A hardware monitor circuit includes an electronic control circuit coupled to a processing unit. The electronic control circuit generates multi-bit protection codes and directs operations of the hardware monitor circuit. A bus interface is coupled to an address bus of the processing unit, and the bus interface passes signals associated with a stack structure of the processing unit. The stack structure is arranged to store the multi-bit protection codes in an internal memory coupled to the processing unit. Comparators in the hardware monitor circuit are arranged to accept values from the internal memory and gating logic coupled to the comparators is arranged to generate an error signal when it detects that an address on the address bus read via the bus interface is equal to an address stored in the internal memory. Upon generating the error signal, the processing unit is placed in a secure mode.