The standard SDR (Single Data Rate) SDRAM, commonly termed simply SDRAM, are often labeled PC66, PC100, or PC133. The number denotes the optimum speed at which the memory will run and is equal to the system’s FSB or front-side bus, that is, PC66 will operate at 66 MHz, PC100 at 100 MHz, and PC133 at 133 MHz. Older systems, usually with Pentium II processors, were designed to use these types of memory.

DDR (Double Data Rate) SDRAM modules are much faster than SDR in that they can transfer data on both the rising and falling edge of a clock signal, hence doubling the data throughput. These modules are typically labeled PC1600, PC2100, PC2700, or PC3200. The number indicates the module bandwidth or the amount of data that can be transferred in one second, that is, 1.6GB/sec, 2.1GB/sec, 2.7GB/sec, and 3.2GB/sec respectively. Sometimes, these modules are also referred to as DDR200, DDR266, DDR333, or DDR400 respectively, the number denoting the bandwidth frequency in MHz. Newer systems already use DDR memory. Still, some systems have been designed to accept both SDR and DDR, but they can not run simultaneously together.

SDR DIMMs and DDR DIMMs are not designed to be interchangeable. They differ in voltage rating and the number of pins is not the same so that their sockets are also different.

All SDRAMs are designed for backward compatibility, meaning, modules of different speeds can be mixed. However, when you do this, keep in mind that memory will always default to the lower speed. Also, it is always recommended that you read your system’s technical reference manual for any limitations before you perform any upgrade.