(Synchronous DRAM) A type of dynamic RAM (DRAM) memory chip that has been widely used since the late 1990s. SDRAM chips eliminated wait states by dividing the chip into two cell blocks and interleaving data between them. While a bit in one block is accessed, a bit in the other is prepared for access.
Double Data Rate (DDR)
DDR SDRAM doubles transfer rates by transferring data on both the rising and falling edges of the clock. DDR uses a 184-pin DIMM module in contrast to the 168-pin DIMM of earlier SDRAMs. Laptops use 200-pin SODIMM modules.
DDR2 and DDR3
DDR2 chips increase data rates using various techniques such as on-die termination, which places the terminating transistors that eliminate excess signal noise on the chip itself. DDR3 offers a moderate speed improvement over DDR2, owing to 90 nm fabrication (see feature size).
DDR2 and DDR3 use 240-pin DIMM modules. DDR, DDR2 and DDR3 modules are all keyed differently so that they cannot be inserted into the wrong motherboard slots.
Dual Channel
Chipsets on the motherboard may support two independent memory controllers, which allow access to two memory modules simultaneously (upstream data on one 64-bit channel; downstream data on the other). Channels can be configured as two 64-bit or one 128-bit. Modules must be installed in matched pairs unless the chipset has an option for mismatched modules. See memory module and SGRAM.
In the following list, the single channel speeds are given. If dual channel is implemented, the memory speed and data rates are doubled, and many memory products use the doubled rate in their chip designations.
Single
Channel Single
Memory Channel DIMM
Type Speed Symbol Data Rate Module
DDR3 800 MHz PC3-12800 12.8 GB/sec 240-pin
DDR3 667 MHz PC3-10600 10.6 GB/sec 240-pin
DDR3 533 MHz PC3-8500 8.5 GB/sec 240-pin
DDR3 400 MHz PC3-6400 6.4 GB/sec 240-pin
DDR2 533 MHz PC2-8500 8.5 GB/sec 240-pin
DDR2 500 MHz PC2-8000 8.0 GB/sec 240-pin
DDR2 400 MHz PC2-6400 6.4 GB/sec 240-pin
DDR2 375 MHz PC2-6000 6.0 GB/sec 240-pin
DDR2 333 MHz PC2-5300 5.3 GB/sec 240-pin
DDR2 266 MHz PC2-4200 4.2 GB/sec 240-pin
DDR2 200 MHz PC2-3200 3.2 GB/sec 240-pin
DDR 266 MHz PC-4200 4.2 GB/sec 184-pin
DDR 200 MHz PC-3200 3.2 GB/sec 184-pin
DDR 166 MHz PC-2700 2.7 GB/sec 184-pin
DDR 133 MHz PC-2100 2.1 GB/sec 184-pin
DDR 100 MHz PC-1600 1.6 GB/sec 184-pin
Single Channel
Speed
SDRAM 133 MHz 1.1 GB/sec 168-pin
SDRAM 100 MHz 800 MB/sec 168-pin
Matched Slots for Dual Channel
Motherboards that support dual channel use color coded memory slots so that module pairs are inserted properly. This example shows three pairs (one orange and two lavender). (Image courtesy of Giga-byte Technology Company Ltd., www.giga-byte.com)
A Little Nostalgia
This is 12 bits of magnetic core memory from the Whirlwind computer of the early 1950s and is about a quarter inch square in size. Today, that much space holds hundreds of millions of bits. (Image courtesy of The MITRE Corporation Archives.)
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