SDRAM Memory Guide

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.