(Alpha and I64 only.) The Get Previous Invocation Context routine
    gets the previous invocation context of any active procedure.

    A thread can obtain the invocation context of the procedure
    context preceding any other procedure context using the following
    function format.

    Format

      LIB$GET_PREV_INVO_CONTEXT   invo_context

1  –  Returns

    OpenVMS usage:longword_unsigned
    type:         longword (unsigned)
    access:       write only
    mechanism:    by value

2  –  Argument

 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.