RAM Fundamentals

There are two fundamental elements to RAM speed: frequency and latency. Frequency was introduced above; but, in relationship to RAM, frequency defines how quickly the motherboard will try to pass data to the RAM. Latency refers to how quickly RAM reads and writes that data to and from the motherboard's data bus.

In thinking about this, you can think of FSB as determining how fast the motherboard data bus will offer to try to move data to/from your RAM, and latencies as how efficiently RAM can react to those requests. The lower the latency, the faster the RAM can read/write data offered from the data bus. There is a trade off, however. As you overclock, and increase the FSB, you may have to compensate a bit and increase the latency of your RAM. It is rare RAM that can both work at high FSBs and low latencies, very expensive and well made RAM generally.

In this regard, Intel based systems operate quite differently from AMD based systems. In Intel based systems, data exchange between the CPU and all other components is controlled by the Northbridge chip. In AMD systems, the data controller is built into the CPU itself, and is called HyperTransport Technology or "HTT" for short. These two very different concepts of how to interface data each have advantages and disadvantages. It is not my purpose to argue the relative merits, advantages or disadvantages of each design. However, having said that, the Intel method offloads data management to the Northbridge, thereby taking that load off of the CPU; and, the AMD method shortens and speeds up the data path producing faster data management. Thus, Intel's method provides more effective CPU cycles at the expense of some data speed; and, AMD's method increases the data speed at the expense of some CPU speed. Take your choice, both methods have their supporters and detractors. The important thing from our point of view is that each require somewhat different RAM and CPU settings.

Common to both Intel and AMD systems is that you must set three things: FSB, latencies and ratio. Ratio? Right, ratio. RAM can work in either synchronous or asynchronous mode. In synchronous mode, the RAM and CPU operate in a 1:1 ratio, i.e., with a 200 mHz FSB, the RAM operates at 400 mHz (DDR) and the CPU at 800 mHz (QDR). That introduces some inherent efficiencies, since the data cycles are in tandem: 1 cycle for the motherboard = 2 cycles for the RAM = 4 cycles for the CPU. On the other hand, for reasons discussed later in this Guide, you may want to operate the RAM asynchronously, or out-of tandem, at a lower percentage than the 100% or 1:1 FSB ratio.

For example, you may want or need to operate the RAM at 80% of the 1:1 speed, at a 5:4 ratio or at a DDR rate of 320 mHz assuming a 200 mHz FSB. Here the RAM speed is 2 x 200 x .8 = 320 mHz. The ratio is set in the motherboard's BIOS, and sometimes it is expressed as an overt ratio, other times it may be expressed as a frequency relative to a 200 mHz FSB. Thus, 2:1 or 50% of the synchronous rate may be expressed as a "2:1", "1:2" or "200", depending on your motherboard's BIOS settings nomenclature. It is very important to know and understand what each setting in your motherboard's BIOS really means, and how each should be interpreted. Remember the old saying, "If all else fails, read the instructions!"? It is even more important to heed this advice if you intend to overclock safely.

Latencies are generally expressed in RAM specifications as a 4 or 5 number expression such as 2.5 3 2 6 1T. Without getting technical as to what each of these numbers represent (and several RAM manufacturers have some excellent "white papers" explaining what each number represents - see the OCZ site for one such paper), each number represents the timing for one part of the RAM read/write cycle. The lower the numbers, the lower the latency of the RAM, the faster the RAM will read or write to or from the data bus. Normally, the manufacturer's specification timings are for the tested speed of the RAM. PC3200 RAM is guaranteed to run at its' specified latency timings at DDR 400 mHz. Similarly, PC3500 Ram at 433 mHz, etc. Most motherboards' BIOSes have either open or hidden settings that permit manually entry of the latency timings, in addition to the ratio and FSB settings. Often, there are default settings of "Auto" that must be changed to "Manual" in order to open a BIOS sub-menu in which these settings can be entered. Again, you must consult your motherboard manual to determine if your motherboard supports these manual settings.

One very important tip is to download and carefully read a motherboard manual before buying any motherboard and see exactly what can and cannot be manually set in the BIOS. If you don't take the time to do that, don't complain later that your brand new, shiny, glow in the dark motherboard lacks capabilities, or is incompatible with your other hardware. Remember this, it is always easier to make something look good, than to make it work properly. The Edsel, for those that might remember it, was a classic example of that principle.

One other critical setting affects your RAM's performance - vDIMM, the voltage supplied to your RAM by the motherboard. The larger your physical RAM, the more sticks you have, the faster you run the RAM, the more vDIMM is usually required. Most motherboards can only supply limited vDIMM, usually 3.0v or less. Most modern RAM is made to work with voltages supported by modern motherboards; however, there have been some very notable exceptions. RAM made with the fabled BH-5 memory chips loved high levels of vDIMM. In fact, two 512 mB sticks of BH-5 based RAM usually required at least 2.80v or more just to operate at specified speeds. In order to significantly overclock BH-5 RAM, voltages well in excess of 3.0v were often required, and most motherboards simply can not supply vDIMM at that level. Clearly, this is an important fact to know and understand before you commit to purchasing either a motherboard or RAM to go with it.

One further point on overall performance differences between Intel and AMD: for Intel based systems FSBs are generally more important than latencies; for AMD it is the opposite, and latencies are generally more important than FSBs. Note well, I said "generally" and "more", not "much more". Given the choice between differing brands and models of RAM, if I were building an Intel system, I would look for RAM that could support a couple of mHz of FSBs more, at the expense of some latency. With an AMD build, I would prefer the opposite trade-off and look for lower latencies, perhaps at the expense of not being able to push FSBs quite as high.

All RAM comes pre-programmed with "SPD" settings. These are standard speed and latency settings pre-programmed on to the RAM sticks by the manufacturer. If "Auto" settings are used in the BIOS, the pre-programmed SPD settings will be read and used by the BIOS to set RAM timing options. In many cases, SPD settings use higher latencies than those specified by the manufacturer. Manufacturers do this to ensure that systems built by OEMs for non-overclockers will always work with "Auto" settings. It is always wise to start your overclocking adventures by manually setting your BIOS to the manufacturer's specifications for your RAM sticks rather than depend on the pre-programmed SPD "Auto" settings.