The symbiont uses the SMB$SEND_TO_JOBCTL routine to send messages
    to the job controller.

    Most messages the symbiont sends to the job controller are
    responses to requests made by the job controller. These responses
    inform the job controller that the request has been completed,
    either successfully or with an error. When the symbiont sends
    the message, it usually indicates that the request has been
    completed.

    In such messages, the request argument corresponds to the
    function code of the request that has been completed. Thus, if
    the job controller sends a request using the SMBMSG$K_START_
    TASK code, the symbiont responds by sending a SMB$SEND_TO_JOBCTL
    message using SMBMSG$K_START_TASK as the request argument.

    The responses to some requests use additional arguments to send
    more information in addition to the request code. The following
    table shows which additional arguments are allowed in response to
    each different request:

    Request                Arguments

    SMBMSG$K_START_STREAM  request
                           device_status
                           error
    SMBMSG$K_STOP_STREAM   request
    SMBMSG$K_RESET_STREAM  request
    SMBMSG$K_START_TASK    request
    SMBMSG$K_PAUSE_TASK    request
    SMBMSG$K_RESUME_TASK   request
    SMBMSG$K_STOP_TASK     request
                           error (See footnote.)

    Footnote: This is usually the value specified in the SMBMSG$K_
    STOP_CONDITION item that was sent by the job controller with the
    SMBMSG$K_STOP_TASK request.)

    In addition to responding to requests from the job controller,
    the symbiont can send other messages to the job controller. If
    the symbiont sends a message that is not a response to a request,
    it uses either the SMBMSG$K_TASK_COMPLETE or SMBMSG$K_TASK_STATUS
    code. Following are the additional arguments that you can use
    with the messages identified by these codes:

    Code                   Arguments

    SMBMSG$K_TASK_         request
    COMPLETE
                           accounting
                           error
    SMBMSG$K_TASK_STATUS   request
                           checkpoint
                           device_status

    The symbiont uses the SMBMSG$K_TASK_STATUS message to update the
    job controller on the status of a task during the processing
    of that task. The checkpoint information passed to the job
    controller with this message permits the job controller to
    restart an interrupted task from an appropriate point. The
    device-status information permits the symbiont to report changes
    in device's status (device stalled, for example).

    The symbiont can use the SMBMSG$K_TASK_STATUS message to request
    that the job controller send a stop-stream request. It does this
    by setting the stop-stream bit in the device-status argument.

    The symbiont can also use the SMBMSG$K_TASK_STATUS message
    to notify the job controller that the symbiont has paused in
    processing a task. It does so by setting the pause-task bit in
    the device-status argument.

    The symbiont uses the SMBMSG$K_TASK_COMPLETE message to signal
    the completion of a task. Note that, when the symbiont receives a
    START_TASK request, it responds by sending a SMB$SEND_TO_JOBCTL
    message with SMBSMG$K_START_TASK as the request argument. This
    response means that the symbiont has started the task; it does
    not mean the task has been completed. When the symbiont has
    completed a task, it sends a SMB$SEND_TO_JOBCTL message with
    SMBMSG$K_TASK_COMPLETE as the request argument.

    Optionally, the symbiont can specify accounting information when
    sending a task-completion message. The accounting statistics
    accumulate to give a total for the job when the job is completed.

    Also, if the symbiont is aborting the task because of a symbiont-
    detected error, you can specify up to three condition values in
    the error argument. Aborting a task causes the remainder of the
    job to be aborted.