The following terms are used in this section.  Please review the definitions if you do not know what they mean.

Some people like to know a lot about computer systems they own - or considering buying.  Many people get confused when a salesperson talks about memory and technical terms.  This section will hopefully give a consumer knowledge about what memory is and the types that are out there.

RAM memory is to a computer as short term memory is to a human.  All programs and data which the computer needs to operate are stored in RAM.  With more RAM, the computer can work on more tasks more efficiently, requiring less swapping of data to the hard drive which is much slower.

The first SIMMs transferred 8 bits of data at a time. Later, as CPUs began to read data in 32-bit chunks, a wider SIMM was developed, which could supply 32 bits of data at a time. The easiest way to differentiate between these two different kinds of SIMMs was by the number of pins, or connectors. The earlier modules had 30 pins and the later modules had 72 pins. Thus, they became commonly referred to as 30-pin SIMMs and 72-pin SIMMs.

Another important difference between 30-pin and 72-pin SIMMs is that 72-pin SIMMs are 3/4 of an inch (about 1.9 centimeters) longer than the 30-pin SIMMs and have a notch in the lower middle of the PCB. The graphic below compares the two types of SIMMs and indicates their data widths.

At one time, FPM was the most common form of DRAM found in computers. In fact, it was so common that people simply called it "DRAM," leaving off the "FPM". FPM offered an advantage over earlier memory technologies because it enabled faster access to data located within the same row.

In 1995, EDO became the next memory innovation. It was similar to FPM, but with a slight modification that allowed consecutive memory accesses to occur much faster. This meant the memory controller could save time by cutting out a few steps in the addressing process. EDO enabled the CPU to access memory 10 to 15% faster than with FPM.

In late 1996 DIMMs (SDRAM) which closely resemble SIMMs began to appear in systems.. Like SIMMs, most DIMMs install vertically into expansion sockets. The principal difference between the two is that on a SIMM, pins on opposite sides of the board are "tied together" to form one electrical contact; on a DIMM, opposing pins remain electrically isolated to form two separate contacts.

DIMMs transfer 64 bits of data at a time and are typically used in computer configurations that support a 64-bit or wider memory bus. Some of the physical differences between 168-pin DIMMs and 72-pin SIMMs include: the length of module, the number of notches on the module, and the way the module installs in the socket. Another difference is that many 72-pin SIMMs install at a slight angle, whereas 168-pin DIMMs install straight into the memory socket and remain completely vertical in relation to the system motherboard.

Unlike previous technologies, SDRAM is designed to synchronize itself with the timing of the CPU. This enables the memory controller to know the exact clock cycle when the requested data will be ready, so the CPU no longer has to wait between memory accesses. SDRAM chips also take advantage of interleaving and burst mode functions, which make memory retrieval even faster. SDRAM modules come in several different speeds so as to synchronize to the clock speeds of the systems they'll be used in. For example, PC66 SDRAM runs at 66MHz, PC100 SDRAM runs at 100MHz, PC133 SDRAM runs at 133MHz, and so on. Faster SDRAM speeds such as 200MHz and 266MHz are currently in development.

DDR SDRAM, is a next-generation SDRAM technology. It allows the memory chip to perform transactions on both the rising and falling edges of the clock cycle. SDRAM only carries information on the rising edge of the signal.  Basically this allows the DDR module to transfer data twice as fast as SDRAM.  For example, with DDR SDRAM, a 100 or 133MHz memory bus clock rate yields an effective data rate of 200MHz or 266MHz.  DDR SDRAM are also called DIMMs, but the come with 184 pins instead of 168 pins used by SDRAM DIMMS.  The module itself looks almost identical to the older SDRAM, but it has only one notch instead of the two found in SDRAM DIMMs.

DDR modules, like their SDRAM predecessors, are called DIMMs.  They use motherboard system designs similar to those used by SDRAM; however, DDR is not backward compatable with SDRAM-designed motherboards.  DDR memory supports both ECC and non-parity.

Direct Rambus is a new DRAM architecture and interface standard that challenges traditional main memory designs. Direct Rambus technology is extraordinarily fast compared to older memory technologies. It transfers data at speeds up to 800MHz over a narrow 16-bit bus called a Direct Rambus Channel. This high-speed clock rate is possible due to a feature called "double clocked," which allows operations to occur on both the rising and falling edges of the clock cycle. Also, each memory device on an RDRAM module provides up to 1.6 gigabytes per second of bandwidth - twice the bandwidth available with current 100MHz SDRAM

RIMM is the trademarked name for a Direct Rambus memory module. RIMMs look similar to DIMMs, but have a different pin count. RIMMs transfer data in 16-bit chunks. The faster access and transfer speed generates more heat. An aluminum sheath, called a heat spreader, covers the module to protect the chips from overheating.

Another factor to consider about memory is CAS Latency.  This is the speed at which you access a single column of RAM.  The best SDRAM chips on the market right know have a CAS Latency of 2, CL2.  SDRAM that is average in quality is CL3 or CL2.5.  CL2 has a few advantages over the other two.  The first is that over the long run, your SDRAM will be more stable in your system as things such as internal temperature and power requirements change. The second is that, in most motherboard BIOSes, the CAS latency can be manually adjusted (or automatically detected) to a CL2 setting thus allowing you system to run faster. The third, for CPU over-clockers, is that you would probably be able run CL2 RAM at speeds in excess of the rated speed of the DIMM. And finally, another reason to get CL2 SDRAM is that some operating systems can be tougher on the memory in your system than others. In particular we strongly recommend that you use CL2 SDRAM in a Windows NT, Windows 2000 and Windows XP computer systems since those operating systems are much more memory dependent. For Windows 98 and ME systems, CL3 memory will generally work just fine. Of course, there are some people that absolutely want to get the best memory available, even if their system doesn't really require it to better insure the long-term stability of their PC and the obvious choice in that case is CL2 SDRAM.