DDR RAM

DDR RAM

Double-Data-Rate Synchronous Dynamic Random Access Memory, better known as DDR SDRAM or DDR RAM for short, is a type of very fast computer memory. It's based on the same architecture as SDRAM, but uses the clock signal differently to transfer twice the data in the same amount of time.

In a computer system, the clock signal is an oscillating frequency used to coordinate interaction between digital circuits. Simply put, it synchronizes communication. Digital circuits designed to operate on the clock signal may respond at the rising or falling edge of the signal. SDRAM memory chips used only the rising edge of the signal to transfer data, while DDR RAM transfers data on both the rising and falling edges of the clock signal, making it essentially twice as fast as SDRAM.

RAM speed works in conjunction with the front side bus (FSB) of a computer system. The FSB is the two-way data channel that sends information from the central processing unit (CPU) throughout the motherboard to the various components, including the RAM, BIOS chips, hard drives and PCI slots. Therefore, a computer system with a FSB of 133MHz running DDR SDRAMwill essentially perform like a 266MHz machine.

Chips and modules

Standard name	Memory clock (MHz)	Cycle time[4] (ns)	I/O bus clock (MHz)	Data rate (MT/s)	VDDQ (V)	Module name	Peak transfer rate (MB/s)	Timings (CL-tRCD-tRP)
DDR-200	100	10	100	200	2.5±0.2	PC-1600	1600
DDR-266	133⅓	7.5	133⅓	266⅔		PC-2100	2133⅓	2.5-3-3
DDR-333	166⅔	6	166⅔	333⅓		PC-2700	2666⅔
DDR-400A DDR-400B DDR-400C	200	5	200	400	2.6±0.1	PC-3200	3200	2.5-3-3 3-3-3 3-4-4

Note: All above listed are specified by JEDEC as JESD79F.^[5] All RAM data rates in-between or above these listed specifications are not standardized by JEDEC—often they are simply manufacturer optimizations using tighter-tolerance or overvolted chips.

The package sizes in which DDR SDRAM is manufactured are also standardized by JEDEC.