I'm reading an article on the DDR2 Memory, and now I'm totally confused. What does this line means

"Since the DDR2 internal clock runs at half the DDR external clock rate, DDR2 memory operating at the same external data bus clock rate as DDR results in DDR2 being able to provide the same bandwidth but with higher latency."

  • What is the difference between Internal and External clock rate in reference to the RAM?
  • What is I/O clock rate, Memory clock rate and Bus clock rate ?

This is a comparison chart of different types of RAM from the Wikipedia .

Module type   ChipType   Clock speed   Bus speed   Transfer rate (byte/s)

PC2-6400    DDR2-800    400MHz         800 MT/s    06.4GB/s

PC3-6400    DDR3-800    400MHz         800 MT/s    06.4 GB/s

How can be the transfer rate of DDR2 and DDR3 RAM with the same clock speed of 400 MHz are same ?

2 Answers 2


DDR,DDR2 and DDR3 memories follow the DDRxxx/PCyyyy classification.

xxx indicates the maximum clock speed the memory chips support.eg DDR400 memories work at 400 MHz at the most,DDR2-800 can work up to 800 MHz and DDR3-1333 can work up to 1,333MHz.Though this is not thr real clock speed of the memory.The real clock of DDR,DDR2, and DDR3 memories if half of the labeled clock speed.Therefore DDR400 memories work at 200 MHz,DDR2-800 memories at 400 MHZ.

yyyy number indicates the maximum transfer rate that the memory reaches in MB/s.

The bus clock rate is the actual speed of ur FSB(The FSB connects the processor (CPU) in your computer to the system memory)


The various generations of DDR RAM have an internal datapath that transfers data from a small capacitor array to the internal I/O buffer. The internal clock rate is the frequency needed to match the external clock rate. So DDR types with larger data-depth require less internal transfers per second to match the external speed.

The above is a quick summary of info at hardwaresecrets.com The image below is also from there.

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Disregarding latency differences the above memory chips will have equal performance, but lower internal frequency chips have less power consumption.

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