Open Watcom FORTRAN 77 Language Reference

Language Reference
Example:
* From the quadratic equation
*
* 2
* ax + bx + c = 0
*
* we derive the following two equations:
* ____________
* + / 2
* -b - \/ b - 4ac
* x = ---------------------
* 2a
*
* and express these equations in FORTRAN as:
X1 = ( -B + SQRT( B**2 - 4 * A * C ) )
$ / ( 2 * A )
X2 = ( -B - SQRT( B**2 - 4 * A * C ) )
$ / ( 2 * A )
1.4 Order of FORTRAN Statements and Lines
The first statement of a program unit may be a PROGRAM, FUNCTION, SUBROUTINE, or BLOCK
DATA statement. The PROGRAM statement identifies the start of a main program and there may only be one
of these in an executable FORTRAN program. Execution of a FORTRAN program begins with the first
executable statement in the main program. The other statements identify the start of a subprogram. If the
first statement of a program unit is not one of the above then the program unit is considered to be a main
program.
Although you may not be familiar with all of the terms used here, it is important that you understand that
FORTRAN 77 has specific rules regarding the ordering of FORTRAN statements. You may wish to refer
to this section at later times. In general, the following rules apply to the order of statements and comment
lines within a program unit:
1. Comment lines and INCLUDE statements may appear anywhere.
2. FORMAT statements may appear anywhere in a subprogram.
3. All specification statements must precede all DATA statements, statement function statements,
and executable statements.
4. All statement function statements must precede all executable statements.
5. DATA statements may appear anywhere after the specification statements.
6. ENTRY statements may appear anywhere except between a block IF statement and its
corresponding END IF statement, or between a DO statement and its corresponding terminal
statement. Open Watcom FORTRAN 77 extends these rules to apply to all program structure
blocks resulting from the use of statements introduced to the language by Open Watcom
FORTRAN 77 (e.g., WHILE, LOOP, SELECT).
7. IMPLICIT statements must precede all other specification statements, except PARAMETER
statements. A specification statement that defines the type of a symbolic constant must appear
6 Order of FORTRAN Statements and Lines