[en] Experience at SLAC demonstrates that the criteria for selection and use of microprocessors in modifying an existing control system may differ from the criteria that apply during installation of the control system of a new accelerator. Considerations such as cost of individual projects, progressive installation without disruption of operations and training of on-board personnel can outweigh ''obvious'' goals such as standardization of hardware, uniformity of software, or even a rigid specification of link protocols with the main computer system