How does Fortran 90 relate to FORTRAN '77?

With a few minor exceptions, Fortran 90 is a superset of X3.9-1978 FORTRAN. But this does not mean that all "77" codes will port sans changes. Many (if not most) programmers employed constructs beyond the '77 standard, or rely on unspecified behavior (say, assuming that an OPEN of an existing file will position the file pointer to just past the last record already written) which has changed (that is to say, has become specified).