What Is Memory, or RAM? System memory, frequently called main memory or RAM (Random Access Memory), is a type of computer memory that can be accessed randomly. That is, any byte of memory can be accessed without touching the preceding bytes. RAM is the "working memory" storage area within the computer. All data on the computer is stored on the hard drive, but in order for the CPU to work with the data during normal operations, the data the computer uses and works with is read into the working memory, which is the RAM chips. Upgrading Memory — Where to Start Before you can upgrade your system memory, you have to first find out what type of memory is supported by your motherboard. This information is readily available in your motherboard manual, or online at the motherboard manufacturer's Web site.  Another issue you will need to consider is how much memory you will need. Again, the motherboard manufacturer will offer information on memory size, and you will also need to find out how much memory is required, at the very minimum, for the operating system you are using.

Key Terms To Understanding RAM RAM DIMM SO DIMM RIMM form factor Related Articles on Webopedia: DRAM Memory Guide Different Types of Memory Cards Webopedia Memory Category

Desktop PC Memory (DIMM)
There are a large number of RAM types available for desktop computer systems. The most commonly used types today are DIMMs (Dual In-Line Memory Module) and these are the small circuit boards that holds memory chips. DIMMs are standard in desktop computers, and common types of DIMMs include the following;

SDRAM - Synchronous Dynamic Random Access Memory
Short for Synchronous DRAM, this is a type of DRAM that synchronizes itself with the CPU's bus. SDRAM, until recently, was the memory standard for modern PCs. When looking at SDRAM The number following "PC" indicates the speed of the system's front side bus. (example: The PC100 SDRAM is designed for systems equipped with a 100 MHz front side bus.)

DDR SDRAM - Double Data Rate Synchronous Dynamic Random Access Memory
Short for Double Data Rate-Synchronous DRAM, a type of SDRAM that supports data transfers on both edges of each clock cycle (the rising and falling edges), effectively doubling the memory chip's data throughput. DDR-SDRAM also consumes less power, which makes it well-suited to notebook computers. DDR-SDRAM is also called SDRAM II. and DDRAM. DDR-SDRAM (and subsequent DD2 and DD3) as well as RDRAM are the technologies which are replacing SDRAM.

DDR2 SDRAM Double Data Rate Two (2) Synchronous Dynamic Random Access Memory
DDR2 SDRAM is the next step up from DDR SDRAM. DDR2 SDRAM offers new features and functions that enable higher clock and data rate operations. DDR2 transfers 64 bits of data twice every clock cycle. DDR2 SDRAM memory is not compatible with current DDR SDRAM memory slots.

DDR3-SDRAM - Double Data Rate Three (3) Synchronous Dynamic Random Access Memory
The third generation of DDR-SDRAM that improves upon DDr2-SDRAM by offering reduced power consumption, a doubled pre-fetch buffer, and also offers more bandwidth because of its increased clock rate.

Note: A RIMM is a type of memory developed by Rambus, Inc. RIMMs are similar to DIMMs but they have a different pin count.

You're SO DIMM — Notebook/Laptop Memory Is Different
Something you may encounter is that there are more notebook(or laptop computer) manufacturers, who use proprietary memory modules. However, most commonly supported by notebook manufacturers is the SO DIMM (Small Outline DIMM), which is a smaller version of the DIMM modules that are used in desktops.

What differentiates the notebook RAM from desktop RAM is its form factor; that is, its physical size and its pin configuration. A full-size DIMM has 100, 168, 184, or 240 pins and is usually 4.5 to five inches in length. In contrast, a SO DIMM has 72, 100, 144, or 200 pins and is smaller — 2.5 to 3 inches.  There is also SO RIMM, which is similar to SO DIMM, but like DIMM/RIMM, it uses Rambus, Inc. technology and also has a different pin count.
 

DID YOU KNOW... A RAM Timeline 1997 (SDRAM) PC66 SDRAM 66MHz 1999 (RDRAM) RDRAM 800MHz 2000 (DDR-SDRAM) DDR SDRAM 266MHz 2004 (DDR2-SDRAM) DDR2 SDRAM 533MHz 2007 (DDR3-SDRAM) DDR3 SDRAM 1066 - 1333MHz (Source: Kingston Technologies)

By Vangie Beal
Last updated: September 10, 2008

Webopedia Memory Category
Webopedia's terms listings and information about computer memory.

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