Every computer needs memory to work properly. The integral computer memory is called the Random Access Memory or also known as RAM for short. It is named this due to the fact that the data stored in it may be accessed in any or random sequence. It is generated on tiny circuit boards in the form of sticks that are usually called as sticks of memory. Information is saved in the memory as it waits to be processed by the central processing unit or popularly known as the CPU. It is critical to have the correct type and speed of memory to guarantee the optimal operation of your computer system.

Currently, the basic type of memory implemented in your desktop computer system is Synchronous Dynamic RAM (SDRAM). Although, since the past few years, Double Data Rate SDRAM (DDR SDRAM or simply DDR) has been the standard for a majority of systems, with previous memory now being called to as Single Data Rate (SDR SDRAM). Sometimes, certain systems use Rambus Dynamic RAM (RDRAM) or DDR2 RAM that are a bit different and are not within the scope of this article.

SDRAM is not similar to other types of asynchronous memory because it waits for each computing cycle or clock before responding to inputs. These clocks occur very quickly in computers (millions of times per second) but still the computer bus eventually bound the speed of the memory. This is the wiring on the motherboard or mainboard in the system, which connects the entire component together. The memory and processor can both manage high-elevated clock speeds, although if the speed of the system bus is at a low level, they will be bounded by the lowest possible speed. It is crucial that they all work at comparable speeds to operate with efficiency. As an example, SDR SDRAM are available in PC66, PC100 and PC133; these numbers correlates to the clock speed of the RAM measured in megahertz (a million cycles per second): 55 MHz, 100MHz, and so on. In case the system bus speed rate is at 100 MHz and the memory is at 66 MHz, thus the memory is not working as fast as it may be and it is recommended to purchase a PC100 memory. Of course, if PC133 was bought for the system above, the additional 33 MHz would automatically be wasted because the system bus can only manage 100 MHz.

Double Data Rate (DDR) memory is quite similar to conventional SDRAM except it transfers data two times during each clock cycle. This increases the transfer rate of the memory by twice as much while the system bus remains the same. Thus a 100 MHz system bus that manages DDR memory can successfully utilize 200 MHz RAM. DDR are available in various speeds, each of them with two designations. Somewhere in the specifications or on the box, the memory will be labeled as DDR-xxx, where the xxx is substituted by the suitable clock speed of the memory like 200 in the example above. Also, it will list a PC-xxxx number, where xxxx signifies the bandwidth of the whole memory stick. This is not mainly essential to understand for beginners. The most usual are: DDR-200/PC-1600 that works at a clock speed of 100 MHz (effectively 200 MHz), DDR-266/PC-2100, which works at 133 MHz (effectively 266 MHz), DDR-333/PC-2700, working at 166 MHz (333 MHz) and DDR-400/PC-3200, which works at 200 MHz (400 MHz).

It is quite essential to know which type (SDR or DDR SDRAM) and speed of memory your system utilizes and to consistently buy this type when replacing or upgrading your memory. You can generally locate this information in technical manual or by contacting the manufacturer.