Comment
In the mainframe environment, a **comment** is a non-executable line or block of text within source code, JCL, or other script files, intended solely for human readability and documentation. It is completely ignored by compilers, assemblers, interpreters, or operating system components during processing. A comment in the mainframe context refers to non-executable lines of text embedded within source code (like COBOL, Assembler), JCL, REXX scripts, or configuration files. Its primary purpose is to provide documentation, explanation, or temporary disabling of code, making the system easier to understand, maintain, and troubleshoot for human readers.
Key Characteristics
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- Non-executable: Comments are entirely bypassed by the language processor (compiler, assembler, interpreter, JCL reader) and have no impact on the program's logic, execution flow, or resource consumption.
- Language-specific Syntax: The method for denoting a comment varies significantly across different mainframe languages and utilities. For example, JCL uses
// *in column 1, COBOL uses*in column 7, REXX uses/* ... */for blocks and--for lines, and DB2 SQL uses--for lines or/* ... */for blocks. - Purpose: Primarily used to explain the purpose of code sections, document complex algorithms, provide author and modification details, or temporarily disable code segments without deleting them.
- Readability and Maintainability: Comments are crucial for improving the readability and long-term maintainability of mainframe applications, especially in large, complex systems with extended lifespans and multiple developers.
Use Cases
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- Explaining Complex Logic: Documenting intricate COBOL paragraphs, Assembler routines, or REXX scripts to clarify their function, the business rules they implement, or the reasoning behind specific technical implementations.
- JCL Job Documentation: Providing clear descriptions for JCL steps,
DDstatements, orPROCparameters to make job streams understandable for operations, support staff, and other developers. - Temporary Code Disablement: Commenting out lines of code in a COBOL program, JCL step, or REXX script during debugging or testing to isolate issues without permanently removing the code.
- Version and Author Information: Including header comments in source files to record the program's name, author, creation date, and a chronological history of modifications and their purposes.
- Database Object Documentation: Adding comments to DB2 DDL statements to describe tables, columns, indexes, or views, aiding database administrators and developers in understanding the database schema.
Related Concepts
Comments are fundamental to the entire mainframe software development lifecycle, closely intertwined with source code (COBOL, PL/I, Assembler, REXX), Job Control Language (JCL), and database definition languages (DDL). They enhance the value of these artifacts by providing human-readable context that compilers and interpreters ignore. Comments are crucial for program maintenance, debugging, and knowledge transfer among development teams, making complex systems more accessible and reducing the effort required for future modifications or troubleshooting. They are a form of internal documentation, complementing external design documents and specifications.
- Keep Comments Up-to-Date: Ensure comments accurately reflect the current state of the code; outdated comments can be more misleading and detrimental than no comments at all.
- Explain "Why," Not Just "What": Focus comments on *why* a particular approach was taken or *what* a complex section's purpose is, rather than merely restating obvious code that can be understood by reading the statements themselves.
- Use Consistent Style: Adhere to established team or organizational standards for comment formatting, placement, and content to maintain uniformity and improve readability across projects.
- Avoid Excessive Commenting: Do not comment every line of obvious code; well-written, self-documenting code is often preferable, with comments reserved for non-obvious logic, business rules