Paragraphs, sections and PERFORM

So far your programs have been one long list of statements. Real COBOL programs run to thousands of lines, so the PROCEDURE DIVISION is split into named blocks called paragraphs, and you run them with PERFORM.

Paragraphs

A paragraph is a name in Area A (column 8) followed by a full stop. It contains every sentence from there until the next paragraph name.

       IDENTIFICATION DIVISION.
       PROGRAM-ID. PARAS.
       PROCEDURE DIVISION.
       0000-MAIN.
           DISPLAY "START"
           PERFORM 1000-GREET
           PERFORM 1000-GREET
           DISPLAY "END"
           STOP RUN.

       1000-GREET.
           DISPLAY "HELLO FROM 1000-GREET".

Output:

START
HELLO FROM 1000-GREET
HELLO FROM 1000-GREET
END

This is an out-of-line PERFORM: control jumps to the paragraph, runs it, and comes back to the statement after the PERFORM. It works like calling a function, except there are no parameters or return values: every paragraph shares the same WORKING-STORAGE.

The number prefix (0000-, 1000-) is a shop convention, not syntax. It tells a reader where a paragraph sits in the hierarchy and makes it easy to find in a 5,000-line listing.

Fall-through

COBOL runs paragraphs in source order unless told otherwise. If you forget STOP RUN, control simply drops into the next paragraph:

       0000-MAIN.
           DISPLAY "START"
           PERFORM 1000-GREET.

       1000-GREET.
           DISPLAY "HELLO FROM 1000-GREET".

This prints the greeting twice: once from the PERFORM, once by falling through. That is why the main paragraph always ends with STOP RUN (or GOBACK).

Sections

A section groups paragraphs. Its header ends with the word SECTION. Performing a section runs every paragraph in it, then returns:

       PROCEDURE DIVISION.
       0000-MAIN SECTION.
           PERFORM 1000-REPORT
           STOP RUN.

       1000-REPORT SECTION.
       1000-HEADER.
           DISPLAY "HEADER".
       1000-BODY.
           DISPLAY "BODY".
       1000-FOOTER.
           DISPLAY "FOOTER".

Some shops use sections everywhere, some only paragraphs. Follow whatever the existing program does.

PERFORM THRU and EXIT paragraphs

PERFORM A THRU B runs every paragraph from A down to B in source order. In older code you will see it paired with an exit paragraph, whose only statement is EXIT (which does nothing):

       0000-MAIN.
           PERFORM 2000-VALIDATE THRU 2000-EXIT
           DISPLAY "BACK IN MAIN"
           STOP RUN.

       2000-VALIDATE.
           IF WS-AMOUNT < ZERO
               DISPLAY "NEGATIVE AMOUNT - REJECTED"
               GO TO 2000-EXIT
           END-IF
           DISPLAY "AMOUNT OK".

       2000-EXIT.
           EXIT.

With a negative amount this prints NEGATIVE AMOUNT - REJECTED then BACK IN MAIN. The GO TO 2000-EXIT is an early return: it jumps to the end of the range, and the PERFORM finishes there.