Question about organization of gfortran source code

[L R <grabmylongswordho at yahoo dot com>]

Hi, I am a high school student working on a project involving fortran.
Since I am focusing more on the optimization side of a compiler, I do
not want to spend ages writing a parser that will work for all of
fortran's weird gotcha's. I am trying to figure out from the sources
where the parser ends and the rest of it begins, but I am having a
little trouble understanding everything without reading 80% of the
code. Could someone please provide some insight into how the sources
are organized. I realize parse.c obviously deals with a lot of the
parsing, but I still am having trouble figuring out the execution flow.
Could someone please tell which sections of the program first deal with
the tokenization of raw strings all the way to when the program is
finally in the form of an expression tree. Thanks a lot!