 |
Inside Memory: It's All About The Numbers
by John H., The Component Guru
What's The Secret To Memory Specs?
What do all those memory numbers mean?
Memory speeds, timings, and bandwidth play a critical role in the performance of your computer. Unfortunately, many people don’t actually understand what each number represents and what each actually means to them.
DDR Is Not A Video Game!
The most commonly seen memory specifications are those for SPEED! Numbers similar to PC3200 and DDR-333. While these values represent different specs, they are linked together. For instance, a memory module may be specified as DDR-400 and PC3200 or DDR-333 and PC2700. The “DDR” number refers to the actually frequency of the module (400MHz in the first case), while the “PC” number refers to the transfer rate (3200 MB/s for the same module). You may notice a pattern here. The “PC” is always going to be roughly 8 times the “DDR” number (it's rounded off). So the next time you see a DDR value, you can actually calculate the PC value from it or at least know the specification is correct.
The PC5400/ PC5300 Controversy
The 8x DDR formula holds true for most of the newer types of DDR memory also. For example, DDR2 modules may show you a speed of DDR-800, which gives a transfer rate of PC6400 (8 X 800). However, one type of memory often causes confusion. That memory is DDR-667. Why this one in particular? This module has an actual transfer rate of 5336 MB/s. (667 x 8 = 5336), which seems to be a difficult number to represent. In most cases, the transfer total is simply rounded off. The problem with DDR-667 is that the various manufacturers making these memory modules don't agree on a common, rounded-off figure. Some call it the more accurate PC5300, while others went with the faster-sounding PC5400, thereby confusing nearly EVERYONE without a Ph.D. in computer engineering. People owning computers using this type of memory run into problems when they try to upgrade their memory and can only find one or the other. They open their computer, or use a memory configurator to find out what type of memory they need, and see PC5300. When they attempt to order the memory, all they find is PC5400, not realizing it’s the same thing as PC5300. So remember, when it comes to PC5300 it's exactly the same as PC5400!
Memory Timings (Latency)
The other scary set of memory numbers you’ll often encounter are the memory timings, or latency. This is shown by four numbers, separated by hyphens (x-x-x-x). Each measures the time (in clock cycles, just in case you were wondering) the memory takes to perform certain actions (keep this in mind for later). I’m not going to explain specifically what each number means (for those of you that actually have the computer engineering degrees, you already know this. As for the rest of you – this is esoteric info that only folks who have advanced computer degrees need to know). All you really need to know is this -- a lower number is generally faster. The number most often mentioned in the specs is the first digit, known as the CAS latency. In DDR memory, values of 2-3 are common, while in DDR2 it is usually around 5, and can be as high at 10 for DDR3.
Lets Be Clear - Smaller Is Better!
Okay, it’s likely that I’ve thoroughly confused you by telling you that a lower number is faster, rather than what you thought was the fastest, and actually has the slowest timings. Let me clear this up! Going back to my explanation that the numbers measure the number of clock cycles it takes the memory to perform its tasks: Because the clock speeds of DDR3 are so much higher than DDR or even DDR2, it can complete each individual clock cycle A LOT faster. In other words, while it may take 9 cycles to do a task, that’s still a lot quicker than the 2 or 3 cycles it took the DDR memory to do the same task. Keep that in mind as you shop for memory. The timings only make a good comparison between modules if you are looking at two of the same types, i.e. two DDR3 modules, but are meaningless when comparing DDR to DDR2 or DDR3.
|
