mvs concepts lesson 1

8/11/2019 MVS Concepts Lesson 1

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Copyright Concept Solutions Corporation

MVS Concepts LessonHistory and Basics

by

Philip L. Yuson


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Learning Objective

By the end of the course, the learner will be able to:

1. Describe the general components of a computer system2. Define virtual storage

3. Describe why multitasking is important


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Basic Components of a Computer

ProcessorMemory

All computers have a processor, memory and ioutput devices.


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The input/output devices are

interfaces that the user uses tointeract with the processor

The data can be usedimmediately or stored and used

whenever it is needed.


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Processor

The processor is responsible

for controlling everything in thecomputer system.

It executes commands to

perform specific functions.




Memory

The memory contains data

that the processor uses



Mainframe Computer

A mainframe computer functions the same way as a basiccomputer.

It has a processor, it has memory and it also has lots ofinput and output devices.

Its processor may not be as simple as a basic

computer

Input output devices are a lot more complex than a

basic computer It is used in many organizations especially those that

require fast and large processing capacity.



Operating System

Some basic functions in a computer system can be repeated.

Examples of these will be reading or writing to an I/O device,

managing memory or managing the workload within a system.These functions are controlled by programs that directly

operate the processor and hardware. These programs are

called operating systems.

An operating system therefore is a set of programs that provide

common functions to other programs. It also controls how the

processor operates and manages tasks.The mainframe operating system has undergone several

iterations. It is currently called z/OS. It used to have several

versions but the more popular name is MVS which stands for

Multiple Virtual Storage.



Virtual Storage

ProcessorMemory

Remember this diagram? The memory is limited by itsphysical size.

In a PC, when you buy 4 GB of memory, that is the

physical size of your memory.



Virtual Storage

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4 GBReal Memory

263BVirtual StorageTransformed to



Virtual Storage

For our course, it is sufficient to say that the

operating system manages the resources thatallow you access to memory that is more than

your physical memory.

4 GBReal Memory

8 GVirtual StoragTransformed to


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This virtual storage is allocated between allthe tasks running in the system

Single Virtual Storage

Other operating systems

may be able to give you

virtual storage, but this

virtual storage is allocated

among all the tasks in the

system.

Let us say you have 1000

tasks running in your

system, and your system

gives you a virtual storageof 4GB, that 4GB is split

between the 1000 tasks

running.


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Multiple Virtual Storage

MVS does not only give

you virtual storage, itgives you multiple virtual

storage.

If you have 1000 jobs in

your system, each of

these jobs can

theoretically have 263 B of

virtual storage!

Each job has its own

address space.

4 GBVirtual Memory4 GB

Virtual Memory4 GB

Virtual Memory8 GB

Virtual Storage

THATS A LOT OF

STORAGE FOR ONJOB!!!


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Multitasking

MVS allows more than one program to execute

at any time. This feature is called multitasking.Multi-tasking is managed by the operating

system.

4 GBVirtual Memory4 GB

Virtual Memory4 GB

Virtual Memory32 GB

Virtual Storage


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Multitasking

This allows the computer to process more

than one program at the same time. On thePC, we see this happening. You may be

surfing the web and later, be typing an email

while your browser is still open.

On the mainframe, you can have several

programs running concurrently. One could be

managing your transactions, another could be

processing batch jobs while another could be

processing your transactions.

This maximizes the use of your computer

system.

ManageTransaction

Batch job

MVS Syst


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How MVS Manages Work

MVS manages work in Jobs. A job is a single unit of work. It

can be made up of several programs executed in sequence.

Jobs are controlled using job control cards. These cards are

coded in Job Control Language (JCL).

When working on the mainframe, you will often hear the term

JCL.

JCL is referred to the language used to create the job control

cards. It is also used to refer to the job control cards itself.

The subsystem that manages the entry and output of jobs is

called the Job Entry Subsystem (JES). The more popular job

entry subsystem is called JES2.

Job

JCL

JES


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Types of Jobs

There are several types of jobs in MVS. All these need JCLs to

execute:

1. Started Task (STC)a. These are tasks that are automatically started by MVS when

it starts up or are started by the operators from the MVS

console

2. TSO User (TSU)

a. These are jobs that control the interactive sessions of

developers. These are started when a user logs on the theMVS system.

3. Batch Job (JOB)

a. These are the regular jobs that are submitted by users

through TSO or internally from another job. These require

JES2 initiators to execute. These are often called batch

jobs.

S

T

B

J


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Job Control

In the early incarnations of MVS:

1. Input is done using punched cards through a physical cardreader.

2. These cards have 80 columns. This is why all JCLs are coded in

80 column records.

3. The cards specify:

a. The sequence of programs to be executed

b. conditions for executing the program

c. The files used by these programs

Today, there are no more physical card readers. JES reads these

cards through a logical reader.

Samp


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Job Entry

1. When a job is submitted:

a. JES checks the JCL for syntaxerrors

b. If there are no errors, the job is

queued in the input queue

2. Every batch jobs has a class and priority JES2 check forsyntax

JCL

JoQ

Submit to JES2


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JES Initiators

Initiators are controlled by JES2. These are basically system

started tasks that look at the input queue and process jobsbased on their class.

1. An initiator can process jobs in one or more classes.

2. When an initiator is available, it scans the input queue of a

specific class. If there are jobs within that class, the job

with the highest priority is executed.

3. If there are no jobs within the class, the initiator is left idle

and will wait until there is a job in the classes that itprocesses.

Pro

jobs

bas

clas


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Job Initiation

1. If an initiator finds a job in a class that it processes, it will

initiate the job

2. When it initiates the job, the initiator will check to make surethat all the files are not on hold. If the files are archived, the

initiator will send a request to recall these files so that the job

can use it.

3. The initiator sets up the environment for the step to execute.

a. Before it passes control to the program in the step, the

initiator will allocate the files used by the program.

i. If files exist and are consistent with what isspecified in the JCL, the initiator will proceed to

pass control to the program.

ii. If there is an error in the file specification, the

initiator will terminate processing of the job.

Job Input

Queue


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Job Execution

1. When control is passed to the program, the program will

run.2. There are three ways the program may end:

a. Normal End. This happens when the program has

no errors. It will end with a zero return code.

b. Error. This happens when the program has errors

that it can handle. This normally will cause a non-

zero return code.

c. Abnormal End (ABEND). This happens when aprogram encounters an error it cannot handle.

Sometimes, the system will cause an ABEND on

error conditions.

Nor

Erro

ABE


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Job Printout

1. In a normal situation, programs will print out reports. In

a PC environment, the reports may be printedimmediately since the printer is probably used by one

person.

2. In a multiple user environment, the reports are placed

in a SPOOL.

3. Reports are placed in a class within the SPOOL.

4. Printers are defined to process outputs from specific

classes in the SPOOL. Reports are printed once theprinters are available.

5. If a report is not printed, it remains in the SPOOL until it

is purged.

Program

SPOOL

Outpu

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