History

How:
- COBOL (Common Business-Oriented Language) was created in 1959 by the CODASYL (Conference on Data Systems Languages) committee, heavily inspired by the pioneering work of Rear Admiral Grace Hopper (who developed Flow-Matic, the first English-like compiler).
- The goal was to establish a portable, readable, and standardized programming language tailored specifically for business data processing (accounting, billing, and transactional ledger tracking).
- Standardized by ANSI in 1968, and subsequently updated across major milestones: COBOL-74, COBOL-85 (which added structured programming like END-IF), COBOL-2002 (adding Object-Oriented structures), and COBOL-2014.
- Today, COBOL remains the engine behind critical financial institutions, managing over 70% of global transaction processing on enterprise mainframes (IBM z/OS).
Who:
- Grace Hopper (technical advisor) and the CODASYL committee.
Why:
- Designed to provide a readable, machine-independent, English-like syntax optimized for complex decimal calculations and high-performance sequential file/record processing.

Introduction

Advantages

Decimal Arithmetic Precision — Avoids floating-point rounding errors (common in C/C++/Java binary floats) by performing exact math using packed decimal (COMP-3), which is critical for financial ledger books.
Self-Documenting English-like Syntax — High readability for non-programmers or business auditors, utilizing explicit keywords like ADD, SUBTRACT, and PERFORM.
Massive Record I/O Throughput — Designed from the ground up for reading and writing massive flat-file databases (sequential, relative, and indexed) on mainframe channels.
Long-term Backward Compatibility — Extremely stable; business logic code written in the 1970s runs seamlessly on modern IBM enterprise compilers.

Disadvantages

Extreme Verbosity — Simple operations require multiline declarations across several required divisions.
Fixed-Format Constraints — Traditional fixed-format demands strict column alignment (historically derived from 80-column punch cards), causing compile failures for minor spacing errors.
Lack of Modern Abstractions — Missing templates, dynamic reflection, generics, and native pointer arithmetic.
Dwindling Talent Pool — The developer community is aging, making maintenance of legacy systems expensive.

Remember Points

Four Divisions are Required — Every COBOL program must contain: Identification, Environment, Data, and Procedure divisions in strict order.
Hierarchy Level Numbers — Structural fields are grouped hierarchically using level numbers from 01 to 49.
Periods Terminate Units — Periods (.) are critical syntax markers; missing or extra periods can alter control flows or trigger compiler crashes.

Fixed-Format Layout

Column Reference Guide

Mainframe compilation historically checks strict column rules (80-column cards format):

Columns      Area Name        Description
1 - 6        Sequence Area    Used for card sequence numbers (frequently ignored by modern compilers)
7            Indicator Area   Determines line parsing behavior:
                              '*' = Comment line
                              '/' = Form feed (starts new page in printed listings)
                              '-' = Continuation of previous line string literal
                              'D' = Debugging line (active only if debugging mode is enabled)
8 - 11       Area A           Reserved for start of divisions, sections, paragraphs, and level 01 / 77
12 - 72      Area B           Reserved for executable statements, sentences, variables, and nested levels (02-49)
73 - 80      Program Name     Identification tags (ignored by compiler)

Program Divisions

Structural Outline

Every program requires the following sequence:

       IDENTIFICATION DIVISION.
       PROGRAM-ID. HELLO-WORLD.
       AUTHOR. VR RATHOD.
 
       ENVIRONMENT DIVISION.
       CONFIGURATION SECTION.
       INPUT-OUTPUT SECTION.
       FILE-CONTROL.
--       (Links physical files on disk to logical descriptors)
 
       DATA DIVISION.
       FILE SECTION.
--       (Defines layout of records read from/written to files)
       WORKING-STORAGE SECTION.
--       (Declares program variables and storage constants)
       LINKAGE SECTION.
--       (Declares arguments passed to this program by another calling routine)
 
       PROCEDURE DIVISION.
       MAIN-PROCEDURE.
           DISPLAY "Hello, Mainframe!".
           STOP RUN.

Data Representation

PICTURE (PIC) Clauses

PICTURE clauses define data storage formats using masks:

-- 9: Numeric character
-- A: Alphabetic character (A-Z, a-z)
-- X: Alphanumeric character (any symbol)
-- V: Implicit decimal point (not stored physically, used for math alignment)
-- S: Operational sign (stores + or - state)
 
01  WS-AGE         PIC 9(3).       -- Stores 3-digit integer (e.g. 025)
01  WS-NAME        PIC X(20).      -- Stores 20-character string
01  WS-BALANCE     PIC S9(5)V99.   -- Stores signed float (+/- 12345.67)

Storage Formats (Usage Clauses)

-- DISPLAY (Default) - Stores values as raw ASCII/EBCDIC text (1 byte per char)
01  WS-TEXT        PIC 9(4) USAGE DISPLAY. -- Takes 4 bytes
 
