*** TEST 21 ***
-----  TESTING THE NULL DRIVER CHECKS  -----
TA1 - io_initialize - NULL DRIVER SUCCESSFUL
TA1 - io_open       - NULL DRIVER SUCCESSFUL
TA1 - io_close      - NULL DRIVER SUCCESSFUL
TA1 - io_read       - NULL DRIVER SUCCESSFUL
TA1 - io_write      - NULL DRIVER SUCCESSFUL
TA1 - io_control    - NULL DRIVER SUCCESSFUL
-----  TESTING THE I/O MANAGER DIRECTIVES  -----
TA1 - io_initialize - STUB DRIVER SUCCESSFUL
TA1 - io_open       - STUB DRIVER SUCCESSFUL
TA1 - io_close      - STUB DRIVER SUCCESSFUL
TA1 - io_read       - STUB DRIVER SUCCESSFUL
TA1 - io_write      - STUB DRIVER SUCCESSFUL
TA1 - io_control    - STUB DRIVER SUCCESSFUL
-----  RETURNING INVALID MAJOR NUMBER -----
TA1 - io_initialize - INVALID_NUMBER
TA1 - io_open       - INVALID_NUMBER
*** END OF TEST 21 ***