Abstract: A method and apparatus for permitting an existing computing system to understand and operate upon applications programming, which has been prepared using the instruction set of another computing system. Selected memory units located in the central processing unit of the existing computing system are removed and apparatus containing a circuit is inserted into the electrical connections previously occupied by such memory units. The inventive circuit contains advanced micro-circuits which provide additional memory capability for containing an inventive microprogram which contains the instruction set of the computing system being emulated. The invention, after installation, permits an existing computing system to operate on applications programming written either in its original language or in the language of the machine being emulated.