What z/OS does for you
z/OS is the operating system. Like any operating system it decides who runs, who gets memory, and who is allowed to touch what. The differences from Windows or Linux are in the priorities: isolation is stronger, resource control is finer, and the system expects to run for years without restarting.
Its five jobs
| Job | What it means in practice |
|---|---|
| Isolation | Each unit of work runs in its own address space and cannot see another's memory |
| Dispatching | Deciding which piece of work gets a processor next, guided by business-level goals |
| Storage | Managing real and virtual memory, and paging when demand exceeds supply |
| I/O | Driving disk, tape and network through channel hardware |
| Security | Every access to a dataset or resource is checked by an external manager such as RACF |
The idea that matters most: the address space
An address space is z/OS's unit of isolation. Every running thing — a batch job, a TSO user, a CICS region, DB2 itself — gets one. Each has its own view of virtual memory starting at zero, and no address space can reach into another's private storage by accident. This is why one runaway program cannot corrupt the rest of the system, and why a CICS region failing does not take DB2 with it.
Work is classified, not just queued
z/OS does not simply run things first-come-first-served. The Workload Manager (WLM) is given business goals — 'this CICS transaction should complete in 0.3 seconds 90% of the time', 'this batch class is discretionary' — and continuously shifts processor, memory and I/O priority to meet them. You rarely configure WLM as a beginner, but you will hear its vocabulary constantly: *service class*, *goal*, *importance*, *velocity*.
Common mistakes
Cancelling a CICS region or a DB2 address space has enormous blast radius. Cancels in production follow a documented procedure and usually an approval.
Modern systems have very large real memory. Paging problems today usually mean something is misbehaving, not that the system is undersized.
What you will see at work
- Address space names are how everyone refers to running things. Learn your site's naming convention for CICS regions and DB2 subsystems in week one.
- You will hear 'started task' (STC) for long-running system work started by the operator, as opposed to batch jobs you submit.
- WLM service class names show up in performance reports and in arguments about why a job ran slowly.
Key terms
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