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i want to ask what factors effect data transfer rate in computer what is external clock frequency and its
effect on data transfer rate

afaik, the external frequency is the frequency of the bus between the CPU and the memory. Now, the data transfer rate will be a sub-multiple of this value, but telling which sub-multiple depends on how much external cycles a memory access requires (usually 2 or 3, but it could be lower if the bus enters the "burst" mode ...)