What Types of PC Memory are There to Choose From?

So you’re looking for PC memory to upgrade your system with, but aren’t quite sure what you’re looking for. Hopefully we can help you with some information about the various types of memory that are available to choose from.

The first type of memory to see widespread use is the 72-pin SIMM. Unlike the 30-pin SIMM that preceded it, the 72-pin version had a 32-bit data bus, requiring only one or two memory modules to fill the system’s data bus, as opposed to the four or eight 30-pin SIMMs that had been required before that. The 72-pin SIMM these days will need to be installed in pairs, as almost all systems have a 64-bit data bus, and come in sizes ranging from 8MB to 128MB of memory. So if you want to add only 128MB of PC memory to your system, remember to get two 64MB memory modules. Otherwise, make sure what type of 72-pin SIMM you need, as there are two formats – FPM (Fast Page Mode) and EDO (Extended Data Out), and not all motherboards will work with both types, not to mention that you will need to make sure that if you are adding RAM to your system, you install the same type as is already being used.

The replacement for the 72-pin SIMM PC memory is the 168-pin SDRAM. The 168-pin SDRAM has a few advantages over the 72-pin SIMM, but the greatest advantage is the 64-bit data bus used by the memory. This means that each module communicates with the system’s data bus without needing to pair memory together, and allows for upgrading the memory one piece at a time. In addition, SDRAM operates at a higher speed then 72-pin SIMMs, operating at 66MHz, 100MHz or 133MHz. The size of memory available in 168-pin SDRAM varies widely, from 32MB up to 1GB modules. The most common sizes in use these days are 256MB and 512MB.

Rambus RAM entered the PC Memory market between SDRAM and the introduction of DDR memory, in 1999. Rambus operates at 800MHz or 1066MHz by sends data twice during a clock cycle through a 16-bit bus or 32-bit bus, as opposed to SDRAM’s slower 100MHz or 133MHz sending data once during a clock cycle through a 64-bit bus. What this means is that Rambus sends data much more quickly to the system, but has a much narrower channel to use. If you want to visualize it, Rambus sends data four times as fast as SDRAM, but only has a two lane highway. SDRAM moves slower, but has eight lanes to send data on. This reduces Rambus’ speed advantage, and combined with Rambus’ higher cost, both in motherboards that take use Rambus memory and its own price, limits Rambus’ share of the market. With the advance of SDRAM technology and the emergence of the faster DDR memory, Rambus has become slower yet more expensive then it’s competitors. Rambus PC memory is still on the market, but generally goes for a higher price then DDR and DDR2 memory. Rambus memory comes in sizes ranging from 64MB to 512MB, and must be installed in pairs if it uses 16-bit memory, or can be installed one at a time if the Rambus memory uses a 32-bit memory bus. One final thing to note with Rambus memory is that all sockets that do not have memory in them must have a continuity spacer in their place, so all memory sockets are filled with either RAM or spacers.

DDR memory is the successor to SDRAM PC memory, and like Rambus sends data twice during a memory clock cycle. This allows DDR to effectively send twice as much information as SDRAM along the same bus. DDR memory has come in a number of different speeds, but the highest official DDR speed is DDR400, which operates at 200MHz and can send up to 3.2GB of data per second. While higher speed DDR RAM is available, it is generally only useful if you plan to overclock your system, which is setting components like the memory and processor to run above their rated speed, which risks causing damage to your memory and other parts of your computer. DDR memory comes in sizes ranging from 128MB to 2GB per module. It is still a fairly common type of memory, though DDR2 has since replaced it as the current standard for PC memory.

DDR2 memory is the current standard in the PC industry, and as the name suggests, is an extension of DDR memory. DDR2 operates by setting the data bus twice as fast as the memory, thus allowing the memory to send more information in the same time period. While this initially gave DDR2 higher internal latencies then DDR, technology has advanced to the point where DDR2 memory operates faster and with the same latencies. DDR2 memory ranges in speed from 400MHz to 800MHz, and modules range in size from 256MB all the way up to 4GB modules.

So which type of PC memory is right for you? Though there are a few general tips, such as any computer purchased in the last year or two is likely to use DDR2 memory, probably the best way to find out is to use our memory checking utility below. If you’re currently using the computer you plan to upgrade, click the ‘Check Your Computer’ button and it will report back to you what type of RAM your computer uses, how many slots it has and which are open, and also the best options for adding RAM. If you know the model of the system you want to upgrade, you can enter it in the model number field and find the best upgrades for that model of computer. Finally, feel free to take a look around – in addition to PC memory, we also carry memory for Macintosh computers as well as memory cards for cell phones, PDAs, and cameras.

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