Your symbiont calls SMB$READ_MESSAGE to read a message that the
    job controller has sent to the symbiont.

    Each message from the job controller consists of a code
    identifying the function the symbiont is to perform and a number
    of message items. There are seven codes. Message items are pieces
    of information that the symbiont needs to carry out the requested
    function.

    For example, when you enter the DCL command PRINT, the job
    controller sends a message containing a START_TASK code and
    a message item containing the specification of the file to be
    printed.

    SMB$READ_MESSAGE writes the code into a longword (specified by
    the request argument) and writes the accompanying message items,
    if any, into a buffer (specified by the buffer argument).

    See the description of the SMB$READ_MESSAGE_ITEM routine for
    information about processing the individual message items.