This routine performs a SCSI Test Unit Ready command, or
    equivalent if some other I/O architecture is supported. It is
    used by the mrd_startup(3mrd) and the OpenVMS implementation
    of mrd_ready(3mrd). Since it accepts a robot_info_t structure
    associated with an open medium changer it can used to perform
    Test Unit Ready command without having to re-open the medium
    changer each time.

    The robot_info_t is the address of a robot_info_t that has been
    opened by mrd_startup(3mrd). If the medium changer isn't opened,
    the Test Unit Ready Command will fail with the operating system
    error for trying to use an unopened device.

    The dev_status_t structure includes the code, os_status, and SCSI
    error fields. The following describes how to decode errors with
    the dev_status_t structure.

    SCSI Errors

    SCSI errors are indicated when the value of the valid field of
    the SCSI error is not equal to 0. The key, asc, and ascq fields
    provide additional information to help determine the cause of the
    error.

    The code usually maps the Additional Sense Code and Additional
    Sense Code Qualifier (ASC/ASCQ) values to an MRD error. The asc
    and ascq values are copied from the request sense data returned
    by the target.

    The Additional Sense Code (asc) indicates further information
    related to the error or exception condition reported in the sense
    key field. The Additional Sense Code Qualifier (ascq) indicates
    detailed information related to the additional sense code. For
    more information, consult the SCSI-2 Specification.

    Operating System Errors

    Operating system errors are indicated when the value of the valid
    field of the SCSI error is equal to 0 and the value of the os_
    status field is not equal to 0. This result is most likely caused
    by an operating system error, and probably has a mapped error in
    MRD.

    MRD Errors

    MRD errors are indicated when the value of the os_status field is
    0, and the value of the valid field of the SCSI error is 0. This
    result is most likely caused when MRD encounters its own failure.