PURPOSE: To prevent the malfunction for the microcomputer system by giving monitor via the external circuit to the return address register after the branch process, the flag stack and stack pointer each.
CONSTITUTION: In case microprocessor 1 containing the RAM and ROM performs the writing of the data to the stack and the reading of the data from the stack, address latch 7 latches address signals A 0 WA 15 via the signal sent from gate 2 or 3. Comparator 8, to which the highest- and lowest-rank addresses of the memory allotted as the stack, compares the value of these addresses with the latched address and then gives the control to reset or not processor 1 based on the result of the comparison. Processor 1 has the same operation even though the process of processor 1 is carried out through the writing of the external memory. In such constitution, such malfunction as to destroy the contents of the memories excepting the stack can be prevented owing to the data to be stored in the stuck.