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MVS

What is an Operating System? What are its functions? An operating system consists of two sets of programs designed to facilitate

the operation of computer. The 1st set called the control programs are used to schedule and supervise work done by computer and the 2nd set called processing programs consisting of language translators, service programs, and utility programs. The heart of the OS is the Resident Supervisor module, which acts as the main control module. The functions of OS are (1) To manage various devices, (2) Control program execution, (3) Interpreting and executing the commands entered at the prompt, (4) Maximize throughput and minimize turnaround time resulting in increase of efficiency. The control programs can't be accessed directly by users. Instead the user codes JCL statements that provides the necessary specifications to process a Job. The set of cards punched on JCL statements are referred as Program control cards.

What are the functions of the major components of the Computer? (1) Central Processing Unit is used to execute all the instructions of the

system. (2) Main Storage unit is used for storing the data and programs, while they are being processed. (3) Programs and Data are permanently stored in Auxiliary Storage such as magnetic tape or magnetic disks called DASD. (4) Card readers, Punches and printers are called as Unit Record Devices. (5) The Unit Record devices located at far away distances and connected to the Computer System by communication lines make a Remote Job Entry. (6) The RJE can be replaced by terminals at remote locations by a display screen and keyboard. (7) Multiple terminals using the same System at the same time is known as Time sharing. (8) A device such as an RJE or a terminal, which can be used to submit programs and data and receive the results are called as work stations.

Explain Virtual Storage Operating System. Virtual Storage is a technique that lets the system processor to simulate a

large amount of Main (Real) Storage, which actually has a smaller amount of real storage. To do so the Computer uses the Auxiliary Storage (DASD) as an extension of the Real storage. VS OS is used to efficiently use the Virtual Storage technique, by well controlled Paging and Swapping process by the System, which is transparent to the user. An address space can provide the entire 16M range of address for 24-bit processing and 2G range for XA mode using the 31-bit processing, for each user under MVS. MVS not only simulates the main (real) storage than is actually present, but also uses the (main) real storage to simulate multiple address spaces, each of which are independent of the other.

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What is meant by Paging and Swapping? Each 4K section of VS are called as Pages. 4K section of Main storage are

called as Page Frames. DASD areas of VS of 4k size are called as Page Slots. If a required storage location for a program is not in the Main (real) storage, a Page fault occurs. Data is then transferred from auxiliary storage to main storage is called as Page In, vice-versa is Page out process. Either way of processing Page-in and Page-out is called as Paging. The transfer of entire address space from Main storage to auxiliary storage to provide space for other program process is called as Swapping. Swapped out pages are stored on special datasets on DASD called Swap datasets.

Explain Multiprogramming. The concept of more than one program running at the same time on a

Computer system is called as multi-programming. The system does it by simply reclaiming the CPU during the idle period of a processor to let other programs use it. It is also transparent to users.

Explain Spooling. It is managed by JES, and is the process used to provide shared access to

devices such as printers, which can not be accessed by multiple programs at the same time. It manages the applications output by interpreting it and directing it to a DASD and send it for processing by that device when ever it is free.

Explain the use of file directory, Control Interval and Control Area under MVS. File directory maintains the Relative Block Address and Physical Block

address of a file. CI is the unit of data movement between Main and Auxiliary storage's (equal to page size). Control area is a fixed length area on the DASD aside by VSAM to accommodate the CI for a particular file, it's the unit of file allocation during the file extension.

Explain the differences between VSAM and Non-VSAM files. (i) The ISPF panel or a standard JCL is used for creation of a NON VSAM

dataset. The AMS program is used to create the VSAM files. (ii) The information about the Non-VSAM files is stored as dataset labels in the VTOC, while the information of VSAM files is stored in Catalogs. (iii)The dataset organization's in Non-VSAM files are sequential (physical), Indexed Sequential, Direct and partitioned. The dataset organization's in VSAM files are Key sequenced, Entry sequenced and Relative record datasets. A dataset is a named collection of physical records stored on a disk or tape, and are normally identified by MVS with special records called labels. In JCL statements we need to supply the label information.

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What are the Features of MVS. (i) It is a Virtual Storage Operating system that support uni-processor, multi-

processor, and attached processors. (ii) It is provided with integrated Time Sharing facilities with individual address spaces. (iii) Provides System resource manager for dynamic work load management. (iv) Provides Virtual I/O for temporary datasets. (v) Functional recovery routine for system component. (vi) Provides cross memory services for inter-address space communication , that reduces the assigned for common virtual storage and sub-system overhead.

Explain VTOC and the various catalogs used to store information about the

VSAM and Non-VSAM files. A. Volume table of Contents is a special file that contains the File Labels of the datasets on the volumes, also called as Dataset Control Blocks. The volume label is always in the same place of the disk, the third record of Track 0 on Cylinder 0, containing the volser id number and disk address of VTOC. Types of catalogs are OS catalogs or CVOLS and VSAM catalogs and Integrated catalog facility catalogs, they can be further classified as Master and User catalog. An access method is an interface between application program and physical operation of storage device. To issue a Open instruction to a dataset, appropriate methods are to be used. For Non-VSAM files the connection is made through Data Control Blocks and VSAM files through Access Method Control blocks. Both are simply tables in storage that contain vital information of a dataset as it is processed.

Explain Batch Process. The processing involved, when a series of commands are to be processed

together is called batch processing. A JCL describes a job by providing information that identifies programs to be executed and data to be processed.

What is meant by Structured Programming? Structured programming means the collection of principles and practices that

are directed towards developing a program, which are easy to understand and maintain. The programs are build by using three structures - Simple (imperative statements), IF - THEN - ELSE and DO-WHILE (through PERFORM in COBOL) structures.

What are the various steps in Programming process? A. Task Analysis, Flow charting, Program Coding, Program testing, Documentation of program and finally preparation for production run.