A method operates a system with a program-controlled unit. The program-controlled unit reads and executes data that are stored in a memory device and that represents instructions. According to the method, a check is made during the reading of data from the memory device to determine whether the relevant data are error-free. When it is ascertained that the relevant data are not error-free, the execution of an interrupt service routine is initiated. The method is distinguished by the fact that, when it is ascertained, during the execution of the interrupt service routine, that the data that are to be read from the memory device for this purpose is not error-free, the execution of the interrupt service routine is interrupted or ended, and the execution of an interrupt service routine stored at a different location is initiated. The memory device described has an error detection device that outputs an interrupt request signal if the stored data are erroneous, and that outputs a different interrupt request signal if it again discovers an error during the execution of the interrupt service routine.