Here is more than you want to know ---
SDRAM was first used in 1996 with support for a PC processor bus (also called front-side bus or FSB) speed of 66 MHz, but by 1998 it had advanced to support the 100-MHz FSB. SDRAM certified to run at 100 MHz was dubbed PC100 SDRAM, and it is available in 168-pin dual in-line memory modules (DIMMs). Since each DIMM offers a 64-bit data bus, the peak bandwidth for SDRAM is 800 MBps (8 bytes x 100 MHz). This means a PC100 SDRAM DIMM can ideally pass up to 800 MBps between the DIMM and motherboard (though speeds rarely get this high in actual practice). By 1999, SDRAM was available to support a 133-MHz FSB speed, and this was termed PC133 SDRAM. With the same 64-bit data bus on a DIMM, PC133 SDRAM DIMM can theoretically handle up to 1.1 GBps (8 bytes x 133 MHz).
DDR SDRAM. The issue with SDRAM is that each data line passes only one bit per clock cycle (resulting, for a 64-bit memory device, in 64 bits per clock). SDRAM creators developed memory that would perform two operations per clock cycle. This memory is called double data rate or DDR SDRAM. For a 100-MHz FSB, DDR SDRAM provides twice the bandwidth (8 bytes x 100 MHz x 2) or 1.6 GBps. For a 133-MHz FSB, DDR SDRAM can reach a peak bandwidth of 2.1 GBps (8 bytes x 133 MHz x 2). With a 166-MHz FSB, DDR SDRAM can offer a peak bandwidth of 2.7 GBps (8 bytes x 166 MHz x 2). These are called DDR200, DDR266, and DDR333 for 100-MHz, 133-MHz, and 166-MHz speeds, respectively. But memory makers often name the modules based on their bandwidth, such as PC1600 (1.6 GBps), PC2100 (2.1 GBps), and PC2700 (2.7 GBps). A DDR SDRAM module uses 184 pins (like a RIMM).
Above is a brief extract from a Ziff-Davis on-line publication.
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