Any data exchanged between the processor and main memory uses the memory bus, sharing it with data exchanged between I/O devices and main memory. If the processor and a device try to transfer data at the same time, an impact can be seen on the processor as well as on the device. As a result, the execution time of an application on the processor may increase due to the memory-bus load generated by I/O devices. In real-time environments, this impact can result in missed deadlines and a behavior that is different to that intended by the designer of the system. This paper gives a method for describing and quantifying the impact of such load on applications executed by the processor.