Introduction to z/OS
z/OS is IBM’s enterprise-grade operating system designed for mainframes, known for its reliability, scalability, and security. It runs critical workloads across industries like banking, healthcare, and government.
This training introduces essential z/OS concepts, components, and the role of middleware in enterprise environments. By understanding how z/OS manages work and supports applications, you'll gain a solid foundation in mainframe operations.
What is an Operating System (OS)?
An Operating System is essential system software that manages computer hardware and provides services for running applications. It helps perform tasks such as:
- Managing processes (programs in execution)
- Allocating memory
- Controlling input/output (I/O) devices
- Ensuring system security and stability
What is Middleware?
Middleware is software that acts as a bridge between the OS and applications. It provides:
- Communication tools
- Transaction processing
- Database access
- Application integration
Together, the OS and middleware enable the system to support complex, interconnected enterprise applications, such as those in banking, airlines, or government.
Basic z/OS Architecture – How the System Flows
To understand how z/OS works behind the scenes, it's helpful to see the workflow of how jobs and applications move through the system.
🔄 High-Level Workflow
- Users or applications submit work (jobs, transactions, or online requests).
- Job Entry Subsystem (JES) receives the input and prepares it for execution.
- Workload Manager (WLM) decides how to prioritize and allocate resources (CPU, memory, etc.) based on policies.
- The z/OS Kernel executes the work by launching programs and managing system calls.
- Middleware handles specific functions such as transaction processing or database access.
- Output (like reports, logs, or data updates) is sent to printers, storage, or returned to the user.
Think of it like a production line where each component (JES, WLM, middleware, etc.) plays a role in ensuring smooth and efficient processing.
Understanding z/OS
z/OS is IBM’s flagship mainframe operating system designed for high-volume, secure, and reliable processing. It supports modern and traditional workloads across industries. Here are few Key z/OS Terminologies
IBM Docs - zOS Programming Constructs
How z/OS Handles Work (Instruction Flow)
These are the general steps z/OS performs to manage and execute tasks:
- Accept incoming work (from users or batch jobs)
- Interpret the work using JCL (Job Control Language) or TSO commands
- Allocate system resources like CPU, memory, and storage
- Schedule the job using workload management
- Execute the task
- Monitor its progress
- Handle the output, e.g., printing or storing results
Modules
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A module is a collection of related programming instructions.
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Modules are reusable and can be combined to build larger systems like:
- Payroll systems
- Inventory management
- Database management
System Components
When modules are assembled together, they form system components, each with a specific role.
Common z/OS components include:
| Component | Description |
|---|---|
| WLM (Workload Management) | Distributes system resources to jobs/tasks based on priority |
| RTM (Recovery Termination Manager) | Handles system recovery during failures |
| JES2/JES3 | Manages batch jobs – submission, scheduling, output |
| RACF (Resource Access Control Facility) | Manages security – user authentication and access |
| SMS (System Managed Storage) | Automates storage management |
Macros
- Macros are shorthand commands that perform complex operations.
- They replace long sequences of instructions with a single command.
Example Uses:
- Opening a file
- Sending a system message
- Requesting a system service (like GET or PUT for file I/O)
Mostly used in Assembler programming on z/OS.
Control Blocks ("Mainframe Sticky Notes")
Control Blocks are internal memory structures z/OS uses to track system activity.
They are grouped by what they track:
| Type | Purpose |
|---|---|
| System-related | Info about the system (e.g., # of processors) |
| Resource-related | Tracks CPU, memory, I/O devices |
| Job-related | Monitors individual jobs (batches) |
| Task-related | Monitors tasks within a job (steps or subtasks) |
Examples:
- TCB (Task Control Block): Tracks each running task
- SRB (Service Request Block): Manages special service requests
- ASCB (Address Space Control Block): Represents a program’s memory space
Control blocks are often linked, helping the OS quickly find and manage data.
Middleware in z/OS
🔎 What Middleware Does
Middleware provides services that the OS doesn't do directly, such as:
- Managing database access
- Processing transactions (banking, payments)
- Handling communication between systems
- Running web applications
🧱 Common Middleware Examples on z/OS
| Middleware | Purpose |
|---|---|
| DB2 | A relational database used for storing and retrieving data |
| CICS (Customer Information Control System) | Manages high-speed transaction processing (e.g., ATMs) |
| IMS | Hierarchical database and transaction manager |
| IBM MQ | Message queuing system for reliable system-to-system communication |
| WebSphere Application Server | Runs Java applications, supports APIs and web interfaces |
| Java Virtual Machine (JVM) | Runs Java programs on any platform, including z/OS |
These tools run on top of z/OS and extend its capabilities to handle business-critical tasks efficiently.
General Abbrevation to remember
| Name | Full Form |
|---|---|
| TSO | Time Sharing Option |
| ISPF | Interactive System Productivity Facility |
| SDSF | System Display and Search Facility |
| JCL | Job Control Language |
| COBOL | Common Business Oriented Language |
| PL/I | Programming Language 1 |
| REXX | Restructured Extended Executor |
| CLIST | Command List |
| HLASM | High Level Assembler |