-- COMP (Computational) - Stores values as binary integers
01  WS-BIN-INT     PIC 9(4) USAGE COMP. -- Takes 2 bytes (16-bit integer)
 
-- COMP-3 (Packed Decimal) - Stores 2 decimal digits per byte (using BCD) with a sign nibble
-- Highly recommended for financial balances to prevent decimal point rounding errors!
01  WS-CASH        PIC S9(7)V99 USAGE COMP-3. -- Takes 5 bytes

Hierarchy Level Numbers

Level numbers establish structured groupings:

-- 01: Group header level
-- 02 - 49: Nested element fields
-- 77: Independent variable (cannot contain subfields)
-- 88: Condition Name variable (defines boolean condition test cases)
 
01  WS-USER-RECORD.
    05  WS-USER-ID       PIC 9(5).
    05  WS-USER-NAME.
        10  WS-FIRST-NAME PIC X(10).
        10  WS-LAST-NAME  PIC X(10).
    05  WS-USER-STATUS   PIC X(1).
        88  STATUS-ACTIVE   VALUE "A". -- True if WS-USER-STATUS is "A"
        88  STATUS-BLOCKED  VALUE "B".
        
77  WS-TEMP-COUNTER      PIC 9(4) VALUE 0.

Control Flow & Subroutines

Conditionals: IF and EVALUATE

-- 1. IF-ELSE Statement (always close with END-IF in modern COBOL)
IF WS-AGE >= 18 THEN
    DISPLAY "ADULT"
ELSE
    DISPLAY "MINOR"
END-IF.
 
-- Testing level-88 conditional variables
IF STATUS-ACTIVE THEN
    DISPLAY "User is active"
END-IF.
 
-- 2. EVALUATE (Switch-case equivalent)
EVALUATE WS-AGE
    WHEN 0 THRU 12
        DISPLAY "CHILD"
    WHEN 13 THRU 19
        DISPLAY "TEENAGER"
    WHEN OTHER
        DISPLAY "ADULT"
END-EVALUATE.

Loops & Paragraphs: PERFORM

In COBOL, code is organized into Paragraphs (labeled code blocks). PERFORM jumps to paragraphs or executes inline loops.

-- 1. Inline Loop
PERFORM VARYING WS-COUNTER FROM 1 BY 1 UNTIL WS-COUNTER > 5
    DISPLAY "Count: " WS-COUNTER
END-PERFORM.
 
-- 2. Out-of-line Paragraph execution
PERFORM PROCESS-DATA. -- Executes paragraph, then returns here
DISPLAY "Paragraph finished".
 
-- Paragraph Definition (Start at Area A, statement ends with a period)
PROCESS-DATA.
    DISPLAY "Processing ledger record...".
    ADD 1 TO WS-RECORD-COUNT.

Arithmetic Operations

Math Verbs vs COMPUTE

-- 1. Traditional Math Verbs
ADD 10 TO WS-TOTAL.
SUBTRACT WS-TAX FROM WS-SUBTOTAL GIVING WS-FINAL.
MULTIPLY WS-QTY BY WS-PRICE GIVING WS-RAW-TOTAL.
DIVIDE WS-SUM BY 5 GIVING WS-AVERAGE ROUNDED.
 
-- 2. COMPUTE Statement (Supports algebraic formulas)
COMPUTE WS-FINAL = (WS-SUBTOTAL * (1 - WS-DISCOUNT)) + WS-TAX.

File I/O Handling

Declaring and Processing Flat Files

ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    -- Links logical file descriptors to physical disk dataset routes
    SELECT CLIENT-FILE ASSIGN TO "clients.dat"
    ORGANIZATION IS LINE SEQUENTIAL. -- Text file line separation
 
DATA DIVISION.
FILE SECTION.
-- File Descriptor (FD) details record structures
FD  CLIENT-FILE.
01  CLIENT-RECORD.
    05  CLIENT-ID       PIC 9(5).
    05  CLIENT-NAME     PIC X(20).
    05  CLIENT-BALANCE  PIC 9(5)V99.
 
WORKING-STORAGE SECTION.
77  WS-EOF-FLAG         PIC X(1) VALUE "N".
    88  END-OF-FILE                  VALUE "Y".
 
PROCEDURE DIVISION.
MAIN-RUN.
    OPEN INPUT CLIENT-FILE.
    
    -- Read loop
    PERFORM UNTIL END-OF-FILE
        READ CLIENT-FILE INTO CLIENT-RECORD
            AT END
                MOVE "Y" TO WS-EOF-FLAG
            NOT AT END
                DISPLAY "Client: " CLIENT-NAME " Bal: " CLIENT-BALANCE
        END-READ
    END-PERFORM.
    
    CLOSE CLIENT-FILE.
    STOP RUN.

Subprograms & Linkage

Passing Data Between Programs

Subprograms read parameters using the LINKAGE SECTION.

