COBOLStudy Notes

CICS Fundamentals for COBOL

Learn how CICS (Customer Information Control System) lets COBOL programs run as online, transaction-based applications, using pseudo-conversational design, EXE…

COBOL and DB2

Learn how COBOL programs embed SQL to interact with a DB2 relational database, using host variables, cursors, and SQLCA/SQLCODE for error checking.

COBOL Best Practices

Practical conventions for writing maintainable, defensive, and performant COBOL that survives decades of maintenance by different teams.

COBOL in Mainframe Systems Today

How COBOL continues to power core banking, insurance, and government systems on IBM z/OS mainframes, and why it remains business-critical in 2026.

COBOL Interview Questions

The core language, file-handling, and mainframe concepts most frequently asked in COBOL developer interviews, with the reasoning behind each answer.

COBOL Program Structure

How a COBOL source file is organized into divisions, sections, paragraphs, and sentences, plus the classic fixed-format column layout.

COBOL Quick Reference

A condensed cheat sheet covering program structure, PICTURE clauses, common verbs, and file I/O syntax for day-to-day COBOL development.

Conditional Logic in COBOL

Master COBOL's IF/ELSE, nested conditions, the EVALUATE statement, and condition-names (88-levels) for expressing readable business rules.

Copybooks Explained

Understand how COBOL copybooks let teams share data layouts and procedural code across programs using the COPY statement, REPLACING phrase, and include-library…

Data Division Basics

An introduction to the COBOL Data Division, the section of a COBOL program where every variable, file layout, and record structure is declared before any execu…

Error Handling in COBOL

Learn how COBOL programs detect and respond to runtime errors using FILE STATUS codes, the ON SIZE ERROR and ON OVERFLOW phrases, and structured exception hand…

File Status Codes

Learn how COBOL's two-character FILE STATUS codes let a program detect and react precisely to the outcome of every file I/O operation.

Indexed File Processing (VSAM)

Understand how COBOL uses indexed files, typically backed by VSAM KSDS on mainframes, to perform fast random and sequential access by key.

Level Numbers and Group Items

How COBOL uses level numbers from 01 to 49, plus the special 66, 77, and 88 levels, to build hierarchical record structures out of group and elementary items.

Loops in COBOL

See how COBOL's PERFORM statement is used in practice to build counting loops, nested loops, sentinel-controlled read loops, and how to exit them safely.

Migrating Legacy COBOL Applications

A practical look at the strategies, risks, and techniques organizations use to modernize decades-old COBOL systems without disrupting business operations.

MOVE and Arithmetic Statements

Learn how COBOL's MOVE statement reformats data between fields and how ADD, SUBTRACT, MULTIPLY, DIVIDE, and COMPUTE perform arithmetic with proper rounding and…

PERFORM Statement Variations

Explore the many forms of COBOL's PERFORM statement, from simple out-of-line calls and PERFORM THRU to TIMES, UNTIL, and in-line PERFORM blocks.

PICTURE Clauses Explained

A detailed guide to COBOL's PICTURE (PIC) clause, the syntax that defines a data item's category, size, and display format.

REDEFINES and OCCURS Clauses

How the REDEFINES clause overlays alternate views of the same storage and the OCCURS clause builds arrays and tables in COBOL, including indexing and variable-…

Report Writer Basics

Explore COBOL's Report Writer feature, a declarative way to generate formatted, control-broken reports without manually coding line spacing and totals.

Sequential File Processing

Learn how COBOL reads, writes, and updates records in a sequential file, the oldest and most common file organization in mainframe batch systems.

Setting Up a COBOL Environment

How to install a free COBOL compiler like GnuCOBOL, choose an editor, and prepare your machine to write and run COBOL programs locally.

SORT and MERGE in COBOL

Learn how COBOL's built-in SORT and MERGE verbs reorder and combine files using the language's native sort work file mechanism.