MVS- Concepts Overview

MVS stands for Multiple Virtual Storage. It is the operating system that runs on IBM mainframe computers.

Today the current version of MVS is called z/OS. When people say MVS today, they usually mean the core of z/OS (the Basic Control Program).

This page explains the big ideas of MVS: what it manages, how it shares the machine between many users, and how batch and online work differ.

What is MVS

• MVS = Multiple Virtual Storage.
• It is an operating system for IBM mainframe computers.
• The key idea: every job and every user gets its own virtual address space.
• MVS grew through MVS/370, MVS/XA, MVS/ESA, OS/390, and is now called z/OS (64-bit).
• z/OS is designed for thousands of concurrent users, heavy I/O, and very large workloads.

IBM mainframe processors

• A mainframe processor has a CPU, main storage, and channels.
• Channels are small I/O processors that move data between memory and devices, so the CPU stays free for programs.
• PR/SM (Processor Resource/System Manager) divides one machine into logical partitions (LPARs). Each LPAR can run its own operating system.
• Processor history: System/360 (1964) -> System/370 -> S/390 -> zSeries/z (64-bit addressing).

Hardware resources managed by MVS

• MVS controls the CPU, real storage, channels, and all I/O devices.
• It shares this hardware safely between hundreds of programs and users.
• It protects one program's memory and data from another program.
• It recovers from most hardware and software errors without stopping the system.

Input/output devices

• DASD (Direct Access Storage Device): disk drives that hold data sets and the operating system itself.
• Magnetic tape drives: mainly used for backup and sequential data.
• Printers: line printers print up to about 1400 lines per minute.
• Display terminals: the 3270 family, or PCs with 3270 emulation software (TN3270 over TCP/IP).
• Channels connect the I/O devices to the CPU.

Virtual storage

• Virtual storage makes the machine look like it has more memory than it physically has.
• The system uses disk space (page data sets) to simulate and extend real storage.
• The part of a program that is running now stays in real storage; the rest waits on disk.
• Virtual storage is divided into 4 KB sections called pages.
• Moving a page from disk to real storage is a page-in; moving it back is a page-out.

Address spaces

• An address space is the complete range of virtual storage addresses available to a user or program.
• Every TSO user, every batch job, CICS, and DB2 each run in their own address space.
• Each address space sees the same range of addresses, but their private areas are isolated from each other.
• A program error in one address space cannot damage data in another address space.

Multiprogramming and multiprocessing

• Multiprogramming: more than one program runs on the processor at the same time.
• Programs spend most of their time waiting for I/O. While one waits, another runs, so the CPU is used efficiently.
• Multiprocessing: several CPUs inside one system share the same memory and I/O.

Batch processing

• In batch processing, jobs run without any user interaction.
• A job is one or more programs run in sequence, described by Job Control Language (JCL).
• A job scheduler (JES) picks jobs from a queue and runs them when resources are free.
• Typical batch work: payroll runs, nightly billing, report generation.

Time sharing (online processing)

• In time sharing, many users interact with the computer at the same time through terminals.
• Each user logs on (TSO/E) and gets commands processed immediately.
• Also called interactive or online processing.
• Example: one user edits JCL in ISPF while another runs an order-entry program.

Spooling

• Spooling means holding printed output on disk before it goes to a real printer.
• Each program writes its output to a spool file on the spool disk.
• JES later sends the spooled output to the correct printer or holds it.
• This lets many programs "print" at once without waiting for a slow printer.

Operating system family: MVS to z/OS

• OS/360 (1964) -> OS/VS1, OS/VS2 (SVS) -> MVS/370 (24-bit) -> MVS/XA (31-bit) -> MVS/ESA (31-bit) -> OS/390 (31-bit) -> z/OS (64-bit).
• Older names you may still see: MFT, MVT, SVS (Single Virtual Storage).
• z/OS can be a 64-bit system with huge address spaces, while old 24-bit MVS could address only 16 MB.

Base services of z/OS

• Basic Control Program (BCP, the MVS core): task, storage, and I/O management.
• JES2 or JES3: job entry subsystem, handles batch jobs.
• Workload Manager (WLM): assigns CPU and storage by performance goals.
• DFSMS: automates storage management for data sets.
• UNIX System Services: lets the mainframe run UNIX programs and files.
• Communication Server: TCP/IP and networking.