Do you run multiple programs at the same time, or play massive games featuring hundreds or even thousands of individual character units? If your system slows down significantly under these circumstances with extremely dull responses and your hard drive LED is blinking at an extremely rapid rate, you may need to add extra memory to your computer.

The memory we refer to here is the main memory in a computer system, also known as RAM (Random Access Memory). It is essentially the computer's workspace - the place where a computer temporarily stores data and programs. More memory allows you to run more programs simultaneously, and to store a greater amount of data for faster access by your computer (mostly the processor). Of course, memory performance is an important factor to consider as well, since faster memory allows more data to be transferred in a given time.

RAM cannot retain data when power is turned off. This volatility means that this type of memory is not used as long-term storage.

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Memory technologies are developing all the time, although not at the pace of processors. There are many different types of memory products offered today, and they aren't always compatible with each other. The memory you buy comes in the form of memory modules, which are constructed of memory chips and PCBs (Print Circuit Board). The most common type of memory module is the DIMM (Dual In-Line Memory Module), which is capable of transferring 64 bits of data per cycle.

System memory support depends on your motherboard (technically, it is the memory controller in the motherboard chipset (Intel) or a processor's integrated memory controller (currently in some AMD products) that determines memory support). It is therefore vital that you determine the memory type and speed support of your motherboard (or processor) before choosing the memory product to purchase.

SDRAM

Synchronous Dynamic Random Access Memory: SDRAM has a synchronous interface. It waits for a clock pulse before transferring data and is therefore synchronous with the computer system bus and processor. This greatly improves performance over asynchronous DRAM. SDRAM is not as popular as it once was and may be used during upgrades. SDRAM modules usually come in the form of 168-pin DIMMs.

RDRAM

Rambus Dynamic Random Access Memory: This is a type of synchronous DRAM created by the Rambus Corporation. RDRAM features an architecture designed to achieve high bandwidth, it is used in the Sony PlayStation 2, early Pentium 4 desktop systems and other applications. The XDR DRAM, RDRAM's successor, is used in IBM's Cell processor and Sony PlayStation 3. RDRAM is also mainly used for capacity expansion of old desktop systems and often come in the form of 184-pin RIMMs/Rambus Inline Memory Modules (16bit).

DDR SDRAM

Double Data Rate: DDR SDRAM sends and receives data twice as often as common SDRAM. This is achieved by transferring data on both the rising edge and the falling edge of a clock cycle. DDR memory is being phased out and replaced by DDR2 memory. DDR memory modules usually take the form of 184-pin DIMMs.

DDR2 SDRAM

Second generation DDR memory provides greater bandwidth and other new features such as On-Chip Termination (OCT). 4 bits of data are moved from the memory array to the I/O buffers (per data line) each core cycle. This can be described as 4-bit prefetch, as opposed to the single-bit fetch in SDRAM and 2-bit prefetch with DDR SDRAM. DDR2 is the dominant mainstream system memory product today, and is replacing DDR in the desktop DRAM market. DDR2 memory modules are 240-pin DIMMs.

DDR3 SDRAM

Third generation DDR memory leaps greatly forward in data transfer rate and power management. DDR3 provides even higher bandwidth than DDR2 due to the 8-bit prefetch buffer (4-bit prefetch of DDR2, and 2-bit of DDR). The advanced fabrication technology allows lower operating currents and voltages (1.5V, compared to 1.8V of DDR2) and thus enhances thermal performance. Successor to DDR2, DDR3 is considered to replace DDR and DDR2 in coming years. DDR3 memory modules take the form of 240-pin DIMMs, and are not compatible with DDR2 memory slots.

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Given a specified memory type, there are still tens or even hundreds of products from different manufacturers available. They are of course different, both in features and in price. Like many other computer products, some of these features may not be that important to certain users depending on their applications and requirements, so it could be helpful to have some basic understanding of the basic memory specifications to help you figure out which features really matter to you.

Capacity

Generally speaking, the larger the capacity the more programs you will be able to run simultaneously (as long as your motherboard and operating system supports it). The capacity you need should depend on your requirements: for most home users, any more than 2GB memory will not result in any performance gains (at least for now). You can read our recommendations in next section for more information.

Speed

Memory speed is a little bit complicated as there are two measurements: operating frequency (or more accurately, the transfer or data rate) and bandwidth. Bandwidth can be regarded as how fast the memory transfers data. And of course, the greater the bandwidth, the better off you are.

SDRAM rated PC100 and PC133 work at 100MHz and 133MHz and provide 800MB/s and 1066MB/s bandwidth respectively. RDRAM utilizes the same rating system as SDRAM - a PC800 RDRAM operating at 800MHz, provides a bandwidth of 1600MB/s. Please keep in mind that SDRAM and DDR/DDR2 SDRAM all use 8-byte (64bit) wide DIMM (transfer 8-byte data per clock cycle), and the common 16bit RIMM utilized by RDRAM is only 2-bytes wide (transfers 2-bytes of data per clock cycle).

