1. A clock circuit for supporting a plurality of memory module types, wherein said clock circuit connects with a first type memory module slot and a second type memory module slot, wherein said first memory module slot has a first type memory clock pin for receiving a first type memory clock signal, wherein said second memory module slot has a second type memory clock pin for receiving a second type memory clock signal, comprising: a clock generator for producing a clock signal; and a clock buffer coupled to said first type memory module slot, said second memory module slot and said clock generator, wherein said clock buffer has a doubly defined clock pin for outputting either said first type memory clock signal or said second type memory clock signal, and said clock buffer also receives said clock signal and outputs a first type memory clock signal to said first type memory clock pin or outputs a second type memory clock signal to said second type memory clock pin.