DDR SDRAM, or double-data-rate synchronous dynamic random access memory is faster than its predecessor SDRAM, because the chips allow data transfer twice for every clock cycle, instead of only once.
As an example, when data transfers at 64 bps (bits per second), DDR memory allows this to be multiplied by 2. Thus, 64 bps x 2 (transfer capability per clock cycle)/ 8 (bits/byte) = 16 bytes per second. At 133 MHz bus frequency, DDR SDRAM provides a maximum rate of transfer of approximately 2100 MB/s.
Besides the more efficient data transfer that DDR offers, it may consume significantly less power. The SDRAM has an operating voltage of 3.3 volts, while the DDR SDRAM uses 2.5 volts.
Chip and Module Specifications
DDR SDRAM has specifications for memory chips and memory modules established by the JEDEC, the standardization body of the electronics industry, which can be seen here:
Standard name Module name Memory clock I/O bus clock Transfer rate
DDR-200 PC-1600 100 MHz 100 MHz 1600 MB/s
DDR-266 PC-2100 133 MHz 133 MHz 2100 MB/s
DDR-333 PC-2700 166 MHz 166 MHz 2700 MB/s
DDR-400 PC-3200 200 MHz 200 MHz 3200 MB/s
These figures are guaranteed data rates of performance; A chip may possibly run at clock rates higher or lower than those specified in the practice of "overclocking" or "underclocking."
SDRAM DIMMs have 240 pins and two notches. DDR SDRAM DIMMs have 184 pins and one notch. DDR SO-DIMMs, for notebooks, have 200 pins and similar notching to DDR2 SO-DIMMs but cannot be used interchangeably, nor can DDR be used with DDR3. SDRAM and DDR SDRAM cannot be mixed either.
Several new chipsets use configurations, known as dual-channel, which effectively double or quadruple throughput, because the dual-channel technology uses a 128-bit path by combining two 64-bit channels.
Important Chip Features
In a system designed for DDR 184 pin DIMM (typically desktops) or 200 pin SO-DIMM (typically laptops), almost all motherboards will recognize only 1 GB modules if they are low density 64Mx8 modules. Using high density 128Mx4 1 GB modules in a desktop the computer may only recognize 512 MB or not work at all. These higher density DDR memory devices are meant for registered memory modules in servers.
DRAM is expressed as 64M x 4, which indicates a chip having 64 million storage units and a data width of 4 bits. DDR chips can be x4, x8, or x16. The latter two tend to be more expensive. The x4 chips offer advanced features like Chipkill, (which scatters bits across multiple chips, so memory can be reconstructed if a chip should fail), memory scrubbing, and Intel SDDC, which is similar to Chipkill.
Important Module Features
Dram devices quantity: Single-sided modules have chips on one side only. 36 (9x4), or 4 multiples of 9 chips, are the maximum chips on a DDR module.
DRAM ranks: Modules have 1, 2, or 4. Only one can be active at a time. If there are two or more ranks, the memory controller will switch between them.
Buffering: Unbuffered or registered
Packaging: DIMM or SO-DIMM
Power consumption: Affected by clock rate