JPS6459542A - Single chip microcomputer - Google Patents

Abstract

PURPOSE:To detect the runaway of a program at the time of fetching data other than an instruction code as the instruction code from a ROM to an instruction register due to a cause such as a noise, etc., and to perform an appropriate processing, by attaching a redundant bit on an incorporated ROM, and attaching a simple gate circuit on an execution control part. CONSTITUTION:On the 8th bit of each word in the incorporated ROM, '0' when data stored in 7th-10th bits are the instruction codes and '1' when other than that are stored in advance. When the 8th bit of an instruction register 4 goes to 101='1' in spite of an instruction fetch signal 9='1', the output of a two-input AND gate 102 goes to '1', and a runaway detecting signal 103 is outputted to an interruption control circuit 13. In other words, when the redundant bit is '1' at the time of fetching the instruction code in the instruction register 4, it is assumed as the runaway of the program, and the runaway detecting signal is outputted. Thus, it is possible to detect the runaway of the program and to make a CPU perform a post processing.