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 HDD LED is blinking insanely, you may need to add extra memory (or upgrade the 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, the performance of the memory is an important factor to consider as well, since faster memory allows more data to be transferred in a given time.

RAM is much faster than any other type of storage device (such as hard drives and optical drives) when utilized as memory. This is especially important in notebook computers which typically utilize hard drives that are slower even in relation to desktop hard drives. Having more memory may also help to extend notebook battery life as the system can interact with the memory more often than with the slower hard drive (hard drive activity consumes more power).

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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. Currently the most common type of memory module used in PCs is the DIMM (Dual In-Line Memory Module), which is capable of transferring 64 bits of data per cycle.

The memory modules used in notebooks can be regarded as smaller versions of desktop memory modules. The most commonly-used type of notebook memory module is the SO-DIMM (Small Outline-DIMM) which is roughly half the size of regular DIMMs.

The memory module type required and the number of modules supported varies by notebook. Therefore, it is extremely important to find out what your notebook can support before entering into an actual 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 improved performance over asynchronous DRAM. SDRAM is not as popular as it once was and may be used during capacity upgrades when keeping the same laptop. Notebook SDRAM modules are usually 144-pin SO-DIMMs.

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 still one of the mainstream products for notebook computers today and usually comes in the form of a 200-pin DDR SO-DIMM (see image above), although it also takes the form of a 172-pin DDR MICRO DIMM (see image below). The latter is currently used in only a few notebooks.

DDR2 SDRAM

Second generation DDR memory provides greater bandwidth and other new features such as On-Chip Termination (OCT). Utilizing a voltage of 1.8V instead of the 2.5V used by DDR memory modules, DDR2 consumes less power than its predecessor, helping to extend notebook battery life. Being a mainstream memory product itself, DDR2 will basically replace DDR in the near future. DDR2 memory modules are 240-pin DDR2 SO-DIMMs.

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Given a specified memory type, there are still tens or even hundreds of products available. They are of course different, both in features and in price. Like many other computer products, some of these features may or may not be that important to certain users depending on their applications and requirements. It can therefore be helpful to have some basic understanding of the basic memory specifications to help you figure out the features that will 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 support the memory capacity you wish to have). The capacity you need should depend on your requirements: for most 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. Please keep in mind that SDRAM and DDR/DDR2 SDRAM all use 8-byte (64bit) wide DIMM (transfer 8-byte data per clock cycle).

Things are different when it comes to DDR and DDR2 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 memory, for example, the DDR2 533 is also called PC2 4200 or PC2 4300 but 'PC2' is used here to refer to DDR2 instead.DDR2 800 is the same as PC2 6400 as well.

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 DDR400 provides 6400MB/s (or 6.4GB/s) bandwidth as opposed to 3200MB/s for single channel DDR400.

CAS Latency

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 usually means better performance, although the performance differences between notebook memory modules is usually quite small.

Other Concerns

Dual Channel Kits

Memory modules are often shipped in the form of dual channel kits – these are essentially two identical memory modules packaged together. This type of product is designed for 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.

Note: A dual-channel kit can be installed in notebooks supporting only single-channel memory operation, but will not be capable of offering the doubled bandwidth of a true dual channel system.

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The most important concern when choosing notebook memory is compatibility – please make sure your notebook can support the memory type you are buying. For example, do not purchase 200-pin DDR2 SO-DIMM memory if your notebook only provides 200-pin DDR SO-DIMM memory slots. You can check your notebook manual or the manufacturer's website/customer service for memory support information (memory type, speed, capacity). In addition, it is very important to note how many empty memory slots are available in your notebook before purchase – you may need to replace existing modules if there are no empty slots.

Important Note: The memory installation process often differs between notebooks. Please make sure to check the notebook manual or turn to the manufacturer for assistance before installation.

Rather unfortunately, in most cases your choice of memory type and speed basically depends on your notebook. This means you are limited in terms of choice, and memory capacity (and brand) may be the only specification that you can determine for yourself.

Here are our recommendations:

Mainstream Users: 512MB or more

A capacity of 512MB is the minimum required to run multiple simultaneous current-day programs. The latest cutting-edge games will also find it difficult to run smoothly when total memory is under 512MB. A capacity of 256MB can still be enough for everyday applications such as Web surfing (a small number of open web browser windows), chatting, and office work.

Business Users: 1GB or more

If you often open and work on many documents (especially large ones) simultaneously, a memory of 1GB is definitely helpful for faster swapping between applications as most application data is stored in RAM (faster) instead of being accessed from the hard drive every time it is needed (relatively slow).

Multimedia Users/Gamers: 1GB or more

The latest 3D games demand a lot from system resources. A memory of 1GB is the recommended minimum if you are a serious gamer. For multimedia processing tasks, which are similarly resource intensive, a memory of 1GB is recommended as well.

Mobile Workstation Users: 2GB

Professional applications are always memory hungry - for image processing, video /audio editing and the most demanding 3D-related work, we recommend at least 2GB memory is installed in your notebook computer in order to avoid system slowdown.

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