The MVS System - IBM Mainframe

We have seen the different components of the MVS system. In this section we will see how the different MVS facilities work together in a typical MVS system. In particular, we will learn about two important activities that are required to establish a working MVS system: system generation and system initialization. Then, we will see the DASD files that are required to support MVS and the different subsystems and other facilities you are likely to find on your MVS system.

System Generation and Initialization

System generation and initialization are activities that are required to establish a working MVS system. System generation is the process of creating an MVS system, and system initialization is the process of starting a previously generated MVS system. Both system generation and initialization are the responsibility of the systems programming staff, so unless you are a systems programmer, you do not have to worry about the overwhelming details of these activities. You will better understand MVS, however, if you have a basic idea of how system generation and initialization work.

System Generation

System generation is the process of creating an MVS system, and System initialization is the process of starting a previously generated MVS system.

The basic components that make up MVS are on a series of tapes, called distribution libraries. System generation selects and assembles the various distribution libraries. To control system generation, normally known as sysgen, a system's programmer codes special macroinstructions that specify how the MVS components from the distribution libraries have to be put together.

have a working MVS before it can create a new one. Since a working MVS is required to execute the macroinstructions. Sysgen is usually used to upgrade to a newer version or to make changes to the current version. For installations which does not have a MVS already. A small, limited function MVS system is setup that can execute the Sysgen for the complete full function MVSsystem.

The macroinstructions that Sysgen uses fall under two categories. The first category of macros defines the system hardware configuration. They are needed because MVS must know about every I/O device that is attached to the system. As a result for every I/O added, the system must be generated again.(Actually smaller, less time-consuming type of Sysgen called an iogen can be used to change the device configuration).

The second category of macroinstructions in a sysgen indicates which options of the operating system should be included. They indicate whether JES1 or JES2 is used, what optional access methods are installed, and so on. The output from a sysgen is a set of system libraries that among other things contains the executable code that makes up the operating system.

System Initialization

Once an MVS operating system has been generated, it can be used to control the operation of the computer system. To begin a system initialization, the system operator uses the system console to start an Initial Program Load, or IPL. That causes the computer system to clear its real storage and begin the process of loading MVS into storage from the system libraries. System initialization is a complicated process and the details are beyond the scope of this book. Just realize that when system initialization is complete, MVS is ready to process your work.

During system initialization, many options can be selected to affect how MVS will operate. In fact, the systems programmers and operators have more influence over MVS at initialization time than they do during sysgen. Initialization options come from one of two sources—the system operator or a special system library called SYS1.PARMLIB. By specifying options in SYS1.PARMLIB, MVS can be initialized with little operator intervention.

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