Dual Data Rate Synchronous Dynamic Random Access Memory, better known as DDR SDRAM or DDR RAM, is a type of very fast computer memory. It is based on the same architecture as SDRAM, but uses the clock signal differently to transfer twice as much data in the same amount of time.
A computer motherboard.
In a computer system, the clock signal is an oscillating frequency used to coordinate the interaction between digital circuits. Simply put, it synchronizes communication. Digital circuits designed to operate on the clock signal can respond on the rising or falling edge of the signal. SDRAM memory chips only used 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.
DDR SDRAM chips can accept more than one write command at a time.
RAM speed works in conjunction with the front side bus (FSB) of a computer system. The FSB is the bidirectional data channel that sends information from the central processing unit (CPU) through the motherboard to the various components, including RAM, BIOS chips, hard drives, and PCI slots. Therefore, a computer system with a 133 MHz FSB running DDR SDRAM will essentially function as a 266 MHz machine.
184-pin DDR RAM Dual Inline Memory Modules (DIMMS) will only work properly on a motherboard designed for their use. Although this RAM comes in various speeds, installing a faster version than the motherboard can handle is a waste of money, as it will only run as fast as the motherboard allows. It is visually distinguished from SDRAM in that it is a 168-pin DIMM with a double notch at the bottom along the pins: one notch just off center and one notch off the side. 184-pin DDR SDRAM has a single off-center notch.
DDR RAM is generally made for 1GHz and faster processors. Designations like PC1600 DDR SDRAM and PC2100 DDR SDRAM match specific FSB and CPU speeds. RAM manufacturers use different schemes to designate processor speed, and the various technicalities in RAM designations and standards can be confusing. Computer users should consult their motherboard manual to see what type of RAM their system supports before purchasing memory.
