The differentiate between RAM

Different types of RAM - SDRAM, DDR, DDR2 and RDRAM

RAM (Random Access Memory) comes in several different types, which are not compatible with each other, and a motherboard will be compatible with one type of RAM. For example, DDR Memory won't work with motherboards designed for only DDR2, SDRAM or RDRAM memory, and vice versa.

	DDR2 DDR2 is the most common type of memory in modern PCs today, having introduced features and functions that go well beyond the DDR SDRAM specification. It has 240 pins and is able to operate at data rates much faster than DDR1. Additionally, DDR2 adds a second port to transfer data packets to the CPU, doubling the transfer rate of DDR1.
	DDR1 Originally known as plain 'DDR', this type of memory has become less common over the years, having been virtually replaced by DDR2. It's an evolution of SDRAM with 184 pins. While SDRAM transfers data once per clock signal, DDR transfers data twice, almost doubling the data transfer rate. DDR is Double data rate (DDR) SDRAM.
	SDRAM SDRAM stands for synchronous dynamic random access memory, so called because it synchronises with a clock signal and therefore the processor. Single data rate SDRAM is the oldest type of SDRAM, and is therefore commonly known simply as SDRAM rather than SDR SDRAM. While older PCs still use SDRAM it is no longer used in new systems, DDR or DDR2 being used instead. SDRAM has 168 pins.
	RDRAM An alternative to DDR SDRAM is Rambus DRAM (or RDRAM). It uses double data rate in a similar way to DDR and has the same number of pins, and is used in some games consoles, but it is not common in PC systems.

Different types of memory module - SIMM, DIMM and SODIMM

Memory modules come in different forms, and it's essential to get the right kind to fit your motherboard. A DIMM module won't fit into a SODIMM slot, and vice versa.

	DIMM Most systems use DIMMs - Dual In-Line Memory Modules. DIMMS have separate electrical contacts on each side, as opposed to the SIMM's redundant contacts. DIMMs with their 64-bit data path can be installed individually and replace the matched pairs of SIMMs. The number of pins depends on the type of DIMM.
	SIMM SIMM stands for Single In-Line Memory Module. These memory modules are installed in pairs, and are the older type of module, having a 32-bit data path and only 72 pins, meaning that 64-bit computing remains an impossibility on SIMM-based PC system.
	SODIMM Small outline dual in-line memory modules (SODIMMs) are a smaller version of a DIMM, being about half the size and with either 72, 144 or 200 pins. They are therefore commonly found in laptop computers while regular DIMMs are found in PCs.

Non-ECC memory

ECC and non-ECC memory can usually be placed in the same system without problem, although it will effectively operate as a completely non-ECC-based system. Non-ECC memory is recommended for home and office desktop PCs. The biggest advantage that non-ECC memory offers to the user, is an increase in speed over any ECC equivalent. Non-ECC is ideal for 99% of computers.

ECC memory

Rarely found in home PCs, Error-correcting code memory is memory that tests the accuracy of data moving in and out, and is able to detect and correct some errors without user intervention. These small errors are normally caused by cosmic rays. ECC memory is only really beneficial if you're going to be processing massive amounts of data (like a server does) and all data needs to be 100% accurate. ECC memory is not normally needed for home systems, but is commonly used in server computers.

Flash memory

Flash memory is a non-volatile form of memory - it doesn't need a power supply to maintain the data stored on it. While it has some applications within PCs, including the BIOS chip, it is also found in devices such as MP3 players, USB memory sticks, memory cards for video game consoles, and the CompactFlash and Secure Digital cards used in many digital cameras.

source:Misco.co.uk, en.wiki.org.

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