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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.
RECOMPILE YOUR FORTRAN PROGRAM ON THE VAX
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
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.
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.
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!
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 New OPEN keywords: BLANK, CARRIAGECONTROL, DEFAULTFILE, EXTENDSIZE,
FORM, INITIALSIZE, IOSTAT, KEY, MAXREC, NOSPANBLOCKS, ORGANIZATION,
READONLY, RECL, RECORDTYPE, SHARED, STATUS, TYPE, USEROPEN.
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]