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Understandably, many PC owners want to get the maximum speed from their equipment. What’s harder to understand is which of the many specifications and numbers make a difference to the real performance of the computer. In this article, we’ll address memory speed and timing, and help you decide where your money is best spent.
(Macintosh owners and laptop owners can largely ignore the debate over memory speed, as the memory buss speeds are usually determined by the computer manufacturer and are not adjustable, so as long as you get the correct memory for your model, you’re set).
There is a relationship between the computer's Front Side Buss (FSB), the memory speed, and the processor (CPU) speed. The FSB carries the signal between the memory and the CPU, so it affects the performance of nearly every function of the computer. FSB speeds range from 66, 100, 133, 167 and 200 Megahertz (MHz) and higher with new processors. The CPU, motherboard and memory will all have to agree on the FSB speed to work efficiently. A CPU will state the FSB speed it supports, usually expressed in a number 4 times the actual FSB speed. Memory also states its speed, depending on its type: SDRAM uses the bus speed directly, Dual Data Rate (DDR, DDR2 and DDR3) RAM lists 2x the FSB value as its MHz rating because it can do 2 operations on each "tick" of the clock. RAMBus (or RDRAM – which is all but discontinued), is labeled at 4 x buss speed. Here is a list of popular speed ratings:
The above is a brief sample only, there is a staggering array of different
processors available from Intel and AMD at different CPU speeds and FSB
Common sense prevails here as well – the effect of higher computer performance is seen mostly when you are using demanding applications, such as digital audio or video production, 3-D games, professional graphics or calculation-intensive programs. A low-demand application like email will show little performance difference. Any upgrades should be measured against the uses you intend to put your machine to.
Rule number one is: Get enough memory.
It doesn't matter how fast your memory is, if you run more software and data at one time than the physical amount of memory, then your machine will swap memory space on and off your hard drive, which is many time slower than memory. Having enough memory for the way you use your machine is paramount. Windows XP and Mac OS X don't start running efficiently until they have 512 Mb or more of RAM. Then, add your programs. An average computer user will need between 1 Gb and 2 Gb RAM, gamers, graphic artists, and those running advanced audio, video or engineering programs will want 2 - 4 Gb of RAM.
The first concept is the speed of the memory in MHz. Contrary to popular belief, the speed of memory is not controlled by the memory chips, but by the memory controller on the computer's motherboard (you’re married to your memory controller, the only way to change it is to replace the motherboard). You can install 400 MHz memory into a machine with a 266 MHz memory controller speed, and you won't get an ounce of extra performance. The machine will just run the RAM at the slower speed. Remember that the speed rating of memory is simply the highest speed that the memory is guaranteed to work at - it's the memory controller that is in the driver's seat.
However, if you install a piece of RAM that is slower than the speed the motherboard is set to, one of two things will happen: all of the memory will slow down to the lowest common speed, or the machine will try to use the slower memory at higher-than-rated speed, potentially leading to data errors and shortened lifespan. Don't under-buy to save $5.00.
So rule number two is: Buy RAM rated at the highest buss speed your motherboard supports, no higher.
The second memory concept, latency, receives less attention than MHz, but may be more important. Latency is the amount of time needed between a memory read or write operation and the next operation. The memory chip needs this time to "recharge" the chips, and if new data comes along before the chip is ready, data errors will occur. The standard latency is 3. This means that after sending a memory operation to the chip, your CPU sits around twiddling its thumbs for two more clicks of the clock, until the memory is ready to accept the next data on the third click. It's not much of an exaggeration to say that 2/3 of your computer's processing time is spent doing nothing, waiting for the memory.
So if we were to lower the waiting time (lower the latency) then your machine would be faster without doing a thing to change the MHz speed of the CPU or the busses.
Most of the next section concerns Dual Data
Rate (DDR) memory, as it is the most popular memory type and the current
(Important Note: lowering latency settings of a motherboard below what the RAM is designed for is an extremely bad idea and will result in instability and data loss.)
The effect of lowering memory latency can be measured. If both your motherboard and your memory support CL2.0 timings instead of CL3.0, you get a memory speed gain of 33%, without the risk and heat involved in overclocking. By comparison, increasing the memory bus from 400 MHz to 500 MHz while remaining at CL3.0 gains 25%. But here's the catch: Faster-rated memory that can survive being clocked to 437 MHz ("PC3700"), or 500 MHz ("PC4000") are almost all CL3, because it is difficult and expensive to build a chip that is both high MHz and low latency. The speed benefits of lowering latency are also influenced by how fast the processor is, and how the L2 and L3 caches, memory controller, and buss interact with the memory.
DDR2: Almost all DDR2 memory is CL5, because faster chips have not reached mass production yet (as of July 2006). Remember that a DDR2-667 MHz module at CAS Latency 5 may run CL4 at DDR2-400 MHz or even CL3 at 400 MHz. That's irrelevant, the only CL value that matters is the one at the speed your machine will operate the RAM.
So rule number three is: buy RAM matching your motherboard bus speed at the lowest latency that you can reasonably afford.
For most modern DDR based machines, PC3200 CL2 RAM will give the best performance, particularly with motherboards based on the popular i865 and i875 chipsets which make automatic timing adjustments.
Most modern motherboards support Dual-Channel memory, which splits the RAM access operations across two separate memory modules in paired memory sockets. This provides a theoretical doubling of memory bandwidth, because one chip can be accessed while it’s “twin” is recharging. Actual performance improvement is not that drastic (6% - 8%), but when you have a choice, install matching pairs of RAM into the corresponding slots (check your motherboard’s manual for the slot layout). Two 512 Mb DDR modules with Dual Channel operation are faster than a single 1 Gb module. However, Dual-Channel is not worth it if it means settling for less RAM.
So rule number four is: If your machine supports it and it has sufficient memory slots, install matched pairs of RAM instead of singles.
More on latency - the tech talk:
There are actually four specific measurements of timing that you will see on DDR RAM, A memory spec might look like “PC3200 CL2: 2-3-2-6”. The 2-3-2-6 number breaks down in four parts like this:
- 1st: CAS (Column Address Strobe) is the most important, and
this is what is referred to as CAS Latency 2 (CL2)
All else being equal, you would choose a 2-3-2-6 CL2 memory over a 2-4-4-8 CL2 memory, but the most important by far is the first figure, CAS Latency.
There is an excellent article on memory timing at www.cooltechzone.com/index.php?option=content&task=view&id=377&Itemid=0 for more information.
Overclocking is the technique of increasing the clock speed of the motherboard and/or altering the ratios between buss speeds, to run components like CPUs, RAM and video cards at higher than their designed speeds. We have to mention that overclocking can void warranties, generate more heat and increase stress on parts, sometimes to the point of instability, and shorten the life of components. Proceed at your own risk.
Lowest latency may not be desirable for overclocking - in order to remain stable as you increase MHz speeds, you commonly have to relax the latency timings from 2.0 to 2.5 or 3.0. This means that what you gain in memory MHz is often lost on longer latency (although there may be other gains in CPU and graphic performance). The key point to overclocking RAM is to choose RAM with high rated speed, then slowly increase the clock speed, testing thoroughly and adjusting latency as you go, and back off on the speed as soon as you start getting instability or errors.
There are plenty of enthusiasts’ sites on the web that go deeply into overclocking, cooling systems, discussions of the overclocking potential of motherboards, CPUs and RAM, so we won't cover that here. There is as much artistry as science in achieving the ultimate combination of processor, bus and memory speeds.
Kingston, Corsair and Shikatronics all have enthusiasts’ memory with either low latency or higher clock speeds, please email us for a quote firstname.lastname@example.org .
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