Data processing system utilizing a unique two-level microcoding technique for forming microinstructions

In a data processing system requiring the generation of microinstructions for performing data processing operations and including:

• a central processor unit having means responsive to macroinstructions for providing decoded macroinstructions for use in generating one or more microinstructions, said central processor unit further including
• first control store means for storing a plurality of first microinstructions, selected fields of which are capable of modification;
• second control store means for storing a plurality of microinstruction portions, a first microinstruction portion identifying selected ones of said plurality of first microinstructions for readout thereof and a second microinstruction portion including two modifier fields of information usable for modifying said selected fields of said plurality of first microinstructions, said second control store means being responsive to said decoded macroinstructions for producing said plurality of microinstruction portions;
• and
• modification means responsive to the selected ones of said plurality of first microinstructions selected by the first microinstruction portion provided by said second control store means and responsive to at least one of said modifier fields for providing output microinstructions one or more selected fields of which are modified by said one or more modifier fields.

In a digital data system, a method for providing microinstructions for controlling data system operations directed by macroinstructions, comprising the steps of:

• storing one or a sequence of more than one microinstruction control words in a microinstruction control means, each one of said sequences related to one of said macroinstructions and each one of said control words having an address field and a modifier field;
• storing said microinstructions in a control store means;
• addressing said microinstruction control means with said macroinstructions to obtain one or a sequence of more than one of said control words;
• addressing said control store means with the address field of said each of said control words to obtain microinstructions corresponding thereto;
• selectively modifying said corresponding microinstructions with the modifier field of its corresponding control word;
• and
• providing said selectively modified microinstructions to said data system to control said data system operations.