Multiple Virtual Storage (MVS)

First let us start with an insight to the topic Multiple Virtual Storage. MVS stands for Multiple Virtual Storage and this is the technology used mainly with IBM mainframe systems. MVS origin is interesting to know. The first developed technology was MVT which stands for Multitasking with a Variable number of Tasks which supported the multitasking facility.

Then this got improved to a technology called SVS also called as Single Virtual Storage in which several tasks share the same adddress space. From Single Virtual Storage only the so called Multiple Virtual Storage got developed. But in contrast to Single Vurtual Storage the Multiple Virtual Storage has the facility to store several tasks in different address space and hence the name. As the name suggests Multiple Virtual Storage have the feature to create limitless virtual storage. In other words with Multile Virtual Storage has the facility for huge number of address spaces to be created.

Having got an idea about Multiple Virtual Storage let us see what are the interfaces or communication media used with Multiple Virtual Storage.

Interfaces in Multiple Virtual Storage

The most popularly used interfaces for Multiple Virtual Storage are namely

JCL also called as Job Control Language
TSO which stands for Time Sharing Option
ISPF which stands for Interactive System productive Facility
CICS which is called as Customer Information Control System

Let us have a brief look on each of the Interfaces used in Multiple Virtual Storage

JCL: This is also called as Job Control Language. This is the interface that is used for batch programs and hence termed as batch interface. Job control language has statements in it to process a job.

TSO : This is the acronym of Time Sharing Option and this is a timesharing interface that can be used in interactive mode.

ISPF : This stands for Interactive System productive Facility . This has various menus to carry out functions in a interactive way and the functionalities are in similar line as that of Time Sharing Option.

CICS: This is acronym for Customer Information Control System. CICS is an online interface that runs on its own address space and this facility supports multiprogamming with its own address space.

Some of the functionalities performed by MVS are similar to that of programming technologies but the terms used differ in Multiple Virtual Storage. Let us see some of the terms used with Multiple Virtual Storage.

Data Set: Multiple Virtual Storage have concept of data set to store data which is similar to the usage of files in programming methodology. In other words dataset in Multiple Virtual Storage is collection of related data.Each dataset has number of members in it. That is each dataset has number of smaller units called records or members.

Organsataion of Data set in Multiple Virtual Storage

There are two ways of organisaing datasets in Multiple Virtual Storage. They are namely:
             Non-VSAM
             VSAM

VSAM: This is the acronym for Virtual Storage Access method. In this VSAM method there are three ways of organising data sets namely

Entry-Sequenced Data set :  This is also called ass ESDS. In this it is possible to access the records only in the same order as it was created.
Key-Sequenced Data set:  This is also called as KSDS. In this records can be accessed either sequentially or in a random order using key value.
Relative Record Data Set:  This called ass RRDS. In this records are accessed by identifying records relative to the number representing starting of the data set.

Non-VSAM: In this dataset are organiszed in the following four ways namely:

Physical Sequential:  This is similar to ESDS
Indexed seqeuential: This is same as KSDS
Direct: Here data sets members can be accessed randomly using member disk location
Partitioned: This is called as partitioned data set or PDS shortly. This PDS has number of subdivisions called as members.

Catolog: The data sets in Multiple Virtual Storage can be found in catalogs. Multiple Virtual Storage supports two types of catalogs namely
             Master Catalog
             User Catalog

Master Catalog has details that are required for the operating system to operate that is details like system files information. Because of the above reason each Multiple Virtual Storage system has only one master catalog. 
User catalog is one in which details about user data sets are present. Since there are number of users in Multiple Virtual Storage, MVS can have number of user catalog. Also each user catalog must be defined in master catalog.

VTOC : This is the acronym for Volume Table of Contents . If a user wants to list all dataset then Volume Table of Contents can be used in Multiple Virtual Storage. When VTOC is used it outputs the lists of datasets only and not the members present inside the dataset.

Task: This is the smallest unit of information that is supported by Multiple Virtual Storage. So Multiple Virtual Storage has the feature of handling multiple tasks.

Job: Execution of one or more related programs in order is called a job. Each program that gets executed in job is called job step. As seen above JCL is used to process a job.

Multiple Virtual Storage and Multiprogramming

Multiple Virtual Storage can support multiprogramming. Let us see how multiprogramming gets implemented using MVS. Each job which runs in background is assigned separate address space. And if a job wants to communicate with other job or in other words if a job wants to pass control to other job this is done in Multiple Virtual Storage by passing processors control to the address space of the other job. So by using address space concept the multiprogramming gets implemented in Multiple Virtual Storage.

There are many other facilities and features available in Multiple Virtual Storage (MVS) but the above gives an overview of MVS technology