Patent US 4,460,972

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 XI MODE input pin signal causes the microprocessor to take its instructions from the external instruction store via 12 XI input pins instead of from the on-chip ROS. A BR DECISION output pin signal from the microprocessor, which indicates that the branch conditions have been met, causes the external instruction counter to be loaded with a branch address from the external instruction register instead of being stepped by external sequencing logic. A WAIT output pin signal causes the external instruction feature logic to suspend operations while the microprocessor is in its wait state.