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Not All Memory is Created Equal

Part II: Speed and Latency

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Memory Speed

The speed of the memory will determine the rate at which the CPU can process data. The higher the clock rating on the memory, the faster the system is able to read and write information from the memory. All memory is rated at a specific clock rate in megahertz that the memory interface talks to the CPU with. Newer memory classifying methods are now starting to refer to them based on the theoretical data bandwidth that the memory supports which can be confusing.

All the versions of DDR memory are referred to by the clock rating, but more frequently memory manufacturers are starting to refer to the bandwidth of the memory. To make things confusing, these memory types can be listed in two ways. The first method lists the memory by its overall clock speed. Thus 400MHz DDR memory would be referred to as DDR400. DDR2 memory would be referred to as DDR2-400.

The other method of classifying the modules is by their bandwidth rating in megabytes per second. 400MHz memory can run at a theoretically speed of 3.2 gigabytes per second or 3,200 megabytes per second. Thus DDR400 memory is also referred to as PC3200 memory. 400MHz DDR2 memory would be listed as PC2-3200. Here is a short conversion of some of the standard DDR memory that can be found:

  • DDR266 = PC2100
  • DDR333 = PC2700
  • DDR400 = PC3200
  • DDR2-400 = PC2-3200
  • DDR2-533 = PC2-4200
  • DDR2-667 = PC2-5300
  • DDR2-800 = PC2-6400
  • DDR3-800 = PC3-6400
  • DDR3-1066 = PC3-8500
  • DDR3-1333 = PC3-10600
  • DDR3-1600 = PC3-12800

It is possible with most memory to use memory of a higher or lower rating than the CPU bus speed. This is not the best way to use the memory in a computer. Memory that is faster than the motherboard and CPU can handle will result in unused potential. Memory that is slower than the motherboard and CPU are capable of handling will result in slower performance of the system. This is why it is important to try and get the best speed module that the system can handle.

Latency

For memory there is another factor that impacts the performance, latency. This is the amount of time (or clock cycles) it takes the memory to respond to a command request. Most computer BIOS and memory manufacturers list this as either the CAS or CL rating. DDR is typically rated at 2.5 but can also be found rated at 2. DDR2 has longer latencies due to the increased number of reads per clock cycle and will tend to have latencies of 3 to 5. DDR3 is still in its early releases and ranges in the 7 to 9 latency ratings.

The lower the latency, the faster the memory is at responding to instructions. Most people cannot tell the difference between the higher and lower rating because they are so close together. Generally this will show up in very memory specific applications such as graphic rendering or cutting-edge 3D applications.

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