Abstract: A test apparatus and method for fully testing the functional aspects of a computer system at the system level through one or more of its I/O ports. A test system is connected to an externally available I/O port of the computer under test for accessing its system ROM and a jumper is installed on the computer under test to disable its system ROM. The jumper also bypasses the PCMCIA controller, if necessary, to provide the test system direct access to the I/O port and thus the system ROM. The computer under test is booted from diagnostic test code stored in memory of the test system, where the diagnostic test code has complete control and performs a series of tests on the computer under test. Other I/O ports of the computer under test, including its serial ports, parallel port, keyboard and mouse ports, video port and a docking port may also be connected to the test system, if desired.