Things are different when it comes to DDR, DDR2 and DDR3 memory. Initially, DDR used the same rating system as SDRAM, e.g. PC266. This has now been changed to DDR266 instead, which still means the memory works at 266MHz, providing 2100MB/s bandwidth, which is where the PC2100 designation comes from. DDR400 memory, by the same rule, is called PC3200 for its 3200MB/s bandwidth. The same rule applies to DDR2 and DDR3 memory, for example, the DDR2 533 is also called PC2 4200 or PC2 4300 but 'PC2' is used here to refer to DDR2 instead. DDR3 1066 can be called PC3 8500. 'PC3' here also refers to DDR3.

The bandwidth we refer to here is for single channel scenarios. When memory is used in dual channel mode, the bandwidth doubles - for instance, dual channel DDR2 800 provides 12800MB/s bandwidth as opposed to 6400MB/s for single channel DDR2 800.

CAS Latency/Timing

CAS is the abbreviation for Column Address Strobe, which is a signal sent by the processor to the DRAM indicating a column address. DRAM stores data in a matrix of columns and rows and data is retrieved from DRAM through CAS and RAS (Row Access Strobe) signals – just like coordinates on a map.

CAS Latency (or CL), it is the amount of time it takes between a CAS signal assertion and the initial transfer of the data stream. The CAS Latency is measured in clock cycles. For example, a CAS Latency of 2 or CL2 means the data is available 2 clock cycles after the CAS signal prompting. As with any latency parameters in the computer domain, a smaller CAS Latency value means better performance.

There are other memory latency parameters as well, such as tRCD (Row-to-Column Delay), tRP (RAS Precharge) and tRAS (minimum bank cycle time), these parameters affect memory performance as well, but generally not as much as CAS Latency. We often call all these latency parameters "timing" as well, and a "loose timing" means high latency parameters, in contrast to "tight timing".

Other Concerns

ECC/Registered/Unbuffered

ECC stands for Error Checking and Correction. ECC utilized by memory modules uses single bit error correction, which is capable of detecting and correcting single-bit errors. It will also detect two-bit and some multiple bit errors, but is unable to correct them. This feature needs a motherboard's support, and is usually applied in workstation and server products.

Almost all system memory in PCs today is unbuffered memory. With increasing system memory, stability and performance deterioration is inevitable since the memory controller has to address each memory chip on all modules directly. To solve this problem, higher density systems use registered memory instead, which contains registers as buffers to temporarily hold data for one clock cycle before it is transferred. This increases the reliability of high-speed data access to high density memory but sacrifices some performance. Registered memory modules are typically used only in servers and other mission-critical systems where it is extremely important that data is properly handled. Motherboard support is necessary as well.

Dual Channel Kits

There are many memory modules shipped in the form of dual channel kits – these are essentially two identical memory modules in one package, one for each channel. This type of product is designed for motherboards/systems that support dual channel mode, with two 64-bit wide channels to provide double the bandwidth of single-channel memory systems. Two identical memory modules are preferred for dual channel usage because this symmetric architecture causes less compatibility problems and delivers the highest performance. A dual channel kit product marked as "1GB (512MB x 2)" means it contains two identical 512MB memory modules.

Heatspreaders

Almost all manufacturers add heatspreaders to their high-end memory products for better cooling. As we all know, the surface area exposed to air is what really matters when you are trying to dissipate heat, and that is where the heatspreader comes into play – it simply spreads heat over a larger surface area than the memory chips, thus providing more efficient dissipation. This really makes sense for high-end products since cooling is always vital in high performance rigs.

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The most important concern when choosing system memory is compatibility. Make sure your system can support the memory you are buying. For example, do not purchase DDR2 memory if your computer only provides DDR memory slots. For brandname PC users, check your PC manual or manufacturer website/customer service for memory support information (such as memory type, speed, capacity), and go to your motherboard manual or manufacturer if your PC was self-built (DIY users). You can also check the memory module maker's website for this type of information no matter what type of user you are. Last but not least, figure out how many empty memory slots there are on your motherboard before purchase - you'll have to replace existing memory if all the slots are full.

Mainstream Users

For most users, compatibility may be the only thing that requires attention. Choosing memory that is compatible for your motherboard/system is enough.

 

A capacity of 512MB is the minimum for today's computers, and a larger capacity should be more useful than faster timing/latency parameters when the total memory is under 1GB.

Gamers/Enthusiasts

Performance is vital for gamers pursuing the highest performance. Timing/latency numbers mean a lot here, as are higher speed ratings, which are definitely preferred – make sure your motherboard supports them.

The latest 3D games are very demanding in terms of memory, so 1GB is the standard choice.

Overclockers

Overclockers should look for products with the highest speed ratings (and the fastest latency parameters). The highest supported voltage should be taken into account as well since you'll have to increase voltage when overclocking the memory to the limit.

Workstation/Server and Other Crucial Applications

Reliability tops all for these crucial applications since you cannot afford a system crash here. Workstation and server must usually runs 24 hours a day without a break, so the ECC function is definitely required for these systems.

Professional software is always memory hungry - especially for 3D related work. 2GB is absolutely necessary for workstation users. This desire for memory is even truer for servers. Simply put, the more memory the better - 2GB is just the starting point. Registered memory is always preferred for servers – actually, most server motherboards require this, and it is also recommended for workstations for memory capacities above 2GB.

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