Written by James Newcomb
Tuesday May 5, 2009
Page 3 of 5

All Computers are not created equal - RAM

Another set of specifications that most people probably have not heard of comes from the system's Random Access Memory (RAM). The job of the computer's RAM is to temporarily store program information. Every time you run a program, some of it gets loaded into RAM. Similar to a CPU's cache, system RAM stores frequently used program code, which speeds up the time it takes for a program to complete the tasks it was written to perform.

When you see a computer advertised as having 4 gigabytes (GB) of RAM, this does not automatically mean the computer will be fast. Quantity does not trump speed in some cases, and here's why - system RAM has a few specs that are usually not advertised. The first and probably most important spec is the bus speed of the RAM. As stated on Wikipedia: "A bus is a subsystem that transfers data between computer components inside a computer". The faster this memory bus, the faster your programs can access your system RAM to read and write data to it.

When you see a memory spec such as DDR2-1066, the "1066" is the effective memory bus speed in Megahertz (MHz.). I say effective bus speed because we're dealing with DDR (Double Data Rate) memory, so in this example the actual bus speed is 533 MHz, but with DDR memory we are doubling the memory's throughput by performing twice as many operations per cycle when compared to a standard memory bus, thus it is rated at 533 MHz. x 2, or 1066 MHz. When ever you see DDR memory bus speeds, you're always seeing the effective speed, not the actual. DDR2 memory comes in many different speeds, starting at 400 MHz. and going all the way up to 1200 MHz. The impact memory bus speeds has on overall computer performance is so great that 2 GB of DDR2-1200 would considerably out-perform 4GB of DDR2-400!

The second most important memory spec is CAS latency. Also stated on Wikipedia: "CAS latency is the delay time which elapses between the moment a memory controller tells the memory module to access a particular column in a selected row, and the moment the data from the given array location is available on the module's output pins". In other words, CAS latency is the time delay between the memory requesting a read or write, and the memory submitting the result back to the computer. The higher your memory's CAS latency, the slower your memory is going to respond to read or write requests from programs. Since all programs (even your operating system - Windows for most people) use memory, the higher your memory's CAS latency, the slower your computer gets in everything it does.

A third thing to look for when purchasing a computer is how the memory is configured. Most computer systems in today's market have more than one memory slot, so in the best interest of your computer's performance you should take advantage of that and have your memory set up in what is called dual-channel mode. Dual-channel memory configurations will work with any memory bus speed, and any CAS latency, but require a motherboard with dual-channel support.

Dual-channel memory configurations pair up two memory modules and in essence double the performance of the memory controller. This does not mean that you will double your computer's performance, or even your memory performance. Doubling the performance of the memory controller simply reduces bottlenecks that occur when processing large amounts of data. Simple tasks such as browsing the web or using a spread sheet will most likely see no performance gain from a dual-channel configuration, but for someone using a lot of processing power, such as a computer drafting professional or a PC gamer, dual-channel memory configurations have the ability to offer considerable performance increases.

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