Definition: SDRAM

(Synchronous Dynamic RAM) The type of dynamic RAM memory chip in wide use since the mid-1990s. SDRAM divides the chip into two cell blocks and interleaves data between them. While a bit in one block is accessed, a bit in the other is prepared for access. See dynamic RAM.

DDR SDRAM (Double Data Rate SCRAM
Since 2000, DDR is the common dynamic RAM standard. DDR SDRAM doubled the data transfer rate by using both the rising and falling edges of the clock cycle, and more speed is achieved with each generation (see below).

Memory Channels
Motherboards may support one, two or four channels. More than one channel requires installing modules in a prescribed order (see image below). See GDDR.

         Clock     Transfer          DIMM
         Speed      Rate            Module
  DDR    (MHz)     (MT/s)   (Volts)  Pins

  DDR5 1600-3200  3200-6400   1.1    288
  DDR4  800-1600  1600-3200   1.2    284
  DDR3  400-1200   800-2400   1.5    240
  DDR2  200-533    533-800    1.8    240
  DDR   100-200    200-400    2.5    184
  SDRAM  66-133     66-133    3.3    168

Matched Slots for Dual Channel

Dual-channel motherboards use color-coded sockets so that DDR memory modules are inserted in pairs. This motherboard supports two pairs of DIMM modules (see memory module).

DDR RAM Modules Are "Keyed"

This example shows the notching of three DDR generations. Modules and slots are "keyed" to prevent the wrong DDR version from being inserted. See memory module.

A Little Nostalgia

These 12 bits of magnetic core memory from the Whirlwind computer of the early 1950s take up about a quarter of a square inch. Today, that much space holds billions of bits. (Image courtesy of The MITRE Corporation Archives.)

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Definition: SDRAM