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Computer Memory Types

There are 3 primary types of RAM in use today, Synchronous DRAM (SDRAM), Double Data Rate SDRAM (DDR) and RDRAM (Rambus DRAM). RDRAM is the higher priced commonly promoted.
Synchronous Dynamic Random Access Memory

Almost all systems used to ship with 3.3 volt, 168-pin SDRAM DIMMs. SDRAM is not an extension of older EDO DRAM but a new type of DRAM altogether.
SDRAM started out running at 66 MHz, while older fast page mode DRAM and EDO max out at 50 MHz. SDRAM is able to scale to 133 MHz (PC133) officially, and unofficially up to 180MHz or higher. As processors get faster, new generations of memory such as DDR and RDRAM are required to get proper performance.
SDRAM  168pins

Currently, aside from a few motherboards that support both SDRAM and DDR-SDRAM, you cannot purchase new systems that use SDRAM. The huge volumes of older computers on the market ensure that SDRAM modules will be manufactured for at least a few more years, however.
Each module of ddr memory has one notch on the connecting side and two notches on the side that attach to latches on the PC motherboard to hold it in place. DDR RAM can come in either single-sided or double-sided chip configurations. Desktop DDR has 184 pins, as compared to 168 pins on SDRAM.

There are three major types of ddr ram. DDR or DDR1, has clock speeds ranging from 100Mhz up to 200 Mhz. The 240-pin DDR2 supersedes DDR1 and quadruples the speed of SDRAM, peaking at 266MHz. DDR3, a 240-pin module, uses less power and transfers data eight times faster than SDRAM. DDR3 is also the most expensive and least common of the ddr memory types.

Currently all but extinct in the standard desktop PC market, RDRAM is a proprietary memory standard, developed by the RAMBUS company. RDRAM originally made a big splash in 1998 with its adoption by Intel to provide memory support for their high-end Pentium III boards and the early Pentium 4 models.
RDRAM is a serial memory technology that arrived in three flavors, PC600, PC700, and PC800. PC800 RDRAM has double the maximum throughput of old PC100 SDRAM, but a higher latency. RDRAM designs with multiple channels, such as those in Pentium 4 motherboards, are currently at the top of the heap in memory throughput, especially when paired with PC1066 RDRAM memory.
Double Data Rate SDRAM

Double data rate synchronous dynamic random access memory, or DDR SDRAM for short, is a type of computer memory used in Pentium III or higher computers. DDR RAM is the computerís primary working memory, storing program instructions and data for use by the central processing unit (CPU) through the bus controller. DDR memory belongs to the dual in-line memory module (DIMM) architectural family and is a faster form of SDRAM.
DDR1 RAM  184pins
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