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9. Programs

9. Programs

This short chapter assembles the pieces from the previous chapters into a whole executable program and describes the order in which its statements run (§9).

What a program is made of

An executable program is a collection of statements, comment lines, and end lines that completely describes a computing procedure (§9.1). Its anatomy, from the inside out:

  • A program part is the run of statements containing at least one executable statement (and optionally FORMAT and DATA statements), possibly preceded by statement function definitions (§9.1.1).
  • A program body is the specification statements and/or FORMAT statements, followed by the program part, followed by an end line (§9.1.2).
  • A main program is simply a program body (§9.1.5) — the one unit not headed by FUNCTION, SUBROUTINE, or BLOCK DATA.
  • A subprogram is a SUBROUTINE or FUNCTION unit (a program body with a heading), or a BLOCK DATA unit (§9.1.3, §9.1.4).
  • An executable program is one main program plus any number of subprograms (§9.1.6).
      PROGRAM AREA
      READ (5,100) R
100   FORMAT (F10.0)
      A = PI(R)
      WRITE (6,200) A
200   FORMAT (1X, F12.4)
      STOP
      END
C
      FUNCTION PI(R)
      PI = 3.14159 * R * R
      RETURN
      END

Here the main program AREA and the function subprogram PI together make one executable program. Each unit ends with its own END line.

How a program runs

Execution begins with the first executable statement of the main program (§9.2). From there, statements run in the order written, except when a statement explicitly redirects the flow:

  • a GO TO, an arithmetic IF, a RETURN, or a STOP;
  • reaching the terminal statement of a DO (which may loop back).

A subprogram, when referenced, starts at its first executable statement and runs the same way until a RETURN sends control back to the caller. Nonexecutable statements (FORMAT, DIMENSION, DATA, …) are never "run" — they are in effect throughout, but the flow of control passes around them.

forterp notes. forterp finds the main program structurally — the unit that is not a FUNCTION, SUBROUTINE, or BLOCK DATA — so a leading PROGRAM statement is optional. It begins execution at that unit's first executable statement, exactly as §9.2 prescribes, and the units of a multi-file program are linked first (see Command-line tools).