Parameter Info Block (PIB), all offsets in pointer-length words
   Name          Offset  Meaning
   buffer_size   0       Size of the destination buffer
   
      The total number of bytes in all of the segments that require a
      buffer to be created in the destination address space.  This is
      specified so that the kernel can allocate one large buffer for all
      segments before traversing the segment list.  When returning from a
      method, the buffer size only includes buffers allocated by the
      caller; "inout" segments where the caller specified a non-NULL ptr,
      and the callee did not increase the length, are not included
      (because the kernel does not need to allocate a caller-side buffer
      for them).  The kernel may throw an exception if the actual
      size is greater than specified in this field.

   objlist_ptr   1       Pointer to the object list
   objlist_len   2       Length of the object list

      The object list is a special segment that contains object IDs
      rather than arbitrary data.  Each object ID will be translated
      into the destination ID-space, allocating new IDs when
      necessary.

   num_segments  3       Number of data segments
   segment.ptr   4+n*4   Pointer to data segment
   segment.len   5+n*4   Length of data segment in bytes
   segment.flags 6+n*4   Attributes of data segment
   reserved      7+n*4   Reserved for future use, and to
                         ensure power-of-two indexing
   
      Each segment describes data being transmitted to and/or from the
      callee.  For out segments, the caller may designate a buffer to hold
      the data, or it may leave the ptr field NULL.  The caller may
      replace an out segment pointer with its own (it must do this if it
      was NULL), and it may change the length of the segment.  Except when
      flags such as Push or Shared force the kernel to map, rather than
      copy, the data, it will choose which method to use based on the
      size, page-alignment, and overmap status of the segment.

Segment Flags (see doc/orb/memory-management for more details):
   In            0x01    Data is copied/mapped from caller to callee.
   Out           0x02    Data is copied/mapped from callee to caller.
   Shared        0x04    A permanent shared mapping is created.
   Push          0x08    The region is unmapped from the source and
                         transferred to the destination.
   Inline        0x10    The callee cannot change the length of an
                         Out segment.  Ignored for In segments.
   Immutable     0x20    The segment is to be mapped read-only in
                         the destination.  Ignored unless Shared is
                         set.
   Copy        0x8000    The segment is permanently copied into the
                         destination address space, with read/write
                         access.