Learning just a little bit of Fortran-90 can help you a lot.
	------------------------------------------------------------

New Free Format
****************************

*.f files abide by the old f77 punch-card format (coding in columns 7-72).

*.f90 files use the new free-format.  Commands begin in any column.
Comments start with "!" only (c in column 1 no longer works).  "&" at the
end of of a line is used to indicate continuation to the next line (and an
optional "&" can be used at the beginning of the continued line).

To convert *.f format files to a format which can be read either as the old
fixed format or the new format, do the following steps:

1.  Convert "c" or "C" in column 1 to a "!", using:

	sed -e "s/^c/!/" -e "s/^C/!/" sub.f > sub.f90

2.  Search for tabs and convert them all to blanks.

3.  Convert old continuation characters in column 6 to "&".

4.  Put a "&" in column 73 (or the end, which ever is greater) of lines
    which are to be continued.

5.  Either make sure that no line exceed 72 columns, or just before using
fixed-format, used sed to strip off any "&" at the end of a line.

There are utilities here (convert.f90, and ftof90), but you might need to
be careful using them...



MODULES
*****************************

Can replace all common blocks with "Module" statements.  (Unless there is a
reason for variables to be stored in a particular order, the actual
"common" statements can be deleted, leaving just their variable
specifications):


MODULE MODELS
   COMPLEX           :: GTX(100, 6)
   REAL              :: X(100)
   COMMON   /BLOCK1/ GTX, X   ! This statement is usually not needed.

   REAL, ALLOCATABLE :: Y(:), Z(:, :)
   INTEGER              CRX, GT, MR2
END MODULE


Then a subroutine could access the data in module MODELS with the
statements:

USE MODELS	! access to all variables in the module
USE MODELS, ONLY: X, Y   ! access only to X and Y

USE MODELS, T => Z   ! T is the local name which points to Z
		     ! All other variables still have their original name



NAMELISTS
*****************************

Namelists are now standard with Fortran-90, including "!" delimited
comments in Fortran-95.  But the format of previous namelists may be
incompatible.  The standard form for a Fortran-95 namelist is:

&listname
 x(3)= 1, 2, 3
/

**********************************

Synonyms for relational operators:

f77	f90

.lt.	<
.le.	<=
.eq.	==
.gt.	>
.ge.	>=
.ne.	/=



Makefiles and f90
=================

With GNU make, add the following pattern rule to the end of a make file:

%.o : %.f90
        $(FC) $(FFLAGS) -c $<

With the other make, use the old style suffix rules:

On Suns
-------
.SUFFIXES:
.SUFFIXES: .f90 $(SUFFIXES)
.f90.o:
        f90 $(F90flags) $<

On SGIs
-------
.SUFFIXES:
.SUFFIXES: .f90 .o
.f90.o:
        f90 $(F90flags) $<