Information Systems

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Index
• Index is a separate entity
• It is organized similar to data component (CI and CA)
• It has a different CI size
• VSAM builds index when data is loaded
• Index is organized as inverted binary tree
• VSAM compresses key to conserve space
• Can have several levels of indexes
• Lowest level of index is called ‘sequence set’. One sequence set for one CA. These are always in the key order whereas data CI need not be in key order. Each entry has the highest key (or “one” less than the first key of the next CI so that the sequence set need not be updated when higher key added to CI
• Next level is called ‘Index set’. It contains primary keys and pointers to the sequence set. More than one level of index set based on number of sequence set records. Top level index is always one record
CI and CA split
• CI split happens while adding new records or extending an old record and not enough room in the CI to complete the operation
• CI split may trigger a CA split
• Splits generally degrades performance
• Specify free space to reduce CI and CA splits
CI size
• VSAM can select an efficient size if you ignore
• Large CI size for batch sequential processing
• Small CI for random processing
• All are done automatically by VSAM

Cluster
• It is a collection of physical data sets that makeup one logical data set
• The concept of a cluster is more suited for a KSDS
• A KSDS cluster has two data sets. One holds actual data records and other contains an index component
• Index component permits direct retrieval of data
• With an ESDS and RRDS and an LDS, the cluster name and data set component name both refer to same data set and a cluster consists of only a single physical data set, the data component
Control Interval (CI)
• A Control interval is the amount of data transferred between device and virtual storage
• When a record is read from or written to a data set, VSAM groups individual data records into larger units of storage. These units of storage are called CI
• Size of CI should be minimum 512 bytes and increased by multiples of 512 upto a limit of 8K. beyond that it increments should be 2K upto a maximum of 32K
CI has 4 components
• Data – contains the actual data processed by the program
• CIDF – contains information about the free space within the CI. Its of 4 bytes length
• RDS – contains information about the records within the data space. It is a 3 bye field that is used to define the location and length of a record or a group of records. RDFs immediately precede the CIDF, at he end of control interval
• Freespace – These are the remaining unused space

Entry sequential data set (ESDS)
• Records in an ESDS are stored in the order in which they are written and are retrieved by addressed access
• Records are loaded irrespective of their contents and their byte address cannot be changed
• ESDS is also referred to as a sequential VSAM data set. This is because records in an ESDS are normally processed sequentially
• ESDS is best suited for applications where most processing is done sequentially
Key Sequential data set (KSDS)
• Records in an KSDS are stored in key sequence and are controlled by an index, determine the order in which records are stored
• In a KSDS, records can be processed both sequentially and randomly using their key field values
• Advantages – sequential processing in useful retrieval of records in sorted form
• Random or direct processing of records useful in online applications
Relative record data set (RRDS)
• Records in an RRDS are loaded into fixed-length or variable length slots
• These records are represented by relative record numbers (RRNs) of their slots
• A processing program uses RRN to provide random access
Linear data set (LDS)
• Data set containing contiguous string of data bytes with no intervening control information E.g., Source control data set (SCDS), Active Control data set (ACDS), Communication data set (COMMDS)
• An LDS is divided into blocks. These blocks can be sequentially retrieved by a processing program in physical order
• Processing program can group several logical records together in to a single block
• LDS can be kept permanently in store for enhanced performance

VSAM is a high-performance access method used in OS/VS & ZOS operating systems. VSAM software resides in virtual storage along with the program that needs its services for the manipulation of data on a direct access storage device (DASD).
VSAM acts an interface between Operating system and application program
The access method is used to organize data and maintain information about the data in a catalog
Catalog management: The catalog contains information about data sets
Record management: VSAM data sets can be defined and catalogs can be established using a multifunction services program called AMS (Access Method Services)
• VSAM is Virtual Storage Access Method
• VSAM is a high-performance access method used in OS/VS and ZOS operating systems
• It is a method used to move data between disk and main storage
• VSAM operates in virtual environment
• VSAM acts as interface between operating system and application program
• All VSAM data sets reside on DASD devices
• All data sets must be defined in a catalog
VSAM does more than just replace non-VSAM access methods with VSAM access methods
• It also provides efficiency improvements and a comprehensive catalog facility that centralizes information about all VSAM data sets
• In addition, the multi-functional utility program – AMS – has a variety of file related functions for VSAM as well as non-VSAM files

Instream procedures
• Completely contained within a JCL JOB
• String of JCL statements appearing between a proc and pend statement within a JOB
• Available to only one JOB
• Upto 15 procedures can be defined within a JOB
• Within a JOB, can be invoked any number of times

Cataloged Procedures
• A procedure that you catalog in a library is called a cataloged procedure
• A cataloged procedure may consist of these JCL statements: CNTL, command, DD, ENDCNTL, EXEC, IF/THEN/ELSE/ENDIF, INCLUDE, OUTPUT JCL, and SET. Optionally, a cataloged procedure can begin with a PROC statement and end with a PEND statement. If coded, PROC must be the first statement in the procedure

Symbolic parameters
• A symbolic parameter is a symbol preceded by an ampersand that stands for a parameter, sub parameter or value
• Use it for values that can change with each execution of the procedure
• Makes the procedure more flexible and general purpose
• Can be 1-7 characters, alphanumeric or national, preceded by and a keyword parameter that can be coded on the EXEC statement like time, cond, parm cannot be a symbolic parameter
• Default values can be specified for a symbolic parameter in the operand field of the proc statement
Overriding parameters
• To override a parameter on the EXEC statement, when invoking the proc, follow on the proc statement with: PARAMETER.PROC STEPNAME=VALUE

