Abstract: A system for controlling an activity (program) switch on a scientific processor having an external executive control program. It includes the sequencing of the hardware at a macro level rather than at the more detailed lower levels previously used. This sequencing and control is accomplished by providing a macro code control, a macro code store and an instruction buffer write data selector interconnected with the existing macro logic and the main storage of the scientific processor to provide a system for starting, running and stopping the scientific processor by an activity switch which responds to interrupt signals to selectively shift from the receipt of data from the main storage to the receipt of data from the macro code store.