VMS Help RTL Routines, LIB$, LIB$GET_PREV_INVO_CONTEXT, Argument  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 invo_context

    OpenVMS usage:invo_context_blk
    type:         structure
    access:       modify
    mechanism:    by reference

    Address of an invocation context block. The given context block
    is updated to represent the context of the previous (calling)
    frame.

    For the purposes of this function, the minimum fields of an
    invocation block that must be defined are those IREG and FREG
    fields corresponding to registers used by a context whether the
    registers are preserved or not. Note that the invocation context
    blocks written by the routines specified in these sections define
    all possible fields in a context block. Such context blocks
    satisfy this minimum requirement.