Abstract: Apparatus for carrying out a set of macroinstructions by means of a microprogram in firmware. The apparatus controls the operating system of a digital computer which is adapted to carry out a set of macroinstructions. The macronistructions are carried out by means of a microprogram in firmware having a fetching phase and an execution phase. Both fetching and execution of macroinstructions are carried out by means of microprograms. One ROM is provided for storing a fetching and execution digital-word, associated with a macroinstruction and contains information relative to each phase respectively. Another ROM is provided for storing the actual microprograms for fetching and executing macroinstructions respectively.

Abstract: The system discriminates between "allowed" and "forbidden" machine instructions by interpreting coded digital words found in an auxiliary memory. "Allowed" instructions found in the microprogramming memory are directly executed. "Forbidden" instructions lead to the execution of exception routines, which determine if the instruction is an "additional" instruction which has a microprogram for its execution in working memory. If so, working memory is accessed and the machine instruction is executed.