Apparatus and method whereby a first digital computer system (231) may be controlled by a second digital computer system (233). The two digital computer systems are connected by data transfer apparatus (213, 210). A program (227) in the second digital computer system constructs a call protocol (215) specifying a routine to be called in the first digital computer system. The data transfer apparatus transfers the call protocol to the first digital computer system, and a program (211) in the first digital computer system performs the call. Also disclosed are a return protocol (217) which may be employed to return data from the called routine to the second computer system, call protocols containing arguments, the use of call and return protocols to call system and user routines, and preferred forms of the call and return protocols.