23 tháng 4, 2008

Overclocking Memory

DDR2 memories are already supported on high-end motherboards. We compiled below a short list with the main differences between DDR2 and DDR memories.

* DDR memories are officially found in 266 MHz, 333 MHz and 400 MHz versions, while DDR2 memories are found in 400 MHz, 533 MHz, 667 MHz and 800 MHz versions. Both types transfer two data per clock cycle. Because of that the listed clocks are nominal clocks, not real ones. To get the real clock divide the nominal clock by two. For example, DDR2-667 memory works in fact at 333 MHz.
* DDR2 memories have a lower power consumption compared to DDR memories.
* DDR memories are fed with 2.5 V while DDR2 memories are fed with 1.8 V.
* On DDR memories the resistive termination necessary for making the memory work is located on the motherboard, while on DDR2 memories this circuit is located inside the memory chip. This is one of the reasons why it is not possible to install DDR2 memories on DDR sockets and vice-versa.
* DDR modules have 184 contacts, while DDR2 modules have 240 contacts.
* On DDR memories the “CAS Latency” (CL) parameter – which is the time the memory delays delivering a requested data –, can be of 2, 2.5 or 3 clock cycles. On DDR2 memories CL can be of 3, 4 or 5 clock cycles.
* On DDR2 memories, depending on the chip, there is an additional latency (AL) of 0, 1, 2, 3, 4 or 5 clock cycles. So in a DDR2 memory with CL4 and AL1 the latency is 5.
* On DDR2 memories the write latency equals to the read latency (CL + AL) minus 1.
* Internally the controller inside DDR memories works preloading two data bits from the storage area (task known as “prefetch”) while the controller inside DDR2 memories works loading four bits in advance.

There are two modes of configuring the memory clock: synchronous, were the memory clock is tied to the CPU external clock, and asynchronous, where the memory clock can be configured independently from the CPU clock. The mode your computer has depends on its motherboard.
The synchronous mode is usually found in entry-level motherboards while asynchronous mode is usually found in high-end motherboards. Also, notice that many low-end motherboards don't have any overclocking configuration at all.

So, how do you know what mode your motherboard uses? Enter setup, go to the "Frequency/Voltage Control" menu and pay attention on the available options. If you find any option to change the memory clock ("Memory Clock" or "Memory Frequency", sometimes under a sub-menu called "DRAM Configuration" or "Memory Configuration"), your memory is running at asynchronous mode. Otherwise it is running at synchronous. Also pay attention that in order to change clock configurations you may need to change the option that allows you to do so. This option can have several different names, like "Clock Control", "System Performance" or "DDR Timing Setting by". Sometimes memory configuration options are under "Advanced Chipset Setup" and not under "Voltage/Frequency Control", so you need to also take a look on this menu and look for memory configuration options there.

Synchronous Mode
http://www.hardwaresecrets.com/article/152/3
The main disavantage of the synchronous mode is that in order to overclock your memory, you will need to overclock your CPU. Usually that's ok since maybe you are trying to overclock both memory and CPU. But in many cases the maximum external clock your CPU is reaching is limited by the memory or vice-versa.
For example, by trial-and-error basis you found out that the maximum external CPU clock you can configure on your system is 180 MHz. So, your memory is also working at 180 MHz ("360 MHz" since DDR memories are rated with two times its real clock) or more, depending on the host/memory ratio settings that some synchronous motherboards have. For example, on the motherboard of Figure 4 the memory can be set to run with the external CPU clock multiplied by 2 or by 2.5. This motherboard was targeted to Pentium 4 CPU, so when the external clock was set to 133 MHz ("533 MHz"), the memory clock could be configured at 266 MHz or 333 MHz. Of course setting it to 333 MHz would make more sense if you use DDR333 or DDR400 memories. But when we overclocked the CPU external clock to 180 MHz, at the 2.5 ratio shown the memory would run at 450 MHz.
So, on motherboards with synchronous mode, when you find out the maximum CPU external clock you cannot know for sure what is limiting you to increase the CPU external clock. Can be a CPU limitation or can be a memory limitation. On motherboards with the host/memory ratio configuration shown on Figure 4 you can change the ratio to check which component is limiting the overclock. For example, we would lower the ratio from 2.5 to 2 in order to make the memory tun at 360 MHz instead of 450 MHz and then try again to increase the CPU external clock. If you really cannot increase it over the 180 MHz limit (in our example), it means that the CPU is the component limiting the clock increase. On the other hand, if after lowering the ratio you now can increase the CPU external clock a little more, it was the memory that was limiting the overclock. Then you can see the new maximum external CPU clock you can achieve with this new configuration.

When the CPU is limiting the clock increase, your have to face a terrible truth: your memory could achieve a higher clock rate, but you cannot configure it to use this higher clock rate because your motherboard doesn’t have a configuration to do so. That's one disavantage of using cheap motherboards and that's why overclockers perfer high-end motherboards.

So, this problem doesn't affect motherboards with asyncrhonous memory configuration. Since you have two different configuration sets for CPU and memory, the maximum clock the CPU can reach is not limited by the memory, and also the maximum clock the memory can reach is not limited by the CPU, which is far better for overclocking.

Asynchronous Mode
http://www.hardwaresecrets.com/article/152/4
As we explained, some motherboards allow you to increase the memory clock separately from the CPU external clock, which is the best option for achieving a high overclocking.

There are two ways of configuring the memory clock on asynchronous motherboards: using predetermined fixed value or typing in the clock you want. This option will depend on the motherboard model.

The motherboard we shown on Figure 3 the memory clock options are fixed. As you can see, you can only choose the memory clock from a predetermined list of settings. This is not the best case but it is better than not having any memory overclocking option at all.

The best scenario is to have a motherboard where you can type in the memory clock you want, like you can see on Figure 5. Usually you need to change a configurarion called "DDR Timing Setting", "Clock Control", "System Performance" or similar from "Auto" to "Manual" in order to change the memory clock.
As we mentioned before, you will need to increase the memory clock, save the configuration, reboot your PC and test if the memory overclocking worked or not. If it worked, you need to repeat the process again trying a higher clock. If not, you need to repeat the process again trying a lower clock. You will need to repeat the whole process several times until you find out the maximum clock rate your memory accepts.

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