A method of operating a computer system is disclosed that facilitates the creation and testing of application system software by utilizing an autonomous environment to emulate the application system environment. The emulation environment allows the user to call into view sequences of standard input-output (I/O) screen format pairs normally used to submit information to and receive information from the application system. In the emulation mode, these screens are prepared off-line and stored until the user desires to exercise the application system. Each input format is filled with information that will serve as actual input to the application system when it is exercised. Each output format is filled with information that comprises expected results when the application system is actually exercised. The expected results are compared to actual results after execution of each I/O pair and further application system processing is controlled by the comparison results. The stand-alone emulation environment provides editing and control routines and library functions to aid the user in defining and modifying the I/O sequences.