invo_context
    OpenVMS usage:invo_context_blk
    type:         structure
    access:       modify
    mechanism:    by reference
    Address of a valid invocation context block.
 index
    OpenVMS usage:index
    type:         longword
    access:       read
    mechanism:    by value
    Index into the IREG array of the invocation context block.
 gr_copy
    OpenVMS usage:integer value
    type:         quadword
    access:       write
    mechanism:    by value
    Address of a quadword that contains the value to be written to
    the invocation context block.