g77 treats an exclamation point (`!') in column 6 of a fixed-form source file as a continuation character rather than as the beginning of a comment (as it does in any other column) when the -fvxt option is specified.
The following program, when run, prints a message indicating whether it is interpreted according to GNU Fortran (and Fortran 90) rules or VXT Fortran rules:
C234567 (This line begins in column 1.) I = 0 !1 IF (I.EQ.0) PRINT *, ' I am a VXT Fortran program' IF (I.EQ.1) PRINT *, ' I am a Fortran 90 program' IF (I.LT.0 .OR. I.GT.1) PRINT *, ' I am a HAL 9000 computer' END
(In the GNU Fortran and Fortran 90 languages, exclamation point is a valid character and, unlike space (<SPC>) or zero (`0'), marks a line as a continuation line when it appears in column 6.)