Self-Assessment Quiz: Reading FORTRAN 77

Twenty questions to confirm you can read fixed-form legacy code and map each old construct to its modern replacement. Aim for 16 or more. Answers and a topic map are at the end — try the whole quiz first.

Question 1

In fixed-form source, a statement label goes in columns: - A. 1–5 - B. 6 - C. 7–72 - D. 73–80

Question 2

A nonblank character in column 6 of a fixed-form line means: - A. The line is a comment - B. The line continues the previous statement - C. The line is a statement label - D. The line is ignored

Question 3

Which character in column 1 makes an entire fixed-form line a comment in strict FORTRAN 77? - A. ! - B. # - C. C or * - D. //

Question 4

Under implicit typing, a variable named NSTEP is: - A. REAL - B. INTEGER - C. CHARACTER - D. undeclared and therefore an error

Question 5

A COMMON block associates the variables of different routines: - A. By name - B. By position (storage order) - C. By type only - D. Alphabetically

Question 6

Two routines declare COMMON /B/ X, Y, N and COMMON /B/ X, N, Y. The result is: - A. A compile-time error - B. A link-time error - C. Silent data corruption — N and Y are swapped in memory, unchecked - D. Correct sharing, because the names match

Question 7

The only program unit that may initialize a named COMMON block is: - A. The main program - B. Any subroutine - C. A BLOCK DATA unit - D. A FUNCTION

Question 8

EQUIVALENCE (A, B) asserts that: - A. A equals B in value - B. A and B occupy the same memory (they are the same bytes) - C. A is a copy of B - D. A and B have the same type

Question 9

The modern intrinsic that safely expresses "reinterpret these bits as another type" (an EQUIVALENCE use) is: - A. reshape - B. transfer - C. merge - D. pack

Question 10

GO TO (10, 20, 30), K transfers control to label 20 when K is: - A. 0 - B. 1 - C. 2 - D. 3

Question 11

The arithmetic IF statement IF (E) 10, 20, 30 jumps to label 20 when E is: - A. negative - B. zero - C. positive - D. undefined

Question 12

A statement function must be placed: - A. Anywhere in the routine - B. After the declarations and before the first executable statement - C. Only in the main program - D. Inside a BLOCK DATA

Question 13

The modern replacement for a statement function is: - A. A COMMON block - B. An internal (contained) procedure - C. A GO TO - D. An EQUIVALENCE

Question 14

An ENTRY statement gives a subprogram: - A. A second entry point sharing its body and local state - B. A return value - C. A comment - D. A new COMMON block

Question 15

-std=legacy is the gfortran flag that: - A. Optimizes for speed - B. Accepts obsolete features and reads fixed-form source - C. Enables OpenMP - D. Turns on all warnings

Question 16

The Hollerith constant 4HHEAT represents: - A. The integer 4 - B. The four characters HEAT - C. A hexadecimal number - D. A label

Question 17

In the PLATE kernel, the separate array TNEW — filled with new values computed from the old T, then copied back after the sweep — tells you the relaxation method is: - A. Gauss–Seidel - B. Jacobi - C. Successive over-relaxation - D. Conjugate gradient

Question 18

The COMMON-based data model of a large FORTRAN 77 program is best described as: - A. A set of components each owning private state - B. One global pool of untyped memory that any routine may reinterpret - C. A relational database - D. A stack of local variables

Question 19

Reading the PLATE kernel, the four-line "heart" that implements the physics is: - A. The SETBC boundary loops - B. The IMPLICIT DOUBLE PRECISION declaration - C. The stencil update TNEW(I,J) = AVG(T(I-1,J), …) plus the DIFF/DMAX change-tracking - D. The WRITE statements in OUTPT

Question 20

True or false: modernizing a validated legacy code should begin by rewriting it from scratch to be sure it is clean.

Answer Key

Q Ans Why
1 A Labels live in columns 1–5.
2 B A nonblank in column 6 continues the previous statement.
3 C C or * in column 1 flags a whole-line comment (no ! in strict F77).
4 B Names starting I–N are integer by default.
5 B COMMON associates by position/storage order, not by name.
6 C Reordered declarations silently swap the bytes; separate compilation hides it.
7 C Only BLOCK DATA may initialize named COMMON.
8 B EQUIVALENCE shares memory — the variables are the same bytes.
9 B transfer(source, mold) reinterprets bits, typed and portable.
10 C The computed GOTO selects the K-th label; K=2 → the second, 20.
11 B Arithmetic IF jumps to the middle label on zero.
12 B After declarations, before the first executable statement.
13 B An internal procedure — checked interface, intent, multi-line.
14 A A second entry point sharing the routine's body and locals.
15 B It accepts obsolete features and treats the source as fixed-form.
16 B nHtext names n characters; 4HHEAT is HEAT.
17 B Computing a separate TNEW from the old T (not updating in place) is the signature of Jacobi.
18 B COMMON is a global untyped memory pool, reinterpretable per routine.
19 C The stencil average plus change-tracking is the numerical core.
20 False Modernize/refactor to preserve validated numerics; never rewrite first.

Topics to review by question

  • Q1–3, 15 → §17.1 (fixed-form columns; the -std=legacy build).
  • Q4, 16 → §17.5 (implicit typing; Hollerith).
  • Q5–7, 18 → §17.2 (COMMON, BLOCK DATA, the global-memory model).
  • Q8–9 → §17.3 (EQUIVALENCE and its modern replacements).
  • Q10–14 → §17.4 (computed GOTO, arithmetic IF, statement functions, ENTRY).
  • Q17, 19 → §17.6 (reading the PLATE kernel: Jacobi, the four-line heart).
  • Q20 → the Part IV ethic (legacy is an inheritance; modernize, don't rewrite).

Scored below 16? Reread the flagged sections. The single most important idea is that every legacy construct has a known modern counterpart — once you can name the map, old code stops being frightening.