All computers require memory to operate. The main computer memory is Random Access Memory, or RAM for short. It is called this because the data contained in it can be accessed in any, or random, order. It is produced on small circuit boards in the form of sticks, which are often referred to as sticks of memory. Information is stored in the memory as it awaits being processed by the central processing unit, or CPU. It is essential to have the right type and speed of memory to ensure your system is operating optimally.

The main type of memory used today in desktop computer systems is Synchronous Dynamic RAM (SDRAM). With in the past several years, Double Data Rate SDRAM (DDR SDRAM, or simply DDR) has become the standard for most systems, with older memory now being referred to as Single Data Rate (SDR SDRAM). Occasionally, systems use Rambus Dynamic RAM (RDRAM) or DDR2 RAM, which are slightly different and are outside of the scope of this article.

SDRAM is different from other forms of asynchronous memory because it waits for each clock, or computing cycle, before it responds to inputs. These clocks happen very fast in computers (millions of times a second) but still the speed of the memory is ultimately limited by the computer bus. This is the wiring on the main- or motherboard in the system that connects all the components. The processor and memory can both handle high clock speeds, but if the speed of the system bus is low, they will be limited by the lowest speed. It is important that they all operate at similar speeds to operate most efficiently. For example, SDR SDRAM comes in PC66, PC100 and PC133; these numbers refer to the clock speed of the RAM in megahertz (a million cycles a second): 66 MHz, 100 MHz and so forth. If the system bus speed is rated at 100 MHz and the memory at 66 MHz, then the memory isn't operating as quickly as it could and PC100 memory should be purchased. Of course, if PC133 were purchased for this same system, the extra 33 MHz would simply be wasted because the system bus can only handle 100 MHz.

Double Data Rate (DDR) memory is very similar to traditional SDRAM except it transfers data twice during each clock cycle. This doubles the transfer rate of the memory while the system bus stays the same. So a 100 MHz system bus that handles DDR memory can effectively use 200 MHz RAM. DDR comes in many speeds, each with two designations. On the box or somewhere in the specifications, the memory will be referred to as DDR-xxx, where the xxx is replaced by the effective clock speed of the memory, such as 200 as in the previous example. It will also list a PC-xxxx number, where xxxx represents the bandwidth of the entire stick of memory. This is not particularly important to understand for novices. The most common are: DDR-200/PC-1600, which operates at a clock speed of 100 MHz (effectively 200 MHz), DDR-266/PC-2100, which operates at 133 MHz (effectively 266 MHz), DDR-333/PC-2700, operating at 166 MHz (333 MHz) and DDR-400/PC-3200, which operates at 200 MHz (400 MHz).

It is very important to know which type (SDR or DDR SDRAM) and speed of memory your system uses and to always purchase this type when upgrading or replacing your memory. You can usually find this information in the manual or contact the manufacturer.