A system for checking a program memory) of a processing unit includes a check module, and the processing unit is made up of an instruction counter connected to the check module. The check module has a register connected to a first changeover switch that sets the register content. In a system that allows for the instruction addresses of the entire program memory to be checked, the instruction counter contains an ancillary counter, which runs through the instruction address space of the program memory independently of the program code during normal operation and which is connected to the register.