Single chip microcomputer selectively operable in response to instructions stored on the computer chip or in response to instructions stored external to the chip

Abstract From the Canadian patent

A microprocessor external instruction feature which provides for a single chip microprocessor with on-chip read only instruction store (ROS) that can also be operated with an off-chip instruction store. To accomplish this the microprocessor instruction sequencing logic (instruction store, instruction register, instruction counter and sequencing logic) is duplicated off-chip. An external instruction mode input pin signal causes the microprocessor to take its instructions from the external instruction store via 12 external instruction (XI) input pins instead of from the on-chip ROS. A branch decision output pin signal from the microprocessor, which indicates that the branch conditions have …