This invention relates to the art of computer system emulation and, more particularly, to a computer system emulator in which the functions normally performed by the hardware in a legacy central processor unit are emulated by a software program. The invention is to enhance the emulated instruction set beyond that of the legacy machine such to include as new single instructions a method for invoking operating system functions, with the machine coding of the operating system functions now being performed by machine code native to the new host machine, rather than as a sequence of emulated legacy instructions.