We can buy the  PC memory in the following three performance standards:

1.Clock frequency:
it represents the greatest frequency of  the stable operation of the DDR can be , that is, we usually talk about the PC-1600 and PC-2100 and so on,  it means they can has stable operation in 200MHz and 266MHz clock frequency . In addition, we also note that the traditional name of the memory specification is based on the memory clock frequency, and the current DDR memory transfer rate is based on the naming. In fact PC-1600 and PC-2100 in accordance with the criteria for the classification of SDRAM that is corresponding to PC-200 and PC-266.

2.Access time:
Read access time represents the delay time data. Sometimes, people have a misunderstanding,  thought that  there is a  link of  the access time and the system clock frequency , in fact, the two have a significant distinction  in essence can be described as an entirely different issue. For example ,SDRAM such as PC133 memory, there is the -7 and -6, which were 7ns and 6ns access time , but they have the same  clock frequency of 133MHz. Access time and clock frequency are not the same, the smaller the more gifted. Also in the same DDR memory, you must pay attention to.

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3.CAS delay time:
This refers to the vertical address pulse of the reaction time ,  it  is also measured indicator  in a certain frequency to support different specification, important indicator of memory. We use the CAS Latency (CL) to measure this indicator. For PC-1600 and for PC-2100 memory , the provisions of the CL should be 2 (that is, it read data delay is two clock cycles), that is to say, It must be in the case of CL = 2 can be stable in its operating frequency.