Abstract: An important issue in building operating systems for hard real-time applications is to compute the worst-case execution times (WCET) of the operating system activities. Traditionally, this has been achieved by an exhaustive testing of the operating system, with a careful attention on the testing conditions to reproduce the worst-case execution scenario. In this paper, we explore the alternative approach of using static analysis to predict off-line the WCET of the system calls of a real-time kernel, the RTEMS kernel [11]. We give qualitative and quantitative results on the analysis of RTEMS, and draw some conclusions on the extent to which static analysis can be used on operating system code.