This paper analyzes and compares the effectiveness of various system level design methodologies in assessing performance of embedded computing systems from the earliest stages of the design flow. The different methodologies are illustrated and evaluated by applying them to the design of an aircraft pressurization system (APS). The APS is mapped on a heterogeneous hardware/software platform consisting of two ASICs and a microcontroller. The results demonstrate the high impact of computer aided design (CAD) tools on design time and quality.