What are differences between SDRAM, DDR1, DDR2, and DDR3?
Why can my computer only recognize 128MB capacity when 256MB memory is installed?
What are differences between SDRAM, DDR1, DDR2, and DDR3?

SDRAM (Synchronous DRAM)
A type of RAM that is stored on DIMMS and it synchronized with the system clock. SDRAM runs at the same speed as the system bus. (For example: DDR, DDR2 and DDR3)

DDR (double-data-rate synchronous dynamic random access memory)
DDR achieves greater bandwidth than the preceding single data rate SDRAM by transferring data on the rising and falling edges of the clock signal (double pumped). Effectively, it doubles the transfer rate without increasing the frequency of the clock.

DDR2 (double-data-rate two synchronous dynamic random access memory)
It is a random access memory technology used for high-speed storage of the working data of a computer or other digital electronic device. Its primary benefit is the ability to operate the external data bus twice as fast as DDR SDRAM. This is achieved by improved bus signaling, and by operating the memory cells at half the clock rate (one quarter of the data transfer rate), rather than at the clock rate as in the original DDR. DDR2 memory at the same clock speed as DDR will provide the same bandwidth but markedly higher latency, providing worse performance.

DDR3 (double-data-rate three synchronous dynamic random access memory)
DDR3 memory reduces 40% power consumption compared to current DDR2 modules, allowing for lower operating currents and voltages (1.5 V, compared to DDR2`s 1.8 V or DDR`s 2.5 V). DDR3`s prefetch buffer width is 8 bit, whereas DDR2`s is 4 bit, and DDR`s is 2 bit. The prefetch buffer is a memory cache located on modern RAM modules which stores data before it is actually needed. In addition to increased operation frequencies, decreased heat production, and increased latency, and increased bandwidth, the width of the prefetch buffer is increased with each successive standard of modern DDR SDRAM modules.

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