JCL Basics What is JCL? Job Entry Subsystem Job Processing JCL Statements JCL Parameters JOB Statement Job Statement Parameters for Job Positional Parameters ACCT NAME Keyword Parameters MSGLEVEL MSGCLASS TYPRUN PRTY CLASS RESTART REGION TIME NOTIFY EXEC Statement EXEC Statement Positional Parameters PGM PROC Keyword Parameters COND PARM ADDRSPC DD statement DD statement DD Parameters SPACE SYSIN VOL DCB SYSOUT DSN UNIT DISP |
Job Processing in JCL:
Involves the following five steps: • Job submission by user • Job conversion • Job execution • Processing the job’s output • Job purging Job submission • The JCL is created using a terminal. This JCL has to be first stored on the DASD. Then submitted to JES using the SUBMIT command. • User interaction ends here and job would get submitted to JES. • JES then copies the job and data from the DASD to another DASD file called the JES spool. Job Selection • The second step of job processing is job selection. It is done by scanning the job queue and selecting the job with the highest priority. • The component of JES that selects the jobs for execution is a program called the Initiator. • JES uses two parameters to specify job’s importance, JOBCLASS and PRIORITY. Both these are specified in the job's JCL. • JOBCLASS is more significant than job priority.• The job with the higher class will be executed first. • If two or more jobs have the same class, then JES selects the one with the higher priority.• If two or more jobs have the same class and priority then they are executed in the order in which they were submitted. Job execution in JES • Initiator picks up a job for execution. After the initiator selects a job for execution, it invokes a program called the Interpreter. • The job is then checked for Format errors. This is done by the interpreter. • Initiator invokes the allocation routines for memory allocation for program. It builds the user address space and executes and then invokes the de-allocation routines to de-allocate the memory used. Job purging from Spooler • The last step of job processing is job purging. After all the steps of the job have been executed, JES removes the job from the JES spool, thus freeing space for other jobs. This process is called purging. • When a job is purged from the system, all control blocks associated with the job are also deleted. |
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