-- CALLED-SUB.cbl (Subprogram)
IDENTIFICATION DIVISION.
PROGRAM-ID. CALLED-SUB.
 
DATA DIVISION.
LINKAGE SECTION.
-- Maps to memory location passed by calling program
01  LK-PARAMS.
    05  LK-NUM1       PIC 9(2).
    05  LK-NUM2       PIC 9(2).
    05  LK-RESULT     PIC 9(4).
 
PROCEDURE DIVISION USING LK-PARAMS.
    COMPUTE LK-RESULT = LK-NUM1 * LK-NUM2.
    EXIT PROGRAM. -- Returns control back to caller

Caller invocation:

-- Calling program execution
WORKING-STORAGE SECTION.
01  WS-MATH-BLOCK.
    05  WS-X          PIC 9(2) VALUE 12.
    05  WS-Y          PIC 9(2) VALUE 05.
    05  WS-ANS        PIC 9(4).
...
PROCEDURE DIVISION.
    CALL "CALLED-SUB" USING WS-MATH-BLOCK.
    DISPLAY "Answer: " WS-ANS. -- Prints 60

Mainframe Enterprise Concepts

JCL (Job Control Language)

JCL controls mainframe job submission, telling the OS which programs to run and linking files:

//COMPJOB  JOB (ACCT),'RUN COBOL',CLASS=A,MSGCLASS=X
//STEP1    EXEC PGM=CLIENTPRG
//STEPLIB  DD DSN=MYAPP.LOADLIB,DISP=SHR
//CLIENTS  DD DSN=MAINFRAME.CLIENT.DATASET,DISP=SHR
//SYSOUT   DD SYSOUT=*

VSAM (Virtual Storage Access Method)

VSAM is a high-performance database file storage method on IBM mainframes:
- KSDS (Key Sequenced Dataset): Record lookup by key (e.g. index on Client-ID).
- ESDS (Entry Sequenced Dataset): Records appended sequentially.
- RRDS (Relative Record Dataset): Lookup by relative record index number.

DB2 Database Integration (EXEC SQL)

Mainframe databases use embedded DB2 relational SQL engines:

WORKING-STORAGE SECTION.
-- Declare SQL variables
EXEC SQL INCLUDE SQLCA END-EXEC.
 
EXEC SQL BEGIN DECLARE SECTION END-EXEC.
77  DB-CLIENT-ID     PIC S9(9) COMP.
77  DB-CLIENT-NAME   PIC X(20).
EXEC SQL END DECLARE SECTION END-EXEC.
 
PROCEDURE DIVISION.
    MOVE 1001 TO DB-CLIENT-ID.
    
    -- Query database
    EXEC SQL
        SELECT NAME INTO :DB-CLIENT-NAME
        FROM CLIENTS
        WHERE ID = :DB-CLIENT-ID
    END-EXEC.
    
    IF SQLCODE = 0 THEN
        DISPLAY "DB Record: " DB-CLIENT-NAME
    END-IF.

CICS Transaction Monitors (EXEC CICS)

CICS runs real-time terminal banking transactions:

PROCEDURE DIVISION.
    -- Reads input from terminal screen buffer
    EXEC CICS RECEIVE
        MAP('CLIENTMAP')
        MAPSET('CLIENTSET')
        INTO(WS-SCREEN-DATA)
    END-EXEC.
    
    -- Write response back to terminal screen
    EXEC CICS SEND
        TEXT FROM("Transaction Success")
        LENGTH(19)
        ERASE
    END-EXEC.
    
    EXEC CICS RETURN END-EXEC.

Compilation & Execution

Mainframe vs. Open-Source Compilers

IBM Enterprise COBOL: Standard compiler for mainframes.
GnuCOBOL (open-source): Excellent free compiler translating COBOL to intermediate C code.

# Compile COBOL file using GnuCOBOL
cobc -x program.cbl -o program.exe
 
# Run executable
./program.exe

Table of Contents

Explorer

COBOL

History

Introduction

Advantages

Disadvantages

Remember Points

Fixed-Format Layout

Column Reference Guide

Program Divisions

Structural Outline

Data Representation

PICTURE (PIC) Clauses

Storage Formats (Usage Clauses)

Hierarchy Level Numbers

Control Flow & Subroutines

Conditionals: IF and EVALUATE

Loops & Paragraphs: PERFORM

Arithmetic Operations

Math Verbs vs COMPUTE

File I/O Handling

Declaring and Processing Flat Files

Subprograms & Linkage

Passing Data Between Programs

Mainframe Enterprise Concepts

JCL (Job Control Language)

VSAM (Virtual Storage Access Method)

DB2 Database Integration (EXEC SQL)

CICS Transaction Monitors (EXEC CICS)

Compilation & Execution

Mainframe vs. Open-Source Compilers

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