RAM Defined:

Computer Memory -- Random Access Memory is used for temporarily storing information that is used by the computer's operating system and applications. In a PC, RAM is classified as volatile computer memory. Volatile, meaning its contents are lost when power is removed. The network of electrically-charged points in which a computer stores quickly accessible data in the form of 0s and 1s is called memory. Random access means that the processor can access any part of the memory directly rather than having to proceed sequentially from some starting place.

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Think of RAM as a scratch pad area for data the operating system or an application is currently accessing. The computer's primary storage is the hard drive--where the operating system and applications themselves are stored, along with their associated data. Placing data in this scratchpad area allows the computer to access and process the information far more quickly than if the data had to constantly be read and written to the hard drive.

RAM Types:

There are several types of computer memory (RAM) with which you should familiarize yourself:

SRAM (Static RAM) is random access memory (RAM) that retains data in its memory as long as power is being supplied. Unlike dynamic RAM (DRAM), which stores data in cells consisting of a capacitor and a transistor, SRAM doesn't have to be refreshed. Static RAM provides faster access to data and is more expensive than DRAM. SRAM is used for a computer's cache memory and as part of the random access computer memory digital-to-analog converter on a video card.

DRAM (Dynamic Random Access Memory) is the most common kind of random access memory (RAM) for PCs. DRAM is dynamic in that, unlike static RAM (SRAM), it needs to have its storage cells refreshed or given a new electronic charge every few milliseconds. DRAM stores data in a storage cell consisting of a capacitor and a transistor. DRAM capacitors tend to lose their charge rather quickly; thus, the need for recharging.

SDRAM (Synchronous DRAM) is a generic name for various kinds of dynamic random access memory (DRAM) that are synchronized with the clock speed that the CPU is optimized for. This increases the number of instructions that the processor can perform at a given time. SDRAM speed is rated in MHz rather than in nanoseconds (ns). This makes it easier to compare the bus speed and the RAM chip speed. You can convert the RAM clock speed to nanoseconds by dividing the chip speed into 1 billion ns (which is one second). For example, 133 MHz would be equivalent to 7.5 ns

DDR-SDRAM (Double-Data Rate SDRAM) works the same way as SDRAM, except it works at twice the speed by synchronizing to both the rising and falling of the clock pulse. DDR-SDRAM can also be installed in dual-channels if the motherboard supports it. There is no difference in the actual RAM modules used for dual-channel vs. single-channel DDR, but the RAM modules in each channel must be a matched pair (same size and speed).

DDR2 SDRAM (Double-Data Rate 2 SDRAM)
DDR2 SDRAM is able to operate the external bus twice as fast as DDR SDRAM. Of all of your choices, this is probably the most economical. To see the latest available prices, check out computer memory products at a popular online retailer. But remember, make sure you choose computer memory that is compatible with your motherboard!

DDR3 SDRAM (Double-Data Rate 2 SDRAM)
DDR3 SDRAM is able to run its I/O bus at four times the speed of the memory cells it contains, thus enabling faster bus speeds and higher peak throughputs than earlier technologies. Also, the DDR3 standard allows for chip capacities of 512Mb to 8Gb, effectively enabling computer memory modules of maximum 16Gb in size.

DDR SDRAM

Standard name	Memory clock	Cycle time	I/O Bus clock	Data transfers per second	Module name	Peak transfer rate
DDR-200	100 MHz	10 ns	100 MHz	200 Million	PC-1600	1600 MB/s
DDR-266	133 MHz	7.5 ns	133 MHz	266 Million	PC-2100	2100 MB/s
DDR-333	166 MHz	6 ns	166 MHz	333 Million	PC-2700	2667 MB/s
DDR-400	200 MHz	5 ns	200 MHz	400 Million	PC-3200	3200 MB/s

DDR2 SDRAM

Standard name	Memory clock	Cycle time	I/O Bus clock	Data transfers per second	Module name	Peak transfer rate
DDR2-400	100 MHz	10 ns	200 MHz	400 Million	PC2-3200	3200 MB/s
DDR2-533	133 MHz	7.5 ns	266 MHz	533 Million	PC2-4200 PC2-43001	4266 MB/s
DDR2-667	166 MHz	6 ns	333 MHz	667 Million	PC2-5300 PC2-54001	5333 MB/s
DDR2-800	200 MHz	5 ns	400 MHz	800 Million	PC2-6400	6400 MB/s
DDR2-1066	266 MHz	3.75 ns	533 MHz	1066 Million	PC2-8500	8533 MB/s

DDR3 SDRAM

Standard name	Memory clock	Cycle time	I/O Bus clock	Data transfers per second	Module name	Peak transfer rate
DDR3-800	100 MHz	10 ns	400 MHz	800 Billion	PC3-6400	6400 MB/s
DDR3-1066	133 MHz	7.5 ns	533 MHz	1066 Billion	PC3-8500	8533 MB/s
DDR3-1333	166 MHz	6 ns	667 MHz	1333 Billion	PC3-10600	10667 MB/s
DDR3-1600	200 MHz	5 ns	800 MHz	1600 Billion	PC3-12800	12800 MB/s