how to do overlays in v7???

[drew at genix.UUCP]

After grepping the documentaion on our system this is all I was
able to come up with:

  from ld(1):

     -O   This is an overlay file, only the text segment will be
          replaced by exec(2).  Shared data must have the same
          layout as in the program overlaid.

  from exec(2):

     Exec in all its forms overlays the calling process with the
     named file, then transfers to the entry point of the core
     image of the file.  There can be no return from a successful
     exec; the calling core image is lost.

  from a.out(5):

     #define A_MAGIC4 0405       /* overlay */

Hmm.  That's not much to go on.
What do I have to do to ensure the shared data has the same layout?
How dow I distinguish shared from non-shared dat?
Are that any tricks which will let me load the overlay then continue
executing in shared code.

Some trivial examples sure would help.
-- 
Drew Einhorn
{csu-cs,gatech,lanl-a,convex,pur-ee,ucbvax}!unmvax!genix!drew
P.O. Box 781, Tijeras, NM 87059, USA.
505/281-1122  505/898-9666