Several processes per address space?

[Doug Gwyn]

I am not sure what you mean by processes sharing an address space.
After a fork(2) call on any UNIX system, there are two processes
that in some sense share an address space (the instruction space
may really be shared, under some circumstances).  On BSD systems,
a vfork(2) will actually share data space.  On UNIX System V, there
is "shared memory" support, which is supposed to be in BSD some day.
One guesses that you are really interested in "shared memory".