While most PC users pay close attention to their CPU and video card performance, the same may not be true when it comes to basic system memory settings. In terms of overall system performance, memory is still a bottleneck to achieving the highest possible speed. Given the right set of circumstances, a performance bottleneck like this can also be viewed as an opportunity for potential gains through adjustments to the system memory.
This guide will illustrate the basics of SDRAM performance settings, such as system BIOS memory options and memory frequencies, as well as some tips on how to determine the optimum settings for your PC. There are also sections dealing with SDRAM ratings, memory tweaking and overclocking, as well as the requisite selection of reference memory benchmarks. As the content provided here may be old hat to experienced users, this guide has been written with the novice or intermediate user in mind.
Each stick of SDRAM has specifications relating to the timing and frequency it can operate at, much the same as a CPU has a certain MHz rating. The most common types of SDRAM are PC100 and PC133, which basically state that the memory can run on a 100 or 133 MHz frequency. Looking a bit deeper, there are also more detailed specifications for CAS latency, RAS-to-CAS delay, and RAS precharge time. These CAS latency, RAS-to-CAS delay, and RAS precharge time specs are commonly displayed in a shorter, 3-digit format (X-X-X) such as 3-2-2 or 3-3-3.
In staying with the non-technical focus of this guide, suffice it to say that CAS (Column Access Strobe) is a ratio determined using both the memory's column access timings and the current system clock. Since the column access timings are static, the faster the internal clock speed (66, 100, 133+) the harder a lower CAS rating is to achieve. RAS (Row Access Strobe) precharge time and RAS-to-CAS delay are also memory latency ratings that are derived from the memory access times. For each of the CAS latency, RAS-to-CAS delay, and RAS precharge time options, the lower the number, the faster the memory timings and potential performance.
Each memory module also has a ns (nanoseconds) speed rating, which denotes its rated clock speed. The minimum speed of PC100 SDRAM is 10 ns, although the 8 ns variety is more common these days. PC133 SDRAM will have a speed rating of at least 7.5 ns, with higher-grade modules featuring 7 ns modules. This clock speed rating can be ascertained by examining the physical DIMM itself (look for a -10, -8, -7.5, etc. marking on the module) or through memory-specific software programs. Although there are a few slightly different formats, the following is a picture of one popular SDRAM marking arrangement.
There are also a few programs that can read and display this information, such as SiSoft SANDRA. Here is a screenshot from the Motherboard Information section, which displays the CAS and RAS ratings, as well as the highest memory frequency it supports.
It is important to remember that although the clock speed specification of a given DIMM may relate to the CAS/RAS timings, it is in no way a consistent link. For example, PC133 units may be rated at timings of 3-2-2 or 3-3-3, regardless of the fact that they all have the same 7.5 ns clock speed. We have tested a high-grade 128 MB stick of 6 ns SDRAM that will hit a 145 MHz memory frequency at 3-3-3, but refuses to work even as low as 133 MHz using 2-2-2.