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File: HLP:VAX5.HLP                                Written:  8-Jul-82/SLP
Length: 4 pages                                   Updated: 13-Jul-82/SLP

   How to Convert DEC-10 Fortran Programs to VAX-11 Fortran Programs

Modifying your DEC-10 Fortran programs to run on the VAX computer should
be very easy. In fact, it's quite possible  that  many  of  your  DEC-10
Fortran programs will run on the VAX without any modifications at all.

The  purpose  of this help file is to alert you to potential differences
in the Fortrans on the  two  systems.  .  .and  what  to  do  about  the
differences you encounter in your programs.


The  first  thing to do is to transfer your Fortran program source files
to the VAX. You can do this with the TVAX command procedure.  Type  HELP
TVAX or HELP VAX3 for details.

The  quick  and  easy  way to convert your Fortran programs is to simply
recompile them  on  the  VAX.  Then  make  any  necessary  modifications
indicated  by  the compiler's diagnostic messages. Feel free to call the
DEC-10/VAX consultant (378-6554) to explain any diagnostic messages  you
don't understand.

There  is  a  "Portable  FORTran"  validation  program on the VAX called
PFORT. It can flag all the statements in your Fortran source  file  that
do not conform to "standard Fortran." Since there are many extensions to
standard Fortran allowed in both the DEC-10 and VAX-11  Fortrans,  PFORT
will probably flag a lot of code that's really okay.

Now for the major differences between DEC-10 and VAX-11 Fortran.

                        HARDWARE CONSIDERATIONS

A subtle, but important difference is imposed by the hardware of the two
computers. A "word" on the DEC-10 is thirty-six bits long; on the VAX, a
longword is thirty-two bits long. This affects (1) the precision (number
of digits of accuracy) of numerical values and (2) the number  of  ASCII
characters that can be stored in a word.

                        DEC-10                  VAX-11

INTEGER precision       11 digits               10 digits
  neg. limit            -34,359,738,368         -2,147,483,648
  pos. limit            +34,359,738,367         +2,147,483,647

REAL precision          7 to 9 digits           6 to 8 digits
  range                 0.14E-38 to 3.4E+38     0.29E-38 to 1.7E+38
(double precision)      15 to 17 digits         15 to 16 digits

ASCII chars/word        5                       4
How to Convert to VAX-11 Fortran                                  Page 2

Generally,  you  can see that the DEC-10 carries about one more digit of
precision for INTEGERs and REALs and can store one  more  character  per
word. On the VAX, you may need to use DOUBLE PRECISION real variables in
some instances where INTEGER and single precision REAL  were  sufficient
on  the DEC-10.(For example, 5 billion is too big to store as an INTEGER
on the VAX.)

Programs dependent on storing "five characters per word" will need to be
reworked to run on the VAX. The  best  solution  is  to  redeclare  such
variables  as  "CHARACTER*5".  (CHARACTER  is a new data type defined in
Fortran-77; see the VAX-11 Fortran Manuals for details.)

On  the nicer side, the VAX has what is called G_floating and H_floating
REAL data types. They provide for up to 33 digits  of  precision  and  a
magnitude   range  as  great  as  +/-0.84E-4932  through  +/-0.59E+4932!
CHARACTER type variables can hold up to 32,767 characters!

                        SOFTWARE CONSIDERATIONS

VAX-11  Fortran  is  based  on  American  National  Standard FORTRAN-77,
X3.9-1978. As a result, it contains certain incompatibilities with  DEC-
10  Fortran,  which  is  based  on the previous standard, X3.9-1966. The
areas affected are:

   o Minimum  number  of  DO  loop  iterations. "Zero-trip" DO loops are
     possible in Fortran-77. For example, "DO 20 I=1,0" will execute the
     loop once under F-66, but skip the loop under F-77.

   o EXTERNAL  statements.  In certain cases, F-77 requires functions to
     be declared "INTRINSIC" instead of "EXTERNAL."

   o The  OPEN  statement  keyword  BLANK. In F-77, if a logical unit is
     opened with an OPEN statement, BLANK defaults to BLANK=NULL (blanks
     in an input numeric field are ignored). If a logical unit is opened
     without an explicit OPEN statement, BLANK  defaults  to  BLANK=ZERO
     (embedded  blanks  are  treated  as zeroes). F-66 does not have the
     BLANK keyword, but it behaves as though BLANK=ZERO.

   o T  and X format descriptors. In F-77, T and X do not actually write
     blanks, they merely position the record pointer.  Only  the  actual
     data  sets  the record size, so that (A5,100X) writes a 5-character
     record whereas (A5,100X,' ') writes a 106  character  record.  F-66
     writes a 106 character record in both cases.

   o Random access files. In F-77, random-access files have no end. Thus
     the END= branch will never be taken.

   o Random  access  files.  In  F-77,  "#" is not allowed as the record
     number specifier in random-access  data  transfer  statements.  You
     must use a "'" or REC=.
How to Convert to VAX-11 Fortran                                  Page 3

The   VAX-11   Fortran   compiler   selects   the   Fortran-77  language
interpretations by default. You can direct the compiler to use  some  of
theFortran-66   interpretations   listed   above   (DO  loops,  EXTERNAL
statements, and OPEN statement keyword BLANK) by specifying the  FORTRAN
command  qualifier  /NOF77.  (See Appendix C of the VAX-11 Fortran Users
Guide for details.)

There  are  a  few  other differences between the two Fortrans, although
they are not due to differences between Fortran-66 and -77.

   o Logical   two-branch   IF  statement.  This  is  a  DEC-10  Fortran
     extension. It is not available in VAX-11 Fortran at all.

                     NEW FEATURES OF VAX-11 FORTRAN

There  are  MANY new features in Fortran-77. In general, Fortran-77 is a
superset of Fortran-66. You don't need to  rewrite  your  existing  (and
working!)  Fortran-66  programs  just  because  you  use  the Fortran-77
compiler. But it's a good idea to find out what the new capabilities  of
Fortran-77  are  and  use  them  in  any new VAX-11 Fortran programs you
write. Check the VAX Fortran Manuals for details. (You could  also  look
through the ANSI FORTRAN-77 X3.9-1978 standard.)

Here's a sampling:

   o IF-THEN-ELSE and DO WHILE for structured programming.

   o CHARACTER data type and string manipulation functions.

   o Expressions are allowed in an output list.

   o New  format  descriptors: BN, BZ, SP, SS, S, Iw.d, Z, TL, TR, Q, :,
     and variable format expressions (such as "FORMAT(I<J+1>)").


   o INQUIRE statement. Find out about specified properties of a file.

   o I/O statmenta can take keywords in their control information lists:
     READ (UNIT=1, FMT=100), TABLE.

   o SAVE  statement.  Retains  values of local variables after a return
     from a subprogram.

   o IMPLICIT NONE. Requires all variables to be declared.
How to Convert to VAX-11 Fortran                                  Page 4

                          FOR MORE INFORMATION

If you need any help converting your Fortran programs to the VAX, please

      Stan Peters       424A Clyde Building     378-6554
or    Matt Hawkes       424  Clyde Building     378-4941

[End of HLP:VAX5.HLP]