This paper describes the IC design language BIT‐I implemented on a personal computer, and the bit‐map CAD system employing the language. The system is composed of several programs covering the mask pattern design, design rule check, extraction of circuit parameters, function simulations, and conversion of data formats. Actual data processing in the program of design rule check and extraction of circuit parameters consists of the combinations of basic logic operations. The basic logic operations are described in the subprograms and their combinations are described in the main program. Therefore when new programs are to be developed for different water fabrication process or different devices, only the development or modification of new main programs can manage them.