Abstract: A data processing system includes a first and a second central system wherein the central processing unit (CPU) of the first central system is operatively connected to the system control unit (SCU) of the second central system and the CPU of the second central system is operatively connected to the SCU of the first central system, each central system including a service processor and a timing subsystem for providing clocking signals. Each central system includes an apparatus for distributing the clocking signals such that each central system is clocked from predetermined ones of the clocking signals in response to control signals from the service processors. The apparatus permits the distribution of the clocking signals to be switched dynamically while the central systems are operating, the switching of the clocking signals being accomplished without affecting the operation of the central systems. The apparatus comprises a generating element for generating local clocking signals.