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Appendix E. Differences from FORTRAN 66

Appendix E. Differences from FORTRAN 66

FORTRAN 77 is mostly a superset of FORTRAN 66, but not entirely: a number of changes mean a program that conformed to the 1966 standard can behave differently — or stop being valid — under the 1978 standard. The standard itself catalogues these in its Criteria, Conflicts, and Portability appendix (X3.9-1978 Appendix A); this chapter restates that list in plain terms and, because forterp implements both dialects, notes what forterp does with each.

This is the bridge between the two manuals. If you maintain old code, read it alongside the FORTRAN 66 reference manual — the same source can run under both F66 and F77, and what follows is exactly what changes when you switch.

Throughout, forterp notes use: enforced = forterp applies the F77 rule on the F77 dialect; gated = the behavior differs by dialect (or a knob); lenient = forterp still accepts the old form even though strict F77 forbids it (an accept-more extension that cannot break a conforming program).

The headline change: the zero-trip DO

The single most consequential difference. In FORTRAN 66 a DO loop is one-trip — its range always runs at least once. In FORTRAN 77 the iteration count is computed up front as MAX(INT((m2 - m1 + m3) / m3), 0), so a loop whose limit is already passed runs zero times (§11.10.4).

      K = 0
      DO 1 I = 1, 0
      K = K + 1
1     CONTINUE
C     -> F77: K = 0 (loop skipped).  F66: K = 1 (one trip).

forterp: gated. Under F77 the loop is zero-trip; under F66/FORTRAN10 it is one-trip. The zero_trip_do dialect knob flips either default. This genuinely changes results, so always know which dialect you are running.

Conflicts catalogued by the standard

The 1978 standard lists these as known conflicts with 1966 (Appendix A). Grouped by area:

Source form

# Change in FORTRAN 77 forterp
1 An all-blank line (cols 1–72) is a comment; in F66 it could be the initial line of a statement. enforced
2 Columns 1–5 of a continuation line must be blank. enforced
8 A labeled END is disallowed (it could clash with an F66 initial line). enforced

Arrays and storage

# Change in FORTRAN 77 forterp
4 Each subscript must be within its declared bound — A(11,1) is invalid for A(10,5). F66 allowed it if the total offset stayed within the array. lenient by default (out-of-bounds read → 0); bounds_check makes it a trap
5 Only an array declared one-dimensional may use a one-dimensional subscript in EQUIVALENCE. enforced
6 A name may not be given an explicit type more than once. enforced
24 An executable program may contain at most one unnamed BLOCK DATA. enforced (the BDU diagnostic)

Hollerith and text

# Change in FORTRAN 77 forterp
3 Hollerith constants and Hollerith data are removed. F66 allowed nH constants in DATA/CALL, non-character list items with A editing, and non-character arrays as formats. (The H edit descriptor survives — it is not a Hollerith constant.) lenient — forterp still accepts Hollerith under F77; use CHARACTER for new code
14 Reading into an H edit descriptor is prohibited. enforced

Control flow

# Change in FORTRAN 77 forterp
7 The extended range of a DO is gone: you may not transfer into a loop's range from outside. follows F77

Input / output

# Change in FORTRAN 77 forterp
9 No record may be written after an endfile record in a sequential file. follows F77
10 A sequential file may not mix formatted and unformatted records. follows F77
11 Negative unit identifiers are prohibited. follows F77
12 Parentheses around more than one I/O-list item must mark an implied DO (redundant parens are removed). follows F77
13 An entity associated with an input-list item is defined at the same time as the list item (F66 delayed it to end-of-statement). follows F77

Formatted output

# Change in FORTRAN 77 forterp
15 The scale factor for E/D/G output is restricted to reasonable values. follows F77
16 A numeric output field must not be a negative zero (F66 required it for negative internal values). follows F77
17–18 I and F output must not produce unnecessary leading zeros. follows F77
19 A + or - is required before the exponent in E/D output (F66 allowed a blank for +). follows F77

Procedures and intrinsics

# Change in FORTRAN 77 forterp
20 An intrinsic used as an actual argument must appear in an INTRINSIC statement, not EXTERNAL. (The F77 intrinsic class absorbs F66's "basic external functions".) follows F77
21 A conflicting type-statement no longer removes a name from the intrinsic class. follows F77
22 29 new intrinsic names were added and may clash with your subprogram names: ACOS, ANINT, ASIN, CHAR, COSH, ICHAR, INDEX, LEN, LGE/LGT/LLE/LLT, LOG, LOG10, MAX, MIN, NINT, SINH, TAN, and the D… variants. provided
23 The argument/result types and ranges of the intrinsics are now specified (they were not in F66). follows F77

Items that inhibit portability (Appendix A3)

Even within FORTRAN 77, the standard flags things that vary between processors:

  • Collating sequence is not fully specified, so character relational expressions can differ between processors. Use the LGE, LGT, LLE, LLT intrinsics for portable character comparison.
  • Character data, H/apostrophe edit descriptors, and comment lines may contain characters one processor accepts and another rejects.
  • File names, unit numbers, and unit capabilities are processor-dependent.

On forterp these resolve to the target: the NATIVE and PDP10 value models differ in word size and character packing, which is where character comparisons and any Hollerith arithmetic can diverge.

See also