SDRAM

(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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SDRAM - Synchronous Dynamic Random Access Memory