how many changes about backend mode from 7.2.2 to 8.4.0?

hi, all
NOTE: fedora 17 x86_64, 7.2.2 can not be compiled in the env, I choose
8.4.0 because I think old version is easier to understand than newer
version!

I want to study pg's external sort (in tuplesort.c )through 8.4.0's source
code, and use ddd to do it, according to usingddd-postgres
<http://www-inst.eecs.berkeley.edu/~cs186/fa04/usingddd.pdf> 's description:

In the backend mode, you don’t create a separate postmaster process and
don’t use psql at all. Instead, you start up postgres from the
command line and directly interact with it.

and after starting postgres in ddd, a backend interactive interface works
as doc's Figure 5, and I found version 7.2.2's source postgres'c has
backend interactive interface, but my version is 8.4.0, which seemly has
not backend interactive interface, so I want to ask

(1) whether I must use another method to debug relative external sort code
(in tuplesort.c ), as some post said that I can execute "psql, select select
pg_backend_pid(), and gdb pid" , any advice will be appreciated!

BEST REGARDS
Dillon Peng