University of Phoenix's Virtual Instructor's Manual

Introduction to Operating Systems

Section Three   Mainframe Operating Systems

  1. List of Objectives

This section begins with the 1964 announcement of IBM System/360 to the development of a single operating system for a planned family of computers. The original OS/360 was a multiprogramming batch system and it led the way to later versions including MVT (Multiprogramming with a Variable number of Tasks) which was released in 1969 and OS/SVS (Single Virtual Storage) introduced in 1972. Both of these operating systems helped to advance the concepts of virtual memory and job scheduling.

In response to the growth of computer program code to accommodate larger, more complex applications, IBM introduced MVS. MVS provided programmers with 16MB per job. That is, each job has its own dedicated virtual memory of 16MB. MVS maps blocks of virtual memory into real main memory and is able to keep track of the separate virtual memories for each job.

Learn about the following four most important factors that have determined the design of MVS:

Learn how MVS implements a multitasking structure and identify the following key components:

Define and discuss MVS facilities for enforcing mutual exclusion with respect to the use of resources: enqueing and locking. Enqueing is concerned with user-controlled resources such as files, whereas locking is concerned with MVS system resources. Section One discussed the phenomena of deadlocks; MVS locking provides a strategy to prevent this condition.

MVS is designed to provide a layered I/O facility that allows programmers to ignore the many details of I/O operations or to bypass or add to some phases of the I/O operation. Use Figure 3-7 to illustrate the logical structure of I/O within MVS. Take some time to follow the nine steps of the typical MVS I/O sequence.

  1. Chapter Summary

Describe how MVS is stored in virtual memory and how the contents of virtual memory are physically stored in real memory and on an external paging device. Review the complexity of concurrently tracking the contents of both types of memory. Fortunately, we can study the virtual memory model as though it were real, and that simplifies understanding the operating system.

Learn the difference between a job and a task. Define the functions performed by the various job management modules, and review the basic control blocks. Go through the example in the text (Figure 3-1).

In Section One, multiprogramming was introduced. Now use your basic knowledge to add details that are based upon MVS techniques.

3. Short Essay Questions

    1. Distinguish between job management and task management.

      2. Answer: Job management is concerned with job-to-job and task-to-task transitions. Task management supports programs as they run by handling interrupts.

    2. What are the functions of task management?

      3. Answer: Task management supports tasks as they are executed by handling interrupts.

    3. MVS is designed to be a general-purpose operating system. What does this mean?

      4. Answer: It is intended to handle a variety of different tasks.

    4. Briefly describe IBM’s strategy for the MVS operating system.

      5. Answer: This section provides a brief history of MVS. Have students relate the details of how MVS evolved from the System/360’s OS/360.

    5. Use Figure 3.1 to identify and define the building blocks of the MVS Operating System. If necessary use the Internet to locate IBM’s product descriptions of MVS.

      6. Answer: Students can refer to Figure 3.1 to review the basic structure of MVS. Encourage their use of the Internet to research IBM and other industry reports on the functionality that is built-in to the various structures shown in the diagram.

    6. What do we mean by the term access methods in MVS operations?

      7. Answer: Access Methods provide users of MVS with a range of access options in file, database control and telecommunications. Have students research, at a minimum, the VTAM and ISAM options.

    7. Relate the terms address space control block (ASCB) and address space extension (ASXB).

      8. Answer: These two MVS structures collectively encompass much of what has been referred to in Section One as process control blocks. Have students review Table 3.1 and any other valuable resource to relate ASCB and ASXB to the management of address space in MVS.

    8. MVS is designed to provide a layered I/O facility that allows programmers to ignore the many details of I/O operations or to bypass or add to some phases of the I/O operation. Use Figure 3.7 to explain a typical I/O sequence of operations.

      9. Answer: Pages 139–141 provide a typical sequence of I/O steps. These steps are graphically illustrated in Figure 3.7.

    9. Use the Internet to research the Virtual Telecommunications Access Method (VTAM). Provide a brief report on the features and functions of VTAM. Describe how VTAM would serve organizations interested in providing access to a corporate database.

    Answer: Students should visit IBM’s web site or other related material to produce a brief report on VTAM.

    1.10 Discuss the role of the Indexed Sequential Access Method (ISAM) in MVS.

    Answer:

    In ISAM, one or more fields, called keys, in a record uniquely identify the record. Records can be accessed either directly, by supplying a key, or sequentially in key order.

    1.11 Discuss the role of the MVS ENQUE Facility. How are users affected by ENQUE operations?

    Answer:

    The ENQUE facility is used to regulate access to shared data resources. Table 3.2 indicates the rules used by MVS to determine the action when an ENQUE requested is issued.

    1.12 Relate the MVS use of task control block (TCB) to Section One in this text and knowledge you gained there about the function of a control block.

    Answer: This is an excellent exercise to help students begin to link fundamental information in Section One with real-world task control structures (in MVS).

    1. Testbank
    1. Under MVS, ________ management is responsible for handling interrupts.
    1. job
    2. data
    3. task
    4. all of the above
    1. Under MVS, the ____________ is the dispatcher.
    1. master scheduler
    2. job entry subsystem
    3. supervisor
    4. initiator
    1. Under MVS, tasks are loaded into memory by the ________.
    1. initiator/terminator
    2. master scheduler
    3. job control program
    4. job entry system
    1. Under MVS, task control blocks are physically stored in the communication vector table. True/False?
    2. Under MVS, a complete list of all I/O devices attached to the system is maintained in the IOB. True/False?
    3. A task is a single program or routine that has been loaded into memory and thus is available to run. True/False?
    4. The master scheduler is the MVS scheduler. True/False?
    5. VTAM is used in conjunction with telecommunications architecture known as SNA. True/False?