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Definition: SDRAM
(Synchronous DRAM) 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 (Double Data Rate) Since 2000
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.
Matched Slots for Dual Channel
DDR RAM Modules Are "Keyed"
A Little Nostalgia
DDR (Double Data Rate) Since 2000
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) (V) 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
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).
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.
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.)