In the early 1990's, most PCs were equipped with EDO-RAM. Although EDO memory was a very good type of memory, it also had some very big inefficiencies. Suddenly, just making the EDO-RAM faster was not good enough. EDO-RAM is asynchronous, which means that it does not necessarily run at the same clock rate as the rest of the computer. This means that a clock would have to be used just for the memory, plus the CPU would spend much of its time doing nothing but waiting for the data to get sent.

To fix this problem, SD-RAM (Synchronous Dynamic RAM) was created. This type of RAM was able to run at the same frequency as the rest of the computer.

The early type of SD-RAM used a 2-clock. This means that it was set up so that each clock cycle it could get access to two of the chips from each memory stick. To improve performance, the SD-RAM was altered so that it used a 4-clock.

Another improvement that had to be made was the clock speed of the memory. When Intel CPUs started migrating to 100MHz buses, SD-RAM had to keep pace. Currently, speeds of SD-RAM are also available in 133Mhz, as well as 150 Mhz and 166MHz for high performance workstations. Today, SD-RAM is rated by the highest speed it can reliably run on. For example 133MHz SD-RAM would be referred to as PC133 SD-RAM.

Mushkin's 128MB high performance PC133 SDRAM
© 2001 by www.mushkin.com

If there is a weakness to SD-RAM, it would have to be its latency. Although Memory modules seem to be getting faster and faster, latency, which is the real performance problem, is getting bigger and bigger. In order to overcome this problem, some manufacturers have actually added a small amount of high speed memory that acts as a Cache to the memory module. This effectively lowers the latency significantly, and improves the performance of the memory. Just like the CPUs cache, the goal of the memory Cache is to hold the most frequently used information. This performance gain does have a price, as ESD-RAM can be up to 4 times as expensive as regular SD-RAM modules.

As you can see, ESD-RAM's added cache gives it an excellent performance gain over standard SD-RAM.

There have not been many higher performing types of SD-RAM at a relatively low price. One type is quickly becoming popular, theoretically being twice as fast as standard SD-RAM. It is called DDR SD-RAM, and the way it achieves the double data rate is actually quite simple and will be demonstrated in the next section.