Overriding DD statements
• Recode the DD statement with the DD name proc stepname. DD name
• No need to recode the entire DD statement. Just code the changes.
• If overriding more than one DD statement, put the overriding statements in the same order as they appear in the procedure
• To add new DD statements, put them after all the overriding statements

JOBLIB
• Used to identify a program library to search first when attempting to locate programs executed during job’s life
• Must be placed after the JOB statement and before the first EXEC statement in the JOB
• More than one program library can be concatenated after the first one on a joblib

JCLIB
• JCLLIB is used only on MVS/ESA version 4 JES2 systems or higher
• Used to identify private library or a system library from which include groups and JCL procedure are to be retrieved.
• Only one JCLLIB statement is permitted in a JOB, it must appear after the JOB statement and before the first EXEC statement in the job

STEPLIB
• The steplib DD statement is used to identify a program library to search when attempting to locate programs executed during the JOB step
• The steplib can be placed anywhere in the step’s JCL

JOBCAT
• The Jobcat DD statement is used to identify a VSAM catalog to search first when attempting to locate cataloged data sets during the job’s execution
• Jobcat must be placed after the JOB statement and before the first EXEC statement in the job

STEPCAT
• The stepcat DD statement is used to identify a VSAM catalog to search first when attempting to locate cataloged data sets while the step is executing
• The stepcat can be placed anywhere after the EXEC statement in the JOB step’s JCL

Job Control Language or JCL is a set of control statements that provide the specifications necessary to process a job. Communication with the OS is by typing commands or Job Control Language statements.
Role of JCL
We do not use JCL to write computer programs. Instead, it consists of control statements that introduce a computer job to the operating system, provide accounting information, direct the operating system on what is to be done, request hardware devices, and execute the job.
JCL tells the operating system everything it needs to know about a job’s (I/O) requirements. We code the JECL (Job Entry Control Language) to specify on which network computer to run the job, when to run the job, where to send the resulting output. IBM provides two job entry systems for Z/Os: JES2 for decentralized control and JES3 for highly centralized control of several computers.
Purpose of using JCL
JCL functions are
• Identifies users which is essential for security
• Identify programs
• Specify device requirements
• Run-time intervention
There are 3 basic JCL statements in every job – JOB, EXEC, and DD. The first statement in every job is a JOB statement to furnish necessary data to identify the job. EXEC statement has to be coded to indicate the name of the program to be executed. Within each step, DD statements have to be coded for every file that is to be processed by the program
Format:
Identifier[name][operation][operand][comments]

Z/Os architecture includes all of ESA/390 and the following extensions
• IBM’s high-end server OS
• Robust IBM e-server Z series mainframe OS for e-business
• Highly secure scalable high performance base to deploy internet + java technology-enabled applications
• Z/OS takes advantage of latest open s/w technologies like EJB/XML/HTML/UNICODE/IP network etc.
• Cryptographic services and distributed print services, storage management, and parallel sysplex availability
• 64 bit general registers and control registers
• 64 bit addressing mode in addition to 24 bit and 31 bit addressing modes of ESA/390 which are carried forward to Z/OS architecture
• The program status word (PSW) is expanded to 16 bytes to contain larger instruction address
• Upto three levels of DAT tables called Region Tables for translating 64 bit virtual address
• Trimodal addressing as against bimodal addressing
• Ability to switch between 24, 31, and 31 bit addressing
Logically a system consists of the main storage, one or more CPUs and channel subsystems.
Addressing: Absolute, real and virtual
Address translation converts virtual addressing to real addressing and prefixes converts real addressing to absolute addressing
A 24 bit or 31 bit virtual address is expanded to 64 bits by appending 40 or 33 zeroes on the left before it is translated by means of DAT process and 24 and 31 bit absolute address is expanded to 64 bits before its is transformed by prefixing. A 24 or 31 bit absolute address is expanded to 64 bits before main storage is accessed

Disk operating systems/360
• Single task (only one program at a time)
• Simple OS, with minimum storage and CPU requirements
OS/360 for system/360
• PCP (Primary control program) ran only one task at a time
• Multiple fixed tasks – up to 15 tasks, with fixed time-slices for each task
• Partitions have to be configured before start-up
MVS (Multiple virtual storage) for system/370 (originally OS/VS2)
• Multiple variable tasks – number of tasks, and their time-slices can change dynamically
• Each user/task can have virtually al the memory of 16MB
• This concept of multiple users/tasks using the virtual memory is called MVS
• This brought in the concept of address space – area where the program runs
MVS/XA (extended architecture) for system 370/XA
• Each user/task can have virtually all the memory of 2GB
• Concept of data spaces – areas for storing huge files in memory
MVS/ESA (Enterprise system architecture) for system 370/ESA and system 390/ESA
• Uses hyperspace – data buffering to avoid file I/O
• Huge files are stored in memory, and written to disk only when not needed
Z/OS architecture is an evolutionary upgrade of 390/ESA architecture
• It is IBM’s high end server OS
• Robust IBM e-server Z-series mainframe OS for e-business
• Highly secure, scalable, high performance base to deploy
• Internet + Java technology-enabled applications