Abstract Cache memories are widely used to improve computer performance, but their inherent unpredictability presents new problems when cached systems must be analysed. In preemptive, multitask real-time systems, the cache memories have been analysed from two complementary points of view. First, calculating the Worst Case Execution Time (WCET) of each task considering no preemptions. Second, making the schedulability analysis considering the effect of cache when tasks are preempted. Both aspects of the same problem (performance variation due to cache) have been historically treated independently. This paper presents a new approach to deal with both problems altogether when a direct mapped cache is used for instructions. Solving both problems jointly provides greatter accuracy in the schedulability analysis.