MB/s
DDR333
333 MHz
2,700 MB/s
5,400 MB/s
DDR400
400 MHz
3,200 MB/s
6,400 MB/s
DDR2-400
400 MHz
3,200 MB/s
6,400 MB/s
DDR2-533
533 MHz
4,264 MB/s
8,528 MB/s
DDR2-667
667 MHz
5,336 MB/s
10,672 MB/s
DDR2-800
800 MHz
6,400 MBs
12,800 MB/s


With single channel configuration we were unable to match our 2.4 GHz Pentium 4 transfer rate of 4,264 MB/s, but with DDR Dual Channel we can match this speed by installing two DDR333 memory modules! You can go ahead and install DDR400 memories if you want to.


As for Pentium 4 CPUs with 800 MHz external bus, we need a 6,400 MB/s transfer rate and two DDR400 memory modules configured as Dual Channel are capable of giving that speed grade.


As we mentioned, if you want, you can install a memory configuration higher than required.


DDR Dual Channel Installation


To use DDR Dual Channel configuration in order to achieve the maximum performance your CPU is capable of giving you, you will need a motherboard with this feature and two identical memory modules.


There is one exception, though: the Athlon 64 processor. On this processor family the memory controller is embbeded in the CPU, not in the chipset. Therefore with this CPU the use of DDR Dual Channel configuration depends on the CPU, not on the motherboard. Socket 754 Athlon 64 processors aren't capable of this configuration. So if you have this kind of CPU you can't use DDR Dual Channel. Socket 939 Athlon 64 processors have an embbeded Dual Channel memory controller, thus allowing this processor to use this configuration. The installation steps are the same for other kinds of processors, as we will explain below.


As we said, the motherboard must have this feature – actually, the chipset must have it. There are several chipsets with this feature. For the AMD platform we can mention nVidia nForce 2 and VIA KT880, and for Intel platform we can mention Intel 865, Intel 875, Intel 915, Intel 925, SiS 655 TX, SiS 655 FX, SiS 656, VIA PT880 and VIA PT894, just to name a few.


The instalattion, however, is tricky. You have to install each module on a different channel in order to work. The problem is that usually the modules aren't installed in a sequential order. Usually you need to install the first module on the first socket and the second module on the third socket (not the second, as you may think).


The motherboards manufacturers, in order to facilitate the memory installation, usually uses sockets with different colors for better identifying where you must install the memory modules. Usually you need to install the modules on sockets with the same color. Some manufacturers like MSI do the opposite, you must install the memory modules on sockets with different colors! And there are several motherboard models with DDR Dual Channel support that have all sockets with the same color!


So, the best way to avoid errors is to find sockets 1 and 3 and install the modules there.


We give some examples at random on the next pictures.


DDR Dual Channel
Figure 3: Colored sockets on a Gigabyte
GA-8IPE1000 Pro 2. Just install the modules on a socket with the same color. If you install one module on a purple socket, install the other on the other purple
socket
.


DDR Dual Channel
Figure 4: Colored sockets on a Soltek SL-75FRN
Golden Flame. Just install the modules on the yellow sockets.


DDR Dual Channel
Figure 5: Pay attention with MSI. On 865PE Neo2
you should install the modules on sockets with different colors (not same color as it happens with other motherboards).









click to enlarge



Figure 6:
Two memory modules correctly installed using DDR Dual Channel configuration (the motherboard is a Chaintech CT-9CJS Zenith).



                    
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