A number of factors can affect real-time performance in a system. One of the most common factors, yet most difficult to characterize, is the effect of device drivers and interrupts on system timings. Different device drivers will have differing requirements as to how long interrupts are suppressed, for example. The eCos system has been designed with this in mind by separating the management of interrupts (ISR handlers) and the processing required by the interrupt (DSR handlers). However, since there is so much variability here, and indeed most device drivers will come from the end users themselves, these effects cannot be reliably measured. We have attempted to measure the overhead of the single interrupt that eCos relies on, the real-time clock timer. This should give the user a reasonable idea of the cost of executing interrupt handling for devices.