Abstract: An accelerator board for use in replacing the microprocessor of a computer system board having a system clock. The accelerator board includes a high frequency oscillator which may be stopped and started under the influence of a clock enable signal. The clock enable signal is synchronized with the phase and logic state of the system clock, to form a clock select signal which serves to toggle a clock selector. When a system access is requested, clock enable stops the high frequency oscillator in a specific phase and logic state, and also toggles the clock selector to select the system clock as the clock speed for all accelerator board operations, giving a smooth transfer. When system access is no longer requested, the high frequency oscillator is restarted and is selected by the clock selector for all accelerator board operations. No time delay is encountered in switching from low to high speeds.