Learn

COBOL programs can be split into multiple files. The CALL statement invokes another program.

       *> Main program (main.cbl)
       IDENTIFICATION DIVISION.
       PROGRAM-ID. MAIN.
       DATA DIVISION.
       WORKING-STORAGE SECTION.
       01 INPUT-VAL    PIC 9(5).
       01 RESULT       PIC 9(7).
       PROCEDURE DIVISION.
           MOVE 5 TO INPUT-VAL.
           CALL "SQUARE" USING INPUT-VAL, RESULT.
           DISPLAY "squared: ", RESULT.
           STOP RUN.

       *> Subprogram (square.cbl)
       IDENTIFICATION DIVISION.
       PROGRAM-ID. SQUARE.
       DATA DIVISION.
       LINKAGE SECTION.        *> data passed FROM caller — no allocation
       01 N            PIC 9(5).
       01 R            PIC 9(7).
       PROCEDURE DIVISION USING N, R.
           COMPUTE R = N * N.
           GOBACK.             *> return to caller

Key details:

• LINKAGE SECTION declares parameters — no memory; aliased to caller's data
• PROCEDURE DIVISION USING ... lists what the subprogram receives
• GOBACK returns to caller (use instead of STOP RUN in subprograms)
• CALL "NAME" invokes by program-id (compile-time linked OR dynamic)

Parameter passing:

• BY REFERENCE (default) — caller and callee share the same memory
• BY CONTENT — pass a copy (changes don't propagate back)
• BY VALUE — modern; pass-by-value (numeric types only)

CALL "SUBPROG" USING BY VALUE INPUT-VAL
                     BY REFERENCE OUTPUT-VAL.

Static vs dynamic calls:

• Static — compile-time linked; faster, larger executable
• Dynamic — CALL VARIABLE-NAME lets you decide at runtime; loaded on first call

For Judge0 (single-file submissions), embed everything in one program. Real mainframe systems have hundreds of small programs called by orchestrators.