RAM Performance For Enthusiasts
Enthusiasts will tweak and overclock their PC. That means actually delving into the BIOS and changing the settings to push the memory modules to their limits. If you are such a person, then you will need to take the time and effort to select memory modules that are particularly good for tweaking and/or overclocking.
As such, it pays to understand what these RAM terminologies mean. Knowing them will help you decide what memory module you should buy based on overclocking and tweaking results posted by other tweakers. Also, knowing them by heart will help you tweak and overclock your memory modules better.
Timing – Timing is the measure of how much time (usually in clock cycles) a memory module needs to do something. It is synonymous with term, latency. When it comes to memory modules, their timings are usually given in this format : 2-2-2-5 1T. This series of numbers list the rated CAS-tRCD-tRP-tRAS and tRC timings for the memory module. What do they mean? Let's take a look.
CAS Latency – The first number of the sequence always denotes the CAS latency. CAS stands for Column Access Strobe. Data is stored in memory cells that are arranged like the cells of an Excel spreadsheet, with rows and columns. To read a memory cell, the appropriate row is selected and then the column selected. The contents of the cell is then read out to the sense amplifier before being forwarded out of the module. CAS denotes the time it takes for the memory module to finally select the column and start the data transfer. The lower the CAS latency, the better the module's performance. For more information on CAS Latency, click here.
RAS-to-CAS Delay (tRCD) – Also known as the RAS to CAS Delay. This is the second number in the JEDEC sequence and denotes the time (in clock cycles) between the selection of the row (RAS) to the selection of the column (CAS). The lower this number is, the better the performance of the memory module. For more information on RAS-to-CAS Delay, click here.
Row Precharge Time (tRP) – This denotes the number of clock cycles it takes for the module to switch from one activated row to another row. Again, the lower this number is, the better the performance of the memory module. For more information on Row Precharge Time, click here.
Row Active Time (tRAS) – This is the minimum row active time, which determines how long a row can remain active for data transfers. If the tRAS period is too long, it can reduce performance by unnecessarily delaying the activation of other rows. However, if the tRAS period is too short, there may not be enough time to complete a burst transfer, reducing performance with the possibility of lost or corrupt data. Generally, use the lowest value possible for maximum performance. For more information on Row Active Time, click here.
Row Cycle Time (tRC) – This determines the time it takes (in clock cycles) for a row to complete a full cycle from row activation to precharge. It is determined by adding the row active time (tRAS) to the row precharge time (tRP). Generally, the lower the row cycle time, the better the memory module will perform. For more information on Row Cycle Time, click here.
Command Rate – This is the time (in clock cycles) the memory module takes from the time a command is given to it, till the time it is actually being carried out. Obviously, the shorter the delay, the faster commands are carried out. However, it is not so simple as just using the shortest possible command rate. I will delve into this later in the guide, but for now, it's simpler to say that users of Intel processors do not really need to bother with this, while some AMD processor users (especially before Socket AM2) need to look into this. For more information on Command Rate, click here.
Frequency – This is the frequency that the memory module is rated for. The higher the frequency, the faster the memory module and the more data it can transfer over time. However, this does not mean the memory module will actually run at this speed. JEDEC only specifies up to a maximum clock speed of 400 MHz (800 MHz DDR) for DDR2. Even high-performance memory modules capable of higher clock speeds will only run at this DDR2 speed, unless you manually select the clock speed. So, don't just slot in your expensive high-performance memory module and expect it to work right out of the box. You may need to manually set the clock speed if it exceeds JEDEC specifications.
Voltage Tolerance – Here, lower rated voltages are better. Manufacturers of high-performance modules often achieve the high clock speeds and/or tight timings by increasing the rated voltage. For such modules, you need to manually increase the voltage to achieve the rated clock speeds and timings. Of course, a module that runs at a lower voltage to achieve the same performance is better than one that requires a voltage boost. Not only does the first module have a greater headroom for you to play with, the lower voltage also reduces the effect of electromigration (which destroys the chip over time), Johnson Nyquist noise (due to the increase in current draw) and thermal output. So, you should favour memory modules that run at standard operating voltages over those that require higher voltages.
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