This paper describes a high-level system dynamics model of a real-world software evolution process. This process is evolving the kernel (low-level routines) of VME, the operating system of a long-lived and successful series of ICL mainframe computers. The model has been developed as part of the continuing FEAST project, which is investigating the role and impact of feedback in the global software process. The modelling approach is top-down, which has resulted in a simple model reflecting elements of the global software process at a high level of abstraction. The actual behaviour of the software evolution process being studied, in terms of enhancements completed and modules added to the kernel over a period of 13 years, is simulated by model outputs. The work reported here, together with the previous construction and calibration of an equivalent model for a different type of software product being evolved in a different context, provides support for the FEAST hypothesis, which states that software evolution processes are feedback systems. The models also suggest that feedback pressures from outside technical software processes have an important influence on the dynamics of the overall software processes, and on resultant evolutionary product and process trends.