Modular design is one of the key factors in building new-generation computer systems. Three principal classes of system construction models are data flow, control flow and the combination of data flow and control flow. Based on the project AIDS, the paper presents an extended control-flow-data-flow model applying to both software and hardware system development, and then describes the system design language SDL. This is a very high-level language, which provides a natural way for users to design systems exploiting concurrency for the combined model. The language, implemented in Prolog, is considered to be precise, flexible, readable and easy to use. A case study is given for comparison with the previous language G