1. In a computer of the type with read-write working memory and a microprogrammed control unit provided with a read-only microprogramming memory containing microprograms for executing so-called "allowed" machine instructions, a system for extending interior decor capability which comprises: transcoding means coupled to said working memory for receiving operating codes of machine instructions stored in said working memory and for providing an output signal of a first level when the received operating code is one of said "allowed" machine instructions, and for providing an output signal of a second level when the received operating code is not one of said "allowed" machine instructions; memory means in said working memory for storing therein so-called "additional" microprograms for executing a set of so-called "additional" machine instructions; control means coupled to said transcoding means and responsive to said output signal of a first level for initiating fetching and execution of said "allowed" machine instructions in said microprogramming memory and responsive to said output signal of a second level for accessing said working memory to determine if there is an "additional" microprogram in said working memory corresponding to said received operation code; and circuit means coupled to said memory means and responsive to said control means for initiating fetching and execution of said "additional" microprogram.