1. In a data processor having an execution unit controlled by microprograms, one of said microprograms being read from control storage means and applied to said execution unit for execution thereby in response to a corresponding instruction read from a storage unit, the improvement comprising: information storage means for storing a plurality of words correlating said instructions to respective microprograms, at least some of said words containing a plurality of instruction codes indicative of respective equivalent functions in a plurality of different system architectures, said words being stored at locations in said information storage means addressed by operation codes of said instructions; mode information storage means for storing system administrative information for designating one of said system architectures; selector means for selecting and supplying to said control storage means one of said instruction codes from each word read from said information storage means in response to the system administration information from said mode information storage means.