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Perfect Timing: DDR Performance Analysis

What Does Memory Access Look Like?

In practice, this is what happens: in order to read or write to a memory point, the memory controller first transmits the row number of the desired address, and the RAS signal is activated. However, it takes a few clock cycles (the RAS-to-CAS delay time) until the columns can be accessed. Likewise, after the CAS signal is activated, further clock cycles must take place. With standard SDRAM based on the PC133 standard, this is always two or three; with DDR RAM, four or five clock cycles. In the DDR process, the "real" CAS latency times are two or two and a half clock cycles.

The RAS-to-CAS time depends on the technology, and, since it's mostly between five to seven clock cycles, this is the fundamental factor for delays. You could also say that CL2 memory is by far superior to slower models with CL2.5 (DDR) or CL3 (SDRAM). Theoretically, this is true, but other factors put the advantages of fast data transfer in another light - for one thing, modern processors have a high hit rate with cache memory, which means that they seldom have to read directly from the memory. Secondly, the rows are also switched most of the time, so that RAS-to-CAS time is also needed, thus delaying the read process. Finally, the burst reads will occasionally happen - in this case, many of the memory cells that are directly adjacent to one another are read, but here the CAS latency occurs only once, and is therefore insignificant.

Buying Memory: Always Go For Speed?

Experience dictates that the advantages of fast memory are worth the slightly higher price that you have to pay to get it. However, many retailers still offer high-end PCs that make do using the slower memory with which they are equipped. This makes it entirely possible for a new Pentium 4 PC with a hefty 2 GHz to end up lagging behind the neighbor's 1.8 GHz computer in the benchmarks. But don't forget that the chipset also plays an important role here. Aside from Gigaherz and Gigabytes, the inner values (especially in the memory interface of the chipset) contribute considerably to higher performance.

When buying memory, if the price for CL2 memory is only a little bit higher, then we recommend that you go for it. However, you should keep in mind that the slowest unit in the group will be the performance bottleneck. For instance, if you use three DIMMs with CL2 and one with CL2.5, then all four will run with only CL2.5

The most important thing is the price/ brand comparison: make sure that you always distinguish the brand name modules (Infineon, Micron, Samsung, Hyundai, NEC, etc.) from the no-name modules. In terms of price, this usually translates to about a 30% difference!