context

    OpenVMS usage:context
    type:         longword (unsigned)
    access:       write only
    mechanism:    by reference
    Value identifying the data stream that DCX$EXPAND_INIT
    initializes. The context argument is the address of a longword
    containing this value. After DCX$EXPAND_INIT initializes this
    context value, you should not modify it. You can define multiple
    context arguments to identify multiple data streams that are
    processed simultaneously.

 map

    OpenVMS usage:address
    type:         longword (unsigned)
    access:       read only
    mechanism:    by reference
    Compression/expansion function (created by DCX$MAKE_MAP). The map
    argument is the address of the compression/expansion function's
    virtual address.

    The map argument must remain at this address until data expansion
    is completed and context is deleted by means of a call to
    DCX$EXPAND_DONE.