file   : newdrvint.text
    date   : 15-March-1983
    author : Keith Ball


    The problem :

    Make the spool option built into the system and transparent to
    applications.  This allows programs to send data to the printer
    and have the system send it to a pipe to be remotely printed.
    Currently, a driver cannot be built to perform spool because no
    open and close calls are passed through to the driver. Therefore,
    the driver cannot open and close the pipe.

    Ideally, the solution should be a mechanism which allows any other
    unblocked device to be emulated transparently by a blocked device.

    A solution :

    Add a unitopen and unitclose call to the driver interface.

    When the file system does an open it calls the drivers unitopen
    procedure.  It passes the file name without the volume name to the
    driver as the only parameter.  For the spool driver, the file name
    is the pipe name to open.  When the file system does a close it
    calls the driver unitclose procedure.  This allows the spool
    driver to close the pipe.  Pipe writes occur when the unitwrite
    call is made.

    This mechanism allows a driver to manage a resource which needs
    preparation before and after each use.

    Unfortunately, this solution does not allow multiple pipes to be
    open and managed by the driver.  Since no space is available in
    the FIB to save an integer returned by the driver and no parameter
    in the other unit commands is available for sending back that
    integer the driver cannot distinguish between unit operations to
    different resources, i.e. multiple pipes.

    Sytem changes :

    1) unitopen call in file system

    2) unitclose call in file system

    3) unitopen and unitclose added to OS unit interface code

    4) unitopen and unitclose entry in the system vector table

    5) Addition of unitopen and unitclose to the driver configuration
       byte in the driver header

    6) Addition of unitopen and unitclose handling in all drivers.
       These for all current drivers are stubs with an RTS.

    7) Addition of unitopen and unitclose to compilers.  This is not
       necessary.