Introduction to Files in Mainframe

If you’re coming from a PC or Unix background, you’re used to working with "files." But in the mainframe world, things are a bit different. This handout explains what a data set is, how it differs from regular files, and why understanding this difference is important when working with z/OS.

What Is a Data Set?

• A data set is the mainframe equivalent of a file—but structured to hold records.
• A record is like a line or entry of structured data—similar to a row in a spreadsheet or a line in a log file.
• Records can be fixed or variable-length, and are read or written by programs directly.
• Some datasets store text (e.g., source code), while others store binary data (e.g., executables or load modules).

Key Differences

Concept	PC/Unix File	Mainframe Data Set (z/OS)
Basic unit	File	Data Set
Structure	Byte stream	Record-based
File types	.txt, .exe, .csv, etc.	SEQ, PDS, PDSE, VSAM, etc.
Storage method	Stored in folders	Managed by z/OS, often cataloged
Access method	Byte-based (e.g., stream)	Record-based (fixed or variable)

Cataloged vs. Non-Cataloged

Term	What It Means	Why It Matters
Cataloged	z/OS knows where the data set is stored	You can access it using just its name
Non-Cataloged	You need volume and device info	Harder to locate manually

Think of the catalog like a search index on your PC—it tells the system where each data set is. In z/OS, the catalog is a core system feature that makes it easy for programs, JCL, and utilities to locate datasets without needing physical storage details.

For a deeper dive, see: Catalog Handout

Ways to Manage Data Sets in Mainframe

You can work with datasets in multiple ways depending on whether you're doing it interactively or through automation:

• TSO Commands: Command-line interface for allocating, deleting, or copying datasets in real-time (e.g., using ALLOCATE, DELETE, etc.).
• JCL (Job Control Language): Batch method where you define dataset actions in a job script and submit it for processing—ideal for repeated or scheduled tasks.
• ISPF Panels: Menu-driven, interactive environment where you can browse, edit, or manage datasets without typing full commands—user-friendly for beginners.

Each method has its use: TSO is quick for ad hoc work, JCL is best for automation, and ISPF is great for hands-on exploration. We have seperate handsout for each of them.

Common Misconceptions

Misconception	Reality
“All files are readable”	Load modules (executables) aren’t meant to be opened
“Data sets are folders like on Windows”	Only PDS/PDSE have members; other datasets don't
“Catalog = Folder”	Catalog = Index; it helps find the data, not hold it
“You can read records like lines in Notepad”	Only if it's a text-based dataset with readable encoding