...'t satisfy Fortran 77 unless the compiler can be sure that the loop will execute at least once. (Easy with constants.) It was common for Fortran 66 compilers, though. (Some believe that the S/360 BXLE instruction was invented for this case.) Note that there is also: 3) Use some variable or register as the loop index. Assign to the DO variable at the appropriate time. In case (3), ...