A microprogrammed control structure for an integrated circuit data processor which employs a two-level control store designated as a micro control store and nano control store. An instruction decoder decodes each macro instruction to be executed by the data processor and causes a series of micro word addresses to be input to the micro control store. In response to such input, the micro control store outputs a corresponding number of nano address words for addressing the nano control store. The nano control store when addressed by the nano address words, outputs a control word to an execution unit for executing the macro instruction.